Seasonal and Nutritional Influences on Growth Hormone and Thyroid Activity in White-Tailed Deer

Blood samples were drawn monthly for 21 months from 4 penned adult female white-tailed deer (Odocoileus virginianus) on high diet, 6 does on low diet, and for 6 months from 3 does semistarved in winter and spring. Growth hormone (GH) showed no difference (P < 0.01) among diets, but was elevated in late spring and summer in all groups. Thyroxine (T4) and triiodothyronine (T,) increased significantly in late spring, presumably in conjunction with advanced pregnancy. Levels of T4 and T3 between winter and summer were not different for either the highor low-diet deer. Does on high diet exhibited greater amounts of T4 and T, during winter and early spring compared to low-diet animals, however. The T4 and T3 levels dropped sharply throughout winter in the semistarved does. These findings suggest that decreased circulating thyroid hormones may serve as a mechanism for reducing energy requirements when deer are confronted with acute winter malnutrition. Measurement of serum T3 appears to provide a particularly valuable indicator of nutritional stress during winter, and probably other periods. J. WILDL. MANAGE. 45(1):140-147 Seasonal changes in white-tailed deer include biannual pelage molts, reduced appetite and metabolism (hence cessation of growth) in winter (Silver et al. 1969, Ozoga and Verme 1970), antler regeneration in spring, lactation during summer, and fat deposition in autumn (Verme and Ozoga 1980). Synchronization of these events is probably cued by photoperiod and mediated by the pineal gland (Plotka et al. 1979), although temperature or nutrition may modulate the exact timing (French et al. 1956, 1960; Verme 1965; Holter et al. 1975). The endocrine interactions associated with these physiological circumstances are poorly understood. Histological and anatomical characteristics of deer endocrine glands show seasonal alterations that suggest diminished thyroid and pituitary function during winter (Hoffman and Robinson 1966). Silver et al. (1969) demonstrated that the fasting metabolic rate of deer in summer was 1.5 times greater than in winter. Seal et al. (1972) reported that thyroxine levels in pregnant does were lower in winter compared to spring, and lowest among animals on poor nutrition. Seal et al. (1978) found that triiodothyronine among fawns on 4 dietary planes in autumn varied markedly according to energy content of their feed. Reported seasonal variations in growth hormone concern only antler growth in male whitetails (Bubenik et al. 1975). Several studies (LeResche et al. 1974, White and Cook 1974, Seal et al. 1978) have indicated that blood assays could eventually be used routinely to diagnose the nutritional status of deer if baseline criteria were established to distinguish shifts in physiology from those caused by trauma or disease. To help elucidate these differences we studied growth hormone (GH), thyroxine (T4), and triiodothyronine (T3) to identify year-round en1 Present address: Department of Pharmacology, St. Louis University School of Medicine, St. Louis, MO 63104. 140 J. Wildl. Manage. 45(1):1981 This content downloaded from 157.55. 39.253 on Wed, 08 Jun 2016 06:04:32 UTC All use subject to http://about.jstor. org/terms GROWTH HORMONE AND THYROID ACTIVITY IN DEER ? Bahnak et al. 141 docrine patterns of penned deer while on different diets; these data are presented here. Serum nitrogen parameters, packedcell volume, and body-weight trends for these animals were reported previously (Bahnak et al. 1979). This paper is in part a contribution from Michigan Federal Aid to Wildlife Restoration Project W-117-R and the Cusino Wildlife Research Station. Bovine pituitary hormones were a gift from the National Institute of Arthritis, Digestive and Metabolic Diseases. We thank M. Tipton and D. Pesola for performing laboratory analyses, and D. DeLisle and C. Bienz for husbandry and handling of animals. T. Naegle helped with the computer analyses. The assistance of D. E. Ullrey in formulating the low diet is gratefully acknowledged. U. S. Seal reviewed the manuscript.