Photoperiod control of birth timing in the harbour seal (Phoca vitulina)

Abstract

With 4 figures in the text)The harbour seal (Phoca uitulina) has delayed implantation, precise annual birth timing, and significant latitudinal variation in birth timing. The birth timing patterns of four distinct groups of seals, including colonies of P . D. citulina and colonies and captive individuals of P. c. richardsi, were examined using population-based photoperiod analysis to assess the role of photoperiod in setting annual birth timing. This analysis simultaneously determined the time, relative to birth, at which photoperiod response was likely to occur and the critical photoperiod.Despite marked differences in birth timing patterns, a high level ofagreement was found among groups for the timing of photoperiod response. The two subspecies, however, demonstrated significantly divergent critical photoperiods. Eastern Atlantic harbour seals were exposed to a common critical photoperiod of 11.7 h/day on the 268th pre-partum day. Wild Pacific harbour seals were exposed to 14.3 h/day on the 283rd pre-partum day. These times corresponded to the estimated occurrence of blastocyst implantation.Using the above information, three small captive populations were subjected to artificially prolonged photoperiods during the period of embryonic diapause to test whether subsequent birth timing could be delayed. Technical difficulties invalidated results at two sites. At the third and largest colony, the mean pupping date of six individuals was significantly delayed by 10.7 days.The precision and latitudinal variation of annual birth timing in the harbour seal are due to a response to photoperiod which occurs immediately prior to blastocyst implantation. The critical photoperiod, however, is divergent among subspecies and, thus, has probably evolved allowing seasonal adaptation. Similar environmental signalling has been described for California sea lions and northern fur seals and represents the likely timing mechanism for most pinniped species.

Earlier pupping in harbour seals, Phoca vitulina

“…Most pinnipeds have highly synchronized annual reproduction, however, variation in the mean date of parturition may occur. Photoperiod at the time of implantation is an important factor in the timing of reproduction in seals (Temte 1994). However, there has been no change in day length in the Wadden Sea over the period of study (KNMI Database).…”

Maternal and newborn life‐history traits during periods of contrasting population trends: implications for explaining the decline of harbour seals (Phoca vitulina), on Sable Island

“…In mammals, photoperiod primarily controls the timing of birth (Follett, 1982). In pinnipeds, including harbour seals (Temte et al, 1991;Temte, 1994), photoperiod is thought to initiate embryonic implantation and all females in a population should implant at about the same time to ensure synchronized reproduction (Boyd, 1991). Although photoperiod plays a key role in the timing of these events, parturition date can be influenced by food availability through changes in gestation length.…”

Median pupping date, pup mortality and sex ratio of fur seals at Marion Island

“…The onset of implantation is a photoperiod response, with the physiological reaction leading to implantation probably being mediated through the pineal gland (Daniel 1974;Daniel 1981;Boyd 1991). Implantation is known to vary with latitude for a number of species of pinnipeds, with populations from lower latitudes generally pupping earlier that those from higher latitudes (Temte 1985;Temte & Temte 1993;Temte 1994). The median pupping dates of the 16-20 December, determined for Subantarctic fur seals, and 5-7 December, determined for Antarctic fur seals (Table 1), differ by a maximum of only three days from those described by Kerley (1983) for these two species at this site for the 1980/81 season.…”