The largest mammal on Earth – the whale – has long captured the imaginations of humans. Indigenous North Americans hunted whales for thousands of years, using them for food and fuel, before commercial harvesting began in the early 1600s. The huge demand for products derived from bowhead whales (primarily lamp oil made from blubber, and buggy whips, umbrella ribs, and corset stays made from baleen, the long keratin plates that bowhead whales have instead of teeth) led to a drastic decline in their worldwide population, from 30,000-50,000 individuals before commercial whaling began to 3,000 in 1921, when large-scale hunting was banned.
Today, commercial whaling is still largely banned, but a small number of bowhead whales are taken in subsistence hunts each year.
Despite humanity’s long relationship with bowhead whales, there are some areas in which our basic knowledge of their life history is lacking, including reproduction. Scientists estimate that mature female bowhead whales go about three or four years between having calves. That estimate is largely based on three direct observations – in other words, an extremely small sample size.
Recent research conducted at the New England Aquarium in Boston suggests that a new technique might allow us to further investigate bowhead whale calving rates. A team of scientists analyzed discs of baleen, taken from 16 bowhead whales harvested during subsistence hunts in Alaska, for levels of progesterone, a hormone that can indicate pregnancy.
Because baleen grows longer over time (just as our fingernails do), the tissue closest to the gum line – the most recently grown tissue – reflects recent hormone levels, while tissue further out can serve as a record of the whale’s hormone levels in the past. The research team estimated that baleen from bowhead whales could record hormone levels from as long as 25 years ago, depending on the age and size of the animal.
Of the seven mature female bowhead whales the scientists studied, four were pregnant at the time of harvest and had elevated progesterone levels in their most recently grown baleen. All of the other whales – the non-pregnant females, immature females, and males – had lower levels of progesterone in the same area.
The researchers also found elevated progesterone levels in older sections of baleen from two of the mature females who weren’t pregnant when they were harvested, which suggests that they might have been pregnant in the past. (The researchers analyzed up to four samples from each whale, so they likely missed some baleen sections that might have shown evidence of additional past pregnancies.)
The method isn’t perfect – the scientists point out that baleen can grow at different rates in different individuals, and estimating calving intervals from baleen would require knowing the growth rate of the mother’s baleen. However, given the current scarcity of information on bowhead whale reproduction, analyzing baleen could be a fruitful area of research. It could also help scientists figure out if reproduction rates have changed since historical times, an important question with implications for conservation efforts. As the researchers note, “of particular interest is the availability of historical baleen samples in museum archives (samples collected during the era of commercial whaling) that could be used for comparisons with present-day population data.”
Though still nowhere near as large as it once was, the bowhead whale population has rebounded since the commercial whaling moratorium in 1921; currently, there are 7,000-10,000 bowhead whales worldwide. Baleen harvest was one of the primary motivations behind the commercial exploitation of whales in the past – perhaps now we can use baleen to inform our conservation efforts, rather than in our umbrellas.