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All animals need to eat. In fact, as you may remember from GenBio, eating is one of the things that defines animals -- animals are obligate heterotrophs that obtain energy by ingestion. Feeding is fundamentally important to every aspect of an animal’s biology: how big it gets; how fast it grows; where it lives; how it lives; how it moves; whether and how it reproduces; whether and how it cares for young; whether and how it avoids becoming food for something else. Feeding fundamentally links organisms together in ecological systems. Feeding is how carbon gets transferred from plants up through food webs, and feeding is how carbon is returned to the earth. Understanding the physiology and ecology of feeding is therefore central to understanding life on earth.
If all animals must eat, the logical next question is what do they eat? And how do they make feeding decisions? Presumably, maximizing nutritional intake should be a priority. All else being equal, we’d predict that most animals should favor higher-value food (i.e. more calorie-dense) over lower-value food. But, in nature, many factors could complicate foraging choices. Circumstances could favor choosing a lower-quality food, such as:
Handling Time: how long does it take to eat the food? Macadamia nuts are among the most calorie-rich tree nuts, but they also have extremely hard shells. So is it better to spend time breaking into a macadamia nut? Or to eat a larger quantity of lower-calorie nuts that are easier to open?
Search Time: How long does it take to find food? This can be related to how large the food is, how visible it is, where it’s located in a habitat (underground? high in a tree?), and also the habitat itself -- search time is likely higher in a dense forest than in an open field. Eating a low-quality food might make sense if it’s very abundant and easy to find. Searching for food takes energy, so an unsuccessful search costs more than just time.
Predation Risk: A chipmunk may be safest deep in its burrow … but if it stays there forever, it will starve! Foraging is inherently risky. Animals must balance the benefits of foraging in any given habitat against the risks of becoming lunch.
Just from these three simple concepts, we can begin to get a sense for potentially complex series of behavioral ‘decisions’. How do animals evaluate cost-benefit tradeoffs between positive and negative outcomes? How should a chipmunk balance its need for food against its risk of becoming food? And do those tradeoffs depend on the quality of food available, on the type of habitat available, on the chipmunk's physiological state -- i.e. how hungry it is? Or on perceived threats -- i.e. the presence of hawk calls?
Optimal foraging theory predicts that evolution should favor animals which optimize foraging trade-offs, maximizing energy intake while simultaneously minimizing costs. There is a rich literature exploring these concepts using both experimental and theoretical approaches.
For the pilot experiment, you will explore trade-offs between nutritional benefit and food handling time for a common backyard critter of your choice. You will not be trapping or handling wild animals -- rather, you will observe their behavior at feeding stations. You’ll later build on this approach in your main experiments, developing your own tests of optimal foraging theory for animals that share our suburban landscape.
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