Grey seals are land-breeding, marine mammals. This means that unlike whales and dolphins which spend their entire lives in the ocean, seals come ashore at various times. Usually, they will come ashore to rest between foraging trips; this is typically called ‘hauling out’. While seals will haul out throughout the year in small groups, once a year adult seals come ashore in large numbers for what is termed the breeding season: the period where pups are born and nursed, and when females mate with males. Our research has focused primarily on the breeding behaviour of seals, as this is an excellent time to observe and track individuals within and between years, and to learn about these animals.
Female grey seals become sexually mature aged 3-5 and their reproductive longevity can exceed 25 years! Males become socially mature at around 8 years old, although some appear on the breeding colonies before this age, and have potential breeding spans of up to 15 years. Grey seals in UK waters breed during the autumn, during which time they gather, typically at remote offshore islands such as North Rona and the Isle of May (Scotland). Although the entire breeding season spans approximately 8 weeks, individual females will spend 18-20 days ashore, during which time they each bear and suckle one pup, come into oestrus towards the end of lactation (approximately 16 days after giving birth!) and are mated. Weaning occurs abruptly when the female returns to the sea. At this point, the pup is large from its mother’s fat-rich milk but will face new challenges as it enters the ocean on its own. We know a bit about how these newly weaned pups tackle the difficulties of the ocean in their first few years, but there is still much to learn!
Grey seals breeding colonies are incredibly varied in their topography and habitat. Some are jagged and covered in rocks while others are simply large, sandy beaches. In the Baltic sea, some grey seals still are known to breed on ice! Each breeding colony offers pupping sites of varying suitability, but females seem to have preferences for particular habitat characteristics. On islands like North Rona, females prefer low slope, low elevation and easy access to or from the sea and to pools of water (Pomeroy et al. 2000a, Twiss et al. 2001). Females aggregate around these features, with local variation in density related to habitat suitability (Twiss et al. 2001). Known adult females on North Rona exhibit ‘site-fidelity’. This means that they return to the almost exact location on the colony each year: on average this is within 55m of their previous pupping site(Pomeroy et al. 1994, 2000b); adult males show similar site fidelity (median distance = 53m, Twiss et al. 1994). Whilst some dispersal from breeding sites occurs, philopatry (when adults return to breed on the same colony they were born on) seems common, as indicated through resightings (Pomeroy et al. 2000b) and by differences in microsatellite allele frequencies between colonies (Allen et al. 1995). At colonies where seals breed inland, such as North Rona, mothers tend to remain at their birth location throughout the day and overall season.
Grey seals are ‘capital breeders’. This is a term meaning that not only do they only spend a short amount of time with their offspring before weaning, but also that during their stay on the colony, both females and males fast, obtaining all their energetic requirements from the metabolism of fat reserves (blubber). Their fast can exceed 20 days for females and over 50 days for males!
So far we’ve mostly discussed the female seals, but male grey seals are incredibly fascinating too! The larger of the two sexes, males come ashore after the females. Unlike the impressive harems and fighting seen in the closely related elephant seals, male grey seals do not defend resources or harems but compete to maintain loose (non-exclusive) territories (perhaps more accurately defined as Home Ranges) amongst the groups of females (Twiss et al. 1998). The mating pattern is polygynous: which means that a single male may mate with multiple females and that typically only a few males monopolize the majority of the mating. It seems that duration of male tenure (how long he stays on the colony) and social dominance are the primary predictors of male mating success, and most males appear to have sufficient energy reserves to remain ashore for the entire breeding season. Males provide no parental care!
We hope this gives you a brief look into grey seal breeding behaviour. We’ve discussed some of our current understanding of grey seal breeding ecology here, but there are still MANY questions and unknowns to pique our curiosity. Stay tuned for more information on how the local environment influences breeding behaviours, genetic relatedness of seals, long-term social relations between seals, effects of climate change, and what we know about seal ‘personalities’. If you are interested in more details right now, check out our references, links or feel free to contact us with questions!
Allen PJ, Amos WA, Pomeroy PP & Twiss SD (1995). Microsatellite variation in grey seals (Halichoerus grypus) shows evidence of genetic differentiation between two British breeding colonies. Molec Ecol 4: 653-662.
Pomeroy PP, Anderson SS, Twiss SD & McConnell (1994). Dispersion and site fidelity of breeding female grey seals (Halichoerus grypus) on North Rona, Scotland. J Zool Lond 233: 429-447.
Pomeroy PP, Twiss SD & Duck CD (2000a) Expansion of a grey seal (Halichoerus grypus) breeding colony:change in pupping site use at the Isle of May, Scotland. J Zool Lond. 250:1-12.
Pomeroy PP, Twiss SD. & Redman P. (2000b) Philopatry, site fidelity and local kin associations within grey seal breeding colonies. Ethology 106: 899-919.
Twiss SD, Pomeroy PP & Anderson SS (1994). Dispersion and site fidelity of breeding male grey seals (Halichoerus grypus) on North Rona, Scotland. J Zool Lond 233: 683-693.
Twiss SD, Anderson SS & Monaghan P (1998) Limited intra-specific variation in male grey seal (Halichoerus grypus) dominance relationships in relation to variation in male mating success and female availability. J Zool Lond.246: 259-267.
**Twiss SD, Thomas CJ & Pomeroy, PP. (2001) Topographic spatial characterisation of grey seal Halichoerus grypus breeding habitat at a seal’s perceptual spatial grain. Ecography 24: 257-266.