![]() That may suggest that individual sharks specialize on feeding in certain areas or on certain foods. ![]() There was a lot of variation between individuals, something that Prebble and Dove both found intriguing. (Learn about the chaotic world of whale shark tourism.)īut not all the animals showed signs of big gaps between meals. “Food availability in the open ocean is very patchy, so it would be a good strategy for them to gorge themselves when food is abundant, then use that energy to sustain them through periods of travelling or searching to find their next seafood buffet,” she says. Prolonged fasting makes a lot of sense when you think about the animals’ environment, says Prebble. That’s probably because they don’t eat while on the move, Wyatt says-perhaps because there isn’t anything worth stopping for along the way. But the researchers found that the beasts can also go for four months or more without eating. You’d think it takes a lot of food to support so much body mass-and it does. They can grow to be almost 40 feet long and weigh nearly 50,000 pounds. Like the marine mammals they’re named after, whale sharks are huge. “It’s an epic amount of work that really nicely takes what we can learn from animals in aquariums and applies it to what we want to know about that same species in its natural setting,” says Dove. But this study was able to use five captive whale sharks with known diets from the Okinawa Churaumi Aquarium to ground-truth their data from eight wild animals. While stable isotopes are frequently used to infer animal diets, the method often relies on making assumptions about how the animals process the nutrients from their foods. There’s usually a bit of a catch, though. Watch them snack on an unusual bait: seagrass. Like whale sharks, bonnethead sharks also aren't true carnivores. The proportion of these isotopes differs in different food sources like algae, zooplankton, and fish, so looking at the ratios of these atoms in the sharks’ tissues can tell researchers a lot about what the animals are eating. To determine what the animals eat, the research team measured the different forms, or isotopes, of key atoms like nitrogen and carbon in the blood and tissue samples. “It's a great addition to knowledge of whale shark foraging and diet,” says Clare Prebble, a senior scientist with the Marine Megafauna Foundation. And that’s why Wyatt and his team’s approach is so appealing-it uses samples of blood and other tissues that can be opportunistically collected to look back at what an animal has been eating. Tagging studies have elucidated the movement piece of the puzzle, but it’s basically impossible to follow these very mobile fish around all the time to see what they’re eating. Those making conservation decisions need concrete information about a species’ habits, including details like how individuals move around and what they need to eat, to determine the best ways to protect it. Studying what an animal eats is “fundamental stuff,” says whale shark biologist and vice president of research and conservation at the Georgia Aquarium Alistair Dove, but “it’s also central to the sort of population models that are necessary when you are trying to develop enlightened conservation plans for an endangered species.” “I am very keen to contribute to an improved understanding of the species.” “Whale sharks are a very charismatic creature that is globally threatened, but we still don’t know enough about their ecology for effective conservation,” Wyatt tells National Geographic. While previous studies had found seaweed in whale shark stomachs, this is the first study to suggest they might ingest such algae as a dietary staple. The research team, led by University of Tokyo biologist Alex Wyatt, used a combination of samples from captive and wild sharks to demystify the feeding habits of these enigmatic ocean travellers. Careful investigation of blood and tissue samples from over a dozen whale sharks suggests that they actually have a pretty omnivorous diet that includes plants and algae. That’s not what an intriguing new study published this month in the journal Ecological Monographs found, though. ![]() ![]() ![]() Still, they are sharks, so it’s long been believed these gentle giants rely almost exclusively on animal protein. Whale sharks are the largest fish in the sea, though as filter feeders, they don’t have the same bloodthirsty reputation as their kin. ![]()
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