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’ SEX LIFE
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 EVOLUTION OF GENITALIA
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.
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.
FISH GENITALIA
Although in most fish the fertilization is external, some have structures or pseudopenises to put the sperm inside the female.
SHARKS AND RAYS
They have the pelvic fins modified into two appendages called pterigopods or claspers, with which they introduce the sperm into the female.
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.
POECIILIDAE FISH
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.
PRIAPIUM FISH
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.
Other curiosities in fish are possession of both sexes (hermaphroditism) or sex change, as in clown fish.
LOPHIIFORMES 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.
BIRD GENITALIA
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.
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.
WATERFOWL GENITALIA
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.
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).
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.
RED-BILLED BUFFALO WEAVER GENITALIA
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.
OSTRICHES AND RELATIVES
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.
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.
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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REFERENCES
- The evolution of animal genitalia
- It’s official: genitals *do* evolve faster!
- Descubren un pez cabeza-pene en Vietnam
- Extraños penes animales
- Why do sharks have two penises
- New species of priapiumfish
- Anglerfish (National Geographic)
- Lef-handed bears and androgynous cassowaries
- Avian reproduction
- Scienceblogs: The birds do it…
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Sex War and Promiscuity: Why Ducks Have the Largest Penises in the World
- Cover image
During fertilization, when the ovum and the sperm fuse together (both haploid cells, with just one copy of chromosomes, n chromosomes) a new individual is formed with a unique genetic combination, with DNA from its father and its mother (diploid, with two copies of each chromosome, 2n chromosomes in each cell). Triploid (3n) or tetraploid (4n) individuals only appear in asexual hybrid species, and most cases are non-viable. Images by 
Colony of Carniolan honey bees (Apis mellifera carnica), a subspecies of hony bee from Eastern Europe. Photo by 
Diagram about the life cycle of a rotifer, in which parthenogenetic asexual reproduction during good environmental conditions is alternated with sexual reproductions with a haploid male during adverse conditions. Image extracted from 
Desert grassland whiptail lizard (Cnemidophorus uniparens) which, as its scientific name implies, is a parthenogenic species in which all specimens are female. Photo by 
Baby Komodo dragon (Varanus komodoensis) born by accidental parthenogenesis at Chester Zoo. Photo by 
Scheme of the formation of polar bodies during oogenesis, which may help parthenogenetic reptiles to regain their diploidy. Scheme by 
Brahminy blind snake (Ramphotyphlops braminus), the only known unisexual ophidian, in which all specimens found to date are females. Photo taken from 
Three species of whiptail lizards. The middle one, Cnemidophorus neomexicanus is an unisexual parthenogenic species, originated from the hybridization of two bisexual species, C. inornatus (left) and C. tigris (right). Photo by 
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