Circulating Testosterone and Inhibin Levels at Different Ages in the Male Beluga (<i>Delphinapterus leucas</i>)

Abstract

ABSTRACT. This study is the first report on circulating testosterone and inhibin levels in a species of whales, the beluga. Circulating testosterone and immunoreactive (ir-) inhibin levels in two captive male belugas ("Nack", originally from Canada and "Duke", from the Okhotsk Sea) were measured every month for 9 years between 1995 and 2003. Assuming that clearly increased testosterone levels in the circulation indicates that the belugas had reached sexual maturity, at the ages of 10 ("Nack") and 11 years old ("Duke"). Their testosterone levels before the significant increase (pre-pubertal) were 0.42  0.07 ng/ml (n=18) and 0.35  0.10 ng/ml (n=18) and, those of after the increase (maturity) were 1.65  0.14 ng/ml (n=74) and 2.06  0.14 ng/ml (n=74). Circulating ir-inhibin levels before sexual maturity were 0.78  0.04 ng/ml (n=18) and 0.64  0.04 ng/ml (n=15) and, after sexual maturity were 0.52  0.02 ng/ml (n=56) and 0.43  0.02 ng/ml (n=67). Seasonal changes were observed in the testosterone levels after sexual maturity and the levels increased during March and April in Canadian origin "Nack", and peaked in February in Okhotsk origin "Duke". Circulating ir-inhibin level gradually decreased as they aged. A negative correlation between the circulating testosterone and ir-inhibin was observed. No seasonal changes were observed in the ir-inhibin levels after sexual maturity. These data will surely correspond to clarification of endocrinology and the successful reproduction of the beluga.

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

“…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.…”

“…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).…”

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

“…To date, there are only a few studies that have been dedicated to investigating cetacean inhibin levels. Katsumata et al [ 13 ] determined serum testosterone and inhibin levels in two captive belugas ( Delphinapterus leucas ) and demonstrated that these two hormones are negatively correlated and inhibin levels were higher in the sexually immature stage than in the mature stage. In the present study, changes in testicular functions during the growth process of male captive bottlenose dolphins ( Tursiops truncatus ) were revealed by measurements of circulating gonadotropins (FSH and LH) and testicular hormones (testosterone and inhibin) in blood samples taken from two male captive bottlenose dolphins reared at Kamogawa Sea World in Japan for 8 and 12 years.…”