Unit Eight. The Living Environment
37.8. A Cost-Benefit Analysis of Behavior
One important way in which behavioral ecologists examine the evolutionary advantage of a behavior is to ask if it provides an evolutionary benefit greater than its cost. Thus, for example, a behavior may be favored by natural selection if it increases the intake of food by parents. This is a clear adaptive benefit when it increases survival of offspring, but it comes at a cost. Searching for food or defending food supplies can expose a parent to predation, decreasing the probability that the parent will survive to raise its offspring. To understand these sorts of behaviors, it is necessary to carefully evaluate their costs and benefits.
For many animals, the food that they eat can be found in many sizes, and in many places. An animal must choose what food to select, and how far to go in search of it. These choices are called the animal’s foraging behavior. Each choice involves benefits and associated costs. Thus, although a larger food may contain more energy, it may be harder to capture and less abundant. In addition, more desirable foods may be farther away than other types. Hence, an animal’s foraging behavior involves a trade-off between a food’s energy content and the cost of obtaining it.
The net energy (in calories) gained by feeding on each kind of food available to a foraging animal is simply the energy content of the food minus the energy costs of pursuing and handling it.
At first glance, one might expect that evolution would favor foraging behaviors that are as energetically efficient as possible. This sort of reasoning has led to what is known as the optimal foraging theory, which predicts that animals will select food items that maximize their net energy intake per unit of foraging time.
Is the optimal foraging theory correct?
Many foragers do preferentially use food items that maximize the energy return per unit time. Shore crabs, for example, tend to feed primarily on intermediate-sized mussels, which provide the greatest energy return (as discussed in the Inquiry & Analysis at the end of the chapter). Larger mussels provide more energy, but also take considerably more energy to crack open. Many other animals also behave to maximize energy acquisition.
The key question, however, is whether increased energy resources acquired by optimal foraging leads to increased reproductive success. In many cases, it does. In a diverse group of animals that includes ground squirrels (figure 37.8), zebra finches, and orb-weaving spiders, the number of offspring raised successfully increases when parents have access to more food energy.
Figure 37.8. Optimal foraging.
Optimal foraging in this golden mantled ground squirrel pays off with increased reproductive success.
In other cases, however, the costs of foraging seem to outweigh the benefits. An animal in danger of being eaten itself is often better off to minimize the amount of time it spends foraging. Many animals alter their foraging behavior when predators are present, reflecting this trade-off between food and risk.
Animals often move over a large area, or home range. In many species, the home ranges of several individuals overlap, but each individual defends only a portion of its home range and uses it exclusively. This behavior is called territoriality.
Territories are defended by displays that advertise that the territories are occupied, and by overt aggression. A bird sings from its perch within its territory to prevent invasion of its territory by a neighboring bird. If the intruding bird is not deterred by the song, the territory owner may attack and attempt to drive the invader away.
Why aren’t all animals territorial? The answer involves a cost-benefit analysis. The actual adaptive value of an animal’s territorial behavior depends on the trade-off between the behavior’s benefits and its costs. Territoriality offers clear benefits, including increased food intake from nearby resources (figure 37.9), access to refuges from predators, and exclusive access to mates. The costs of territorial behavior, however, may also be significant. The singing of a bird, for example, is energetically expensive, and attacks from competitors can lead to injury. In addition, advertisement through song or visual display can reveal one’s location to a predator. In many instances, particularly when food sources are abundant, defending easily obtained resources is simply not worth the cost.
Figure 37.9. The benefit of territoriality.
Sunbirds, found in Africa and ecologically similar to hummingbirds, increase nectar availability by defending flowers. A sunbird will expend 3,000 calories per hour chasing intruders away.
Key Learning Outcome 37.8. Natural selection tends to favor the evolution of foraging and territorial behaviors that maximize energy gain, although other considerations such as avoiding predators are also important.