Stable isotopes reveal pygmy right whales’ secretive behavior

Researchers have discovered intriguing information about pygmy right whales (Caperea marginata) through stable isotope ratios in their baleen. Unlike their larger relatives, they don’t undertake seasonal migrations to Antarctic regions. Instead, they remain near southern Australia year-round, where they feed on krill and copepods. These whales are the smallest and least studied among baleen whales, and their elusive behavior is likely due to their small size, sparse distribution, and inconspicuous nature.

Whalers historically didn’t target them, and our knowledge mostly relies on beached specimens. Dr. Tracey Rogers, a professor at the University of New South Wales, led a study confirming their homebody behavior, focusing on regions like the eastern Great Australian Bight and the area dominated by the Bonney Upwelling, where they find an abundant food supply.

A pygmy right whale (Caperea marginata) in the wild. Credit: Henry Cordell

Beached whales

In their study, the researchers utilized stable isotope ratios (δ15N and δ13C values) from the baleen plates of 14 adult pygmy right whales to understand their diet and habitat preferences. The baleen, made of keratin like our fingernails, continuously grows throughout the whales’ lives, making it an ideal long-term indicator of their food and environment.

By examining the isotope ratios in the baleen, which reflect those of their prey, the researchers could determine the species consumed by pygmy right whales. The isotope ratios increase consistently across trophic levels within the food web, from phytoplankton with the lowest values to apex predators with the highest values. The study included both male and female pygmy right whales that had stranded on the Tasmanian or southern Australian coast between 1968 and 2019. The baleen plates used in the research were borrowed from the South Australian Museum in Adelaide. The findings shed light on the diet and habitat use of these enigmatic whales.

The first author, doctoral student Adelaide Dedden, with baleen from pygmy right whales in the foreground. Credit: Adelaide Dedden

Not like other baleen whales

The study revealed that the stable isotope ratios in the baleen of pygmy right whales closely match those of copepods and krill species found in zooplankton-rich regions off Australia. This finding confirms that these whales primarily feed on Australian krill and copepods and don’t undertake seasonal migrations to the Antarctic, unlike many other baleen whale species.

The absence of correspondence with isotope ratios of Antarctic krill and pelagic fish indicates that pygmy right whales do not rely on these prey items for their diet. Instead, they have a specific feeding preference for krill and copepods.

This restricted, mid-latitude range of pygmy right whales and their dependence on particular prey expose them to risks. The researchers cautioned that being large-bodied mammals relying on tiny prey makes them vulnerable to changes in their local environment. With the warming of temperate oceans in the southern hemisphere, the authors expressed concern about how these whales might respond to such environmental changes. Further study is needed to understand the potential impacts on pygmy right whales in the face of this alarming rate of ocean warming.

Source: Frontiers

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