Whale sharks are the world’s biggest omnivores
Whale sharks are the biggest shark species in the world, and now scientists have found that the giant sharks are even more prodigious eating machines than thought. In addition to gulping down enormous mouthfuls of krill, whale sharks also swallow huge helpings of seaweed, enabling the aquatic giants to officially dethrone Kodiak bears as the world’s largest omnivores.
Researchers made the discovery by analysing whale shark (Rhincodon typus) skin samples collected near Western Australia’s Ningaloo Reef. These gargantuan sharks are the largest fish in the sea, weighing up to 36 tonnes and growing to about 12 metres long on average. Scientists thought that the gentle giants were primarily filter feeders, gaping their cavernous mouths wide to gulp in roughly 600 cubic metres of water every hour. By straining the water out through their gills, the sharks are left with mouthfuls of plankton, shrimp, tiny fish and crustaceans to swallow down. But the new discovery has given scientists important new information to chew on.
“This causes us to rethink everything we thought we knew about what whale sharks eat” and calls into question other aspects of the sharks’ behaviour “out in the open ocean,” said Mark Meekan, a fish biologist at the Australian Institute of Marine Science in Queensland. Meekan said that the discovery contradicts the common assumption that large land creatures are typically herbivores, but those that live in the sea occupy a different niche on the food chain, feeding on small shrimp and fish. “Turns out that maybe the system of evolution on land and in the water isn’t that different after all,” Meekan said.
For their study, the scientists collected the sharks’ possible food sources, ranging from tiny crustaceans and plankton to large clumps of seaweed, and then chemically analysed the samples to reveal their amino and fatty acids. After cross-referencing these acids with those found in skin samples taken from whale sharks, the researchers identified high concentrations of sargassum – a type of brown seaweed made up of thousands of microscopic algae – in the sharks’ diets.
The scientists think this omnivorous diet could be a result of the sharks evolving to digest accidentally swallowed seaweed, saving them the energy cost of spitting it back out. “We think that over evolutionary time, whale sharks have evolved the ability to digest some of this sargassum that’s going into their guts,” Meekan said. “The vision we have of whale sharks coming to Ningaloo just to feast on these little krill is only half the story. They’re actually out there eating a fair amount of algae too.”
Having a broader range of food sources might sound like good news for whale sharks, as it could help them withstand potential upheavals to their marine ecosystems brought about by climate change. However, it’s possible that the sharks’ propensity for swallowing most of what is swept into their mouths could make them far more likely to swallow copious amounts of ocean-borne plastics. Whale sharks can pass some plastics through the gut, but ingesting small or large plastic pieces could cause the sharks to vomit up their meals, and could reduce their gut capacity and interfere with digestion.
Did you know?
The biggest whale shark on record was 18.8 metres long
Did you know?
Tarsiers eat insects, reptiles, frogs and even birds
Tiny monkey-like creatures called tarsiers sing duets together in the style of opera singers – but those who fail to hit the high notes may also flop at attracting mates. With their large, pointed ears and expressive eyes, nocturnal tarsiers carry a striking resemblance to the diminutive Jedi master Yoda from the Star Wars films. But while Yoda never demonstrated any operatic ability, tarsiers are energetic singers who may exert themselves vocally as a form of sexual selection or to signal to each other that it’s time for all members of a troop to gather together to sleep.
To learn more about these vocal performances, scientists eavesdropped on tarsiers in Tangkoko National Park in Sulawesi, Indonesia, in July and August 2018, capturing 50 recordings of 14 pairs of Gursky’s spectral tarsiers (Tarsius spectrumgurskyae) singing their morning duets. Researchers from Universitas Sam Ratulangi in Sulawesi and Cornell University in Ithaca, New York, used machine learning to distinguish and classify notes and musical phrases in the tarsiers’ songs. The research suggests that these performances are so taxing that not all tarsiers can hit the fast and high notes and duet proficiently. Measuring just 9 to 15 centimetres tall and weighing no more than 200 grams, Gursky’s spectral tarsiers live only on Sulawesi, an Indonesian island east of Borneo. They were described as a separate species from other tarsiers in 2017 and are classified as vulnerable by the International Union for the Conservation of Nature’s (IUCN) Red List of Threatened Species. About 13 species of tarsiers inhabit islands in Southeast Asia.
The researchers captured the tarsiers’ duets using a handheld digital recorder and autonomous recording devices that were installed in the tarsiers’ jungle habitats. When the scientists analysed the intricate duets, which were performed between male and female tarsiers, they found that the performances resembled coloratura – a singing style that produces many notes very rapidly and that’s used by opera singers during arias to showcase their control and virtuosity.
“The duets exhibit acoustic trade-offs in note rate and note bandwidth – the range of frequencies within a note,” said Isabel Comella, a researcher at the K. Lisa Yang Center for Conservation Bioacoustics. The tarsiers that sing more slowly do so with the widest range of frequencies within a note, while the tarsiers that repeat notes more quickly only appear capable of a narrower range of frequencies within a note. Only a minority manage both simultaneously. The researchers hypothesise that rapidly singing notes containing a broad range of frequencies during a duet may be more physiologically and neurologically taxing for the singer, with only physically fit individuals able to do so.
Exactly why tarsiers duet in such a complex and physically taxing way is unknown, largely because the animals are rarely studied. Other primates are known to sing duets, including a type of lemur called the indri (Indri indri), titi monkeys in the Callicebus genus and eastern gray gibbons (Hylobates funereus). Prior studies into primate duets suggest that this behaviour may be used for finding or guarding a mate, defending territory or strengthening social bonds, though further research is required to determine exactly why tarsiers are performing these duets.