A new DNA analysis of stranded Gray's beaked whales, a species rarely observed alive, demonstrated with the highest accuracy yet that the population is large and has high genetic diversity, factors that will help it avoid being threatened by global warming and other ecosystem changes like many other whales are.
The Gray's beaked whale is one of many poorly understood whale species in the deep sea around Antarctica, and a better understanding of its behaviors and conservation statuses can inform upcoming regulatory efforts, according to the researchers behind the new study. Their paper was published March 24 in Royal Society Open Science.
Beaked whales are a collection of 23 species and are "the least known family of mammals on the planet" because they live in poorly explored deep-ocean habitats, according to Kirsten Thompson, a population biologist at the University of Exeter and the paper's lead author. They are named for their pair of tusk-like teeth, which males are believed to use to spar over females. They are also extreme divers, and one of the few well-described species has been observed diving nearly 3 kilometers deep and spending hours underwater.
First described in 1876, Gray's beaked whales inhabit much of the waters north of Antarctica in the Southern Hemisphere and have been found beached in Australia, New Zealand, Africa and South America. They are roughly 5 meters long and slender, but the little information known about them is mostly derived from bodies stranded onshore.
"How they live, and exactly where they live, and how many there are and what their social structure might be — all of that has been previously unknown," said Thompson, who also conducts research in association with Greenpeace.
Thompson and her coauthors used new genetic data from six stranded Gray's beaked whales found in South Africa and southern Australia in addition to 16 previously analyzed whales that were beached in New Zealand and western Australia. The research team analyzed part of the whales' DNA in addition to the entire DNA in their mitochondria, a separate set of genetic information inherited only from the mother that allowed the scientists to analyze the whales' demographic history.
"Part of the work that I've been doing over the last 10 years is … to use as many tools as we can to try and find out more about their ecology, and that's always going to be a difficult task because you can't go out and observe them," Thompson said.
By comparing mitochondrial DNA between the whales and approximating the rate of their genetic mutations, the authors estimated a current effective population size of 270,500 females, or about 541,000 Gray's beaked whales. The actual population may be even higher due to the methodology behind the figure, Thompson said.
This also allowed the researchers to find that the population of the female whales has been stable for the last 1.1 million years — except for 250,000 years ago, when their population size doubled during an interglacial period that created more favorable living conditions.
The DNA analysis revealed information about the whales' social behavior. They have no population structure, or genetic differences between groups caused by non-random mating. Most whales and dolphins do have population structure, often due to populations remaining separate in different oceans or individuals regularly returning to the same area to breed or feed.
Gray's beaked whales also have a high genetic diversity, which indicates that they spend much of their time with unrelated individuals. This was previously suspected because groups of beached whales have been found to be unrelated, Thompson said.
The study is a marked improvement over a 2015 analysis of partial mitochondrial DNA of 94 Gray's beaked whales, which Thompson was also involved in. It predicted a female population of 460,000 female whales, but with a large margin of error; it also picked up on their high genetic diversity and preliminary signs of low population structure.
"It's amazing what you can do from only 22 samples," Thompson said. Inexpensive high-throughput sequencing "is a new thing to be able to do on species like this, and it's very powerful for species that are hugely elusive, like Gray's beaked whales."
She and her coauthors said in the paper that Gray's beaked whales are expected to be "resilient" to climate change and other changes to their ecosystem. Their large population size and highly diverse genetic pool are important factors in conservation, and they do not seem to be close enough to human activity to be disrupted. Thompson said she expects their habitat to increase as the oceans warm.
The population biologist would like to see tools developed that use acoustic waves to track Gray's beaked whales deep underwater, given that recent research better informs scientists on where to look for them. Being able to pin down and place a tag on beaked whales would lend new insights into where the species lives and how it behaves.
More needs to be learned about other mysterious whales and marine species around Antarctica, according to Thompson, especially as the United Nations prepares to adjust its regulations of international waters. The stated goals of overseeing "the conservation and sustainable use of marine biological diversity" in these areas will be best served with a better understanding of the local animals' conservation needs, she said.
"To put it in the context of land, people would be appalled that we didn't know anything about a land mammal that was six meters long that lived in a forest, that'd never been mapped," Thompson said. "But essentially, that's the situation in the oceans."
The study, "Ocean-wide genomic variation in Gray's beaked whales, Mesoplodon grayi," published March 24 in Royal Society Open Science, was authored by Michael Westbury, Marie Louis, Andrea Cabrera, Mikkel Skovrind, Jose Castruita and Eline Lorenzen, University of Copenhagen; Kirsten Thompson and Jamie Stevens, University of Exeter; and Rochelle Constantine, University of Auckland.