Circulating gonadotropins and testicular hormones during sexual maturation and annual changes in male bottlenose dolphins (<i>Tursiops truncatus</i>)

Abstract

To reveal the reproductive biology in male bottlenose dolphins (Tursiops truncatus), circulating gonadotropins (follicle stimulating hormone [FSH] and luteinizing hormone [LH]) and testicular hormones (testosterone and inhibin) were monitored for 8−12 years in 2 captive bottlenose dolphins (Mars and Regulus). During the study period, Mars was undergoing sexual maturation, whereas Regulus was already mature at the beginning of the study. Assuming that Mars had reached sexual maturity when the significant increase in circulating testosterone levels was observed, serum concentration of inhibin was higher in the sexually immature stage than in the mature stage, whereas the serum concentration of FSH was higher in the sexually mature stage than in the immature stage. No difference was observed in the LH levels between pre- and post-sexual maturation. There was a significant increase in serum concentration of testosterone during spring in both animals. These results suggest that the mechanism responsible for regulating FSH secretion by inhibin functions during the sexually immature stage in this species.

Circulating inhibin and testosterone during sexual maturation and reproductive seasonality of captive male killer whales (Orcinus orca)

“…In contrast, there was no seasonal effect on circulating inhibin. These seasonal changes are consistent with the results of studies on bottlenose dolphins (Katsumata et al, 2017, Schroder, 1990 and existing evidence on killer whales (Robeck et al, 2004. According to the latter, seasonal fluctuations in testosterone secretion within each animal had no bearing on sperm production.…”

“…The circulating inhibin concentrations in Oscar were high in the juvenile stage and declined abruptly after sexual maturity. This dramatic change in circulating inhibin concentrations showed a clear negative correlation with testosterone during the mature stage, confirming previous reports in beluga (Delphinapterus leucas) (Katsumata et al, 2012) and bottlenose dolphins (Tursiops truncatus) (Katsumata et al, 2017). Importantly, the same high concentrations of circulating inhibin prior to sexual maturity coupled with low concentrations thereafter have been observed also in terrestrial mammals, such as bull calves (Kaneko, et al, 2001, Matsuzaki, et al, 2000, 2001a, 2001b, boars (Kanematsu et al, 2006), and horses (Dhakal et al, 2011(Dhakal et al, , 2012.…”

“…Importantly, the same high concentrations of circulating inhibin prior to sexual maturity coupled with low concentrations thereafter have been observed also in terrestrial mammals, such as bull calves (Kaneko, et al, 2001, Matsuzaki, et al, 2000, 2001a, 2001b, boars (Kanematsu et al, 2006), and horses (Dhakal et al, 2011(Dhakal et al, , 2012. Analysis of male bottlenose dolphins (Katsumata et al, 2017) revealed that while follicle-stimulating hormone (FSH) concentrations were low in the immature stage, they were high in the mature stage, showing an opposite pattern to inhibin. Such negative relationship between FSH and inhibin during the developing stage has been documented also in bull calves (Kaneko et al, 2001, Matsuzaki et al, 2000, 2001, boars (Kanematsu et al, 2006), and horses (Dhakal et al, 2011(Dhakal et al, , 2012, suggesting that inhibin functions as a suppressor of FSH secretion in male killer whales.…”

“…However, one of the testicular hormones, inhibin has not been investigated in male killer whales. Inhibin is a glycoprotein hormone secreted by the gonads in mammals (Medan et al, 2007, Chaichanathong et al, 2017, birds (Akashiba et al, 1988, Yang et al, 2005a, 2005b, 2005c, and some Cetaceans, such as beluga (Katsumata et al, 2012) and bottlenose dolphin (Katsumata et al, 2017).…”

Addressing Combative Behaviour in Spanish Bulls by Measuring Hormonal Indicators

“…However, the serum testosterone concentration in the sexually mature control bull group was significantly lower than that in the calf group. As the testosterone concentration varies throughout an animal's life, specifically at the stage of sexual maturation [70], the high levels obtained in the calf group may be due to the acute stress to which calves are subjected before and during shoeing [67].…”