Calving intervals inferred from progesterone patterns in historic baleen of female fin whales (Balaenoptera physalus)

Reproductive patterns were once a cryptic topic in whale physiology due to the inaccessibility of obtaining endocrine data from mysticete whales. However, recent advanced approaches in measuring hormones in baleen reveal the possibility of obtaining multi-year reproductive hormone profiles from various species of mysticete whales. Hormone analysis of baleen thus offers a tool to evaluate how ecological and anthropogenic pressures impact whale physiology. We investigated progesterone patterns in historic WWII-era baleen of four female fin whales (Balaenoptera physalus) to (1) develop the first longitudinal hormone profiles of fin whales and (2) evaluate gestation periods and calving intervals in a time period when global climate change was minimal and there was a cessation of commercial whaling. Historic baleen plates from the Smithsonian National Museum of Natural History were drilled every other centimeter to obtain a pulverized powder for hormone extraction. Enzyme immunoassays were run to measure progesterone concentrations of each whale (4 females, pregnant at the time of capture) over ~3-4 years. Results indicate a likely one- to two-year calving interval in fin whales, but with notable individual variation. This pilot study helps provide a baseline of female fin whale progesterone patterns, thus offering data that can be compared to modern-day whales that are subjected to the effects of climate change. Future research can assess the influence of modern anthropogenic stressors by identifying abnormal reproductive patterns which can influence recovery efforts and management strategies.

Hi, my name is Piper Thacher, and I am a senior majoring in Environmental Science and Policy. For my URSP project this semester, I was mentored by Dr. Kathleen Hunt to determine possible calving intervals of female fin whales from the late 1940s. This was done by creating the first-ever longitudinal profile of progesterone using historic baleen samples. So, for a brief background on fin whales, they are classified as vulnerable on the IUCN red list so there are around 100,000 left and their main threats are vessel collisions and entanglement. We currently have a knowledge gap on the hormones of fin whales and because of that, we cannot determine if modern-day whales who are subjected to climate change are having altered reproductive cycles. So, this project aims to fill that gap. Typically, blood samples are taken to measure hormones, however, that is not possible for a whale. Scientists can use feces, earwax, and blubber, to look at hormones but that only tells us the hormones of the individual for that specific day and the samples do not remain stable over time. Luckily, within the last decade, advanced approaches to measuring hormones in whales were discovered. This new approach involves baleen. So, for those who do not know what baleen is, it is a filter-feeding apparatus in the mouths of certain whales, and here is a close-up of one baleen plate that was actually used for my project. These baleen plates are made from keratin and grow over time similar to our own fingernails. So as the baleen grows hormones are accumulated within it, so it essentially provides a time series of the whale’s hormones for a said number of years. Now this is a more recent finding, however, it was confirmed that different steroid and thyroid hormones such as cortisol, progesterone, and thyroxine are detectable in baleen which leads us to the core of my project. We hypothesized that there would be periodic spikes of progesterone over the length of the baleen plates that may indicate past pregnancies. So, my first goal was to create longitudinal progesterone profiles of four female fin whales that were captured by Japanese commercial whaling vessels. These four females were all pregnant upon capture in February of 1948. The second goal was to determine what the normal gestation periods and calving intervals were for fin whales. And what I mean by normal is what were their reproductive cycles like at a time when there was a cessation of commercial whaling, and the effects of climate change were minimal. This goal is important because it really helps provide a baseline of female fin whale progesterone patterns, which can, in the future, offer comparable data to modern-day whales that are subjected to those anthropogenic factors. So, there are three major steps in the methods for my project. The first is the preparation and extraction. So, baleen was pulverized into a powder every odd centimeter from the base to the tip. Then we take 20 milligrams of power from every odd centimeter, add methanol, decant, and evaporate every sample to get pure dried hormone extracts. And here is what my setup looks like and in those tubes are the weight-out baleen power. The next step is to reconstitute the samples and make a 1 to 4 dilution because the hormones are so highly concentrated. Now the last step is to put the samples into an enzyme immunoassay and when we compare the readings from the plate reader to a standard curve, we can determine the amount of progesterone in the sample. So, this is a little bit of what the process looks like. I have added a stop solution here to end the 30-minute colorimetric changing stage. And then we take the plate and record the optical density. So, for my results, these are the longitudinal progesterone profiles of the four female fin whales. These results show high progesterone spikes that may indicate a possible one to two-year calving interval with notable individual variation and we are assuming each baleen plate has a 3 to 4-year timeline. We can see that our prediction of progesterone spikes indicating pregnancies is confirmed because all of these whales were pregnant when captured and we can see a spike leading up to their death which is at the 0cm from the base. The one oddball at the top right that had its baleen cut from the base rather than being pulled so we are missing the most recent growth which may be why we don’t fully see a spike there. There is a dramatic individual variation when it comes to the average progesterone levels of each whale some have spikes that go as high as 300 nanograms of progesterone per gram of baleen while another has up in the 3000. So, for the next steps, we first want to use stable isotopes to determine the actual timeline of the baleen and this can help solidify when the breeding seasons were and if they match the progesterone spikes on these figures. We also want to expand the sample size so I will be continuing this work so we can get a larger population size. In the future, we also plan to run stress hormones like cortisol and corticosterone to map any correlations between reproductive and stress hormones. So, I wanted to take a moment to thank some amazing people who made this project possible. My mentor Dr. Kathleen Hunt, I am so privileged to have worked with you and I am excited to continue working with you. Allie Case you essentially were a second mentor to me and I’m so thankful for you and your guidance. Also, all of my co-authors who made this research even possible. The Smithsonian Museum of natural history for letting us use their baleen archives and lastly George Mason’s College of Science and the OSCAR program for supporting this research I am so appreciative of you all. I included a QR code if anyone would like to look at references or my other work. Thank you so much everyone for listening this project was an amazing experience.