Tag Archives: omnivorous

The bonnethead shark. Credit: Wikimedia Commons.

Scientists find the first plant-eating shark — but it still likes to hunt

Jaws would’ve been a heck of a lot less terrifying had it stared the bonnethead shark. According to a recent study, this shark enjoys a diet of bony fish, crabs, snails, and shrimp — but also seagrass. This makes it the first omnivorous shark that we know of.

The bonnethead shark. Credit: Wikimedia Commons.

The bonnethead shark. Credit: Wikimedia Commons.

For some time, biologists have been observing that bonnetheads (Sphyrna tiburo) ingest seagrass in addition to their favorite crustaceans and shellfish. However, because their stomach is almost identical to other meat-eating sharks, being highly specialized for a diet revolving around protein, scientists had always assumed the seagrass was consumed by accident.

But it seems like the bonnetheads really like seagrass, which they ingest for its nutritional value. This was proven by Samantha Leigh at the University of California Irvine and her colleagues, who ran a series of tests to determine how much of this seagrass diet could be digested. What an animal consumes is not necessarily the same as what it digests and retains nutrients from.

First, the researchers planted seagrass from Florida Bay in their lab. To track the plants along the food chain that would eventually reach the sharks, researchers added a specific carbon isotope to the water.

Five bonnetheads were served a diet made up of 90% seagrass and 10% squid in a controlled environment in the lab. Three weeks later, all the sharks had put on weight, suggesting that the animals must have derived calories from the plant-based diet.

To be sure, researchers ran a new series of tests, meant to measure how much of the plant matter was digested and how much of it simply passed through.


Credit: Leigh et al.

Blood tests found high levels of the carbon isotope in both the sharks’ bloodstream and liver, suggesting that the seagrass nutrients were being absorbed.

Finally, Leigh and colleagues found the types of digestive enzymes that break down the plant-based food. Carnivores like white sharks usually have very low levels of enzymes that break down fibers and carbs. The bonnethead sharks, however, had copious levels of these types of enzymes. Over half of the organic matter found in seagrass was digested by the bonnetheads.

“Bonnethead sharks are not only consuming copious amount of seagrass but they are actually capable of digesting and assimilating seagrass nutrients, making them clear omnivores,” the researchers wrote in their study.

“This is the first species of shark ever to be shown to have an omnivorous digestive strategy.”

Approximately 4.9 million individuals swim in the coastal waters of the USA in the Atlantic and Gulf of Mexico. Their great numbers will force researchers to re-evaluate the role that bonnetheads play in seagrass meadows, which are critical ecosystems that provide habitat for thousands of fish species, filter the surrounding water, act as a sink for atmospheric CO2, and produce large quantities of oxygen.

“Considering bonnethead sharks as omnivores, rather than carnivores, in models of seagrass meadow function, and then testing the predictions of those models for management purposes, changes our understanding of the fluxes of nutrients and energy among trophic levels within each part of these ecosystems,” the authors wrote in the Proceedings of the Royal Society B.

The researchers aren’t sure when the bonnetheads started eating plants in their evolutionary history but it’s possible other shark species do it as well.

No web, no worries — spiders also like to eat vegetarian

Spiders’ diets aren’t limited to juicy insect bits; they’ve been shown to occasionally consume fish, frogs or even bats before — but they spice up their menus with vegetarian courses too, zoologists from the US and UK have found.

Young jumping spider consuming a Beltian body (lipid and protein-rich detachable leaflet tips of acacias.)
Image credits Eric J. Scully/ Harvard University.

Spiders are traditionally viewed as insectivorous predators, dining on anything their webs can trap. But scientists are becoming increasingly aware that’s a skewed view of them, and that their diet is more diverse than we imagine. If available, spiders won’t shy away from eating fish, frogs, bats — all kinds of meat. But a team of zoologists from the University of Basel, Brandeis University and Cardiff University has now brought evidence of meat-eating spiders chowing down on plant-based foods too.

“The ability of spiders to derive nutrients from plants is broadening the food base of these animals; this might be a survival mechanism helping spiders to stay alive during periods when insects are scarce”, says lead author Martin Nyffeler from the University of Basel in Switzerland.

They gathered and documented all examples of spiders eating such items from scientific literature they could find. Their collection of data shows that spiders from ten families have been reported feeding on a wide range of plants such as trees, shrubs, ferns, flowers, weeds or grasses. And they aren’t picky, either; they’ll eat anything from nectar, sap or honeydew to leaves, pollen and seeds.

Jumping spider drinking nectar at extrafloral nectaries of a shrub.
Image credits David E. Hill, Peckham Society, South Carolina.

A family of diurnal spiders, the Salticidae, seem to be the most voracious plant-eaters of the Araneae order. These plant-dwelling, highly mobile foragers were attributed with almost 60 percent of the incidents documented in this study.

But such feeding habits aren’t a Salticidae-only thing. Plant-eating in spiders has been reported from all continents except Antarctica, but seems to be more common in warmer areas of the globe. As a larger number of the reports relate to nectar consumption (which has its core distribution in warmer areas where plants secreting large amounts of nectar are widespread) this isn’t too surprising.

“Diversifying their diet with plant is advantageous from a nutritional point of view, since diet mixing is optimizing nutrient intake,” Nyffeler concludes.

Currently, the extent to which different categories of plant-based food contribute to the spiders’ diet is still largely unexplored. But, as there currently is a known species of spider that is mostly herbivorous, the Central America indigenous Bagheera kiplingi, it can be assumed that in a pinch spiders can live on a full veggie diet for some time.

The full paper, titled “Plant-eating by spiders” has been published online in the Journal of Arachnology and can be read here.

Panda poo shows they shouldn’t munch on bamboo so much

Giant pandas love to feast on bamboo – it’s their favorite food, and they usually make quick work of it, using their powerful jaws to peel the plant’s tough bark and get to the tender core. But even though the pandas love it, their stomachs don’t – a new study has revealed that the panda’s stomach is not adapted to a completely herbivorous diet, and still craves for an omnivorous meal, like other bears.

With them being so fluffy and lazy, it’s easy to forget that panda bears are… well, bears. But bears eat both plants and other animals – they have what is called an omnivorous diet – while pandas only eat plants (mostly bamboo). However, the giant panda still has the digestive system of a carnivore, as well as carnivore-specific genes, and thus derives little energy and little protein from consumption of bamboo – which is why it can spend up to 14 hours a day eating bamboo.

A team of researchers in China wanted to see just how well the panda’s stomach gets along with its food, so they took 121 fecal samples from 45 giant pandas — 24 adults, 16 juveniles and five cubs. They then compared the results with those from a previous study on wild pandas. Both studies showed the fact that pandas don’t have plant-degrading in bacteria, and draw very little energy from bamboo.

“This result is unexpected and quite interesting, because it implies the giant panda’s gut microbiota may not have well adapted to its unique diet, and places pandas at an evolutionary dilemma,” said Xiaoyan Pang, a co-author of the study in a press release.

It seems like evolution only went half way – the pandas developed powerful jaws and teeth specifically for eating plants, but they don’t have the digestive system to work with. So the only solution that was left for them was to have bacteria that help it break down the bamboo. The authors write:

“The giant panda appears to have no alternative but to rely on symbiotic gut microbes to adapt to its highly fibrous diet.”

So that’s what the researchers were expecting to find, except they didn’t. Furthermore, the bacterial diversity in the panda’s stomachs was extremely low, compared to other mammals. A high gut bacteria diversity is associated with resilience and adaptation capability, so this means that the panda is highly vulnerable and can’t really adapt to new environments and new diets.

But the biggest surprise was the fact that the dominating bacteria population was represented by Escherichia/Shigella and Streptococcus – something you’d expect to see in meat eaters, not vegetarians. Ruminococcaceae and Bacteroidetes bacteria, generally associated with degrading fiber, were missing.

So why is it then that pandas eat plants? Why did their transformation from omnivorous to plant-eating bear stop half way?

We still don’t know yet, but one thing’s for sure: pandas are passing through an evolutionary stage where they are extremely vulnerable. They can’t adapt to new environment, they can’t properly digest the food they eat, and so they have to spend most of their time eating and not spend much energy. In other words, pandas might just be eating their way to extinction.

Fossils Reveal “Beer-Bellied” Dinosaur

It was about as big as T-Rex, but not quite as fit – new fossils have revealed that Deinocheirus mirificus had quite a beer belly.

Image via Scientific American.

“This is an entirely new body plan” for such dinosaurs, says Stephen Brusatte, a vertebrate palaeontologist at the University of Edinburgh, UK.

Indeed, few scientists would have imagined such a scientific appearance. The first fossils of Deinocheirus mirificus (which means ‘unusual horrible hand’ in a mixture of Greek and Latin) were dug in 1965 in the Gobi Desert, in Mongolia. The fossils were quite significant: a few rib and vertebra fragments and a remarkable set of shoulder girdles and 2.4-meter-long forelimbs, the longest yet found for a bipedal animal of any era (although some flying animals, notably pterosaurs, had longer wings).

After analyzing the bones, paleontologists placed in a group of therapod dinosaurs called ornithomimosaurs – bird-mimicking dinosaurs. They believed Deinocheirus was related to T-Rex and Allosaurs – some of the most fierce creatures to walk on Earth, explains Yuong-Nam Lee, a vertebrate palaeontologist at the Korea Institute of Geoscience and Mineral Resources in Daejeon, South Korea, and a co-author of the study.

However, in a later expedition, Lee and his colleagues unearthed a much more complete skeleton – 95% intact. Some of the bones had been gathered by poachers but were further recovered by Lee from a private collection. These new fossils were shocking.

For starters, the spinal vertebrae have blade-like projections that extended upward and served as anchors for a network of ligaments; but there would not be a need for such a strong and well fitted system unless the animal had a huge belly. Deinocheirus measured about 11 meters long and tipped the scales at more than 6.3 tons. That’s much more than you would expect for this size.

“This is definitely an unusual animal,” says Thomas Holtz, Jr., a vertebrate palaeontologist at the University of Maryland in College Park, who wrote an accompanying News & Views piece. “It had more of a ‘beer belly’ than your typical ornithomimosaur,” he suggests.

There is still no explanation or theory for why it was built like this. What researchers do know is what it ate. With its 1 meter long toothless head, Deinocheirus ate lots of things. The content of its stomach were also found fossilized: fish vertebrae and scales suggest that Deinocheirus also consumed large quantities of aquatic prey. But the way its head is built suggests that it also feasted on vegetation – it was likely omnivorous, eating both plants and other animals.

Paleontologists believe that throughout Earth’s history, several dinosaurs were omnivorous, though that was likely pretty unusual. The physical build also had a lot to do with its diet – it’s likely that the dinosaur initially evolved as carnivorous, only to change its way and to eat more plants. It seems plausible to me that if this was the case, then it would need to eat more plants to satisfy its energy needs, and therefore develop this “beer belly”.

The beast was a slow mover, but it had long feet with hooves, which would have prevented it from sinking into the boggy wetlands where it lived. Commenting on the research Prof John Hutchinson, a palaeontologist from the UK’s Royal Veterinary College, said:

“Many dinosaur fans have seen pictures of the 8ft-long arms and hands, and they really are amazing and wonderful. People were really wondering what the rest of this animal looked like. Now we know, and it’s just so freaking weird – we never would have expected this animal to look so bizarre. It really is shocking to see how many weird features it has. It changes our view of what kind of forms dinosaurs can even take.”

Yep, when scientists call something “freaking weird” – that’s when you know something’s up.