Belding's Ground Squirrel
We are interested in both the long-term fitness consequences of inter-group movements, and the short-term physiological and social consequences of immigration. Using integrated field and laboratory approaches, we are investigating these problems in hyenas and also in free-living Belding's ground squirrels near Yosemite National Park in California. Background Belding's ground squirrels (Spermophilus beldingi) are group-living diurnal rodents that inhabit alpine and subalpine meadows of the western United States. The life history of Belding's ground squirrels is characterized by 4-month active seasons interspersed with 8-month hibernation periods. Their annual cycle is rigidly constrained by extreme seasonal changes in climate, and ontogenetic development is accelerated compared to that of non-hibernating rodents of similar body size. Between weaning and entry into hibernation 7-12 weeks later, body mass of juveniles increases 4-6 fold. This rapid development presumably permits juveniles to attain body mass and fat reserves necessary for overwinter survival.The breeding season of Belding's ground squirrels ordinarily begins 1-2 weeks after emergence from hibernation in the spring, and lasts 3-4 weeks. Adult males are reproductively competent when they emerge from hibernation, whereas yearling males generally do not exhibit descended testes and do not mate. Variance in mating success is high, and only a small proportion of males successfully mate during the breeding season. Body mass of male squirrels is positively correlated with success in winning fights with other males during the breeding season, and also with mating success. Adult and yearling female Belding's ground squirrels emerge from hibernation slightly later than do males, and most females (>60%) begin mating as yearlings. Females exhibit estrous behavior on only one day per year, and appear to exhibit mate choice behavior, accepting the mating attempts of some males while rejecting those of others. Litters are multiply sired, typically by 2-3 males. Gestation lasts 24-25 days, and the lactation interval is 25-28 days. Young remain underground in their natal burrows until weaning. Females maintain maternal territories during gestation and lactation, and often act cooperatively with female kin in defending these territories. Female Belding's ground squirrels are able to discriminate between littermate sisters, non-littermate sisters, and other female squirrels, and this capacity to recognize kin appears to facilitate nepotistic interactions among females. Natal dispersal in Belding's ground squirrels occurs predominantly among males. Most males (40-90%) emigrate from their natal areas as juveniles, and 100% of surviving males disperse by the end of their yearling year. By contrast, most females remain in their natal areas throughout their lives, and only about 8% of females ordinarily emigrate. The probability of natal dispersal by male Belding's ground squirrels is not proximally influenced by social or ecological factors. All males eventually disperse regardless of population density, ectoparasite load, conspecific aggression, the frequency of other social interactions with conspecifics, and the availability of food, space, or burrows. Instead, a suite of behaviors, including increased locomotion, exploration, and boldness, appear to be triggered at some point during ontogeny. These behaviors evidently facilitate emigration from the natal area. The trigger of these behaviors appears to be attainment of a particular body mass or body composition. Our Work With Belding's Ground Squirrels We have studied the dispersal behavior of Belding's ground squirrels in the Sierra Nevada Mountains since 1979. Since 1993 we have been studying a population at Tioga Pass (pictured). Our recent work has focused on the physiological mediation of the natal dispersal behavior. In particular, we are examining three classes of physiological variables as potential regulators of dispersal behavior: availability of energy as reflected in body fat, internal timing mechanisms, and gonadal steroid hormones. We are especially interested in how these variable interact with each other in the mediation of dispersal behavior. Sample abstract from our work with Belding's ground squirrels Body fat and time of year interact to mediate dispersal in ground squirrels Why do female Belding's ground squirrels disperse away from food resources? Mass and fat influence the timing of dispersal in Belding's ground squirrels Proximal causes of natal dispersal in Belding's ground squirrels Hormonal influences on natal dispersal in free-living Belding's ground squirrels Natal dispersal in Belding's ground squirrels Publications on Belding's ground squirrels Holekamp, K.E. (2004) A view from the field: What the lives of wild animals can teach us about the care of laboratory animals. Proceedings of the International Workshop on Development of Science-Based Guidelines for Laboratory Animal Care. ILAR Journal. Institute for Laboratory Animal Research. National Research Council, Washington, DC. In Press. Nunes, S., E. M. Muecke, and K. E. Holekamp (2002) Seasonal effects of food provisioning on body fat, insulin, and corticosterone in free-living juvenile Belding’s ground squirrels (Spermophilus beldingi) Canadian Journal of Zoology.80: 1-6. Nunes, S., E. M. Muecke, H. E. Ross, P. A. Bartholomew, and K. E. Holekamp (2000) Food availability affects behavior but not circulating gonadal hormones in maternal Belding’s ground squirrels. Physiology and Behavior 71:447-455. Nunes, S. T. R. Duniec, S. A. Schweppe, and K. E. Holekamp (1999) Energetic and endocrine mediation of natal dispersal behavior in Belding’s ground squirrels. Hormones and Behavior. 35: 113-124. McElhinny, T. L., C.L. Sisk, K. E. Holekamp, and L. Smale (1999) A morning surge in plasma LH coincides with elevated Fos expression in GnRH-IR neurons in the diurnal rodent Arvicanthis niloticus. Biology of Reproduction. 61: 1115-1122. Nunes, S., C. T Ha, P. J. Garrett, E. M Muecke, L. Smale, and K. E. Holekamp (1998) Body fat and time of year interact to mediate dispersal behaviour in ground squirrels. Animal Behaviour. 55: 605-614. McElhinny, T. L., L. Smale, and K. E. Holekamp. (1997) Patterns of body temperature, activity, and reproductive behavior in a tropical murid rodent, Arvicanthis niloticus. Physiology and Behavior. 62: 91-96. Nunes, S., Zugger, P. A., A. L. Engh, K. O. Reinhart, and K. E. Holekamp (1997) Why do female ground squirrels disperse away from food resources? Behavioral Ecology and Sociobiology. 40: 199-207. Nunes, S. and K. E. Holekamp. (1996) Body mass and body fat influence the timing of natal dispersal in Belding's ground squirrels (Spermophilus beldingi). Journal of Mammalogy.77: 807-817. Holekamp, K. E. and F. Talamantes (1992) Seasonal fluctuations in hormones and behavior of free-living male California ground squirrels (Spermophilus beecheyi). Hormones and Behavior. 26: 7-23. Holekamp, K. E. and F. Talamantes (1991) Seasonal variation in circulating testosterone and estrogens of wild-caught California ground squirrels (Spermophilus beecheyi). Journal of Reproduction and Fertility. 93: 415-425. Holekamp, K. E. (1991) Small mammals. Pp 178-181 in R. R. Baker (Ed.) Fantastic Journeys. Weldon-Owen Publishers. Sydney, Australia. Holekamp, K. E. and P. W. Sherman (1989) Why male ground squirrels disperse. American Scientist. 77: 232-239. Holekamp, K. E. and S. Nunes (1989) Seasonal variation in body weight, fat, and behavior of free-living California ground squirrels (Spermophilus beecheyi). Canadian Journal of Zoology. 67: 1425-1433. Holekamp, K. E., S. Nunes, and F. Talamantes (1988) Patterns of progesterone secretion in free-living California ground squirrels (Spermophilus beecheyi). Biology of Reproduction. 39: 1051-1059. Holekamp, K. E., S. Nunes, and F. Talamantes (1988) Circulating prolactin in free-living California ground squirrels (Spermophilus beecheyi). General and Comparative Endocrinology. 71: 484-492. Colosi, P., K. E. Holekamp, G. Thordarson, J. N. Southard, and F. Talamantes (1987) Purification and partial characterization of prolactin from the California ground squirrel (Spermophilus beecheyi). Biology of Reproduction. 36: 1017-1023. Thordarson, G., K. E. Holekamp, and F. Talamantes (1987) Development of an homologous radioimmunoassay for secreted prolactin from the California ground squirrel (Spermophilus beecheyi). Biology of Reproduction. 36:1186-1190. Holekamp, K. E. (1986) Proximal causes of natal dispersal in Belding's ground squirrels (Spermophilus beldingi). Ecological Monographs. 56: 365-391. Holekamp, K. E., H. B. Simpson, and L. Smale (1985) Endocrine influences on natal dispersal in Belding's ground squirrels (Spermophilus beldingi). Pp. 397-408 in M. A. Rankin (ed.) Migration: Mechanisms and Adaptive Significance. Contributions in Marine Sciences (Supplement). Vol. 27. Holekamp, K. E., L. Smale, H. B. Simpson, and N. A. Holekamp (1984) Hormonal influences on natal dispersal in free-living Belding's ground squirrels (Spermophilus beldingi). Hormones and Behavior 18: 465-483. Holekamp, K. E. (1984) Natal dispersal in Belding's ground squirrels (Spermophilus beldingi). Behavioral Ecology and Sociobiology. 16:21-30. Holekamp, K. E. (1984) Dispersal in ground-dwelling sciurids. Pp. 297-320 in J. O. Murie and G. R. Michener (eds.) Biology of Ground-Dwelling Squirrels. University of Nebraska Press. Lincoln, Nebraska. Holekamp, K. E. (1983) Proximal mechanisms of natal dispersal in Belding's ground squirrels (Spermophilus beldingi). PhD Dissertation. University of California, Berkeley. Holekamp, K. E. and D. B. Reutener (1981) Observation, description, and analysis of the captive behavior of Marmosa murina. JSAS Cat. Sel. Doc. Psych. Vol 11. 69 pp. |