There is a marked difference in nutritional content of seed kernels in the families Fagaceae and Juglandaceae. The kernels of seeds in hickory nuts (Carya spp.) and walnuts (Juglans spp.), among the Juglandaceae, have a relatively high content of protein (>20%) (Wainio and Forbes 1941, Baumgras 1944) and lipids (> 20%) (Wainio and Forbes 1941, Baumgras 1944, Smith and Follmer 1972), giving the kernels a high energy content per gram of dry weight (> 25,200 joules) (Smith and Follmer 1972). Among the Fagaceae are acorns and chestnuts (Casta-nea), which are relatively low in protein (< 10%) (Wainio and Forbes 1941, Baumgras 1944, King and McClure 1944, Ofcarcik and Burns 1971, Short 1976, Short and Epps 1976) and are variable by oak subgenus in lipids (Baumgras 1944, King and McClure 1944, Ofcarcik and Burns 1971, Smith and Follmer 1972, Short 1976, Short and Epps 1976, and Chapter 11), giving them a relatively low, but variable, energy content per gram of dry weight (Smith and Follmer 1972, Havera and Smith 1979). Acorns in the subgenus of white oaks (Leucobalanus) and chestnuts (Wainio and Forbes 1941) have a low lipid content (< 10%) and energy content per gram of dry weight (< 21,000 joules). In contrast, acorns in the subgenus of red oaks (Erythrobalanus) have a relatively high lipid content (varying around 20%) and energy content per gram of dry weight (> 21,000 joules). Although smaller, the seed kernels of species in the Pinaceae are generally similar to those in the Juglandaceae, having proportionately high concentrations of protein (> 20%) and lipids (> 20%) (King and McClure 1944) and energy content per gram of dry weight (> 25,200 joules) (Smith 1968). Kernels of seeds in the Fagaceae are thus less nutritionally desirable for tree squirrels than are the seed kernels from the Juglandaceae and Pinaceae.
The food value that squirrels obtain from different species of nuts is often difficult to measure, because when squirrels feed in the field they sometimes leave behind, uneaten, part of the kernel of hickory nuts when they discard the remains of the nut and they often leave part of the kernel of walnuts. The shells of walnuts and hickory nuts are convoluted and partition the kernel into lobes separated by hard shell. When opening a walnut, squirrels will gnaw through a thin part of the shell and scrape out the part of the kernel that they can reach with their incisors through the hole. Over half of the kernel of some nuts may be discarded. Gray squirrels eating black walnuts in captivity spend an average of 15 minutes on a walnut before discarding it with kernel left uneaten (Smith and Follmer 1972), while a fox squirrel observed on a golf course in late spring took 45 minutes to consume all the kernel from a black walnut (C.C. Smith, personal observation). Nixon and Hansen (1987) calculate that fox squirrels require 16 walnuts or 3 bur oak acorns as a daily ration in winter on the basis of observation of captive squirrels. Smith and Follmer (1972) calculated the caloric content of black walnut kernels to be approximately 60% of the caloric content of bur oak acorn kernels. It is likely that squirrels, as their cached stores run low, will put more effort into finishing the kernel of a walnut.
The efficiency with which red, gray, and fox squirrels metabolize the energy of seed kernels correlates closely with the lipid content of the kernels (Smith and Follmer 1972, Havera and Smith 1979). When presented with shelled seed kernels, gray and fox squirrels show a preference for the kernels of walnut and hickory nuts over those of both red and white subgenera of acorns. They also show a preference for the kernels of one species of red oak over those of two species of white oak acorns (Smith and Follmer 1972). Another factor that influences preference is the extra mass of gut content that results from an acorn diet. This can be an adaptive problem for tree squirrels, which must lift their body weight up trees to acquire food and escape some enemies. Stalheim-Smith (1984), comparing the forces exerted by the muscles of the forelimbs of fox squirrels and of Gunnison's prairie dogs (Cynomys gunnisoni) of similar mass, determined that those of fox squirrels are slightly greater. However, the muscles of fox squirrels fatigue slightly faster than those of Gunnison's prairie dogs. Extra weight is therefore a handicap for tree squirrels but an advantage for prairie dogs, which are moving soil with their forelimbs. It is significant that tree squirrels store very little body fat, cache food, and do not hibernate, while semifossorial sciurids store considerable body fat, usually do not cache food, and undergo periods of torpor.
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