We limit our discussion of food storage, or caching, to the movement of potential food items from one location to another for eating at some later time. This activity occurs exclusively in those animals that bring food to their offspring, and not in those that bring their offspring to food (i.e. that lay their eggs near food in a favorable microhabitat). Provisioning offspring is limited taxonomically to mammals, most birds, and some Hymenoptera. Moving food to a favorable microhabitat was apparently the first transitional step in the evolution of more complex systems of provisioning offspring in hymenopterans (72). Although all species that cache food provision their offspring, the converse is not true. Relatively few of the animals that provision their young with food also cache food, and caching food has no obvious connection to the glandular secretion of milk, which probably initiated offspring provisioning in the evolutionary history of mammals. Repeated traveling from a foraging area to dependent young seems to precondition animals for caching food. One of the goals of our review will be to determine what other conditions among species of birds and mammals and their food favor the evolution of caching. We limit our discussion to birds and nonhuman mammals because of our own backgrounds and because experimental studies have recently been done on these vertebrates (4, 14, 20, 48a, 62, 10la, 103-105, 116, 117, 128), although earlier investigations started with wasps (121). A corollary of provisioning offspring with food is a stable, relatively high metabolic rate (even in the Hymenoptera), which ensures that parents can maintain a steady flow of food to the relatively few, "expensive" offspring

The Evolution of Food Caching by Birds and Mammals

Natural selection and specialized chorusing behavior in acoustical insects

Gray and fox squirrels show similar preferences for various types of natural foods. Their preferences are based on a combination of two factors; the speed with which they can ingest food energy and the digestibility of the food eaten. The two species of squirrels have essentially the same ability to digest their natural foods. However, five species of nuts differ in their digestibility, ranging in percentage of energy assimilated from about 78% for white oak to 95% in shagbark hickory. The squirrels' efficiency in digesting the kernels of different species of nuts is correlated with the lipid content of the kernels. The niche differences between gray and fox squirrels are not based on food preference or feeding efficiency. Instead, they are probably related to differences in foraging behavior and predator escape behavior which adapt fox squirrels to open forests and forest edges and gray squirrels to dense forests. The annual activity pattern of gray and fox squirrels make the hard—shelled nuts of hickory and walnut the most efficient food in fall and spring and acorns the most efficient food in winter. This change in the relative efficiency of foods results in the squirrels acting as dispersing agents for the seeds of both oaks and hickories in mixed stands of trees.

Food Preferences of Squirrels

Only model organisms live in a world of endless summer. Fitness at temperate latitudes reflects the ability of organisms in nature to exploit the favorable season, to mitigate the effects of the unfavorable season, and to make the timely switch from one life style to the other. Herein, we define fitness as Ry, the year-long cohort replacement rate across all four seasons, of the mosquito, Wyeomyia smithii, reared in its natural microhabitat in processor-controlled environment rooms. First, we exposed cohorts of W. smithii, from southern, midlatitude, and northern populations (30–50°N) to southern and northern thermal years during which we factored out evolved differences in photoperiodic response. We found clear evidence of evolved differences in heat and cold tolerance among populations. Relative cold tolerance of northern populations became apparent when populations were stressed to the brink of extinction; relative heat tolerance of southern populations became apparent when the adverse effects of heat could accumulate over several generations. Second, we exposed southern, midlatitude, and northern populations to natural, midlatitude day lengths in a thermally benign midlatitude thermal year. We found that evolved differences in photoperiodic response (1) prevented the timely entry of southern populations into diapause resulting in a 74% decline in fitness, and (2) forced northern populations to endure a warm-season diapause resulting in an 88% decline in fitness. We argue that reciprocal transplants across latitudes in nature always confound the effects of the thermal and photic environment on fitness. Yet, to our knowledge, no one has previously held the thermal year constant while varying the photic year. This distinction is crucial in evaluating the potential impact of climate change. Because global warming in the Northern Hemisphere is proceeding faster at northern than at southern latitudes and because this change represents an amelioration of the thermal environment and a concomitant increase in the duration of the growing season, we conclude that there should be more rapid evolution of photoperiodic response than of thermal tolerance as a consequence of global warming among northern, temperate ectotherms.

I. Muul

Papers

Behavioral and physiological influences on the distribution of the flying squirrel, Glaucomys volans