In 2012, biologists Robert Pitman and John Durban published a paper on the long-distance movements undertaken by type B Antarctic killer whales. They discovered something fascinating: the whales, tracked by satellite tags, swam virtually non-stop nearly 9,400 kilometers round-trip to the subtropical waters off Uruguay and Brazil! Though there are many distinct variations of killer whales, as a whole, the species displays very little regular long-distance movements. So what is going on with the type B killer whales?
The tags on the whales revealed that although they swam at slower speeds in these sub-tropical waters, they did not display any kinds of movements that were indicative of foraging or giving birth, the two most common reasons for migrations in animals. With these behaviors ruled out, biologists concluded there was likely only one explanation for these extreme sprints to the subtropics: the killer whale equivalent of a spa treatment.
The above photos are of the same individual type B killer whale. In the first photo, the whale has a yellow tint caused by the accumulation of diatoms, tiny single-celled algae. Killer whales living in extremely cold Antarctic waters likely prioritize blood flow to vital parts of their body, such as their core, in order to stay warm and functional. However, skin regeneration requires constant blood flow to the epidermis, and thus it likely occurs at a slow rate in these whales, allowing diatom buildup to occur. However, in the second photo, the whale is now “clean” and free of the algae! How did this happen?
Biologists suspect type B killer whales undergo intermittent migrations to the warm subtropics in order to speed up the process of skin regeneration and get rid of the diatoms.
But why shed the diatoms anyway? Why travel such long distances only to turn around and come right back? Migrations like these are almost certainly energetically costly as they are probably not stopping for food along the way.
New research published in the journal Molecular Ecology may hold some of the answers. A team of biologists went through the tedious process of examining and identifying the types of microbes that live on killer whale skin. They had samples from resident killer whales and transient killer whales in the North Pacific as well as samples from type B and C killer whales in Antarctica. They were able to provide genetic evidence from the microbiome of killer whales that supports Durban and Pitman’s hypothesis on type B killer whale movements. Even more interesting, they found that killer whales with high diatom loads also had high levels of a bacteria known as Tenacibaculum dicentrarchi, a pathogen that is known to cause skin lesions and skin rot in fish. They also noted that whales with high diatom loads sometimes were in poor health and had peeling skin and lesions, such as the one below documented in this study:
Photo: Hooper et al. (2018)
However, other cetaceans are known to carry Tenacibaculum sp. without any apparent negative health effects, and more research will be needed to definitively conclude if this bacteria causes poor health in killer whales.
This study provided more support for the “physiological maintenance” hypothesis proposed by Durban and Pitman, and showed some tentative links to a potentially pathogenic bacteria associated with heavy diatom loads. The benefits the whales get from ridding themselves of these diatoms and the bacteria associated with them may outweigh the energetic costs of these long migrations to the subtropics.
J. W. Durban, R. L. Pitman. Antarctic killer whales make rapid, round-trip movements to subtropical waters: evidence for physiological maintenance migrations? Biol. Lett. 2012. DOI: 10.1098/rsbl.2011.0875.
Host‐derived population genomics data provides insights into bacterial and diatom composition of the killer whale skin. Mol. Eco. 2018. DOI: 10.1111/mec.14860