American Society of Naturalists

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“Caching in or falling back at the Sevilleta: the effects of body size and seasonal uncertainty on desert rodent foraging”

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Justin D. Yeakel, Uttam Bhat, and Seth D. Newsome (July 2020)

Foraging rodents balance risk and rewards. A new model explores balancing food choices when monsoonal onset is uncertain

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<i>Dipodomys spectabilis</i>, the largest species of the family Heteromyidae found at the Sevilleta National Wildlife Refuge in Socorro County, New Mexico.<br />(Credit: Jessica C. Johnson, University of New Mexico)
Dipodomys spectabilis, the largest species of the family Heteromyidae found at the Sevilleta National Wildlife Refuge in Socorro County, New Mexico.
(Credit: Jessica C. Johnson, University of New Mexico)

Foraging in an unpredictable environment is risky business, especially for small endotherms unable to carry around significant body fat stores. In deserts throughout the American Southwest, diverse groups of rodents circumvent these risks by either foraging for lower quality fallback foods, such as grass leaves, or by maintaining caches of higher quality grass seeds. While both of these strategies mitigate the uncertainty associated with an increasingly unpredictable monsoonal growing season, the specific fitness advantages of these behaviors depends on both the rodent's body size as well as the variability and associated risks of resource acquisition.

In a new study by Justin Yeakel and Uttam Bhat at the University of California Merced, and Seth Newsome at the University of New Mexico, the authors use a mathematical model to explore the fitness advantages associated with different foraging strategies and caching behaviors in environments with increasingly unpredictable growing seasons. Specifically they examine the role of fallback foods – less preferred foods available during periods of scarcity – and maintaining caches of different sizes for rodent consumers varying in body size by an order of magnitude (10 to 200 grams). Because larger consumers can maintain a larger proportion of body mass as fat storage, the role of fallback foods and cache size is very context-dependent.

The authors find that less productive environments magnify the expected role of fallback foods in consumer diets, regardless of body size, which may lead to increased competition. They also report that, while maintaining a large cache can make up for the uncertainty associated with unpredictable monsoons, the benefits provided by maintaining large caches are primarily realized by large consumers. Consequently, it is the smaller rodent consumers that may be expected to first feel the effects of increasingly unpredictable monsoons in a future defined by a changing climate.


Abstract

Foraging in uncertain environments requires balancing the risks associated with finding alternative resources against potential gains. In aridland environments characterized by extreme variation in the amount and seasonal timing of primary production, consumers must weigh the risks associated with foraging for preferred seeds that can be cached against fallback foods of low nutritional quality (e.g., leaves) that must be consumed immediately. Here we explore the influence of resource-scarcity, body size, and seasonal uncertainty on the expected foraging behaviors of caching rodents in the northern Chihuahaun Desert by integrating these elements with a Stochastic Dynamic Program (SDP) to determine fitness-maximizing foraging strategies. We demonstrate that resource-limited environments promote dependence on fallback foods, reducing the likelihood of starvation while increasing future risk exposure. Our results point to a qualitative difference in the use of fallback foods and the fitness benefits of caching at the threshold body size of 50 g. Above this 50 g body-size threshold, we observe large fitness gains associated with the maintenance of even a modest-sized cache, whereas similar gains for smaller consumers require maintenance of unrealistically large caches. This suggests that larger-bodied consumers that cache may be less sensitive to the future uncertainties in monsoonal onset predicted by global climate scenarios, whereas smaller consumers, regardless of caching behavior, may be at greater risk.