Differential testosterone response to GnRH-induced LH release before and after musth in adult Asian elephant (Elephas maximus) bulls

Abstract

Bull elephants exhibit marked increases in testosterone secretion during musth, and studies have shown a heightened sensitivity of the testis to GnRH-stimulated testosterone production in musth compared to nonmusth males. However, activity of the hypothalamo-pituitary-gonadal axis before or soon after musth has not been studied in detail. The aim of this study was to evaluate LH and testosterone responses to GnRH challenge in nine adult Asian elephant (Elephas maximus) bulls during three periods relative to musth: premusth, postmusth, and nonmusth. Bulls were administered 80 μg of a GnRH agonist, and blood was collected before and after injection to monitor serum hormone concentrations. The same bulls were injected with saline 2 weeks before each GnRH challenge and monitored using the same blood collection protocol. All bulls responded to GnRH, but not saline, with an increase in LH and testosterone during all three periods. The mean peak LH (1.76 ± 0.19 ng/mL; P < 0.001) and testosterone (6.71 ± 1.62 ng/mL; P = 0.019) concentrations after GnRH were higher than the respective baselines (0.57 ± 0.07 ng/mL, 3.05 ± 0.60 ng/mL). Although basal- and GnRH-induced LH secretion were similar across the stages, evaluation of the area under the curve in GnRH-treated bulls indicated that the testosterone response was greatest during premusth (2.84 ± 0.76 area units; P = 0.019) compared to postmusth (2.02 ± 0.63 area units), and nonmusth (2.01 ± 0.46 area units). This confirms earlier reports that GnRH stimulates LH release and subsequent testosterone production in bull elephants. Furthermore, although the hypothalamo-pituitary-gonadal axis is active throughout the year, the testis appears to be more responsive to LH in terms of testosterone production in the period leading up to musth, compared to the nonmusth and postmusth periods. This heightened sensitivity, perhaps as a result of LH receptor up-regulation, may prime the testis for maximal testosterone production, leading to the physiological and behavioral changes associated with musth.

Introduction

Musth is the circannual period of heightened sexual activity typically observed in mature Asian and African bull elephants. Musth comprises physiological, anatomical, and behavioral changes that play an important role in elephant society by helping ensure that dominant bulls produce the most offspring [1], [2]. Musth is typically associated with increases in serum androgen concentrations and temporal gland secretions (TGS) and continuous urine dribbling (UD) [3], [4], [5], [6]. Rajaram (2006) [7] described five stages of musth: premusth, earlymusth, mid-musth, postmusth, and nonmusth. In premusth, volatile compounds are released from the temporal gland. The early- and mid-musth stages are characterized by TGS, which becomes odorous as the period progresses. During mid-musth, TGS and continual UD are observed, and the bull often becomes very aggressive [7] in conjunction with a significant increase in circulating androgens [4], [6], [8], [9], [10], [11]. These signs decrease during postmusth and disappear during the nonmusth period. In Asian range countries, most healthy adult bulls exhibit musth at a predictable time each year although the timing can vary between individuals [8]. In Thailand, musth is most often observed during the late rainy season or early winter (November–January) and lasts from 1 week to 3 months or more.

Musth in elephants appears to be under control of the hypothalamo-pituitary-gonadal (HPG) axis; indeed, exogenous GnRH induces a surge of LH secretion, followed by the release of testosterone [8], [12]. Frequent sampling of Asian elephant bulls has further demonstrated that pulses of testosterone closely follow those of LH (∼1 pulse/3 hours) [13]. In a long-term study (4 years) of one African bull, serum LH increased about 4 weeks before the onset of musth and was maintained for approximately 5 weeks and was considered likely to be responsible for triggering the rise in testosterone [14]. The temporal relationship between aggressive behaviors and androgen production during musth suggests that the two are linked although the regulatory mechanisms are unknown. Temporally, TGS and UD occur one to several weeks after the initial rise in testosterone, with TGS almost always observed before UD [4], [5], [8], [10]. Aggressive behavior also appears to be more intense when testosterone begins to decline [8], [12]. In short, there tends to be a lag between peak testosterone production and musth symptoms, not unlike that described for Soay sheep, where maximal aggression occurs after androgens begin to decrease [15]. Men also exhibit increased aggression, anger, and irritability after a temporary reduction in testosterone [16]. However, mahouts can often tell when a bull is coming into musth based on behavioral changes, which presumably is due to increasing testosterone concentrations.

Based on frequent blood sampling (every 15 minutes for 12 hours), little difference in LH and testosterone pulse frequency was observed between musth and nonmusth Asian bulls [13]; however, pulse amplitude and pulse area were significantly increased during musth. Likewise, in African elephants, the testes of musth bulls appeared to be hyper-responsive to LH with regard to testosterone secretion after GnRH injection, compared to nonmusth males [12]. There are, however, no reports on whether the sensitivity of the elephant HPG axis differs during the more discrete periods of pre and postmusth compared to nonmusth in Asian or African elephant bulls. The objective of this study was therefore to compare the LH and testosterone responses to GnRH administered during the nonmusth, premusth, and postmusth periods in captive Asian elephant bulls.