How do dolphins eat plankton
Lots of food with little effort : Why blue whales don't get any bigger
With a length of up to thirty meters and a weight of 200 tons, blue whales are the largest animals on earth. Why they don't get even bigger is a matter of debate in science. Now scientists working with Jeremy Goldbogen from Stanford University are reporting in the journal "Science": The growth of the individual whale species depends primarily on how they eat and how much food is available to them.
How big an animal species becomes is determined by the balance between the energy it gains by eating and the energy it has to expend on it. On land, this usually means that small animals feed on small prey and large ones feed on large prey.
In the ocean, however, this calculation does not work: there, the largest animals on earth feed on tiny plankton. This paradox wanted to investigate the scientists around Goldbogen.
Every dive for an octopus costs a lot of energy
To do this, they provided 300 toothed and baleen whales - from small porpoises to gigantic blue whales - with sensors for acceleration and diving depth, cameras and microphones. They followed the animals as they moved between Greenland and the Antarctic and were able to evaluate more than 10,000 "feeding events".
They also analyzed the stomach contents of whales that were stranded dead on the coast. From the data, they calculated how energy-efficient it is for each type of whale to eat. In doing so, they encountered significant differences between the species.
Toothed whales - such as dolphins or sperm whales - often dive to great depths to look for food with the help of echolocation, such as squid. Most of the time, the prey they make in the depths provides just more energy than they need to spend on their dives.
If, on the other hand, they were larger, they would have to muster too much energy to survive in the long term. Also, there would simply not be enough squids for a larger body. Clearly, the sperm whales have reached the limits of growth for such lone hunters, state Jeremy Goldbogen and his colleagues.
Open your mouth - pure energy
The larger baleen whales, on the other hand - for example blue or humpback whales - forego the exhausting individual hunt. Instead, they feed on krill and other plankton. In order to catch this prey efficiently, their physique has adapted perfectly: a third to half of their body consists of head, mouth and throat. Behind this is an intestine around a hundred meters long, for example in the case of the fin whale.
Together this results in a huge, precisely balanced feeding machine. Baleen whales only have to swim through the water and open their mouths in order to quickly and efficiently absorb massive amounts of energy in the form of miniature animals. And more than they have to spend for the leisurely hunting method, the researchers write. But if the whales were even bigger, this could have a negative impact on the sensitive system.
Food isn't always there
Another factor is the seasonal availability of food. In the summer months, blue whales filter vast amounts of plankton from the water and in the process eat a layer of fat that provides enough energy for their long migrations. Only a large body allows such a lifestyle with long hunger breaks. But if they were even bigger, the animals would no longer be able to filter enough energy out of the water.
In the face of climate change and overfishing, it is worth understanding what whales really need, University of California biologist Terrie Williams wrote in an independent commentary. Because the giants play an important role in the sea: With their excretions, for example, they ensure that microorganisms can thrive. And even when they die, their bodies feed creatures at every level of the food chain.
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