A study of the factors influencing the timing of breeding in the Grey seal Halichoerus grypus

• Published in: Journal of Zoology
• Main Author: Coulson, J. C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1981
• Subjects: North Atlantic
• Online Access: http://dx.doi.org/10.1111/j.1469-7998.1981.tb04601.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1469-7998.1981.tb04601.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-7998.1981.tb04601.x; https://zslpublications.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-7998.1981.tb04601.x

The Grey seal has a restricted distribution in the North Atlantic and Baltic but within this area there are differences of almost six months in the breeding dates in individual colonies, although there is little variation between years in the same colony. The date of breeding shows an irregular geographical distribution which excludes photoperiod as being the main factor determining the time of breeding. The timing of breeding is most likely to be determined by the factors which terminate the period of suspended embryonic development (“delayed implantation”) and studies of both the year to year variation in the mean date of births on the Farne Islands, Northumberland, and also the geographical differences in the date of breeding are correlated with the sea‐surface temperature in the period just prior to the end of the suspended development. On average, a one degree Celsius lower sea‐surface temperature between colonies at this time results in the date of pupping being 13 days later. Evidence is also presented that the size of the seal colony has an influence on the date of breeding and there is reason to believe that the breeding season on the Farne Islands in the 1930s, when the colony was much smaller than it is now, was at least 10 days later than in recent years. It is suggested that this influence of numbers is possibly a social facilitation effect which slightly modifies the date of termination of the period of suspended embryonic development. It is also suggested that the two days later mean birth date of female pups is caused by a minor alteration to the restart of embryonic development caused by the sex of the embryo. A model of the manner in which the breeding season is determined in the Grey seal is presented and attention is drawn to possible selective advantages stemming from synchronized breeding rather than pupping at a particular time of year.