Even though it’s december and the vast majority of the seals on the Isle of May breeding colony have already raised their pups and returned to the sea, we are still seeing a few newborn pups from mothers that are late season breeders. Most of the colony is quiet now as the majority of adult females have gone, leaving their weaned pups (weaners) in their 2-3 week post-weaning fast (see this blog for more details about the post-weaning fast in seal pups).
We don’t really know why some females give birth so late in the season, but doing so tends to have a big impact on how likely they are to raise their pups to weaning, with late season mothers typically having lower pupping success than early season mothers. While there are very few adult females on the colony, and therefore less problems with aggressive neighboring females, there are still plenty of adult males around looking to mate. The ratio of females to males on the colony changes through out the breeding season, and goes from lots of females to every male in the early season to at least one male per female late in the season, if not more. This means that a female trying to raise a pup in the late season has to deal with a lot more male harassment than the females in the early part of the season, and as a result she spends less time nursing and caring for her pup. There is also evidence that females breeding in the late part of the season are younger and therefore are more inexperienced mothers, but what age females start to transition to become early season breeders, if they do at all, is still a mystery, especially as most of our study mothers seem to give birth exactly the same time each year, to the day in some cases.
Back at the PHATS lab, things are starting to wind down. We had to ship all the samples we had generated so far from the cell culture experiments off the island this week as we had filled the minus 80 freezer on the island! The tissue, media and blood samples we’ve collected over the last 6 weeks will give us plenty to study back on the mainland in the new year. Our daily surveys of the study weaners have shown that two of our weaners, Charlie and Alpha, have left the island and gone to sea! Charlie was 35 days old when she left the colony, and Alpha was 43 days old. Some of our study weaners don’t seem very keen to go, both Bravo and Delta are 43 days old and still happily playing around on the island! Some of our seals are making their way through the colony and the rocky tidal region of the shore towards sea, like Foxtrot who is 40 days old and is on the southern edge of the island.
MEANWHILE the research team on the island has dropped to just 4 people as half the team left the island this week to return home, the remaining two researchers from the Durham University seal behaviour team and two research assistants from the Sea Mammal Research Unit of St Andrews. We also were lucky enought to be visited by one of the bird researchers from the Center for Ecology and Hydrology again, as they keep track of the ringed sea birds (shags) on the island during the winter. During this visit they caught and ringed a wild short-eared owl and were kind enought to show us before they released her, beautiful!