Photoperiod and melatonin effects on growth and endocrine response in white-tailed deer (Odocoileus virginianus)

Abstract

Captive white-tailed deer (Odocoileus virginianus) fawns (4-11 mo) were utilized to establish the role of endogenous versus exogenous melatonin on patterns of growth. A deuterium oxide (D20) dilution system was developed with I I fawns for estimation of body composition in deer. Empty body weight, water, lean body mass, protein, and fat were predicted with R2s of .989,.988,.985,.961,.656, respectively. Lean body mass and its'components, fat, and gut fill, were well predicted with the developed systems. Fawns were fed melatonin (5 mg/d at 2 h before sunset) or no melatonin, and fed high, medium, or low intake. Body composition was measured at four points, melatonin at six time points, and T3, T4, and IGF-I at ten time points during the experiment. Melatonin levels increased, as daylength declined from 10.9 to 10.3 h/d, from 74.3 pg/ml at day 12 to 120.7 pg/rnl at day 83, then decreased to 84.7 pg/ml at day 159, as daylength increased to 12.4 hr. Body fat, initially 2.33 kg, increased in short days to 3.19 kg, and then declined to 2.07 kg at day 159. Daily protein gain averaged 13 g/d in non-melatonin fed fawns but declined from 21 to 3.4 g/d over the length of the trial (P = .0001) in deer receiving melatonin. Melatonin fed fawns maintained (P < .05) elevated melatonin (average 92 pg/ml higher) until day 159. Melatonin concentrations were similar for male and female fawns. Plasma concentrations of T3 were similar for control and melatonin (average 312 ng/rnl, d 12) fed fawns, and generally mirrored daylength, decreasing (196 ng/ml, d 83) and then increasing in parallel with daylength (203.3 ng/dI, d 180). Plasma T4 concentrations averaged 26.1 28.6, and 23.7 gg/dl at day-4, 83, and 180. IGF-I concentrations in controls began at 66.5 pg/ml, (d-4) decreased to 36.4 pg/ml (d 83), and then increased to 67.09 pg/ml (d 180) in parallel with daylength (r--.832). Photoperiod through melatonin regulated fat storage and retrieval, and melatonin treatment reduced rates of protein deposition in deer fawns. Photoperiod, through melatonin, was the driving force in the growth and development patterns in these growing fawns.