Lecture 1. An Evolutionary Approach to Animal Behaviour


The below set of notes are from: APS209: Lecture 1. An Evolutionary Approach to Animal Behaviour

There are two basic questions that a biologist can ask about animal behaviour:

How? (how do they do it; what is the mechanism?)

Why? (what is the benefit of doing it in terms of transmitting your genes to the next generation?)

These questions are not in opposition to each other. Thus, the following two hypotheses can both be correct:

1. Swallows fly south at the end of summer because decreasing day length causes hormonal changes that trigger migration (How does it happen?)

2. Swallows fly south at the end of the summer because those that do so have more offspring (why do they do it?).

 The two hypotheses are not “competing”. They are at different levels of analysis (why and how). If research finds that hormones trigger migration, it doesn’t mean that migration can’t also help the birds to survive the winter and have more offspring as a result. To gain a full understanding of the question “why swallows fly south at the end of the summer” both types of hypothesis need to be investigated.

It is also possible to have hypotheses at the same level of analysis (i.e., why and why, or how and how). In the case of swallows, at the 'how' level questions include how the birds sense changes in day length, how they can navigate to their winter quarters, and many others.

Classic “How” & “Why” studies by Niko Tinbergen

Niko Tinbergen was one of the most influential people in the study of animal behaviour, and in clarifying levels of analysis. His research typically involved studying animals in natural conditions, and in carrying out simple but elegant experiments that gave clear results.

One question he asked concerned the adaptive significance of a behaviour shown by many birds. When eggs hatch, the parents carry the broken shells away from the nest. He hypothesized that broken shells attract predators. In an experiment, he found that broken shells increased predation by crows when placed near black-headed gull eggs. No effect of broken shells on predation would have weakened his hypothesis. Showing that a behaviour improves survival or reproduction is important because it shows that the behaviour is adaptive. He also studied the mechanism by which the bee wolf, a wasp that preys on honeybees, is able to locate its nest burrow when returning with prey. He found that the wasp uses landmarks near the entrance to locate it.

Questions about behaviour: eyespots on moths & butterflies

The how and why levels of analysis can be divided into sub-categories. For example how questions can be “physiological” and “developmental”, so each level of analysis can have several distinct types of questions.

Vallin et al. (2005) studied the defensive role of eyespots on the wings of peacock butterflies. They noted that, if disturbed while resting, the peacock opens its wings revealing the large eyespots, which appear to frighten predators. They showed that the eyespots are effective in scaring off blue tits. The peacock butterfly intimidates the bird by pretending it’s a large animal with large eyes, although it’s actually harmless and edible.

Peacock butterflies have two potential ways of scaring predators: the eyespots and a hissing noise made by rubbing their wings together, which may sound like a snake. Vallin et al. (2005) did an experiment to determine the effects of these potential defences against blue tits. They found that the eyespots reduce predation but that hissing has no effect. This is not the first study of the role of eyespots in scaring predators, but previous ones had shortcomings such as pseudoreplication, meaning the repeated use of the same animals to gather data.

Vallin, A., Jakobsson, S., Lind, J., Wiklund, C. 2005. Prey survival by predator intimidation: an experimental study of peacock butterfly defence against blue tits. Proceedings of the Royal Society of London B. 272: 1203-1207. Hypothesis Testing and Anting

Anting is a behaviour carried out by birds such as jays and crows, in which they put worker wood ants, Formica spp, in their feathers where the ants discharge their formic acid. Anting must be beneficial to the birds or they wouldn’t do it. But what is the benefit? One hypothesis is that releasing formic acid harms parasites living in the bird’s feathers. Another hypothesis is that it makes the ants more palatable by reducing their formic acid content. Both hypotheses address the fitness benefit of carrying out anting behaviour: more food or fewer parasites.

The two hypotheses are at the same level of analysis. But it is possible that both could be correct. There is no fundamental reason why causing an ant to discharge its formic acid into the feathers could not both control parasites and make the ants more edible. If this is the case then there is a double benefit to the bird. How would you test these “multiple competing hypotheses” for the benefits of anting? Which of the following observations would allow you to reject one of the two hypotheses?

1. Birds were never seen to eat the ants after they had wiped them onto their feathers.

2. Birds were usually but not always seen to eat the ants after they had wiped them onto their feathers.

3. Birds were seen wiping ants onto vegetation but not on their feathers before eating them.

4. In the laboratory, formic acid is found not to harm bird lice.

5. Birds are never seen to eat wood ants without wiping them on their feathers or some other substrate.

6. Birds that anted had significantly fewer parasites in their feathers than those that did not.

7. Birds that anted had significantly fewer parasites than those that didn’t; the parasite levels are sufficient to harm the birds.

The scientific method & hypothesis testing

Biologists studying behaviour usually work by forming hypotheses and testing them. Sometimes, there are multiple competing hypotheses at the same level of analysis, as in the anting example. The trick, is to devise tests (i.e., suitable experiments or observations) that can differentiate among the hypotheses, by rejecting some and supporting others. A hypothesis is tested by making a logical prediction that arises from the hypothesis. If the data collected support the prediction, then this is strong support for the hypothesis because the prediction was made a priori (before the data are collected). If the data collected do not follow the prediction, then the hypothesis is weakened or even rejected.

It is possible to break down the scientific approach into four stages

1. Question about a natural phenomenon

2. Hypothesis to potentially explain what has been seen

3. Predictions from the hypothesis

4. Test predictions arising from the hypothesis by gathering appropriate data (field observations, experiment)

You should not normally gather data at random and hope to see what it means at the end. It is much better to ask a well-defined question and to gather data to address that question. Theory is often used to develop a hypothesis and prediction. However, it is often necessary to make general natural history observations to formulate a question.

The word hypothesis means foundation in Greek. It is a tentative explanation for a phenomenon used as a basis for further investigation, or a statement that is assumed to be true for the sake of argument (e.g. a null hypothesis).

Evolutionary history of behavioural traits: number of origins

There are many questions that we can ask about the evolutionary history of behavioural traits. For example, what selective forces caused the trait to be favoured by natural selection at the time of its origin, or how the trait has been modified since then. The lecture slides present an example of another historical question, which is both simple and important: how many times a trait has evolved. Let’s consider three behavioural traits in the Apidae bees: eusociality (living in a colony with queen and workers); swarming and nectar transfer.

The slides show a photo of one species in each of the four subfamilies of Apidae bees: Orchid bees, native to the American tropics, Stingless bees, native worldwide in the tropics, Honeybees, native to Europe, Africa and Asia including Britain, and Bumblebees, native in all continents except Australia but mainly found in temperate regions. Of these, only the orchid bees are not eusocial. The Xylocopinae are another group of bees and are used to “root” the tree. This is known as using the outgroup comparison.

Despite being the best-studied group of eusocial insects, the phylogeny of the Apidae is not yet known for sure. Phylogenies constructed with molecular versus morphological data do not give the same result. The phylogeny presented is probably the correct one, but this is by no means certain.

What can this phylogeny tell us about the evolution of behaviour? First, the phylogeny indicates a single origin of eusociality. This is the most parsimonious (= simple) explanation. A less parsimonious, but possible, explanation is that eusociality evolved several times, once each in honey bees, stingless bees, and bumblebees. Second, the phylogeny indicates that swarming (colony fission in which a colony is formed by a queen plus a large number of workers) evolved twice, once in the honeybees and once in the stingless bees. Bumblebees do not swarm, instead bumblebee colonies are founded by a lone queen. Honeybees and stingless bees also have nectar transfer, in which a forager transfers her nectar to a receiver bee in the nest. In bumblebees a nectar forager stores her own nectar. The phylogeny indicates a double origin of nectar transfer.

If the probable phylogeny is wrong and another possible phylogeny is actually the correct one (in which stingless bees and honey bees are sister groups), what does this allow us to infer about the evolution of these three behaviours? As before, the most parsimonious explanation is that eusociality evolved once, in the common ancestor of honeybees, stingless bees, and bumblebees. But the most parsimonious explanation for both swarming and nectar transfer is now that each only evolved once, in the common ancestor of honeybees and stingless bees.

This example shows that making reliable inferences about the number of origins of behavioural traits, depends on having a good phylogeny. Fortunately, molecular data, especially DNA sequences, are increasingly available and are making phylogenies easier to construct.

Thompson G. J, Oldroyd, B. P. 2004. Evaluating alternative hypotheses for the origin of eusociality in corbiculate bees. Molecular Phylogenetics & Evolution 33:452-456.