In this ‘Behind the Paper’ blog post, we’re doing things the Weddell seal way! In her recently published paper: “Hormones reflect the impacts of reproductive rest on subsequent pregnancy in a marine predator“, author Amy Kirkham, a biologist for the US Fish and Wildlife Service’s Marine Mammals Management program in Alaska, shares interesting perspectives on optimal birth timings for Weddell seals. Read about the trade-off mechanisms that the authors observed and the fascinating (and cold!) research they conducted to learn about this dynamic that leave us asking: “have I ever seen a seal baby ultrasound?”. Amy also offers some sound advice to our young career readers: lean on your support circles and don’t forget to make good notes!

1. About the Paper

Figure 1: An adult female Weddell seal and her pup rest together. (Credit: Amy Kirkham; NMFS MMPA Permit 17411)

Our paper is about how “skipping” a year of annual reproduction affects female Weddell seals, a species of Antarctic seal that is well studied and breeds closer to the South Pole than any other mammal. With this research, my co-authors and I were seeking to learn what factors—like blood hormone levels and body fat stores—impact how likely a seal is to become pregnant and successfully give birth and how recent reproductive history impacts these factors.

We found that mature seals that skip pupping in a given year (“skip females”) looked very different physiologically from female seals that gave birth (“moms”), even months after the pupping period. Along with being fatter, skip females also had distinct hormone profiles which likely help support healthy pregnancies. Skip seals also went into estrus earlier than most moms, allowing them to give birth earlier the next summer or gestate their pups for longer. For seals that give birth late in a particular year, skipping pupping the next year could help shift future births to more optimal times.

Our paper shows how hormonal changes and shifts in reproductive timing can contribute to trade-offs between current and future reproduction in Weddell seals, and these mechanisms are likely important in other species that are more difficult to study and track over time.

2. About the Research

Figure 2: Research team members capture a Weddell seal in a hoop net, with Mt. Erebus in the background. (Credit: Amy Kirkham; NMFS MMPA Permit 17411).

Figure 3: Weddell seal embryo of approximately 35 days gestational age. (Credit: Michelle Shero; NMFS MMPA Permit 17411)

For this research, our team captured seals in McMurdo Sound, Antarctica. Most of the seals in our study were assessed during field work from 2013–2017 as part of a broader project examining annual life history in female Weddell seals, including the relationship between reproduction and the annual molt. During field seasons, we took snow mobiles or helicopters each day from US research base McMurdo Station to Weddell seal colonies, where we looked for specific seals (with flipper tags they were given at birth). When we found a target seal, we caught it in a net, our veterinarian sedated it, and we collected a series of measurements and samples.

We conducted reproductive ultrasound scans during our procedures. These showed which seals were pregnant and provided some exciting images of embryos. Our collaborators surveyed seal colonies the following summer and let us know which of the animals we assessed successfully gave birth. Back in the lab, we measured blood hormone levels using radioimmunoassay’s. One of the original goals of this research was to find a hormonal signal of early pregnancy in Weddell seals, allowing future studies to tell which seals are pregnant with just a blood or urine sample. We didn’t find any hormones that differed enough between pregnant and non-pregnant Weddell seals in the first couple months of gestation to use for this, though—there’s no easy pregnancy test for Weddell seals like there is for humans. We did, however, find a surprising seasonal pattern in the hormone prolactin, which dropped steeply in the middle of the summer in male as well as female seals.

Figure 4: Weddell seals resting on sea ice late in the Antarctic summer. (Credit: Amy Kirkham; NMFS MMPA Permit 17411).

I think important next steps for studying Weddell seal reproductive trade-offs and timing is additional population-level analysis of changes in individual pupping dates across years, along with research on what factors make some female Weddell seals better at producing pups overall (“high quality” or “robust” females). I’m excited that some very neat work on physiological traits linked to Weddell seal reproductive quality is currently in progress.

3. About the Author

Figure 5: A Weddell seal and the author saying hello. (Credit: Skyla Walcott; NMFS MMPA Permit 17411).

I decided to go to graduate school to study physiological ecology after having fascinating undergraduate research experiences. I spent time at Stanford University’s Hopkins Marine Station, where I learned how studies of intertidal and nearshore systems have revealed foundational ecological concepts, and I saw how much you can discover from taking a naturalist’s patient approach to observation in the field.

During my time in graduate school at the University of Alaska Fairbanks, I also became very interested in human dimensions of ecosystems, which led me to complete a Sea Grant policy fellowship in the US Senate in Washington, DC. I currently work as a biologist for the US Fish and Wildlife Service’s Marine Mammals Management program in Alaska, applying both my research and policy interests. One research topic that fascinates me now is phenology at high latitudes—how do marine mammals and other organisms that live at the poles regulate the timing of life history events, even as climate change alters conditions in seasonal environments that are already extreme?

Figure 6: Author Amy Kirkham with one of her future dive buddies. (Credit: Elliot Stiassny).

Outside of research, my favourite hobby is scuba diving, but most of my favourite things to do now involve having fun with my two young children. Young kids are amazing scientists and observers, and I’m learning a lot about the world from watching what they discover. Advice I’ll give them, and would give my younger self, would be to revel in their curiosity—it’s a superpower!

I’d also tell my younger self to keep better notes so I wouldn’t have to redo work after being pulled away from projects for a while by changes like starting a new job or having a baby. I’m proud to be a parent in science, and I’m grateful for the support I’ve gotten for my family needs. I hope that growing support mechanisms in the research community can help ensure that becoming a parent doesn’t create barriers for early career scientists.