A blubber coat and large amounts of fish keep Denmark’s smallest whale, the porpoise, alive in the cold winter

Porpoises are among the world’s smallest marine mammals, but despite their small size they need to maintain a body temperature of 37 degrees year-round, just like all other mammals. We, land-living mammals, would soon succumb to the ice-cold water where the temperature gets down to zero degrees in winter. How do the porpoises meet the challenge?

2018.12.07 | Peter Bondo Christensen

A porpoise with a transmitter

A porpoise with a transmitter attached with suction cups. The transmitter registers every time the animal comes up to breathe. When porpoises are caught in seine nets, scientists are able to catch the animal alive and attach transmitters to their backs before they are released again. Photo: Jonas Teilmann

 

This is a question that Laia Donate-Rojano and her research colleagues from Aarhus University are now able to answer, as they have for the first time ever measured the metabolism of free-living porpoises. Thus, they have unveiled the energy requirements of the small whales and their ability to regulate their internal heat despite the fluctuations in ocean temperatures from zero to more than twenty degrees throughout the year. 

Porpoises eat twice as much as humans to keep warm

The results of the new studies show that porpoises keep warm by consuming energy equivalent to approx. four packs of butter a day. It is twice as much as humans need. 

To the researchers’ great surprise, it turns out that the metabolism of porpoises is not higher in winter than in summer. One might have thought that the cold water required an extra energy bar to retain the internal heat. 

In order to maintain the same body temperature as humans – whether the temperature of the sea is zero or twenty degrees – porpoises change the thickness of their insulating blubber layer. In summer, where the whales should not get too hot, the blubber layer is about 2.5 cm thick, and in winter, where the whales need to keep warm, the blubber thickness increases with approx. 1.5 cm. 

They build up blubber stores throught a fattening cure in late summer, where they eat significantly more than they burn off. Conversely, the animals go on a diet in spring, where they eat less than they burn off, thus reducing the thickness of the blubber layer. 

“It’s the same as when we wear a down jacket in winter. Otherwise, we would lose a lot of heat that takes energy to produce,” explains Ph.D. student Laia Donate-Rojano, who is the main author of the article. 

Porpoises cannot sweat to reduce heat but besides adjustments of blubber thickness, they can open and cut off the blood supply to the tail, dorsal fins and flippers to release heat and thereby maintain a constant body temperature of 37 degrees. 

Measurements in captivity and open sea

Knowledge of the metabolism of wild animals is crucial if you want to understand their role in the ecosystems and how we humans affect them directly or indirectly. But it can be very difficult to evaluate the metabolism of free-swimming whales, and it is almost impossible to transfer measurements from land-living mammals, which do not live with the cooling effect of the water, to the wild animals.

Therefore, the researchers are particularly proud that they finally succeeding in measuring the metabolism of porpoises in captivity in an enclosure at Fjord&Bælt in Kerteminde. The researchers used ”double-labelled water” where they inject a little water that has heavier oxygen and hydrogen molecules than normal water into the animals. In this way, blood tests allow them to measure how fast the ”heavy” water disappears from the body and in this way accurately calculate the metabolism.

In the F&B enclosures, the researchers were able to relate the energy requirements to each breath intake of the porpoises.

”We have developed a technique where we can attach a specially designed electronic sound recorder on the back of a porpoise. The sound recorder is attached with suction cups that do not harm the animal and it registers each time the porpoise comes up to breathe," explains Jonas Teilmann from Aarhus University.

The researchers managed to attach sound recorders to 13 wild porpoises. By correcting for the difference in weight and thus the lung volume between the captive animals and the animals in the wild, the researchers were able to calculate the metabolism of the wild porpoises. It turned out that a wild porpoise eats about 1 ton of fish a year and that they prefer small, typically 3-10 cm long, fish and therefore need to catch a couple of thousands per day.

”The great nutrient demand of the porpoise and the fact that they prefer small fish mean that they have to hunt nearly continuously night and day. If they are scared away from their feeding areas by underwater noise, the porpoises have less time to catch the required amount of fish, and this may affect their survival,” says Professor Peter Teglberg Madsen from the Department of Bioscience, Aarhus University.

 


More information

Professor Peter Teglberg Madsen,
Department of Bioscience, Aarhus University.
Mail: peter.madsen@aias.au.dk 
Mobile.: +45 5177 8771

Senior Researcher Jonas Teilmann,
Department of Bioscience,
Aarhus University.
Mail: jte@bios.au.dk
Mobile: +45 2142 4291.

Department of Bioscience, Public / media, Staff, Alumner, Marine Mammal Research