Reproductive biology of seals

Abstract

The reproductive biology of seals is fascinating in many aspects. As in most mammals, the time of onset of puberty in seals is variable. Once sexually mature, most but not all seals are seasonally mono-oestrous, with highly synchronized breeding seasons. They have evolved as either terrestrial or aquatic copulators, although a few species mate in a variety of habitats. Their mating strategies are diverse, ranging from serial monogamy to extreme polygyny. Gestation in seals is characterized by an embryonic diapause, which is obligate in most species. Reactivation of the blastocyst is followed by a placental gestation. All species of seal require a terrestrial (including ice floes) habitat for parturition. Lactation differs between the two seal families: phocid seals have an intense period of maternal investment, during which the mothers fast; otariid seals have a prolonged lactation during which intense bouts of suckling are interspersed by days of separation from their pups while the mother forages at sea. Although the anatomy and functional morphology of seals has been well described, less is known of the endocrinology of reproduction. This is due mainly to the logistical difficulties that researchers experience in collecting serial samples from a species that is relatively difficult to handle. This article reviews the basic anatomy and physiology, and our current understanding of the comparative aspects of reproduction in seals. Reproductive behaviours as well as the influences of environmental factors, such as photoperiod, nutrition and xenobiotics, are also discussed.

Earlier pupping in harbour seals, Phoca vitulina

“…Typically, timing of birth in most seal species shows little inter-annual variation (Atkinson 1997). Here, we report that harbour seals Phoca vitulina from the Wadden Sea (southeast North Sea) have shortened their yearly cycle, resulting in a birth peak 25 days earlier than in the 1970s.…”

Testosterone and tusks: maturation and seasonal reproductive patterns of live, free-ranging male dugongs (Dugong dugon) in a subtropical population

“…Testosterone secretion may be synchronous with both testicular maturation and testicular activity in male dugongs, with further evidence from post-mortem studies that the relative volume of Leydig cells in dugong testes increases with progressing spermatogenesis of the seminiferous epithelium (Marsh et al 1984a). Similarly, higher circulating testosterone concentrations have been shown to reflect sexual maturation in other marine mammals (reviewed by Atkinson (2008)), including cetaceans (Desportes et al 1994, Kjeld et al 2006, Atkinson & Yoshioka 2007 and pinnipeds (Atkinson 1997) for which maturity has been confirmed by histological examination of the gonads.…”

“…This collection occurred prior to the main peak in testosterone production, which is associated with the commencement of the breeding season. However, seasonal reproduction requires that males have adequate numbers of viable sperm when females enter estrus, and hence, the spermatogenic cycle must be initiated months before breeding (Atkinson 1997). Germ cell proliferation in male dugong testes has been evident from May onwards in north Australian populations (Marsh 1995, Kwan 2002.…”

Patterns of cetacean vaginal folds yield insights into functionality

“…Any seawater that overcomes the physical barrier of the labia minora may encounter additional challenges, which prevent entry into the cranial vagina or upper reproductive tract. For example, the hymeneal folds of pinnipeds and manatees are located more caudally in the vagina compared to cetacean vaginal folds and could potentially form a physical barrier [65]. The cervix functions as a critical barrier to the upper reproductive tract.…”