21 November, 2003
Today was a great day to be a seal-and a seal counter. The past week's low pressure system with its endless grey skies and stiff winds moved on just as we left camp this morning, leaving blue skies, relatively calm winds, and warmish temperatures in its wake. We recorded the largest number of seals for any of our four censuses-1061 seals were out on the ice today. Out of those 1061 animals, we only had four pups that needed tagging. These rather large pups were in a group of seals near South Base that had been missed in previous censuses. While pupping has tapered off, there seemed to be more seals just out on the ice, lounging around holes that have opened up in some of the newer ice cracks. It's a good thing that the seals are black, and lie against a white background. It definitely makes them easier to spot when you're bouncing across the sastrugi on your snowmobile.
An interesting statistic from today's count was the presence of a female who was first tagged as a pup in 1973. In her 30 years, she has given birth to 14 pups that have been tagged. Her first pup was born (or at least the first pup that was tagged) in 1983. This mother is older than most of the people on our seal research team.
The thing that struck me after looking at almost 150 adult seals and pups today was the variability in pup size. While some are almost full-grown mini-seals, others are still pups with long fur and a definite uncoordinated look to their movements. The older pups are attempting to imitate their mothers' vocalizations-making an assortment of grunts and growls that are not quite puplike anymore, but certainly wouldn't pass for an adult's calls.
The pup's weight gain is their mother's weight loss. The estimate is that each kilogram of weight gained by the pup costs the mother 2 kilograms. To put it more simply, a pup that gains 50 kg has caused its mother to lose approximately 100 kg. By the time some of the pups are weaned, they may weigh 100 kg-that translates to a 200 kg weight loss for the mother. If the mother started out around 400 kg, she would be down to around 200 kg by that point in time. Producing a supply of milk with a fat content of 50-65% for a period of 6 weeks is energy-intensive for these mothers and represents a big investment in the survival of their pups.
Wandering through the seal colonies gives plenty of time to think about the questions that are raised by a long-term study such as this. While the database represents 30+ years of tagging information, many of the larger questions are still unanswered. The logistics are incredibly complex for studying a population of seals in an area with such extreme weather as Antarctica-following seals in the winter dark and cold is difficult, to say the least. No one is quite sure where the pups go after weaning-often they are not seen again for at least 3 or 4 years. Another unanswered question is whether there is any correlation between where the ice edge is each year and the numbers of seals at each colony or the number of pups born, as well as the number of pups that survive. Do some colonies have a higher rate of success, measured by pup production and survival, than others? If so, what factors play a part in this success? I am constantly amazed at the fact that an animal population that has been studied as intensively as this one still provides us with so many mysteries and opportunities for future studies.
Black seals on the ice
Visible from far away
Is that just a rock?
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