2 November 2016 Leptonychotes weddellii

Journal

Okay so my friends will say (and I will admit) that I'm not exactly the most touchy-feely guy out there, but today I headed out to Turtle Rock and Hutton Cliffs to get an idea what the team from Montana State University has been up to while doing the Weddell seal census – one of the longest Antarctic projects, having been started in 1968. Any attempt to not be touchy-feely would be ridiculous after having seen this ... so bear with me and enjoy the experience.

The Weddell Seal Population Study

Studied since 1968, the Erebus Bay population of Weddell seals is the southernmost breeding population of mammals in the world and the database contains information on more than 24,200 individual seals. This has been a massive and unique undertaking which has resulted in knowing more about this population of long-living mammals than few others in the biological realm. Although originally a tagging and census project to track their movements – Weddell seals are known to return to the same birthing and breeding grounds year after year – the project has evolved as more data has been collected and additional questions can now be asked as a result.

Weddell seal range — Weddell seals can be found throughout the fast ice of Antarctica. Image source: Wikimedia

Weddell seals in the colonies here travel from the ice edge – this year that is about 40 miles from McMurdo by Beaufort Island (76°56′S 166°56′E). That's a long ways to go if you're holding your breath – and although Weddell seals can hold their breath for up to 90 minutes (AFTER breathing out all the air from the lungs before submerging) that's still too far, so they crack hop along the shore. Fast ice (read the previous journal on sea ice training) will rise and fall with the tides, but a portion of it is also attached to the shore and the area in between the freely floating fast ice, and the shore-attached fast ice will act like a hinge and crack – giving our intrepid Weddell seals ample areas to cut breathing holes as well as get out onto the ice to bask in the sun along the way. And they do cut the holes – they do this with a sawing motion of their head and teeth, wearing them down over time.

The Hutton Cliffs hang heavy with ice and snow as the team works to identify a mother and tag her new pup for the Weddell Seal Population Study.

Why go so far inland along the fast ice? Orcas are the main predator of the Weddell seal and they patrol the ice edge – by heading so far inland, the Weddell seals are given time to birth and feed their pups rapidly, getting them to the right size and weight in order to survive back out in the open water. The juvenile mortality rate is high once the sea ice breaks up and only 1 in 5 of the pups will live long enough to breed – they are easy prey for the fast and efficient Orca. But for that 20%, life is good since 95% of the returnees will continue to live long lives if they can make it back as an adult.

Weddell seal mother and pup — This Weddell seal mother will lose large amounts of weight during the 35 day lactation period in order to bring this little pup up to 300 lbs before the sea ice breaks up.

Spotting and Tagging Weddell seals — Once a Weddell seal is spotted that hasn't been tagged, a plan of action is put into place with a lead for tag process decided upon. There are two tags total – one for each flipper of the pup – with two sides to it. Once tagged, the information is recorded along with the sex of the pup on remote field computers.

Old Project, New Questions

Placenta and Weddell seal mother with pup — The sea ice may look a little bit messy but that's the aftermath of birth you're looking at. The placenta at the bottom of the picture may or may not be from the pup and mother above, but there are more than enough to go around. Now well frozen, the placenta provides easy pickings for the Skua, Antarctica birds.

Now researchers and graduate students from Montana State University in Bozeman, Montana are working to understand more than if the population is just growing or shrinking, With the big data to help support, questions such as the breeding cycles, material effects, and the relationship between mother and pup are being investigated. Based on the data, we now know that as a female gets older, her body mass will decrease and that the pup size correlates to the mother's size. So for example, an older, thinner mother will produce a thinner pup than a younger mother with more mass. However, the pups all generally reach the same size by the time they are ready to be on their own (about the time the sea ice breaks up). This means that the oldest mothers are producing pups that are gaining the weight the fastest. Weddell seal milk basically has the same fat content as Crisco and pups have been recorded as gaining nearly 9lbs a day! With a lactation period of roughly 35 days, pups have to gain a lot of weight from their mothers in not a very long time – especially considering that the pups will weigh around 300 lbs in that amount of time. While I was out with the team, we came across a pup that had been born 20 days ago with a weight around 55lbs and now weighed in at 161lbs – that's an increase of 5lbs for every day.

Weighing a Weddell seal pup in the field requires the use of a spring scale, a duffel bag, and a strong back.

This massive weight gain is essential, and the mother fattens up considerably before giving birth since she will transfer a large amount of her energy to her pup. This transfer is quite efficient at nearly 40 to 50% – which means that a mother who loses 400lbs will pass 200lbs on to her pup. Think about that ... if you or I lost 1/2 of our body mass in basically a month, we would die – the mothers at the end of the lactation period are thin due to this weight loss.

This data comes from weighing the seals over the months of lactation and birthing. How exactly do you weigh a Weddell seal pup? Using a simple spring scale like you'd find in a grocery store is effective enough for a seal pup weighing in a 160 lbs. The pup is placed in a special "duffel" bag with a breathing hole, lifted with a scale, and their weight recorded. This method is all well and good for small pup but how do weigh a 1000 lbs mother out on the fast ice? This is too heavy for two researchers to lift ... so the body mass loss is what we "think" – the researchers have never been able to weigh the mothers ... until recently.

No Fuss Weighing Method

Kaitlin Macdonald, a masters student at Montana State University, has been working to glean the mass of a mother seal with a camera through a method called stereophotogrammetry. Not a new method of viewing something, but certainly a new use and when combined with technology, provides for a method of determining a mothers mass with very little disturbance.

Stereophotogrammetry starts with a series of photos with guide rods, those guide rods will help the program determine where the image was taken in 3D, then the edges of the seal are determined and an approximate model is created that can be used to determine the female's mass. Photos: Kaitlin Macdonald

With the advent of 3D printing, many people are aware of iPhone applications like 123D Catch where individual photos are taken of an object and a 3D wireframe model is created. Using that model, a 3D replica of what ever the object was could be printed. In this case however, Kaitlin is using a similar program to do the same thing and will calculate the volume of the model. Since the average density of a Weddell seal is known, and by rearranging the density equation that my 8th graders know extremely well at this point (right?), we can solve for the mass of the Weddell seal mother ... so that mass = (volume) X (density). And like that another dataset can be added to the already massive dataset of decades of work by the Weddell Seal Population Study.

For more information on the Weddell Seal Population Study, make sure to check out their website. It's full of great information, pictures, videos, and even lessons for those teachers out there following along with me!