Abstract
Male baleen whales have long been suspected to have annual cycles in testosterone, but due to difficulty in collecting endocrine samples, little direct evidence exists to confirm this hypothesis. Potential influences of stress or adrenal stress hormones (cortisol, corticosterone) on male reproduction have also been difficult to study. Baleen has recently been shown to accumulate steroid hormones during growth, such that a single baleen plate contains a continuous, multi-year retrospective record of the whale's endocrine history. As a preliminary investigation into potential testosterone cyclicity in male whales and influences of stress, we determined patterns in immunoreactive testosterone, two glucocorticoids (cortisol and corticosterone), and stable-isotope (SI) ratios, across the full length of baleen plates from a bowhead whale (Balaena mysticetus), a North Atlantic right whale (Eubalaena glacialis) and a blue whale (Balaenoptera musculus), all adult males. Baleen was subsampled at 2 cm (bowhead, right) or 1 cm (blue) intervals and hormones were extracted from baleen powder with methanol, followed by quantification of all three hormones using enzyme immunoassays validated for baleen extract of these species. Baleen of all three males contained regularly spaced peaks in testosterone content, with number and spacing of testosterone peaks corresponding well to SI data and to species-specific estimates of annual baleen growth rate. Cortisol and corticosterone exhibited some peaks that co-occurred with testosterone peaks, while other glucocorticoid peaks occurred independent of testosterone peaks. The right whale had unusually high glucocorticoids during a period with a known entanglement in fishing gear and a possible disease episode; in the subsequent year, testosterone was unusually low. Further study of baleen testosterone patterns in male whales could help clarify conservation- and management-related questions such as age of sexual maturity, location and season of breeding, and the potential effect of anthropogenic and natural stressors on male testosterone cycles.
Highlights
Most mysticetes reproduce seasonally, yet it is unclear whether they show seasonal patterns in reproductive hormones or in the adrenal glucocorticoids
Baleen cortisol and corticosterone are elevated in baleen grown during late pregnancy, and are strongly elevated in baleen grown during known entanglements in fishing gear and during chronic illness (Hunt et al, 2017a; Lysiak et al, 2018). In this first study of baleen hormones in male whales, we studied patterns in baleen testosterone and glucocorticoids along the full length of a single adult plate from each of three whale species: bowhead whales, which have longest baleen; NARW, with next-longest baleen; and blue whale (B. musculus, ‘blue’), selected as a representative of the rorquals (Balaenopteridae) which all have relatively short baleen
Our goals were to determine (1) whether the baleen has regularly spaced testosterone peaks along its length; (2) if so, whether the spacing of these peaks corresponds with estimates of annual baleen growth rate (BGR) and/or with annual cycles in stable isotopes (SI) ratios (Lubetkin et al, 2008; Lysiak, 2008), i.e. could the patterns represent annual testosterone cycles; (3) whether there is any relationship of baleen glucocorticoids to baleen testosterone, regarding potential impacts of annually predictable breeding-related stressors as opposed to unpredictable stressors such as entanglement and disease
Summary
Most mysticetes (baleen whales, Mysticeti) reproduce seasonally, yet it is unclear whether they show seasonal patterns in reproductive hormones or in the adrenal glucocorticoids. The annual cycle of a mysticete whale typically involves a migration between polar or subpolar ‘feeding grounds’ in summer months and subtropical ‘breeding grounds’ in winter months, with females typically producing calves only during a calving season in a portion of the winter months (Lockyer and Brown, 1981). These life history features imply seasonal reproduction, i.e. regular alternation between a reproductively active and a reproductively inactive (non-breeding) state (Bronson, 1991). A clear understanding of male testosterone cycling in a given species can potentially improve accuracy of sex identification of unknown individuals via hormone ratios, more accurate discrimination of sexually active adults from juveniles, better understanding of age of sexual maturity (often difficult to determine in males), potential occurrence of reproductive senescence in older males, and could help determine month and location of the conceptive season—which in turn may inform estimates of gestation length of females
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
