A good part of the work the Weddell seal field team does during the pupping season involves determining the weight of seal moms and pups--their mass--at various points in their lives. Mass matters at all stages of a Weddell seal's life. Much has been learned by the project scientists over the many decades of the study through weighing some of the Weddell moms in the pupping colonies of Erebus Bay, and weighing their pups at three different points during the nursing period.
Measuring adult Weddell seal weight acts as a surrogate for measuring the amount of marine resources--primarily fish, squid and octopus--that the Weddell seal is able to avail itself of and convert to stored energy and, for many females, fetal development. For a nursing pup, measuring pup weight gives scientists information on pup weight at birth and how much energy is being transferred to that pup during the nursing period by its mother in the form of rich, extremely high fat milk. It has been determined by scientists that there is around a 50% efficiency in this transfer: for every 1 lb. of pup weight gained, the pup's mom loses 2 lbs. of weight. Weddell moms don't usually eat, or eat very little, during the approximately 45 day nursing and pup nurturing period. Weddell pups can more than triple their birth weight before being weaned, so there is a very large transfer of energy from mom to pup during this nursing period.
A crucial part of the Weddell seal research team's field work involves weighing pups at 3 different stages during the 5 to 6 week nursing period, with the first weighing taking place a day or so after birth, a second weighing around the mid-nursing period at 20 days, and a third weighing near weaning at around 35 days. The 35 day pup weights range from around 150 to 280 lbs (68 to 127 kg).
Also during the 35 day weighing, researchers will retrieve temperature recording tags from those pups that received small temperature sensor/recorders when they were first weighed at birth.
In the image above, data from one of the small temperature recorders are displayed in graph form. Because sea water temperature under the ice is a constant -1.8° C (28° F) but the surface temperatures on top of the sea ice can vary widely, researchers are able to determine when pups are in the water, and for how long, based on the data recorded by the temperature tags. Over time the scientists will be able to determine the relationship of pup time spent in the water and pup weight to pup survival to adulthood and female reproduction.
The Weddell seal population project has published a number of research papers to date on Weddell seal population and mass dynamics. All of these different findings contained in the project's papers were attained through continuing and furthering the data acquisition accomplished in the field. On the ice.
Weighing Weddell seal moms historically has been difficult. In the early days of the study, back in the late '60s and early 70's, Weddell seal moms were weighed using a heavy tripod rig. Only a few seals could be weighed because of the method. The more recent history of the study saw the introduction of a digital livestock weigh sled that could be moved to various colony locations. The traveling digital weigh sled allowed for the Weddell mom to move onto the scale herself, enticed by her pup waiting at the other side. The mom's weight could be accurately assessed when she was on the sled scale.
Efforts were made in the last decade to use a tall photogrammetric rig to assess the mom's weight using a program that converted the photos into measurements of Weddell seal mass. This photogrammetric rig was bulky and the process took a lot of time to do, as seen in this photo below.
Currently, the research team is using both the digital weigh sled to weigh Weddell moms, and also developing and using a new 3D modeling photogrammetric approach that makes use of photos from small point and shoot digital cameras. The method is quicker and easier to do, and makes it possible to get far more individual Weddell mom weights in one season, as shown in these new photos from the ice this season.
In future research work, the project scientists will be focusing in on how the many different findings by the project about mom and pup mass relate to Weddell seal female pup survival to reproduce, themselves; and what the variations and fixed differences found in Weddell seal moms and pups studied to date mean to the female pups' survival to reproduce. What matters, and how, in this population of long-lived Antarctic marine predator.
- Mary Lynn Price