26.7:
Optimal Foraging
Animals need to forage. That is, find and eat food to survive. Optimal foraging theory postulates that evolution favors foraging behaviors that balance the benefits of getting food with the costs of obtaining it, such as energy expenditure, and the risk of predation.
One example is the Northwestern crow, which picks up whelks, sea snails, then flies upwards and drops them onto the rocks below to crack open the shell, allowing the crow to eat the insides. The crow repeats this process until the shell is broken. If it flies higher, the shell will break with fewer drops.
However, that uses more energy. Scientists have found that the crows, on average, fly to a height that yields the most amount of food relative to the energy used to obtain it.
26.7:
Optimal Foraging
How animals obtain and eat their food is called foraging behavior. Foraging can include searching for plants and hunting for prey and depends on the species and environment.
Optimal foraging theory states that natural selection favors foraging strategies that balance the benefits of a particular food, such as energy and nutrients, with the costs of obtaining it, such as energy expenditure and the risk of predation. Optimal foraging maximizes benefits while minimizing costs.
For the Crows
Optimal foraging theory is supported by evidence from several species. One example is the Northwestern crow’s strategy for eating whelks (sea snails). To eat a whelk, a crow must crack open its shell, which it achieves by flying with the whelk and then dropping it onto rocks beneath. Crows will do this repeatedly until the shell cracks.
Flying higher will break the shell sooner, but requires more energy. By dropping whelks from various heights, scientists calculated the optimal height that will break the shell using the least amount of energy. The crows, on average, fly close to this height to crack whelk shells—supporting the idea that this foraging behavior has evolved to be optimal for energy balance.
Larger whelks also break more easily than smaller whelks, in addition to containing more caloric energy. Crows further optimize their strategy by selecting large whelks and making many attempts to crack a single whelk, rather than expending extra energy to find another whelk.
Mule Deer
The risk of being attacked by predators can be another cost of foraging. Researchers found that mule deer spend more time foraging in open areas although there is slightly less food available than at the edges of the forest. This is due to a lower risk of predation by mountain lions in open areas.
This observation further supports the idea that foraging is a trade-off between benefits and costs, and that evolution favors strategies that are optimized to balance the two.
Suggested Reading
Watanabe, Yuuki Y., Motohiro Ito, and Akinori Takahashi. “Testing Optimal Foraging Theory in a Penguin–Krill System.” Proceedings of the Royal Society B: Biological Sciences 281, no. 1779 (March 22, 2014). [Source]