Risky Rewards

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Animals face many types of risk including starvation and depredation. During risk-aversive foraging, animals opt to feed in patches that supply a consistent rate of food and avoid patches with higher variability. One example of such behavior is with shrews. Shrews are tiny animals with a large surface-to-volume ratio and thus greater heat loss compared to a larger animal with comparable body shape and insulation capabilities. In order to maintain homeostasis, shrews must eat constantly. So they are constantly faced with starvation and face minute-by-minute decisions that impact its survival.

shrewBarnard and Brown (1985) introduced shrews into small tanks with feeding pots. At first, the shrews were provided with a constant supply of all-you-can eat mealworms. They provided 2 feeding stations to each shrew: one that consistent had 1 worm chunk per visit and the other varied between having 2 chunks per visit and no chunks. In both cases, the quantity of food was more than the shrew could eat. The shrews opted for the constant food station.

When the researchers removed both food stations periodically, cutting the shrew to starvation rations, the shrews switched to risk-prone foraging. They opted for the higher-reward, greater risk food station when they were low on energy reserves.

The shrew, like many animals, is risk sensitive, modifying its choice between a risky food vs. constant food patch depending on its physiological state. When food is plentiful, the value of a more profitable but variable gain is diminished relative to a constant reward that appears to satisfy all the animal’s energetic needs. However, when facing the risk of starvation the perceived value of a variable food source increases dramatically. The animal could receive a constant insufficient ration at one patch and surely starve. Alternatively, at the variable, high-reward patch, some animals may be unfortunate and starve. Others may receive an average amount and still starve. But a percentage will receive the high rewards sufficiently to survive.

So, risky rewards can pay off when you have nothing to lose.

In my study system, deer must make decisions about foraging. Those decisions include where, when, and for how long to forage at a particular patch. Food scarcity may lead to changes in risk aversion for instance, immediately after a fire burns the understory, vegetation may be very scarce and deer may exhibit more risk-prone foraging.

Barnard, C. J. et al. “Risk-Sensitive Foraging in Common Shrews: An Interruption Model and the Effects of Mean and Variance in Reward Rate.” Behavioral Ecology and Sociobiology, vol. 18, no. 2, 1985, pp. 139–146. http://www.jstor.org/stable/4599872.

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