Seasonal pattern of luteinizing hormone and testosterone pulsatile secretion in young adult red deer stags (Cervus elaphus) and its association with the antler cycle
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Summary. Blood from stags aged 15 months (n = 6) was sampled at monthly intervals every 30 min for 24 h for 12 months, at 45°S in New Zealand. Three extra samplings each for 24 h were carried out at about the anticipated time of antler casting. All samples were analysed for luteinizing hormone (LH) and testosterone and the resulting data further analysed by the pulsar pulse detection routine. The animals were kept indoors under natural daylength and were fed ad libitum. All animals were weighed, antler status and size recorded and testes diameter was measured on each sampling day. Mean LH and testosterone pulsatility and plasma concentrations varied seasonally. LH pulse frequency was low during autumn (2·5 pulses in 24 h), winter (1·0–1·5 pulses in 24 h) and early spring (1 pulse in 24 h) and lowest in late spring (0·2 pulses in 24 h) before rising in summer (1·0–4·0 pulses in 24 h). LH pulse amplitude and mean plasma concentration were low (<1 ng ml−1) from March to November (autumn–spring); both rose to a peak in January (summer) of 3·4 and 1·6 ng ml−1, respectively. Testosterone pulse frequency was generally similar to LH except that slightly more pulses of testosterone than of LH were detected from March to November and more pulses of LH from November to February (summer). Testosterone pulse amplitude fell from March to November (5·3 ng ml−1 to undetectable) although there was a conspicuous peak in July (midwinter) of almost 5 ng ml−1. Testosterone pulse amplitude remained low until February (late summer) when it rose to 3·7 ng ml−1. Mean testosterone concentrations were high in autumn (2·7 ng ml−1) during the breeding season, when the stags were 15 months old. Mean testosterone concentrations fell gradually during winter and reached a nadir in midspring of 0·2 ng ml−1. Testosterone concentrations were low during late spring to early summer (0·2 ng ml−1), but rose to a peak in late summer (3·7 ng ml−1), when the stags were 26 months old. From 12 weeks before antler casting, LH pulses of low frequency (0·2 pulses in 24 h) and low amplitude (0·1 ng ml−1) were detected, resulting in diminishing mean testosterone. At the time of antler casting, no pulses of LH or testosterone were detected and mean concentrations of both hormones were very low or undetectable. After antler casting, LH increased to 4 pulses in 24 h, but testosterone remained low (<0·6 ng ml−1) until the velvet antler was almost fully grown. High testosterone concentrations (3·7 ng ml−1) were associated with velvet antler cleaning in late summer. This study provides a description of the ontogeny of the seasonal nadir in mean testosterone in red deer stags, which is believed to facilitate antler casting and antler regeneration.
Keywords: luteinizing hormone; testosterone; seasonal variation; antler cycle; deer
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