This is the 2nd half of Part 1 of our new series, "How Do Weddell Moms Matter? Part 1: Cohort Effects and Conditions Before Birth." (Read 1st half of Part 1 here.) The 2nd half of Part 1 continues...
Project scientists suspect (but don’t really know for certain) that a female starts to regain body mass as soon as she weans her pup. But there is a lot of uncertainty about the dynamics of Weddell mom mass gain—whether it occurs quickly over a month or two, or gradually over many months, and whether the summer is when most mass is gained, or the winter. Or perhaps both summer and winter seasons contribute equally.
Delayed implantation of the fertilized egg in the mother's womb is exclusively a summer phenomenon, while overall body mass gain by Weddell moms could be a result of summer or winter conditions, or a combination of conditions spanning both seasons. (Please see the previous post of the 1st section of this writing for more information on Weddell seal females being "delayed implanters.")
One of the main environmental factors affecting the amount of food available for the Weddell females during winter is the amount of sea ice extent. Scientists have hypothesized that the Ross Sea marine ecosystem is strongly structured from the top down, indicating that high level predators such as Weddell seals have a strong influence on the overall ecosystem. There is thought to be substantial predator competition by minke whales, emperor and Adélie penguins, killer whales (orca), snow petrels, and Antarctic toothfish for food resources such as Antarctic silverfish, a major prey of Weddell seals.
Aerial video of helo flight from the Erebus Bay spring sea ice edge to Big Razorback Weddell colony.
Increased sea ice extent limits access to food resources by competing predators such as whales and birds; and the amount of food Weddell moms can obtain during the winter when their unborn pups are developing in utero may be greater. The more mass the moms can put on, the bigger the pups are likely to be when they’re born the following spring, and the bigger the moms. Bigger moms can transfer more energy and mass to their nursing pups after birth. Bigger pups have a greater likelihood of surviving to reproductive age and recruiting—returning to a pupping colony and giving birth to pups, themselves. Project scientists have concluded that annual variation in food resources available to pregnant Weddell females was likely the driver of variation in recruitment probability among cohorts (Garrott et al. 2012).
One of the most significant findings described in Garrott et al. 2012 is that bigger cohorts do better. So when conditions are such that more females do produce pups, those bigger cohorts have a greater proportion of female pups that go on to become mothers, themselves. It will be particularly interesting to see how the most recent record-breaking or near-record pupping seasons—cohorts—fare in the next several years.
So from the beginning, Weddell moms and the varying environmental conditions they encounter make a difference in whether the fertilized eggs implant, how much nutrition the unborn pups gets while in utero, how much energy and mass is available to transfer to the pups after birth while nursing and, thus, the likelihood of whether the pups in the particular annual pup cohort recruit—survive to return to the pupping colonies and give birth to pups of their own.
Go here to read the 1st section of this 2 post article.
Thank you to Weddell project scientists and cohort effects paper authors Drs. Bob Garrott, Jay Rotella, and Don Siniff for their patient assistance reviewing and ensuring accuracy of this post.
Garrott, R. A. et al. 2012. Environmental variation and cohort effects in an Antarctic predator. – Oikos 121: 1027-1040.
Hadley, G. L. et al. 2006. Variation in probability of first reproduction of Weddell seals. – J. Anim. Ecol. 75: 1058-1070.