Animal behaviour and evolution

A mouse which disappears in a hole, a bee heading for a blossom, a blackbird in full-throated song – we are surrounded by animals which interact with their environment in certain ways. In other words, they display behaviour. In contrast to molecular or cell biological processes, animal behaviour can usually be observed with the naked eye. However, whereas we can determine with relatively simple means how an animal behaves, the question why it does so is much more complex. Using scientific methods to get to the bottom of it is one of the central tasks of behavioural biology. Behavioural scientist and later Nobel prize winner Nikolaas Tinbergen already recognised back in the 1960s that we have to take four different levels into account if we want to understand animal behaviour.

The mechanism: how is behaviour controlled?

One of these levels is the underlying mechanism, i.e. the processes which directly control behaviour. Researchers have recently discovered for example that one hormone plays a decisive role in fruit flies’ choice of a partner. Initially, the females are not very choosy when it comes to seeking out a partner for mating – but this changes after they have made their first sexual experiences. On a mechanistic level, this phenomenon can evidently be explained by the fact that the first mating triggers an increased release of the so-called juvenile hormone, which in turn induces the female flies to prefer males which emit especially large amounts of attractants.

The life history: what role does experience play as regards behaviour?

The example of the fruit flies shows that the way an animal behaves can change as a result of experience gained during its individual life history. In the past, the question often cropped up of whether a certain behaviour was innate or learned. Nowadays we know that in most cases it is not possible to make such a rigid distinction because both genetic factors and experience play a role – but in different proportions, depending on the behaviour in question. Many songbirds, for example, are distinguished by singing typical of their species. It is genetically determined that they learn this way of singing during a sensitive phase at the beginning of their lives. However: how they sing, ultimately, depends to a considerable extent on their experience, because they adopt the songs produced by other members of the species in their environment. As a result, even regional dialects can occur in birds – just like in us humans.

The function: why did the behaviour arise?

The function of a specific behaviour is another important level in behavioural research. What lies behind this concept? Like all living things, animals have been formed by evolution. Some individuals have reproduced more successfully than others thanks to a certain behaviour and they passed this on to following generations. So there is a reason why a certain behaviour has developed – and we call this its function.

For example: male orchid bees collect fragrances from orchid blossoms, dissolve them in a fatty substance and store them in special pouches on their rear legs. It wasn’t hard to guess that this self-made “perfume” played a role in the insects’ reproduction. However, it was unclear for a long time whether it served to deter male rivals or to impress females. An experiment carried out in 2023 provided a very clear answer to this question: the perfume leaves other males cold – but leads to considerably increased success with females.

The history of a species: how and when did the behaviour arise?

Behavioural scientists also investigate when and how a behaviour emerged and developed in the course of a species’ evolutionary history. As we cannot take a direct look into the past, researchers sometimes use a trick: they compare the behaviour of species living today and, in this way, try to draw conclusions regarding its evolutionary development. For example, there are more than 200 species of orchid bees today – and in every species the male has pouches on its rear legs for collecting fragrances. Hence, this remarkable behaviour presumably already evolved in an ancestor of this species.

The situation is evidently a different one in the case of Anolis lizards, in which, a few years ago, researchers discovered a fascinating breathing technique. Some of these reptiles sometimes dive into flowing waters to look for food or to flee from predators. There, the animals repeatedly exhale and inhale an air bubble and, in doing so, gradually use up the oxygen still present in it. What enables them to do this is the animals’ water-resistant skin, which results in the air bubble remaining attached to their bodies. But while all Anolis lizards have such skin properties, only certain species carry out this distinct underwater breathing. As all these species are not particularly closely related to one another, we can conclude that this special breathing technique developed independently in different species during evolution – as a form of adaptation to a way of life in which longer dives showed themselves to be of advantage.

Behaviour and evolution

To sum up: if we want to explain a specific behaviour completely, we have to find out what the underlying mechanism is; how the behaviour was formed by experience made in the course of the animal’s life history; what function it fulfils; and when and how it arose in the course of the species’ history. The last two of these levels are related directly to evolution – but the species’ evolutionary past is also important for the other two levels because it plays a decisive role in how behaviour is controlled and is shaped by experience.

This shows what a key role is played by evolutionary biological considerations in researching into animal behaviour. The reason is simple: all organisms living today are the result of evolutionary processes lasting billions of years. If we want to understand them, then it is only possible against this background. The evolutionary biologist Theodosius Dobzhansky once put it in a nutshell. In 1973 he gave an essay the title “Nothing in biology makes sense except in the light of evolution”. This statement is still unreservedly true 50 years later – not only for the behaviour of animals but also for every single one of their other traits.

Further information

• Online-Magazin „ETHOlogisch – Verhalten verstehen“ (in German)
• All articles of the dossier “Evolution”