Lecture 3. Feeding Behaviour

The below set of notes is from APS209: Lecture 3. Feeding Behaviour.

One of the most important things in an animal’s life is obtaining food, which is necessary for both survival and reproduction. Therefore, natural selection should act strongly on foraging behaviour. An animal that forages more efficiently will on average have more offspring. Greater efficiency may enable an animal to grow more rapidly, to feed more young, or to reduce predation. In eusocial insects, such as honeybees and ants, what matters most is the foraging efficiency of the whole colony. Many animals are social (but not eusocial) nesting in aggregations of mainly unrelated individuals. Here natural selection is less likely to favour animals deliberately communicating food location.

Finding Food

Any animal will be better off if it can forage more efficiently. But, sometimes, if you become better at one thing you become worse at another. For example, in larval amphibians one morph may be better at being a carnivore/cannibal and the other at being a herbivore. Another example is the search image. Many moths rest on tree trunks during the day and have cryptic forewings, which blend in to the background. Not all moths have the same patterns. Some birds search for resting moths. Birds develop a search image for a particular pattern. However, if the bird is given several types of moths it does not do as well at finding them as when it has only one type to look for. It seems that the birds, in seeing through one cryptic pattern are then less good at seeing through another. What this suggests is that the bird’s nervous system is constrained in some way. It cannot get better at detecting one type of cryptic moth without getting worse at recognizing another. An important spin off from this is that it may favour rare types or species of moth. Birds are less likely to develop search images for the rare species, so these may be predated less. In cost-benefit terms, natural selection should favour a bird becoming better at detecting a common prey species at the expense of becoming worse at detecting a rare species.

Group foraging and helping others to find food

Many animals forage in groups, and there can be advantages especially when group members cooperate. Group predation has evolved several times independently in the carnivores (e.g., in dogs, cats, hyenas). By working together they can kill larger prey. The same applies to army ants. Nestmate honeybees cooperate indirectly in foraging by dancing to tell each other where the flowers are. What advantage does this confer? There are several alternative hypotheses: finding flower patches quicker, finding better quality patches, being able to monopolize a food patch. Surprisingly, bees that read dances take longer to find the flowers than bees that scout out a patch of flowers for themselves. This is because the recruit only finds the patch advertised by the dance about 1/4 of the time as the dance is imprecise. However, recruits that do find a patch advertised by a dance are more likely to keep foraging on it than is a scout who has located a patch by herself. This confirms that the dance directs bees to better quality patches, because a forager only dances when she is working a highly rewarding patch. It is suggested that the dance also allows a colony to monopolize a patch of flowers because it can recruit foragers quickly. It is certainly true that recruitment can be rapid. However, most flower patches are much larger than those considered in studies, and there are typically dozens or hundreds of bee colonies within range of any patch of flowers. So monopolization may be difficult. Now we know why honeybees pass information on food sources to their nestmates but how do they measure distance in order to direct other workers to flowers? Honeybees bees use the speed at which images pass their eyes during flight, to estimate the distance they’ve flown. This is known as the ‘image motion hypothesis’.

Honeybees and wolves live in closely related groups and helping each other increases their inclusive fitness. Many animals live in groups that are not closely related, such as in a seabird colony. We would not expect these animals to deliberately communicate to each other. But information can still be transmitted incidentally, such as when an osprey learns the direction of a shoal of fish by observing the direction a returning osprey carrying a fish is coming from. Thus, in an osprey colony a bird leaving the colony in search of food is more likely to fly in the direction from which a bird carrying a fish has recently arrived. However quite a few of the information centre studies show that colonial nesting animals do not seem to benefit from being in a group when finding food. Barn swallows feed on insect prey that is ephemeral in time and space. By the time information was passed on, it is likely that the prey would have moved away so information centres would not be adaptive.

Female lions are able to take larger prey when they hunt together in groups, but at times when prey is scarce, small groups of 2-4 females, have lower food intake per day than females that hunt alone. Yet they continue to hunt in groups. Creel and Creel (1995) studied group hunting in African wild dogs and found that they are maximising the difference between energy gained and energy expended during the hunt. The net energy gained increased in large groups.

Creel, S; and Creel, N. M. (1995. Communal hunting and pack size in African wild dogs, Lycaon pictus. Animal Behaviour 50: 1325-1339. Sherman, G., and Visscher, P. K. 2002. Honeybee colonies achieve fitness through dancing. Nature 419: 920-922. 

Srinivasan, M. V., Zhang, S., Altwein, M., Tautz, J. 2000. Honeybee navigation: Nature and calibration of the “odometer”. Science 287: 851-853.