The effects of contrasting (16L:8D vs. 8L:16D) photoperiods on fat accretion and growth in 19 white-tailed doe fawns (Odocoileus virginianus) from 9 September to 16 December 1981 were examined. Fawns exposed to long days (LD fawns) delayed their autumn pelage change, whereas those exposed to short days (SD fawns) underwent a normal molt. SD fawns were heavier and accumulated more fat than their LD counterparts. The combined skeletal muscle and viscera of fawns exposed to short daylength contained 1,195 and 2,269 g of fat on 10 November and 16 December, respectively, as compared with 290 and 621 g of fat for LD fawns. SD fawns also had a higher daily intake of feed and were more efficient in converting dietary gross energy into tissue gross energy than LD fawns. Serum prolactin concentrations were substantially lower in SD fawns. Diminishing daylength appears to set the time frame for physiological events associated with fat accretion in white-tailed deer. J. WILDL. MANAGE. 48(3):776-787 Recently, researchers have become interested in evaluating the effects of photoperiod on certain physiological changes that deer undergo annually. Seasonal changes in pelage and feed intake have been linked to changing daylength in red deer (Cervus elaphus) (Pollock 1974, Kay and Ryder 1978). White-tailed deer, responding to an artificially extended summer photoperiod, delayed pelage change, and a decrease from a long day (16L:8D) to a short day (8L:16D) stimulated increased weight gain and sexual maturity (Budde 1983). Antler growth cycles also appeared to be mediated by photoperiod (Goss 1980). It is not clear how photoperiod induces these effects. Many physiological events are under hormonal control. The pineal gland, a light-sensitive organ, may play a role in mediating various hormone concentrations seasonally, thereby implicating changing daylength in circannual endocrine rhythms (Schulte et al. 1981). In domestic ruminants, serum prolactin concentrations follow a seasonal pattern, with lowest concentrations occurring during winter and highest concentrations during summer (Koprowski and Tucker 1973, Tucker et al. 1974). Similar seasonal patterns in prolactin concentrations have been demonstrated in white-tailed deer (Mirarchi et al. 1978, Schulte et al. 1980). Verme and Ozoga (1980) postulated that lipogenesis is mediated by a decline in prolactin and a rise in adrenocorticotrophic hormone, thereby suggesting a possible relationship between decreasing daylength (autumn), decreasing prolactin concentrations, and increased lipogenesis. The objective of this study was to determine the influence of different photoperiods on the growth and fat metabolism of white-tailed doe fawns as measured by 'Present address: Caesar Kleberg Wildlife Research Institute, Texas A&I University, Kingsville, TX 78363. 776 J. Wildl. Manage. 48(3):1984 This content downloaded from 157.55.39.132 on Thu, 15 Sep 2016 06:06:09 UTC All use subject to http://about.jstor.org/terms EFFECT OF PHOTOPERIOD ON DEER * Abbott et al. 777 changes in body composition and in vitro rates of fatty acid synthesis and lipolysis in adipose tissue. We appreciate the assistance provided by C. L. Bennett, W. S. Budde, T. M. Cooley, P. D. Friedrich, T. F. Reis, and L. L. Swanson of the Michigan Dep. of Nat. Resour. and by M. H. Clapsaddle, M. K. Hodge, G. C. Holt, W. V. Rumpler, G. C. Shurson, J. F. Sikarskie, P. A. Whetter, and J. R. Witwer of Michigan State Univ. This is Michigan Agric. Exp. Stn. J. Article 10797. The study was supported in part by the Michigan Dep. of Nat. Resour. through Federal Aid in Wildl. Restoration Proj. W-117-R.