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Evolution for beginners 2: coevolution

After the success of Evolution for beginners, today we’ll continue  knowing the basics of biological evolution. Why  exist insects that seem orchids and vice versa? Why gazelles and cheetahs are almost equally fast? Why your dog understands you? In other words, what is coevolution?


We know that it is inevitable that living beings establish symbiotic relationships between them. Some depend on others to survive, and at the same time, on elements of their environtment as water, light or air. These mutual pressures between species make that evolve together, and as one evolve as a species, in turn it forces the other to evolve. Let’s see some examples:


The most known process of coevolution is pollination. It was actually the first co-evolutionary study (1859) by Darwin, although he didn’t use that term. The first to use the word coevolution were Ehrlich and Raven (1964).

Insects existed long before the appearance of flowering plants, but their success was due to the discovery that nectar is a good reserve of energy. In turn, the plants found in the insects another way more effectively to carry pollen to another flower. Pollination by the wind (anemophily) requires more production of pollen and a good dose of luck to at least fertilize some flowers of the same species. Many plants have developed flowers that trap insects until they are covered with pollen and then set them free. These insects have hairs in their body to enable this process. In turn some animals have developed long appendages (beaks of hummingbirds, butterflies’ proboscis…) to access the nectar.

Polilla de Darwin (Xantophan morganii praedicta). Foto de Minden Pictures/Superstock
Darwin’s moth (Xantophan morganii praedicta). Photo by Minden Pictures/Superstock

It is the famous case of the Darwin’s moth (Xanthopan morganii praedicta) of which we have already talked about. Charles Darwin, studying orchid Christmas (Angraecum sesquipedale) saw that the nectar was 29 cm inside the flower. He sensed that there should exist an animal with a proboscis of this size. Eleven years later, Alfred Russell Wallace reported him that the Morgan’s sphinxs had proboscis over 20 cm long, and a time later they were found in the same area where Darwin had studied that species of orchid (Madagascar). In honor of both it was added “praedicta” to the scientific name.

There are also bee orchids that mimic female insects to ensure their pollination. To learn more about these orchids and the Christmas one, do not miss this post by Adriel.

Anoura fistulata, murcielago, bat
The bat Anoura fistulata and its long tongue. Photo by Nathan Muchhala

But many plants not only depend on insects, also some birds (like humming birds) and mammals (such as bats) are essential to pollination. The record for the longest mammal tongue in the world is for a bat from Ecuador (Anoura fistulata); its tongue measures 8 cm (150% of the length of its body). It is the only who pollinates one plant called Centropogon nigricans, despite the existence of other species of bats in the same habitat of the plant. This raises the question of whether evolution is well defined, and occurs between pairs of species or it is diffuse due to the interaction of multiple species.


The cheetah (Acinonyx jubatus) is the fastest vertebrate on land (up to 115 km/h). Thomson’s gazelle (Eudorcas thomsonii), the second (up to 80 km/h). Cheetahs have to be fast enough to catch a gazelle (but not all, at risk of disappearing themselves) and gazelles fast enough to escape almost once and reproduce. The fastest gaelles survive, so nature selects in turn faster cheetahs, which are who eat to survive. The pressure from predators is an important factor that determines the survival of a population and what strategies should follow the population to survive. Also, the predators will find solutions to possible new ways of life of their prey to succeed.

Guepardo persiguiendo una gacela. Foto de Federico Veronesi
Cheetah hunting a Thomson’s gazelle in Kenya. Photo by Federico Veronesi

The same applies to other predator-prey relationships, parasite-host relationships, plants-herbivores, improving their speed or other survival strategies like poison, spikes…


Our relationship with dogs since prehistoric times, it is also a case of coevolution. This allows, for example, to create bonds with just looking at them. If you want more information, we invite you to read this post where we talk about the issue of the evolution of dogs and humans in depth.

Another example is the relationship we have established with the bacteria in our digestive system, essential for our survival. Or with pathogens: they have co-evolved with our antibiotics, so using them indiscriminately has favored these species of bacteria to develop resistance to antibiotics.


Coevolution is one of the main processes responsible for the great biodiversity of the Earth. According to Thompson, is responsible for the millions of species that exist instead of thousands.

The interactions that have been developed with coevolution are important for the conservation of species. In cases where evolution has been very close between two species, if one become extint will lead to the extinction of the other almost certainly. Humans constantly alter ecosystems and therefore biodiversity and evolution of species. Just declining one species, we are affecting many more. This is the case of the sea otter (Enhydra lutris), which feeds on sea urchins.

Nutria marina (Enhydra lutris) comiendo erizos. Foto de Vancouver Aquarium
Sea otter (Enhydra lutris) eating sea urchins. Photo by Vancouver Aquarium

Being hunted for their fur, urchins increased number, devastated entire populations of algae (consumer of CO2, one of the responsible of global warming), seals who found refuge in the algae nonexistent now were more hunted by killer whales… the sea otter is therefore a key species for the balance of this ecosystem and the planet, as it has evolved along with urchins and algae.

Coevolutive relations between flowers and animals depend on the pollination of thousands of species, including many of agricultural interest, so we must not lose sight of the seriousness of the issue of the disappearance of a large number of bees and other insects in recent years. A complex case of coevolution that directly affects us is the reproduction of fig.


As we have seen, coevolution is the evolutionary change through natural selection between two or more species that interact reciprocally.

It is needed:

  • Specificity: the evolution of each feature of a species is due  to selective pressures of the feature of the other species.
  • Reciprocity: features evolve together.
  • Simultaneity: features evolve simultaneously.



Cold blood vs warm blood? Neither one nor the other

When we are at school and at science class we are taught about the different groups of animals, we are taught that animals can be divided into “warm blooded” and “cold blooded”. Even though this refers to the different thermoregulation mechanisms found in the different animals, this differentiation between cold and warm blood is not completely right. In this entry we’ll explain, in a more scientific way, the different temperature-controlling mechanisms present in the animal kingdom and we’ll give you examples of different species that cross the line between cold blood and warm blood.


The first thing we must ask ourselves is where body heat comes from. This may come from two different sources:

Endothermy: Endothermy (“endo”, inside) is the mechanism of obtaining body heat by intern production. Endotherm animals have metabolic mechanisms that generate heat (thermogenesis). To generate heat is energetically costly, which means that these animals have high energetic and nutritional requirements.

seal-255522_1280The blue-footed booby (Sula nebouxii) and the Galápagos sea lion (Zalophus wollebaeki) are two good examples of endothermic animals. Photo by Peter Stuart Mill.

Ectothermy: Ectothermy (“ecto”, outside) happens in animals that get their body heat from the environment, basking in the sun (heliothermy) or staying in contact with heat sources like sun warmed rocks (tigmothermy), etc… These animals do not present any metabolic mechanisms to generate inner heat, but ectotherms have many behavioural adaptations to obtain or release heat. Therefore, as they do not spend any energy to generate heat, their energetic requirements are usually lower and they do not need to eat as often as endotherms.

16. Podarcis liolepisA Catalonian wall lizard (Podarcis liolepis) basking in the sun, is a good example of an ectothermic animal. Photo by David López Bosch.

These two concepts only refer to the source of heat, independently of the animal’s ability to regulate or not its body temperature.


The next pair of concepts refers to whether the body temperature of an animal is constant or if it varies over time:

Homeothermy: Homeotherms (“homeo”, same) control their body temperature, making it relatively constant while the environmental temperature varies. As they have constant body temperature, their activity is not conditioned by environmental conditions.

Homeothermy-poikilothermyRange of temperatures at which an homeotherm and a poikilotherm can be. while the temperature of the rat is usually around 37oC, the temperature of the lizard is much more variable. Image by Petter Bockman.

Poikilothermy: Poikilotherms (“poikilo”, varied) present a body temperature similar to the environment. Their inner temperature varies as the environment temperature does, and so their activity is pretty much conditioned by environmental conditions. For example, on the Iberian Peninsula, lizards have their optimum temperature at 33-38oC and snakes at 28-34oC. Still, as they are used to sudden changes in body temperature, they have an advantage over homeotherms in some habitats, as the latter can die if their body temperature increases or decreases a few degrees.

Wiki_stranglesnakeThermogram of a snake aroung a human arm, showing how the snake's temperature is similar to that of the environment. Photo by Arno.

Yet these are relative concepts. Homeotherms, while having pretty constant average body temperature, do not have exactly the same temperature in all their body parts (temperature in the trunk and internal organs is usually higher than in the extremities). Similarly, poikilotherms do not always have exactly the same temperature as the environment, because, as most of them are ectotherms, they can increase their body temperature using external heat sources.

Cold_nose,_warm_touch_-_Thermography_of_CatThermograms of a cat, showing that homeothermic animals do not have exactly the same temperature on different body parts. Photos by Yellowcloud.

Using these four definitions, birds and mammals are endotherms (generate metabolic heat) and homeotherms (constant body temperature) while reptiles, amphibians and the rest of animals are ectotherms (obtain heat externally) and poikilotherms (body temperature varies with the environment). But in practice, this isn’t an impassable line. Hereafter we’ll show you various examples of animals that make the line dividing these four concepts even more blurred.


Heterothermy (“hetero”, different) occurs in animals which are able to switch from endothermy to ectothermy. This usually happens in small birds and mammals with high metabolic rates (quite active and with very high energetic requirements), which decrease their body temperature during inactivity periods. These inactivity periods usually are yearly or daily.

Bear_hibernatingAmerican black bear (Ursus americanus) and cubs hibernating. Photo by National Park Service.

Yearly inactivity periods are usually known as hibernation (or estivation if it happens in summer). Usually hibernators are endothermic mammals such as squirrels, small primates, hedgehogs and many marsupials, and even though bears also hibernate and decrease their metabolism, their body temperature is not so much lowered (only one or two degrees).

Fat-tailed_Dwarf_Lemur,_Kirindy,_MadagascarThe fat-tailed dwarf lemur (Cheirogaleus medius) usually estivates during the malgasy dry season. Photo by Frank Vassen.

During hibernation, the animal’s metabolic rate is drastically lowered and so, they stop regulating their body temperature, making it similar to that of the environment (that’s why hibernator mammals seek refuge in places where the temperature is not as cold as the exterior). When typically homeotherm and endotherm animals aren’t able to find enough food, they pass to a poikilotherm and ectotherm state to save energy.

F1.largeGraphic of body temperature variation during daily torpor on the Syrian hamster (Mesocricetus auratus). Source Fatemeh Talaei et al.

Other homeotherm animals go through daily poikilotherm periods called torpor, during which their metabolism is also greatly reduced. This typically happens in many species of bats, which have a high body temperature during the night (when they are awake) but this decreases during the day (when they are sleeping). Yet as bats sleep during daytime when temperatures are higher, their temperature does not decrease as much as it would if they slept at night.

Cluster_of_hibernating_virginia_big_eared_batsCluster of hibernating Virgina big-eared bats (Corynorhinus townsendii virginianus). Bats usually congregate during hibernation, helping them to stay warm enough during cold weather. Photo by Stihler Craig, U.S. Fish and Wildlife Service.

Hummingbirds can also lower their temperature during night time, but in their case it depends on the quantity of nectar they have consumed during the day. Hummingbirds feed exclusively on nectar, which contains mainly sucrose, which cannot be stored in the organism and passes directly to blood and tissues. This means hummingbirds have an extremely high metabolism and that they need to feed very often to be able to keep up their activity every day.

Purple-throated_carib_hummingbird_feedingPurple-throated Carib hummingbird (Eulampis jugularis) feeding on nectar. Photo by Charles J. Sharp.

If during the day they have been able to consume a lot of nectar, at night they are able to maintain their typical body temperature of 39oC. However, if during the day they haven’t consumed enough nectar, at night they enter a state of torpor, during which their metabolism is greatly reduced and body temperature drops to 12-17oC. This allows them to save energy to be able to look for more nectar the following day.

Video of a hummingbird that fell into torpor during the night in an artificial feeder in Tennessee. Video by Chip Curley.

Next, we have the case of the naked mole rat (Heterocephalus lager), a rodent from east Africa which is the only known poikilotherm mammal. The naked mole rat’s body temperature is the same as the environmental temperature. Yet some scientists have studied that while its body temperature is exactly the same as the environmental temperature from 12 to 36oC, because it is not able to thermoregulate, at 28oC and over its metabolism becomes homeothermic, even though it is not known how it generates or regulates its body heat.

Angry female naked mole rat. Credit: Buffenstein/Barshop Institute/UTHSCSAFemale naked mole rat (Heterocephalus lager). Photo by Jedimentat44.


Finally there are typically ectotherm animals which are able to generate heat and increase their body temperature using various adaptive strategies. Most of these animals use muscle activity to generate body heat.

Many oceanic fish present a complex of veins and arteries called rete mirabile. This complex is found typically in mammals and birds, and is a countercurrent exchange system (artery-vein) which is used to level different parametres (temperature, pH or gas concentrations) in different body parts.

8041255171_bf47050eef_oDrawing about the functioning of the rete mirabile. The thick orange arrows indicate heat exchange that returns to the body. Image by Arne Hendriks.

In many oceanic fish like sharks, tunas and marlins, this system allows them to raise their body temperature because their inner muscles are very strong and their temperature is very high. The rete mirabile allows them to distribute through all the body and to keep the heat generated while swimming, making them practically endotherms.

Rete_Mirabile_04Drawing where we can see the high temperature on the inner muscles of a shark. Drawing by Vittorio Gabriotti.

In fact, the fish called opah (genus Lampris) are the first fish known to be completely endothermic, because thanks to their rete mirabile located in their gills and to a layer of fat covering their bodies and insulating them from the exterior, they can keep their temperature 5oC above water temperature. Yet they are not homeothermic, as their body temperature can still vary depending on the environment.

Opah_6Photo of an opah fish (Lampris guttatus). Photo by USA NOAA Fisheries Southwest Fisheries Science Center.

Muscle-driven heating is not found only in fish. Mammals also shiver when we are at risk of hypothermia, because muscular contraction generates heat. Even if it cannot be said that they shiver, many insects and some reptiles also use muscle activity to raise their temperature. When insects need to activate their metabolism, they flutter violently to increase their temperature. This, together with a counter current system (similar to the rete mirabile), makes the insect’s body temperature raise considerably compared to that of the environment.

Insect_warm-upThermograms of an insect increasing its body temperature by fluttering. Photos by Crespo J.

Similarly, some reptiles can generate heat. Many pythons incubate their eggs after laying them. To do so, they roll around their eggs and start contracting their body muscles voluntarily to raise the temperature of their clutch.

Female_Python_sebae_brooding_eggs_Tropicario,_FINAfrican rock python (Python sebae) brooding eggs. Photo by Tropicario.

We’ve seen mammals and birds reducing their body temperature, fish generating heat, non-thermoregulating rodents, and reptiles and insects that get warmer moving. As you can see, each species is a unique example of adaptation to its habitat and, even if for an elementary school class it may be useful, dividing animals into warm-blooded and cold-blooded not always the most suitable.


The following sources have been consulted during the elaboration of this entry: