Arxiu de la categoria: BASIC BIOLOGICAL TERMS

Beyond red: the color of blood

There are people who remember with great impact the first time they saw their own blood. Even in adulthood and in controlled conditions (for example, during an extraction in a medical center) the vision of the red fluid is not always pleasant. Sometimes more intense, sometimes darker, but always red… or not? Do you know if there are animals with blue, green or maybe yellow blood? Keep reading to find out.


We are used to the color of blood being red, since it is the color of our blood and many vertebrates, like all mammals. The color of the blood is due to respiratory pigments, those responsible for transporting oxygen to cells throughout the body and carbon dioxide to the lungs. As you may remember, the human respiratory pigment is hemoglobin, which is found in red blood cells or erythrocytes.

But other animals have respiratory pigments other than hemoglobin, which endow their blood with colors as varied as green, blue, yellow and even purple.

glóbulos rojos, sangre, eritrocitos, hematíes
Human red blood cells (erythrocytes) seen under the electron microscope. Photo: John Kalekos


As mentioned, the respiratory pigment of mammals and many other vertebrates is hemoglobin, a protein. In its molecular structure, hemoglobin is formed by 4 subunits (called globins) linked to a heme group. The heme group has a central iron atom (iron II) that is responsible for the red color.

sangre color rojo hemoblogina molécula
Representation of the structure of hemoglobin. We can see the globins joined to their corresponding heme group, and an enlargement of the heme group with the iron (II) atom at its center. Picture: Buzzle

The hue of red may vary, depending on how oxygenated hemoglobin is. When it is attached to oxygen (O2), it is called oxyhemoglobin and its color is an intense light red (arterial blood). In contrast, deoxyhemoglobin is the name given to reduced hemoglobin, that is, when it has lost oxygen and has a darker color (venous blood). If hemoglobin is more oxygenated than normal it is called methemoglobin and has a red-brown hue. This is due to the intake of some medications or a congenital disease (methemoglobinemia).

sangre venosa, sangre arterial, rojo intenso, rojo oscuro, color
Color hue difference between venous blood (upper syringes) and arterial blood (lower syringes). Photo: Wesalius

As we have seen, deoxygenated blood is not blue. The blue tone that we see in our veins is due to an optical effect resulting from the interaction between the blood and the tissue that lines the veins.


Some animals, on the other hand, do have blue blood: decapod crustaceans, some spiders and scorpions, horsehoe crabs, cephalopods and other molluscs. When dealing with invertebrates, we must specify that instead of blood its internal liquid is called hemolymph, but in this post we will not distinguish hemolymph from blood for better understanding.

cangrejo herradura sangre azul xfosuro
Ventral view of a wounded horsehoe crab, in which its blue blood can be observed. Photo: Dan Century

The pigment responsible for the blue color of blood in these animals is hemocyanin. Its structure is quite different from that of hemoglobin, and instead of iron, it has a copper (I) atom at its center. When hemocyanin is oxygenated, it is blue, but when it is deoxygenated it is colorless.

molécula hemocianina
Chemical structure of oxygenated hemocyanin. Picture: Chemthulhu


There are some animals with green blood, such as some annellids (worms), some leeches and some marine worms. Its respiratory pigment, called chlorocruorine, gives its blood a light greenish color when it is deoxygenated, and a little darker when it is oxygenated. Structurally, it is very similar to hemoglobin, since it also has an iron atom at its center. Unlike hemoglobin, it is not found in any cell, but floats in the blood plasma.

molécula clorocruorina
Chemical structure of chlorocruorine. Public domain image


sangre color verde
Tube containing green blood from a New Guinea lizard. Photo: Christopher Austin

In the case of vertebrates with green blood (like some New Guinea lizards), the color is due to biliverdin, which results from the degradation of hemoglobin. Biliverdin is toxic, but these lizards are able to withstand high levels in their body. In the rest of vertebrates, if biliverdin levels are high because the liver can not degrade it to bilirubin, they cause jaundice, a disease that gives a yellowish color to the skin and corneas of the eyes. But in species of lizards like Prasinohaema prehensicauda, the high presence of biliverdin could protect them against malaria, according to some research.

lagarto nueva guinea sangre verde
Species of New Guinea lizard with green blood. Photo: Christopher Austin


Tunicates (fixed ascidians) are a type of animals with yellow/yellow-green blood. The pigment responsible for this color is hemovanabine, a vanadium-containing protein, although it not transport oxygen, so its function remains unknown. Similarly, the yellowish, yellow-green and even orange color of the blood (hemolymph) of some insects is not due to the presence of a respiratory pigment, but to other dissolved substances that do not carry oxygen.

Tunicate (Didemnum molle) in Sulawesi, Indonesia. Photo: Bernard Dupont


Some marine invertebrates have violet blood (hemolymph), such as priapulids, sipunculides, brachiopods and some annelids.

priapulida hemeritrina
Priapulus caudatus, a priapulid. Photo: Shunkina Ksenia

The responsible respiratory pigment is hemeritrin, which turns violet-rosacea when it is oxygenated. In its deoxygenated form it is colorless. Like the rest of the respiratory pigments we have seen, hemeritrin is less efficient than hemoglobin when transporting oxygen.

hemeritrina color sangre violeta
Chemical structure of hemeritrin in its oxygenated form. Like hemoglobin, the central element is iron II


Finally, there is a family of fish called crocodile icefish whose blood is transparent. Actually, these are the only vertebrates that have lost hemoglobin. Similarly, erythrocytes are usually absent or dysfunctional. This strange anatomy is because they live in very oxygenated waters and their metabolism is very slow. In order for oxygen to reach all cells, it dissolves in the blood plasma, which distributes it throughout the body.

pez de hielo draco sangre color transparente
Crocodile Icefish (Chionodraco hamatus). Photo: Marrabbio2


To conclude, we have seen that in animals that require a respiratory pigment to deliver oxygen to all tissues, the color of blood (or hemolymph) will depend on the type of pigment that is present. In contrast, other animals that do not require respiratory pigments, have transparent blood or their color is due to other dissolved substances that have nothing to do with breathing.

infografía colores de la sangre
Infographic-summary of the chemistry of the main blood or hemolymphatic respiratory pigments (click to enlarge). Image: compound interest


Cover photo: John Kalekos


Where do names of species come from?

All known living beings have names that allow us to recognize and classify them. However, only scientific names are valid for scientific purposes. Who does assign these names? Has it been always done the same way? And the most important: is there any rule when assigning a name to an organism?

Previously in All you need is Biology, we talked you about classification and phylogeny of organisms. Now, we bring you the answers to all these questions about nomenclature and taxonomy. Keep reading and you will discover some curiosities!  

The value of scientific names

If someone asks us what a dog or a cat is, of course all of us will know the answer. However, these names are not useful from a scientific point of view (despite biologist use them assiduously), especially when making studies and publishing papers. Common names (such as ‘dog’ or ‘cat’) are not constant: every language, every country, even every region, has their own terms to refer to their organisms. Even sometimes they change through time or are used to appoint different organisms (e.g. the red panda, which is near to mustelids, and the giant panda, a bear, don’t belong to the same family despite being called pandas).

As you see, using only common names in science could put you in trouble. If someone publish that has performed a study about reproduction of macaw populations, we could not know of which species they are talking about; the common name of this bird varies among some countries; moreover, there exist different species of macaws. Thus, the study does not make sense without context.

So, the use of scientific names in science is very important: they are constant worldwide (we avoid translation problems) and refer only to one organism with no ambiguity.

Currently, designation of scientific names follows the binomial nomenclature, that is, a scientific name of a species is composed by two terms: the genus (an upper level of classification than the species) and the specific epithet or name (and not the species, as some people tend to confuse). While the first term has validity by its own, the second one only has it if is preceded by the genus.

Thus, and keeping with the example above, the macaws from this study actually belong to the genus Ara, but there are different related species of macaws belonging this genus (Ara ararauna, Ara glaucogularis, Ara militaris…).

Macaw of the species Ara ararauna. Picture by Ralph Daily, CC.

However, how has the way biologists assign names changed through the time?

Linné, the father of binomial nomenclature

For a long time, biologists have tried to classify and give names to every living being they discover. The science of defining and naming groups of organisms according to their shared features is known as taxonomy.

In the beginning, there was not a clear consensus for naming the species. For the first ‘taxonomists’ it was of a big importance to classify and identify poisonous and medical plants, of which there are ancient documents wrote by Egyptians more than 3000 years ago.

The first person who started to formally classify organisms was Aristotle (384-322 AC). He was the first to differentiate between animals and plants, besides starting the classification of organisms according to their ‘parts’: four legs, warm body, etc.

During the Middle Age and the beginning of the Modern Age, most of scientists followed the Aristotle’s system of classification. Thanks to the improving of observing tools, such as the development of the first optical lenses during XVI and XVII centuries, some biologists started to improve their descriptions, and eventually abandoned this system.

However, among taxonomist still didn’t exist a formal consensus for assigning names. Before the instauration of the binomial system, species were named with a term (the genus) followed by a specific epithet or name composed by one or more words which described the species. This system, known as polynomial system, gave room to really long names such as ‘Plantago foliis ovato-lanceolatus pubescentibus, spica cylindrica, scapo tereti‘. Of course, this was not an optimum system.

During XVI and XVII centuries, Caspar Bauhin made the firt steps to simplify this system, sometimes shortening species names to just two terms. However, it was the Swedish botanical Carl von Linné (or Carolus Linnaeus) who formalized the use of the binomial nomenclature in his publication Species Plantarum (1753). Since then, species were given a name composed only by two terms: the genus and a trivial name designated by its descriptor; e.g., Panthera tigris (tiger).

Carl von Linné. Public domain.

The establishment of this system was favored by three reasons:

  • Its economy: there are needed only two words to identify a species with no error.
  • Its diffusion and general use by scientists, who standardize them and promote their use.
  • Its stability: scientists try to preserve the original name of an organism even if its classification changes through time.

How to name an organism: the nomenclature codes

Taxonomy and nomenclature are two different but inseparable concepts. While taxonomy is the science of describing and classifying organisms, nomenclature is the tool that allows taxonomists to assign names to those organisms.

In 1758, Linné stated the basis for an objective classification of species in the 10th edition of one of his most famous publications, Sistema Naturae:

  • Each species must have an own scientific name, unique and universal.
  • When a species is given more than one name by different scientists, the oldest one must prevail.
  • Scientific names are composed by two Latin or Greek terms: the first one corresponds to the genus and the second one, to the species belonging this genus.
  • The first letter of the genus must be written in upper case, while the specific epithet or names must be written in lower case. Moreover, both terms must be written in italic or underlined.
Cover of the 10th edition of Sistema Naturae. Public Domain.

Nomenclature has been getting more and more complex over the years. Nowadays, there are international codes of nomenclature for every group of organisms, like the ICZN (International Code of Zoological Nomenclature) or the ICN (International Code of Nomenclature for algae, fungi, and plants), amongst others. Taxonomist from each branch must obey their own codes when naming an organism.

Two of the most important rules when giving a name are the validity and the availability of the name. Let’s imagine we discover a new species of wasp of the genus Polistes: in one hand, the name (Polistes x) must be available, that is, it must accomplish the needed requirements to be assigned to our species. These requirements are gathered in the international codes, which are based on the Linné’s criteria. Moreover, a name is available when it is accompanied by a formal (published) description. Availability of a name can change under certain circumstances; e.g., a name considered unavailable can be available again if is republished following the code’s criteria.

In the other hand, a name must be valid, that is, it must have not been used to designate another organism, or considered invalid. For example, two taxonomists one before the other describe the same species and give it different names; in this case, the valid name would be the oldest one, so the second one would become a junior synonym according to the priority principle, thus getting invalid for its use.

When giving names gets out of hand…or not

Usually, when giving name to a species taxonomists get inspired by specific features of the organism (Dosidicus gigas (giant squid)), its native location (Synergus mexicanus (gall wasp from Mexico)) or in honor to relatives or other scientists.

However, nomenclatural world is full of curiosities, from scientists that give extravagant names to their species to the ones that get inspired by their favorite characters or TV shows:

  • There exists a genus of moths called La (by Bleszynski, 1966). Its ambiguity with the feminine article ‘La’ in Spanish (‘the’ in English) makes search engines go crazy. Moreover, some of the species belonging this genus were given names like La cerveza, La cucaracha or La paloma (literally, ‘The beer’, ‘The cockroach’ and ‘The dove’ in Spanish, respectively).
  • While some taxonomists give species short names, others prefer them longer: Gammaracanthuskytodermogammarus, Rhodophthalmokytodermogammarus and Siemienkiewicziechinogammarus are genera of amphipods from the Baikal lake given by the naturalist Dybowski. For sure he had much fun with this!
  • During a long time, it was a common practice to use specific epithets and names to insult other scientists (e.g. stupidus). Fortunately, this is currently prohibited.
  • Abra cadabra, Aha ha, Attenborosaurus (dinosaur genus given after the naturalist David Attenborough), Acledra nazgul, Desmia mordor (in honor to the Lord of the Rings), amongst others.

It is important to note that the international codes try to avoid this kind of names; but it is still funny! If you haven’t had enough, take a look to this list. It will not disappoint you!

.           .           .

Do you still think naming an organism is an easy task?


Main picture property of Irene Lobato Vila (author of this post) took at the Smithsonian’s National Museum of Natural History (Washington D.C., EUA).

Biology and extraterrestrial life

Frequently we can read on the news newly discovered planets that could harbor extraterrestrial life. Often we have new information about Mars, other worlds with water and extremely resistant living beings, like tardigrades. But is life possible outside the Earth? What is life? What is needed to sustain life? Astrobiology tries to answer this questions. Do you want to find out more?


Astrobiology is a set of different scientific disciplines that studies the existence of life in the universe. To achieve this it combines knowledge of biology, physics, chemistry, astronomy, ecology, geography, geology, planetary science and molecular biology. Within astrobiology, exobiology studies the possibilities of life outside our planet. It should not be confused with ufology, a pseudoscience. Astrobiology tries to answer such exciting questions as:
– What is life?
– How did life appear on Earth?
– How does life evolve, and what is its adaptability?
– What is the future of life on Earth and other places?
– Is there life in other worlds?

No, neither is this a Martian nor is it astrobiology. Source: Quo


Although it seems like a banal question, life is not easy to define. Apparently, we can recognize if something is alive or not if it can perform certain functions and has certain features. Living beings have vital functions:

  • Nutrition: they can obtain energy from the environment to grow, survive and reproduce.
  • Reproduction: they can create copies similar to themselves.
  • Interaction: they can perceive what is going on the environment and inside themselves.
  • Organization: living beings are formed by one or more cells
  • Variation: variability between individuals allows species to evolve.

Problems begin when with beings that don’t have all the characteristics. The most classic example would be viruses: they are unable to reproduce on their own and lack cellular structure. Another example would be erythrocytes (red blood cells) of mammals, cells without genetic material or mitochondria.

Microphotography of the Ebola virus under electronic microscope (Public photo of the CDC)


We only know one type of life: the terrestrial one. This is why astrobiologists need to take it as a reference to know what to look for elsewhere. Could there be other forms of life different than terrestrial? Maybe, but it would be almost impossible to recognize them. If you do not know what you are looking for, you may find it but do not realize it.

It is considered that in order for life to appear and develop, it is necessary:

  • A liquid where chemical reactions take place: on Earth, it is water.
  • An element with ease to form stable compounds: on Earth, it is carbon.
  • A source of energy: on Earth, it is the Sun.

We are looking for planets or satellites with these characteristics, although other possibilities such as liquid methane (in the case of Titan, a satellite of Saturn), ethane, sulfuric acid, ammonia or acetic acid as solvent are being considered. Life-based on other elements such as silicon, it is a recurring topic in science fiction stories.

Artistic representation of Titan’s methane lakes. Credit: Steven Hobbs


The celestial body has to fulfill a series of characteristics so that life can be sustained:

  • An abundance of chemical elements such as carbon, hydrogen, oxygen, and nitrogen to form organic compounds.
  • The planet/satellite has to be within the habitability area of its star (orbiting at a distance that allows a temperature suitable for life).
planet, star, habitable zone
Habitability area (green) according to the temperature of the star. Red: too hot, blue: too cold. Source: NASA / Kepler / D Mission. Berry
  • A source of energy enough to maintain the temperature and allow the formation of complex molecules.
  • An appropriate gravity to keep an atmosphere and not crush the living beings of the planet.
  • A magnetic field to divert the radiation incompatible with life.
The Earth’s magnetic field protects life from the solar wind. Source: ESA

In our Solar System, the candidates that possibly fulfill these characteristics are Mars, Europe and Ganymede (satellites of Jupiter), Enceladus and Titan (satellites of Saturn) and Triton (satellite of Neptune).


Living beings are formed by cells, and if we reduce the scale, by molecules, and atoms (like all matter). Why is life-based on carbon?

In fact, in the constitution of organisms 26 elements are involved, but 95% of living matter consists of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P) and sulfur (S). We can imagine them as the “bricks of life”: by combining these building blocks, we can obtain complex organisms. These bricks can be joined to others by covalent bonds. Metaphorically, atoms can be imagined as spheres with hands which can be grasped by other hands. For example, the main energy source molecule for all living things is ATP (Adenosine triphosphate, C10H16N5O13P3).

enlaces químcos, moléculas, sulphur, phosphorus, hidrogen, oxigen, carbon, nitrogen, chemical bond
Schematic representation of carbon, hydrogen, oxygen, nitrogen and phosphorus atoms and their valences (possible bonds). Own production based on figure 6.3 of “Life in space” (see references)

The candidate element to sustain life would have to be an abundant element able to form a great amount of bonds with itself and with other elements. The 5 most abundant elements in the universe:

  • Helium: does not form compounds
  • Hydrogen and oxygen: they have 1 and 2 hands: they can only form very simple compounds
  • Nitrogen: can bind to 3 atoms, but no chains of several nitrogen atoms are known.
  • Carbon: it has 4 hands so it can be strongly bonded to other carbons with single, double, or triple bonds. This allows it to form long chains and three-dimensional structures and can still join to other atoms. This versatility allows constructing molecules chemically active and complex, just the complexity that makes life possible.
DNA chemical structure, double helix
DNA chemical structure where we can see the importance of carbon bonding to form rings and chains. Source

Could there be life in another place based on a different atom?



Since establishing 4 links is so useful, silicon is the first candidate for biologists and science fiction writers, even if it is not as abundant as carbon. Silicon (Si) can also form 4 bonds and is abundant on rocky planets like Earth, but …

  • The Si-Si bond is quite weak. In an aqueous medium, life based on silicon would not be sustained for a long time as many compounds dissolve in it, although it could be possible in another medium, such as liquid nitrogen (Bains, W.).
  • It is very reactive. Silane, for example (one silicon atom bonded to 4 hydrogens) spontaneously ignites at room temperature.
  • It is solid at most temperatures. Although it can easily form structures with oxygen (silica or silicon dioxide), the result is almost always a mineral (quartz): too simple and only reacts molten at 1000ºC.
  • It does not form chains or networks with itself, due to its greater size compared to carbon. Sometimes it forms long chains with oxygen (silicones), that perhaps could be joined to other groups to form complex molecules. The alien of the movie Alien has silicone tissues. The beings formed by silicones would be more resistant, which leads to speculate what kind of extreme conditions they could withstand.
Horta, a silicon-based form of life featured in the science fiction series Star Trek. Source


Let’s look at some characteristics of nitrogen and phosphorus:

  • Nitrogen: can only form 3 bonds with other molecules and is poorly reactive.
  • Phosphorus: its bonds are weak and multiple bonds uncommon, although it can form long chains. But it is too reactive.

By combining the two, stable molecules could be obtained, but the beings based on nitrogen and phosphorus would have other problems: the nitrogen compounds, from which they would have to feed, are not abundant in planets and the biological cycle would not be energetically favorable.


The most unlikely biochemistries could be based on these elements:

  • Boron: can form long chains and bind to other elements such as nitrogen, hydrogen or carbon
  • Sulfur: can form long chains, but because of its size is highly reactive and unstable.
  • Arsenic: is too large to form stable compounds, although its chemical properties are similar to those of phosphorus.

In 2010, the journal Science published a scientific research in which researchers claimed to have discovered a bacterium (GFAJ-1) capable of living only in arsenic, lethal to any living being. It broke the paradigm of biology by not using phosphorus (remember ATP and DNA structure) and opened up new study lines for astrobiology. In 2012, two independent investigations refuted the theory of researcher Felisa Wolfe-Simon and his team. Phosphorus remains essential for organisms to live and develop on Earth.

GFAJ-1 bacterium. Source

At the moment, these hypothetical biochemistries are nothing more than speculations, so astrobiologists are still looking for carbon-based life, although we already know that science never ceases to amaze us. Although we could identify life based on other elements if we ever find extraterrestrial life (or vice versa) the revolution will be so great that it won’t matter if they are carbon-based beings.








Animal genitalia: amphibians, reptiles and mammals

After the first post on the genitals of birds and fish, we close chapter on the curiosities of the penises, vaginas and other reproductive organs of amphibians, reptiles and mammals.


As we saw in a previous post, the cloaca is the hole where the digestive, reproductive and excretory systems converge. All amphibians possess cloaca, as well as reptiles, birds and some fish (sharks and rays) and mammals.

Larvae of amphibians are characterized by a great transformation known as metamorphosis .
Do not miss the successful post about amphibious sperm thieves .


The anurans (amphibians without tail, such as frogs) have external reproduction and mating occurs usually in the water. The male, who is smaller than the female, grips the female firmly. This embrace  is called amplexus.

Amplexus of Litoria xanthomera. Photo: Rainforest harley

The contractions of the female when expelling the eggs stimulate the male to spray them of sperm in the same moment that they are expelled. The eggs are joined by a gelatinous mass that takes different forms depending on the species.
The male frogs of the genus Ascaphus have a false tail that is nothing but an extension of the cloaca.

Tailed frog (Ascaphus truei). Photo: Mokele


Almost all urodela (amphibians with tail, such as salamanders and newts) have internal fertilization. The male is placed in front of the female and releases sperm packages (spermatophores) containing the sperm. The female walks over one of them, collects it with the lips of the cloaca and places them in the spermatheca, a cavity where the sperm wait for the eggs to pass through the cloaca to make them fertilize. The female lays the fertilized eggs one by one beating them in aquatic plants, except in some species of salamander, in which the female retains them and they are born live larvae (ovovivivarism).

Salamander spermatophores (Ambystoma sp.). Photo: Placeuvm


Apoda or caecilians are amphibians without legs with internal fertilization, but unlike in anura, internal insemination occurs. This is possible thanks to a pseudo-phallus (phallodeum) that have the males, which they insert in the cloaca of the female for two or three hours. In oviparous species (25%) the eggs are kept by the mother, the rest of species are ovoviviparous (75%).

Caecilia phallodeum. Photo used under permission by: Danté Fenolio

In some ovoviviparous species the offspring are born metamorphosed, in others as larvae. During their stay inside the mother, they feed on oviduct cells, which they scrape with their special teeth. In the case of the oviparous species Boulengerula taitana, the larvae feed on the mother’s skin allowing them to grow 10 times their size in a week.



Scaled reptiles (Squamata order), as lizards and snakes have the penis divided into two: this is known as  hemipenis. It is kept inside the tail and exits to the outside during intercourse thanks to the erectile tissues. In spite of being double, during intercourse they only introduce one of the parts into the female, although they can do it alternately. The ends can be smooth or have spikes or structures to ensure grip to the female’s cloaca.

Viviparous lizard (Zootoca vivipara) showing its hemipenis. Photo: Charlesjsharp



In some sea turtles, the cloaca retains the ability to exchange gas, in other words, to breathe. The water slowly passes through it, which allows to collect the oxygen and take it to the lungs.

The male tortoises have a simple penis that is folded in two in the cloaca, inside the tail, reason why the tail of the males is thicker and longer than the females’. During the erection, it fills with fluid, deploys and exits, reaching a comparatively larger size.

Mediterranean tortoise penis (Testudo hermanni). Source


Crocodiles have a rigid penis (always in erection) hidden inside the body that, shot out like a spring to the outside at the time of copulation and is hidden again at the same speed. According to this study , fibrous tissue and collagen makes unnecessary the erection and detumescence in the American alligator.



Monotremes are the most primitive mammals, with some reptilian characteristics, like the laying of eggs and the presence of cloaca. Platypus and echidnas are the best known representatives.

Monotremes penises have 4 heads, although not all can work simultaneously. It uses only half, that is, two heads at a time. In the case of the platypus only the left side works, since the female only has functional the left ovary.

Echidna penis’. Source


The marsupials are those mammals in which the breeding ends its development in a pouch, a kind of bag that own the females and where the breasts are. The best known marsupials are kangaroos, koalas, opossums and the extinct thylacine.

Opossum’s penis. Photo: Ellen Rathbone

Generally females have two vaginas, which fit with the bifurcated penises of males, which retract into the S-shaped body, In the case of kangaroos, females have three vaginas and two uteri . The two lateral vaginas lead the sperm towards the uterus and the central one is where the brood descends during the delivery.

Reproductive system of marsupial femanel. Photo: National Geographic



In placental mammals, such as humans, the offspring develops in the uterus and is nourished by the placenta. Many placental males have a penile bone (baculum). This bone would allow copulation even if there is no erection. Some placentals have lost their baculum: humans, hyenas, equines (horses, zebras, etc.) and lagomorphs (rabbits, hares …). In them, erection is possible thanks to the blood filling of the corpora cavernosa.

A dog’s baculum.The arrow shows the uretral hollow. Photo: Didier Descouens


In the case of dolphins, their penis is prehensile and sensory. The end is rotatory and it is not uncommon to see them feel the seabed with their penis. This has led to false myths such as that the dolphins are always excited and try to copulate with anything that gets them ahead. This tactile ability would also allow them to strengthen social bonds between them, even among males. This behavior is also observed in orcas.

The dolphin’s vagina is full of folds and corners to make sperm access to the egg difficult, either from rival males or males with which the female did not want to mate. If you want to see how the penis fits in the intricate dolphin’s vagina, click here.


At first glance we could confuse a male hyena with a female. Female spotted hyenas (Crocuta crocuta) have a long vagina that extends into an external clitoris of the same size as the male penis. The offspring must cross this long channel at birth, who suffers from great tears in the first deliveries and sometimes the puppies die because they can not cross it. In addition, the vaginal lips are also large and full of fat, which could seem testicles.

Spotted hyena’s female genitalia. Source: Quora



Animal genitalia: birds and fish

The function of the reproductive system is apparently simple: to transmit the genes to the next generation. Why does it has so different and curious shapes in all kinds of animals? Would not  be enough with a simple sperm-emitting channel and a simple receiver? Find out in this post different shapes and reproductive strategies of birds and fish. If you want to learn about amphibia, reptile and mammal genitalia, click here.


Animals have different breeding strategies. In elementary school we learned that the fertilization can be:

  • External (outside the female’s body)
  • Internal (inside the female’s body)

And according to where the embryo develops the species are:

  • Oviparous: in an egg hatching outside the mother’s body (most fish, amphibians and reptiles)
  • Ovoviviparous: in an egg that hatch inside the mother’s body (sharks, vipers, boas…)
  • Viviparous: in a womb inside the body of the mother.

In high school we learned that there are strategists species:

  • r-strategists: they do not care for the offspring, which suffers a high mortality at birth. To compensate, they lay lots of eggs. They are usually short-live animals that quickly reach sexual maturity (invertebrates, fish, amphibians …)
  • K-strategists: they dedicate more energy to the care of the breeding, reason why they assure their survival and therefore the number of offspring in each laying or delivery is lower. Usually they are animals of longer life (dog, elephant, human …).

But if we analyze the reproduction in detail, it is not as simple as this. The sexual life of invertebrates is full of unlikely strategies to ensure fertilization, but we will talk about them another time and now we will focus on birds and fish.


The reproductive organs seem to have been the most diverse and most rapidly changed throughout evolution, giving rise to structures of almost every shape and size imaginable. If we believe that the only function is to give sperm, to receive it and to transport it to the ovule, we may be surprised by such diversity. In fact, the reproductive apparatus does much more than that and that is why the anatomy is so different between different animal groups.

Some insects, for example, use their penis for courtship, others use it to make sound and transmit vibrations to the female during mating. If the female likes the music, she will allow the male to take care of her offspring. Females also have adaptations to administer the sperm, like some flies, that can keep the sperm of different males in different receptacles of his reproductive apparatus.

Penis with bristles of the beetle Acanthoscelides obtectus. Source

The use of the penis in courtship and mating by the male and the administration of sperm in females would be two reasons that would explain the complexity of animal genitalia. The competition to ensure that a male’s sperm is actually the one that fertilizes all the female eggs would be another, with strategies as radical as plugging the ducts of the female once inseminated so that no other male can access it.

We will focus on this article on the genitals of fish and birds, do not miss the following post on amphibians, reptiles and mammals.


Although in most fish the fertilization is external, some have structures or pseudopenises to put the sperm inside the female.


They have the pelvic fins modified into two appendages called pterigopods or claspers, with which they introduce the sperm into the female.

Male shark (left) and female shark (right). Source

During copulation only one is used, which is filled with water thanks to a structure called siphon to expel it under pressure mixed with the sperm. According to the species, the young can be born from the mother’s body or from an egg.

Birth of a shark and fertilized egg. Source


Fish of the family Poeciliidae (guppys, mollys, platys, xhipos…), well known in aquarism, have the anal fin modified in a copulatory structure called gonopodium. They do not lay eggs, but the offspring are born directly from the mother’s body.

Male and female guppy. Source


It is a family of fish (Phallostethidae) that present the copulatory organ under the head. They mate face to face with the female, a thing almost unique in animals that live underwater. With the priapium they are anchored to the female and fertilize the eggs internally for a longer time than usual in other species.

Phallostethus cuulong male, discovered in Vietnam.. Source

Other curiosities in fish are possession of both sexes (hermaphroditism) or sex change, as in clown fish.


The best known representative of the Lophiiformes is the monkfish. In this order of fish, males are much smaller than females and they latch to females with their teeth, because of the difficulty of finding a mate in the abyssal bottoms. As time passes by, the male is physically fused to the female. He loses its eyes and its internal organs, except the testicles. A female may have six or more males (pairs of testicles) fused in her body.

Lophiiforme with a fused male. Source


Most bird species (97%) do not have a penis and fertilization is performed with cloaca-cloaca contact (cloacal kiss, or cloacal apposition), a conduit that is used both as a reproductive and excretory system.

Left: urogenital apparatus of the male: F) testis, B) vas deferens, A) kidney, E) ureter, C) urodeum of the sewer. Right: urogenital apparatus of the female: A) ovary with mature follicle, F) infundibulum, E) oviduct, B) kidney, C) ureter, D) cloacal urodeum.Source

There are different hypotheses by which birds are believed to have lost their penis during evolution (since their reptilian ancestors did have it): to lighten the weight during flight, to avoid infections, by chance during the evolutionary process or to the females had better control over whom to reproduce. It seems that the latter would be the most accepted, since for example ducks fly long distances and have large and heavy penises.

But some birds do have a penis, which unlike mammals and reptiles, goes into erection by filling it with lymph, not blood.


Ducks, geese and swans are among the few birds that have a penis. The vagina of the ducks is shaped like a spiral clockwise, so when the male penetrates the female with his penis also spiral counterclockwise, if she is not interested flexes his vaginal muscles and penis leaves her body.

Vagina (left) and penis (right) of Mallard (Anas platyrhynchos).Source

The vertebrate with the longest penis in proportion to its body is precisely a duck, the Argentine diving duck (Oxyura vittata). He keeps it rolled up inside, but in erection he can be twice as long (42.5 cm) as his body (20 cm).

Oxyura vittata male with penis out. Unknown author

Duck penises, in addition to their different sizes and curvatures, can be smooth or have spines or furrows. This variability is due to the competitive pressure to overcome the females’ vagina. Both genitals are a clear example of coevolution. If you want to know more about coevolution visit this post.

Vaginas with blind ducts, penises with spines to extract sperm from previous copulations… ducks have a real “war” for reproductive control. In monogamous species such as geese and swans, the reproductive apparatus is not so complex, but in more promiscuous species, such as ducks, they are more complex and with longer penises so that it can be guaranteed that the male who has fertilized the eggs is the one who will take care of the chicks.


This African passerine (Bubalornis niger) has a pseudopenis 1.5 cm long. It does not have blood vessels nor spermatozoa, reason why apparently its function is to give pleasure to the female and to favor the attraction of the male. Males in colonies have longer pseudopenises than those living alone, so the evolution of this appendix could also be explained by male-male competition.

Red-billed buffalo weaver. Photo by Reg Tee


African ostriches (Sthrutio camelus) are from the Ratites family, which also includes kiwis, rheas (American ostriches), tinamous, emus and cassowaries. All of them have a penis, and except for tinamu, they are running birds.

Ostriches about to mate. Source

The genitalia of the cassowary are really very peculiar. We have already discovered in this post how exceptional this animal is. Both sexes have a phallic appendage, but it is not connected to any reproductive organ. In the case of males, it is invaginated in a sort of “vaginal cavity”. At the moment of the copulation, it comes out (as if we turned out the finger of a glove), but the sperm leaves the cloaca, that is, from the base of this pseudopenis, not from the tip. In the case of females, the phallic appendage (sometimes referred to as clitoris) is a little smaller than in males.

Cassowaries mating. You can watch the video here

These male-female characteristics have given rise to rituals and beliefs in the folklore of New Guinea. They consider the cassowary an androgynous creature of mixed genders, therefore powerful because they have the attributes of both sexes. The remote Bimin-Kuskusmin tribe (Central New Guinea) celebrates rituals where intersexual people are considered representatives of these animals, so they are revered and powerful. On the other hand, Mianmin people tells stories about a human woman with a penis that became a cassowary.

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It’s a matter of horns

Some beetles, lizards... have horn-like structures, but mammals have the most diverse horns without doubt. Are all horns the same? What are they used for? Do they have economic value? Find out more in the following post.


Bulls, deer, rhinos… all of them have structures on their heads that we call horns, but they are not all the same. Strictly horns are two bony structures that emerge from the frontal bones of the skull, they are permanent (never fall off) and unbranched. In some species they grow throughout life.

El watusi, el bóvido con los cuernos más grandes del mundo. Foto: Marina Calvo
Watusi (Bos taurus watusii), the bovid with the biggest horns in the world. Photo: Marina Calvo

They are made up of a bony nucleus and an outer coating of keratin (the same protein from our hair and nails).

Horns have different shapes and sizes depending on the species: straight, curved or spiral; flipped, bent or flat; short or wide. All of them have sharp ends.

Diversidad de cuernos de la familia Bovidae. Fuente
Horns’ diversity of Bovidae. Source

All bovids (bulls, goats, sheep, antelopes…) have horns, including the females in many species. However, in general, females have thinner horns while in males they are wider and can withstand more force.


When the horns begin to grow, they do not do it directly from the bone, but from the connective tissue. When growth is complete the horn nucleus ossifies and fuses with the frontal bones of the skull.

Cráneo de cabra en el que se observa el interior óseo del cuerno y la cubierta queratinosa. Fuente
Goat skull showing the bony interior of the horn and the keratinous covering. Source


The pronghorn has different horns than the bovids: they are branched and the keratinized covers change annually, whereas in bovids are permanent.

Berrendo (Antilocapra americana). Fuente
Pronghorn (Antilocapra americana). Source


Antlers are two bony structures that come out of the frontal bones, but they are seasonal (they change every year) and branched .

Antlers only exist in males of the Cervidae family, except for the caribou or reindeer (Rangifer tarandus), in which both males and most of females have antlers.


Unlike horns, antlers do grow out of bony structures (pedicle) found on the side of the frontal bones.

Growth begins in spring (April or May in the Northern Hemisphere), due to hormonal changes and the gradual increase in light hours. The growth of the antlers has several phases:

  • Initial phase: antlers are covered with skin and velvet, so they also have blood vessels and nerves.
  • Intermediate phase: the exterior of spongy bone is replaced by compact bone. The interior is filled with spongy laminar bone.
  • Final phase: the velvet dies and it is removed. To help this removing the animals rub against the trunks and vegetation, leaving the antlers polished and brown.

    A, B, C: 1, 15 y 30 días de crecimiento. D, E: 3 y 5 meses después.F: pérdida del terciopelo. Fotos: A-E, Steve Demarais, F, Dave Hewitt.
    A, B, C: 1, 15 y 30 days of growht. D, E: 3 and 5 months later. F: loss of the velvet. Photos: A-E, Steve Demarais, F, Dave Hewitt

After the reproductive period the hormonal levels fall and the photoperiod decrease, which causes the pedicle to lose calcium, it weakens the union between itself and the horn and the horn ends up falling. The cycle will be repeated the following spring, and will appear one more branch, so the most an antler is branched, the older is the individual.

Alce pediendo su terciopleo. Fuente
Reindeer losing its velvet. Source


As we know, mainly antlers and horns are used by males during the breeding season to compete for females, in fights and exhibitions. Usually the animals collide their horns/antlers together to demonstrate their body strength. Horns, often sharp, are also used as a defense against predator attacks.

Check out this spectacular dispute between two Canadian mouflons:

There are species with small antlers but highly developed tusks, despite being herbivores. This is because they also use them during fights. In contrast, species with larger antlers do not have these developed tusks.

Siberian musk deer (Moschus moschiferus) - does not belong to the family Cervidae-, Muntjac (Muntiacus sp.) And roe deer (Capreolus capreolus)
Siberian musk deer (Moschus moschiferus) -it does not belong to the family Cervidae-, Muntjac (Muntiacus sp.) and roe deer (Capreolus capreolus). Source

For humans, horns and antlers shouldn’t have significance. Unfortunately, its carriers are objective of hunters, for the mere achievement of their “trophy.” In Spain there are more than one million people with a hunting license. According to Fecaza, the hunting business generates 3.6 billion euros a year in Spain.

Trofeos de caza robados incautados por la Guardia Civil. Su valor pudo ascender a 300.000 euros. Fuente
Stolen hunting trophies seized by the Guardi Civil. Its value could amount to 300,000 euros. Source

Spain is also the second importing country of hunting trophies. Thousands of euros are paid (from 2,000 to 80,000) to make hunting safaris in Africa, for example, where the most valuable animal is the one with the largest horns. This results in the elimination of the best breeding males and in the decrease of specimens in general.


Ironically, since their horns have led and are leading to extinction many species, rhinos do not actually have real horns, as they do not have a bony nucleus or a cover. They are an accumulation of corneous fibers, resembling a thick hair, although they are not true hairs. In addition, the horn is placed above the nasal bones, not  in frontal position as in the case of antlers and true horns. Only in species with two horns, the second one rests on the frontal bones.

In females, the horn would help to protect the young, whereas in males to face their rivals.

Sección de un cuerno de rinoceronte cisto bajo lus ultravioleta. Se observa el cartílago nasal, el hueso, la dermis y cómo el cuerno se asienta encima de la dermis. Fuente
Fraction of a rhinoceros horn under ultraviolet light. The nasal cartilage, the bone, the dermis and how the horn settle in above the dermis can be seen. Source

As we have discussed, due to the alleged magical powers of rhinoceros horns in the traditional medicine, we are extinguishing rhinoceroses just like with are doing with the pangolin… for a handful of keratin. On the black market, a kilo of rhinoceros horn can cost from $ 60,000 to $ 100,000, more than gold.

Rinoceronte con el cuerno amputado.
Rhinoceros with its horn amputated. Photo: A. Steirn


As you may assume at this point, no, giraffes do not have true horns, but they also have two structures in the head, males, females and newborns. They are called ossicones. They are permanent, not branched and they are always covered with hair and skin. In fact, they already appear in the fetus as cartilaginous structures and do not merge into the skull until the age of 4, between the frontal and parietal bones.

Female giraffe (Giraffa camelopardalis). Source
Female giraffe (Giraffa camelopardalis). Source

We can tell age and sex of a giraffe by its ossicones: if they are thin and ended up in a tuft of hair they are young ones or females, while males do not usually have hair on its top. Males also have a protrusion in front of the ossicones more sharp than females. At an older age, this protuberance is bigger, since calcium is deposited over time.

Giraffe ossicones are used by males during their confrontations. Surely they played a more important role in its ancestors like the Sivatherium, the largest giraffid that has ever existed. It is possible that they also have some function in thermoregulation.



The mom’s microbiological present

During the gestation, the mothers bring all that necessary for thecorrect development of the baby. Did you know that also implies to microorganisms? For the good maturation of our intestines and immune system, we  need a contribution microbiological from mom. Enter and discover the different bacterial species  that gives us our mother in our first days of life.


For a long time it was believed that the uterus and amniotic sac that containing the fetus are a sterile place without any microbiological presence. Moreover, the mere presence of microorganisms was associated with a disease or a risk to the baby. So, it was believed that the fetus was conducted in a completely sterile environment during 40 weeks of gestation and came into first contact with some type of bacteria during birth.

Today, thanks to technological advances and genetic studies, it has been observed that this dogma was not true. Fetuses are in contact with bacteria throughout gestation. Generally it is non-pathogenic bacteria that are transmitted by the mother during pregnancy and after delivery.

Ultrasound of the placenta. In red are the bacterial communities, while in blue are observed them veins. (Image of Wolfgang Moroder)

This maternal microbiological transmission is a widespread phenomenon in many groups of the animal kingdom, such as Porifera, mollusks, arthropods and chordates. The presence of this phenomenon throughout the animal kingdom and the ease with which these organisms to reach the fetus, show that this transmission is a very old process and represents an evolutionary advantage to organisms.

Different organisms that maternal transmission has been observed. A) Pea Aphid (Acyrothosiphon pisum). b) Common chicken (Gallus gallus domesticus). c) Red Salmon (Oncorhynchus nerka) and d) River turtle Podocnemis expansa. (Image: Lisa Funkhouser).


There are different ways why does the mother get the baby the first bacterial communities. So the baby’s contact with his future microbiome is given for the first time through bacteria of the placenta. Then and during the delivery, some bacterial strains are transferred through the birth canal, skin and finally, through breast milk.

The differents transmission routes of microbiological communities. (Image: Lisa Funkhouser)


Is relatively recently,  that the presence of bacterial communities in this organ is known. Yet it is noteworthy that it is a small microbiome in terms of abundance. Generally, it is non-pathogenic microorganisms, but their variation could be related to common disorders in pregnancy such as premature births.

Initially it was believed that these bacterial communities would be related to the vaginal microbiota of the mother, but it has been observed that placental bacteria are more similar to those of the mother oral microbiota. According to research, the bacteria come from the mouth of the mother to the fetus through the bloodstream. So good oral health is essential for the proper development of the baby.

In the following diagram represent the main bacterial species identified in the human placenta. 

The major bacterial phyla observed in the placenta. Own image.


As it is well known, during labor, a major transfer of bacteria occurs. Most of these bacteria are related to the vaginal and fecal microbiota of progenitor. During pregnancy, the vaginal microbiome of the mother varies and becomes less diverse, being more predominant the presence of bacteria such as Lactobacillus sp.

Still, it is noteworthy that this transmission will vary depending on the type of delivery, that is, babies born vaginally present similar microbiome to that of the vagina of the mother microbiota (Very rich in Lactobacillus sp., Prevotella sp., Bacteroides and Bifidobacterium sp.), while those born by Caesarean section present a more similar microbiome to the microbiota of breast skin, rich in Clostridium sp., Staphylococcus sp., Propionibacterium sp. and Corynebacterium sp.

The first baby’s microbiome depends on the type of chilbirth. In this diagram we can observe the differents bacterial communities that are involved in each one. (Own image)


As in other cases, skin with skin contact produces a transmission of microorganisms between two humans. In this case, it can be through the type of delivery (C-section), by contact with the external vulvar area of mother and by contact with the outer skin of his mother.

Some of the bacteria that are acquired at birth and are commonly in the skin of adult humans are Staphylococcus sp. Corynebacterium sp. and Propionibacterium sp.


Another of the myths about esteril pregnancy was breast milk. Until recently it was thought that breast milk was sterile and bacteria that were in the samples were due to cross-contamination through the skin of the mother and the baby’s mouth. Today, thanks to the discovery of certain anaerobic bacteria, has concluded that the mother also provides certain bacterial communities by human milk.

There are a variety of microorganisms in milk and generally vary depending on the type of feeding and origin of the mother (See the different abundances of microorganisms in different mothers in the figure below). Still, it has been observed that during the early months of breastfeeding, breast milk is rich in Staphylococcus sp., Streptococcus sp. and Lactococcus sp.; while from six months of lactation milk is rich in typical microorganisms of the oral microbiota as Veillonella sp., Leptotrichia sp. and Prevotella sp.

Figure 1
Differences in the abundance of the bacterial species found in the breast milk of 16 analyzed subjects. (Image: Katherin Hunt)

Thus, it is expected that breastfed infants present a different intestinal and fecal microbiota than the artificialfed babys. These bacteria favor the baby against diarrhea, respiratory diseases and reduce the risk of obesity. BE CAREFULL! This does not mean that a child fed with artificial milk is worse than a breastfed, as many of these bacteria also can be purchased by other means.


All these bacterial transmissions by the mother let the baby start the maturation of their immune system and the development of good intestinal microbiota. Our mothers always give it best to us!