The below set of notes is from APS209: Lecture 5. Adaptation and Anti-predator Behaviour.
Being eaten by a predator usually has a major detrimental effect on an animal’s fitness. As a result, anti-predator behaviour is the focus of intense natural selection and presents many striking examples. Animals have a wide range of anti-predator behaviours, including being hard to detect (e.g., being cryptic), to attack (e.g., being vigilant), to capture (e.g., being fast running), or to consume (e.g., the puffer fish swallows water to inflate into a spiny ball). Even when the anti-predator adaptation is not primarily behavioural, it may have a behavioural component. Animals that have warning colours rarely hide away, and animals that are cryptic have to position themselves correctly.
Predation avoidance provides a good opportunity to consider the adaptive value of different behaviours. It is usually easier to investigate the current benefits of behaviours. Many birds mob potential egg predators. This is an effective anti-predator behaviour with current value. In one study Hens’ eggs were placed inside and outside a black-headed gull breeding colony. It was found that Crows searching for eggs to predate were more likely to be mobbed and their predation attempts were less successful, inside the colony. It is also possible to investigate past benefits. For example, in the Automeris moth, an individual has realistic eyespots on its hind wings. But, presumably, these spots were not initially so realistic. By manipulating the eyespots, an experimenter could test the hypothesis that any contrasting shape on the hind wings can startle birds pecking the moth (i.e., to get this anti-predator adaptation started) but that markings more similar to eyespots have a greater anti-predator effect (i.e., selecting for larger or more realistic eye spots). However, not all factors that reduce predation can be considered adaptations. Humans are relatively large and are not eaten by spiders. But large size in humans is not an adaptation to reduce predation by spiders. Alcock gives additional reasons why current traits need not be adaptations. For example, a trait may be a deleterious side effect of something else that is adaptive or may have evolved as an adaptation under conditions that no longer exist.
Many anti-predator behaviours are easy to understand. A rabbit’s ability to run fast, for example, or a skunk’s potential for releasing noxious smells. You should learn examples of these and it is not difficult to understand why they are adaptive. However, some anti-predator adaptations, such as stotting behaviour in Thompson’s gazelle, are more puzzling. On detecting a cheetah the gazelle leaps 0.5m into the air orienting its white rump markings to the predator. The gazelle is deliberately drawing the predator’s attention. Stotting is an anti-predator adaptation that communicates to the predator that the gazelle has detected it and is ready and able to flee. Caro (1986) showed that stotters were less likely to be chased and killed than non-stotters. Note that stotting, by requiring athleticism, provides some accurate information about the stotter’s physical ability. If the gazelle merely raised its tail then any animal, whatever its physical condition, could make the signal. The signal would then not be a reliable marker of escape ability and cheetahs would be selected to pay little attention to it. In general, when signal producer and receiver have different interests, as they clearly do in this case, there must be some mechanism to keep the signal reliable, such as by having a signal that cannot easily be made dishonestly. But when the signal producer and receiver have identical interests, such as waggle-dancing and dance-following honeybees, there is no benefit in giving false information.
Another behaviour with interesting underlying logic is selfish herding, in which prey group together. By doing so they may even make themselves a more tempting or detectable target and so increase overall predation. But grouping will be favoured at the individual level if animals in a group have a lower chance of being predated than those alone. When selfish herding occurs everyone can be worse off. But individuals who do not group do worse than those that do. In whirligig beetles larger groups attract more predators than small groups. But each individual has a lower chance of being predated if it’s in a large group than a small one. Selfish herding works by diluting the risk by providing many targets. However, grouping can also reduce overall predation by increasing vigilance, that is the ability to detect a predator (and then flee). Many birds forage or roost in groups. Having more eyes improves vigilance and reduces overall predation. However, with selfish herding, grouping does not need to improve detection, defence or escape and can in fact worsen these. It merely provides safety in numbers.
Caro T.M. 1986. The functions of stotting in Thomson’s gazelles: ome tests of the predictions. Anim. Behav. 34:663-684.
Kenward R.E. 1978. Hawks and doves:factors affecting success and selection in goshawk attacks on wild pigeons. J. Anim Ecol. 47:449-460.
Kruuk H. 1964. Predators and anitipredator behaviour of the black-headed gull. Behav. Suppl. 11:1-129.
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