As it gets later into the breeding season here on the Isle of May, more and more of the grey seal mothers are weaning their pups and returning to sea to fish. Grey seals, like most species of true seals (called ‘phocids‘, including all earless seals), have one of the shortest dependant periods (the time the infant has to stay with the mother to survive) of all mammals, just 18 days. After that, mothers leave the colony and it’s up to the pups to look after themselves. Left to their own devices, newly weaned pups (called ‘weaners‘) soon find their way out of the colony or into nooks and crannies out of the way of the adult seals where they can rest and play, moulting their white baby fur (called lanugo) so they are ready to go to sea for the first time.
Moulting pups are usually much larger than when they were born despite only being 18 days old. Grey seal mothers produce milk with such a high fat content that pups can put on over 1.5kg a day in blubber tissue. Pups that are born at just over 10kg can therefore easily reach over 40kg in mass at weaning, and for seal pups the fatter they are at weaning the better as it means they are more likely to survive their first year of life. During their first year, a weaner’s body composition will dramatically change and they will go from the fat, round shape like in the above photo to a leaner look, like the yearling in the picture below. As a juvenile, a grey seal’s body composition is only about 12% fat of its total mass, a huge difference to the 45% fat individuals can have as weaners. These big changes in body composition are completely natural for the seals, but it means that it is vital for their fat tissue to function correctly both when they are rapidly generating blubber or mobilising it when they don’t have any food. That’s why the PHATS team are investigating whether the high persistent organic pollutant (POPs) burden that all marine mammals have in their blubber tissue is disrupting the seal’s ability to generate and utilise their fat reserves.
We now have 12 study weaners this year, all named after the phonetic alphabet as their identifying code in our project is a single letter. The largest of our pups is Lima, a male weighing 54kg! I introduced you to Charlie in the last blog when she had just weaned from her mother and still had lots of lanugo covering her body, she’s changed a lot in the last week! She’s moulted completely now and has really pale fur with subtle spots.
The other weaners in our study vary from pups that are almost completely covered in lanugo to fully moulted, and its interesting seeing the different patterns the weaners have hiding under their white baby fur. Here are some more introductions to our new study weaners, you can see Foxtrot (female) and Golf (male) in the photo at the top of the blog, and below are pictures of Alpha (male), the oldest pup in our study, and Kilo (male) snuggled up together and Delta (male) playing around with his flippers.
MEANWHILE away from the seals, we were all very keen to see the super moon, unfortunately the weather on the island did not co-operate and we didn’t get to see it in it’s full glory. We did all manage to see it before the sun rose the following morning and the next night, which was still very impressive!
Even though we didn’t get to see the super moon properly, we still felt it’s impact as we had huge tides on the island, and all the seals near the coast found the ocean encroaching where it doesn’t normally reach! The resulted in lots of seal pups getting swimming lessons with worried mothers in tow, and even some underwater nursing attempts.
Sadly it’s getting to the time in the season where some of the team have to think about leaving the island, including our team leader Dr Kimberley Bennett who has to go back to the Abertay University to continue teaching. It’s likely Holly and I will stay on the island until mid December to complete our work on the weaners, so we’ll be getting to know them very well in the coming weeks!