Seasonal effects of food provisioning on body fat, insulin, and corticosterone in free-living juvenile Belding's ground squirrels (Spermophilus beldingi)

We evaluated the relationship between body fat and the metabolic hormones insulin and corticosterone in free-living juvenile Belding's ground squirrels (Spermophilus beldingi). Some free-living juveniles were provisioned with extra food high in fat and calories, and unprovisioned juveniles were used as controls. Body fat (as a percentage of body mass) of squirrels was regularly evaluated using non-invasive methods. Blood samples were also regularly collected from juveniles and assayed for plasma concentrations of insulin and corticosterone. Provisioned juveniles were fatter than unprovisioned juveniles during most of the active season, but differences in body fat disappeared near the end of summer as juveniles began fattening for hibernation. Thus, our data indicate that although body fat may fluctuate with energy availability early in the active season, it is regulated at precise levels as squirrels prepare for winter. Plasma corticosterone concentrations did not differ between provisioned and unprovisioned juveniles. However, plasma insulin concentrations were greater in provisioned than in unprovisioned juveniles early in the summer. Differences in insulin concentrations disappeared later in the active season, just prior to the disappearance of differences in body fat. Moreover, plasma insulin concentrations were significantly correlated with subsequent changes in body fat. Thus, our data suggest a link between insulin and seasonal regulation of body fat in juvenile S. beldingi.

Use of visual cues in foraging by a diurnal herbivore, Belding's ground squirrel

Ground squirrels' vision has been fairly well characterized, but whether they actually use vision in foraging has not been demonstrated. Belding's ground squirrels (Spermophilus beldingi) were allowed to forage under natural conditions for experimental bait stations to test the hypothesis that they can learn to use visual cues to forage more efficiently. Bait stations with visual cues were more likely to be found, and were found more quickly, than controls. Bait stations in taller and (or) denser vegetation were less likely to be found, suggesting that the value of visual cues may change during the growing season. Visual cues can be used to increase foraging efficiency, but other cues are probably used as well.