Methods to examine reproductive biology in free-ranging, fully-marine mammals

Hormonal biomarkers provide insights into the reproductive biology and pollutants-associated health hazards of endangered dolphins

Contaminants are known to contribute to the reproductive health hazards of wildlife, but pollutants-associated detrimental impacts on the endangered Indo-Pacific humpback dolphins (Sousa chinensis, IPHD) are largely unknown due to a lack of reproductive parameters. Here we validated and applied blubber progesterone and testosterone as reproductive biomarkers to assess reproductive parameters of IPHD (n = 72). The gender-specific progesterone concentrations and progesterone/testosterone (P/T) supported progesterone and testosterone as valid biomarkers in identifying the genders of IPHD. Significant month-to-month variations of two hormones indicated a seasonal reproduction, in accordance with the observation of photo-identification approach, further supporting testosterone and progesterone as ideal reproductive biomarkers. Progesterone and testosterone concentrations showed significant differences between Lingding Bay and West-four region, possibly due to chronically geographic-specific pollutants differences. The significant relationships between sex hormones and multiple contaminants suggested that contaminants contribute to the disruption of testosterone and progesterone homeostasis. The best explanatory models between pollutants and hormones suggested that dichlorodiphenyltrichloroethanes (DDTs), lead (Pb) and selenium (Se) were the major risk factors jeopardizing the reproductive health of IPHD. This is the first study on the relationship between pollutant exposure and reproductive hormones in IPHD and represents a substantial advance in understanding the detrimental reproductive impacts of pollutants on endangered cetaceans.

Reproductive biology of female common dolphins (Delphinus delphis) in New Zealand waters

Reproductive biology was assessed in 106 female common dolphins (Delphinus delphis) examined post-mortem from stranding and bycatch events along the New Zealand coastline between 1997 and 2019. The average age (ASM) and length (LSM) at sexual maturity was estimated at 7.5 years and 183.5 cm, respectively. The total number of corpora in mature individuals increased with age and appeared to persist throughout life. Ovarian asymmetry was apparent, with the left ovary displaying higher rates of ovulation, and a maximum of 19 corpora recorded for a 24-year-old female. The estimated ovulation and annual pregnancy rates for mature females were 0.39 year-1 and 30%, respectively. Conception and calving occurred year-round, with a weak seasonal increase observed in late austral spring and early austral summer. As these data did not clearly show whether seasonality was present, the gestation, lactation, and resting periods were calculated as either 12.6 or 12.8 months based on the presence/absence of seasonality, respectively. Similarly, calving interval ranged from 3.15 to 3.2 years, depending upon whether seasonality was considered. The estimated LSM of the New Zealand population aligns with other populations globally, although the estimated ASM is younger by approximately 6 months. Other reproductive parameters align with Northern Hemisphere populations, although demonstrate variation, which may reflect adaptations to local conditions such as water temperature and prey availability. As the species is subject to anthropogenic impacts including pollution and bycatch, we suggest our findings be used as a baseline with which to monitor trends in population parameters.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00227-022-04139-3.

Genital interactions during simulated copulation among marine mammals

Genitalia are morphologically variable across many taxa and in physical contact during intromission, but little is known about how variation in form correlates with function during copulation. Marine mammals offer important insights into the evolutionary forces that act on genital morphology because they have diverse genitalia and are adapted to aquatic living and mating. Cetaceans have a fibroelastic penis and muscular vaginal folds, while pinnipeds have a baculum and lack vaginal folds. We examined copulatory fit in naturally deceased marine mammals to identify anatomical landmarks in contact during copulation and the potential depth of penile penetration into the vagina. Excised penises were artificially inflated to erection with pressurized saline and compared with silicone vaginal endocasts and within excised vaginas in simulated copulation using high-resolution, diffusible iodine-based, contrast-enhanced computed tomography. We found evidence suggestive of both congruent and antagonistic genital coevolution, depending on the species. We suggest that sexual selection influences morphological shape. This study improves our understanding of how mechanical interactions during copulation influence the shape of genitalia and affect fertility, and has broad applications to other taxa and species conservation.