So, what can Weddell seal researchers learn about these seals by tagging them? Well, it turns out, a lot!
When a small, uniquely numbered tag is attached to the rear flippers of a Weddell seal, the researchers record a substantial amount of data on that animal. Lead scientist on the Weddell project Jay Rotella notes, "For each seal observation, we record the date, location, seal tag numbers, sex, and age class. For females with pups, we also record that they are a pair. For a subsample, we also collect a very small tissue sample for genetics work and record that. If we weigh the seal or outfit it with a swim tag, we also record that information."
Since 1968, over 22,500 Weddell seals have been tagged by the Montana State University based Weddell seal population project, and their information added to the extensive project database. This means that the B-009 Weddell population project has recorded vital information on over four decades, and several generations, of Weddell seals in the Erebus Bay study area, making this one of the longest running population studies ever of a long-lived mammal, and providing a database that is unique in the field of population ecology.
The researchers know how many seal pups survive to return to the pupping colonies as adults (around 20% of female pups survive to return as adults), how long the seals live (the oldest seal recorded in the study to date is 30 years old), how many pups the females have over their lifetimes (anywhere from 0 to 21 pups over a lifetime, recorded to date), and which of those pups survived to return to the study area to have pups of their own.
A great example of how well the researchers can trace the history of Weddell moms and pups using this extensive database can be found in the story of a very old seal very recently re-discovered this season at age of 30, with her record-setting 21st pup!
For well over a decade, many of the Weddell seal pups have been weighed at three different stages of their nursing period, which lasts approximately 45 days. Researchers weigh a select number of pups to determine their mass (weight) at birth, at the mid-nursing period, and at weaning. Some Weddell moms have been weighed during that nursing period, as well. So, the mass of these seals at various points in their lifetimes is also a vital part of the extensive project database, and central to the mass dynamics work of the Weddell seal population project. Unfortunately, due to the delay in the field season start this year because of the government shutdown, very few Weddell pups and moms will be weighed this season due to lack of data regarding accurate dates of pup births to correspond to those animals' weights.
In 2010, the project began a pilot study of Weddell seal pup swimming behavior during the nursing period. Co-Principal Investigator Bob Garrott developed a small temperature tag to attach to a select number of pups' flippers to record the surrounding temperature at six minute intervals over the course of the nursing period. This simple temperature recording tag allows researchers to know how often and for how long pups are in the water during the crucial time in their lives when they are learning to swim and interact underwater with their moms. Here's a project video on Weddell seal pup swimming behavior and survival, and the temperature tag study...
For about a decade, the researchers have also taken genetic samples from a select number of seals, and are developing a genetic archive of this population to enable even more discoveries in the future based on genetic analysis.
By recording the birth date, birth location, re-sightings, and reproductive histories of these seals, and determining a select number of the seals' weights at various stages of their lives, as well as recording the swimming history of a select number of pups, the researchers are able to use all of these data to determine whether there are correlations with environmental changes and variation, and to assess the effects of extreme environmental events on this population of Weddell seals.
A vital aspect of this population study's extensive database, is providing a better understanding of how individual variation or differences among Weddell seals affect the way the population functions. Researchers are gaining a better understanding of how the population responds to environmental changes, as well as to the occurance of extreme environmental events such as the giant iceberg that blocked access to some of the seals pupping colonies and feeding areas in the past decade. Here's a project video on how these Weddell seals fared in a massive iceberg event this past decade...
Recently, the researchers made a breakthrough discovery about individual variation in Weddell seals using a 30 year database of the reproductive history of Weddell females. What they discovered is that there are fixed--real--differences among seals when it comes to frequency of pupping. Co-Principal Investigator Jay Rotella states, "Before this study, it was not known if the difference in the number of pups that different mothers produced over a lifetime was due to random chance or due to fixed differences among the seals. The new findings indicate that it's due to real or fixed differences in individual quality."
Lead author of this fascinating study, project PhD recipient Thierry Chambert notes that, "Interestingly, results also suggest that the expression of these individual reproductive differences tended to remain constant across varying environmental conditions." Females that were "robust" reproducers tend to reproduce at higher than average rates even in harsher environmental conditions. As Jay Rotella puts it, "When the going gets tough, the tough stay--and they keep on producing pups!"
Viewers can learn more about individual variation in Weddell seals in a new project video to be released later this month, so please stay tuned!