From left to right, a harbour porpoise, a white-beaked dolphin, a minke whale, a humpback whale and a blue whale. Photo credit: Maria Glarou.
From left to right, a harbour porpoise, a white-beaked dolphin, a minke whale, a humpback whale and a blue whale. Photo credit: Maria Glarou.
Keeping warm in the cold is challenging at the best of times. In the water, however, heat leaves your body 23 times faster than it does when you're in the air, making it much harder to stay warm. This is the challenge facing marine mammals such as whales, dolphins and porpoises. Without the ability to leave the water, how do these animals manage to keep themselves warmer than the water around them? Generally, marine mammals insulate themselves from the water using a thick layer of fat underneath their skin called blubber. Is this thick blubber layer always necessary to stay warm in the frigid waters surrounding Iceland, or do some species have a different strategy? With this question in mind, Maria Glarou of the University of Iceland, Iceland, along with an international group of researchers from across Europe and the USA, headed to the waters of Skjálfandi Bay, Iceland, to see how much heat some of its marine mammal residents lose to the chilly waters surrounding them.
First, the team needed to find out how thick the blubber is of five species that are frequently seen in the bay, ranging in size from smallest to largest: harbour porpoises (Phocoena phocoena), white-beaked dolphins (Lagenorhynchus albirostris), minke whales (Balaenoptera acutorostrata), humpback whales (Megaptera novaeangliae) and blue whales (Balaenoptera musculus). After combing through old records, the researchers discovered that larger whales had thicker blubber than smaller whales of the same species. When they compared different species, the porpoises and dolphins had relatively thicker blubber for their size than their larger counterparts. Glarou and colleagues suggest that blubber might be more important to insulate the smaller animals from the cold water. However, this begs the question, what are the larger whales doing?
To find out, the team took to the sky. Using a drone, they videoed whales, dolphins and porpoises swimming around the bay to measure their body size. The researchers measured how long and wide the whales were but could only measure their height if the whales turned onto their side while swimming. Luckily, at least one individual of each species gave the researchers what they needed. From this information, the scientists could calculate the volume of each whale and how much of the whale's body was in contact with the cold water compared with its volume. If the whales have a higher ratio of surface area to volume, they should lose heat more quickly. As the researchers expected, blue whales had much lower surface area to volume ratios because of their huge size, while harbour porpoises had high ratios. This suggests that blue whales would lose body heat slowly in the cooler waters while the smaller porpoises would lose heat rather quickly. So, the way that the bigger whales stay warm is simply by being gigantic.
After all this difficult work, the team could finally calculate how much heat these mammals lost while being in the water. When they took the animals’ size into account, the white-beaked dolphins and harbour porpoises lost the most body heat to the water, while blue whales lost the least, suggesting that the dolphins and porpoises need to expend more energy on keeping themselves warm than the larger whales do. Glarou and colleagues point out that this could mean that the dolphins and porpoises need a reliable source of food because they burn more calories just trying to keep their bodies warm. This also means that the porpoises and dolphins are confined to places with lots of food while the larger whales can move to warmer waters with less food if needed. It seems that while blubber is essential in insulating marine mammals from the cold water, being huge certainly helps keep whales warm.
