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Durham University

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Publication details for Dr Sean Twiss

Twiss SD., Thomas CJ. & Pomeroy PP. (2005). Local weather affects the degree of polygyny in breeding grey seals (Halichoerus grypus). 16th Biennial Conference on the Biology of Marine Mammals, San Diego.
  • Publication type: Conference Proceeding

Author(s) from Durham


Individual variation in reproductive success is a major driver of sexual selection and determinant of population genetics. We examined variation in male grey seal mating success in relation to weather conditions at the island colony of North Rona (Scotland) over 9 successive annual breeding seasons (1996 to 2004). Based on previous studies we hypothesised that reduced rainfall during the breeding season would limit availability within the colony of pools of water, close to which females prefer to pup. This would increase spatial clustering of females permitting greater monopolization of females by fewer dominant males, thus increasing mating skew. Contrary to expectations we found a strong positive relationship, such that mating skew was less extreme under drier conditions (Adj. R2 = 0.82, df = 7, p<0.001). Examination of female distribution patterns also failed to show the expected increased clumping with less rain. However, we found that females tended to be more mobile under drier conditions (Adj R2 = -0.83, n = 6, p<0.05). As female grey seals show strong inter-annual site-fidelity, we conclude that females typically return close to previous pupping sites, irrespective of weather conditions, presenting similar spatial distributions across years. Having pupped, females tend to be ‘tied’ to the pupping location. If the season is atypically dry, females without access to pools will commute, sometimes relatively long distances, to gain access to this important resource, subsequently returning to their pups to nurse. This increased mobility of females, we argue is the reason for the decreased ability of dominant males to monopolise matings, allowing a greater number of less dominant males to gain matings. In the context of changes in recent UK weather patterns and projected future changes as a result of climate change, these findings imply that long term population genetics may be strongly influenced by rare weather events.