A new study revealed that analyzing the hormones preserved within the baleen plates of Rice’s whale, one of the rarest whales in the world, may open an unprecedented window to understanding the history of stress and reproduction in this critically endangered species, at a time when the number of its adult individuals is estimated to be no more than fifty whales only.
The study, published May 13 in the journal PLOS One, found that baleen plates — keratin plates that some whales use instead of teeth to filter food from the water — maintain a hormonal record spanning long periods of the animal’s life.
This means that, by analyzing these plates, scientists can reconstruct part of the whale’s physiological history, including pregnancy periods, chronic stress levels, and perhaps the circumstances that preceded death.
Rice’s whale is a relatively new species. Until 2021, it was believed to be just a subgroup of Bryde’s whales, before it was later classified as an independent species. This whale lives year-round in the Gulf of Mexico, with audio recordings and sightings indicating its presence in Mexican waters as well. But it faces great pressures, including ship collisions, pollution, entanglement in fishing gear, and marine noise, making it one of the most endangered species of baleen whales.
Baleen plates as a biological archive
The researchers relied on analyzing the baleen plates of only seven individuals of Rice’s whale, which are all samples available globally at the time of the study. The samples included males, females, and juveniles, a very limited number, but it reflects the extreme scarcity of this species and the difficulty of obtaining samples of it.
The study’s lead author, Rebecca Gee, explains: Effie, a doctoral researcher in evolutionary biology at George Mason University in the United States, said that the team measured four main hormones within the baleen plaques: progesterone and testosterone, which are linked to reproduction, in addition to cortisol and corticosterone, which are hormones associated with stress and physiological stress.
To collect data, scientists used a micro-drill to take sequential samples from the plates at regular distances, so that each part of the plate represents a period of time in the whale’s life that may range from approximately 15 to 30 days. After that, the samples underwent advanced hormonal and chemical analyzes to confirm the presence of these hormones within the keratinous tissue, according to Effie’s statements to Al Jazeera Net.
The importance of this method – according to the researcher – lies in the fact that the baleen plates grow gradually over time, like a long record that preserves signals from the animal’s body. Instead of relying on a single blood sample that reflects only a specific moment, researchers can read the change in hormones over a period of months or years, which is extremely important in the case of rare animals that are difficult to follow or sample while they are alive.
References to stress, pregnancy and hunger
The results showed that two whales, believed to have died due to hunger or chronic disease, recorded a clear increase in the four hormones during the last months of their lives. Researchers interpret this increase as a sign of severe and long-term physiological stress.
One of these whales had swallowed pieces of plastic that caused rupture in its digestive system, while the other suffered from chronic entanglement in fishing gear, which led to serious injuries and malnutrition. In doing so, the baleen sheets reveal not only the presence of stress, but a cumulative trajectory of suffering before death.
The researcher points out that the team was also able to monitor a previous pregnancy in an adult female who was breastfeeding when she died. Her baleen plaques showed a prolonged rise in progesterone that extended across most of the length of the plate, and then began to decline near the base, which the researchers interpreted as a sign of labor.
The study estimated the gestation period at about 13.7 months, which is close to what is known for species close to Bryde’s whales.
Another striking finding is that adult males did not show a clear seasonal pattern in testosterone, unlike many large whales that breed in specific seasons. “This suggests that Rice’s whale may be able to reproduce year-round, or that it does not rely on a strict breeding season,” says Ivey.
The researchers also found that young whales had relatively high levels of reproductive hormones and stress hormones compared to adults, which may be related to the transfer of some hormones from the mother through milk, or to other biological functions in the early stages of development that are not yet fully understood.
Despite the importance of the study, the researcher stresses that its results must be read with caution. The sample size was very small, as it was limited to only seven whales. Also, some time estimates were based on the growth rates of baleen plates in Bryde’s whales that are genetically close, due to the lack of sufficient direct data on Bryde’s whale itself.
Also, explaining hormonal surges is not always simple; Factors such as illness, hunger, childbirth, and chronic stress may interfere, making determining the exact cause of each hormonal change complex. In addition, the baleen plates of Rice’s whale are relatively short, meaning that the available time record often does not exceed one or two years, while other species of whales can maintain longer hormonal records.
In terms of funding, the study received support from the US National Science Foundation, in addition to support from the George Mason University School of Science and the School of Conservation of the Smithsonian-Mason Institution. The researchers confirmed that the funding bodies did not interfere in the study design, data analysis, or decision to publish.
