This work formed my Masters thesis at the University of St Andrews in 2009. It focused on aggressive behaviour, haul-out patterns and anthropogenic disturbance of grey seals (Halichoerus grypus) at Abertay Sands in Scotland.
What did I do?
For several weeks each year, grey seals form large colonies at breeding sites when females will haul out to give birth, suckle a single pup for 18-20 days, and mate. During this time, females compete with each other for access to prime pupping sites, whilst males compete over access to females. Consequently, social relationships and dominance hierarchies form within the sexes – this minimises aggression, as individuals know their likelihood of winning a contest before it begins.
The predictable availability of seals during the breeding season has meant that the majority of grey seal behavioural research has focused on behaviour seen during this period, with considerably fewer studies examining behaviour at other times. I aimed to study the context of aggression in grey seals using a haul-out site outside the breeding season. Specifically, I was interested in whether haul-out space was a contested resource, and whether there was the potential for dominance relationships to exist. I also recorded any disturbance from human activities.
What did I find?
I spent 117 hrs observing seals from a wooden hide situated opposite the tidal sandbars of Abertay Sands. I used a telescope to count the number of seals present and record any aggressive interactions. There was no relationship between the frequency of aggressive interactions and group size. However, aggression was most frequent in the first 30 mins of the haul-out forming, and generally resulted from space invasion or incidental contact between animals. Within same-sex pairs, similarly-sized animals aggressively interacted most often, and with the longest duration. Overall, females were generally the more successful sex, and large animals typically won. However, small females usually won against large males. This suggests that some form of dominance relationships do exist outside the breeding season.
I also observed that the seals used nine different sand bars to haul out upon in this area. The closest inshore site was the first accessible and, consequently, most utilised. However, seals often left this site to transfer to other sand bars as they became available. This appeared to be a result of the high disturbance levels at this inshore site, with dog-walkers frequently passing within a few hundred meters of the sand bar. Aircraft flying overhead also caused disturbance at this inshore site, but not at haul-outs situated further out. Therefore, there is strong evidence that disturbance at haul outs influences their use by both individuals and groups of seals.
What does it mean?
This study suggested that dominance relationships may exist both between (females having dominance over males) and within the sexes (larger animals dominating smaller ones). Whether this is the result of an individual-based hierarchy, where animals construct relationships due to repeated encounters with the same individuals, or a population-based hierarchy with general ‘rules’ of dominance, is yet to be determined. Regardless, understanding how animals interact with each other allows us to study individual relationships and the social structure of animal populations. This information is important for understanding population dynamics, disease transmission, and animal behaviour.
Disturbance appears to influence the use of haul-out sites, with seals being regularly exposed to disturbance events. When disturbance occurred at the inshore sandbar, individuals relocated to more offshore sites. It could be that animals feel ‘safer’ at offshore sites, as they offer direct access to open sea, whereas inshore sites require animals to swim through narrow or shallow channels to reach the sea. As disturbance events can be very energetically costly to animals, this information may be useful for managing human use of this site to minimise impacts on grey seals.