Molecular Ecology (2000) 9, 283 – 292 © 2000 Blackwell Science Ltd Blackwell Science, Ltd Patterns of paternal relatedness in British grey seal colonies J. WORTHINGTON WILMER,* A. J. OVERALL,* P. P. POMEROY,† S. D. TWISS‡ and W. AMOS* *Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK, †Sea Mammal Research Unit, Gatty Marine Laboratory, University of St. Andrews, St. Andrews, Fife KY16 8LB, UK, ‡Department of Biological Sciences, Durham University, Durham, DH1 3LE, UK Abstract Previous studies of breeding behaviour in the grey seal, Halichoerus grypus, have painted conflicting pictures. Behavioural observations suggest a classical polygynous system with a small number of dominant males fathering most of the offspring. However, genetic analysis suggests that many potential fathers spend little time ashore, that some pairs of seals show partner fidelity and that the dominant males are not as successful as their behaviour would suggest. Here we used paternal relatedness between pups with known mothers, sampled over an 11-year period, to show that behavioural dominance leading to enhanced fitness is a feature of only a handful of males located near the centre of the breeding colony. The vast majority of pups are fathered by any of a large number of males who all share approximately equal success, including virtually all those males who have previously escaped our best sampling efforts. As expected, the frequency of full-sibs is reduced in this longer time series relative to the original study. However, absolute estimates of the frequency of full-sibs seem to be confounded by a tendency for females who produce paternally unrelated pups to have conceived to males who are more genetically dissimilar from each other than expected by chance alone. Together, these elements of breeding behaviour would help to maintain maximum genetic diversity and to minimize the effects of inbreeding. Keywords: breeding behaviour, female mate choice, grey seal, microsatellite, paternal relatedness, polygyny Received 16 July 1999; revision received 29 September 1999; accepted 29 September 1999 Introduction The mating system of grey seals (Halichoerus grypus), which breed on the Scottish islands of North Rona (NR) and the Isle of May (IoM), has been described as following the classic pinniped model of terrestrial polygyny. During autumnal breeding seasons spanning ≈ 2 months, females come ashore and form predictable breeding aggregations (Anderson et al. 1975; Boness & James 1979; Pomeroy et al. 1994), each giving birth to a single pup that is suckled for ≈ 18 days. Then, towards the end of lactation, the females come into oestrus, are mated on average 2.9 times by one or more males (Twiss 1991), then return to sea leaving their abruptly weaned pup on land. Males compete aggressively to maintain positions within the established aggregations of females, starting to come ashore only after the first pups have been born (Twiss 1991; Twiss et al. 1994; 1998). Although male grey seals are organized into only an approximate linear hierarchy (Twiss et al. 1998), skew in observed mating success favours those dominant males that remain tenured for longest among the females (Anderson et al. 1975; Boness & James 1979; Anderson & Fedak 1985; Twiss 1991; Twiss et al. 1994). The recent application of molecular genetics to the analysis of paternity within these grey seal colonies has helped to reveal a mating system far richer in complexity than previously assumed. DNA fingerprinting studies have shown that while behaviourally dominant males gain proportionately more paternities than other males, they do not achieve the level of reproductive success pre- dicted by their observed mating success (Amos et al. 1993). Furthermore, the discovery of more full-sibs than could be accounted for simply on the basis of male and female site fidelity, implies that some pairs of seals show mate fidelity (Amos et al. 1995). Most recently, results Correspondence: W. Amos. Fax: +44-1223-336676; E-mail: w.amos@zoo.cam.ac.uk