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Hippos Eat Way More Meat than We Thought, and It Can Make Them Sick

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Hippos are huge animals with fearsome tusks and aggressive natures, but they mainly eat plants. Sometimes they attack people and can tangle with crocodiles, sure, but they’re not predators or carnivores. Right?

But a closer look reveals that hippos aren't so herbivorous after all, says biologist Joseph Dudley. Despite their grass-heavy diets and all the adaptations that make them great grazers, hippos have been known to eat their fair share of meat. There are scattered reports by scientists and amateur observers (note: NSFW gory pictures) of hippos attacking, killing, and eating other animals, stealing kills from predators, and scavenging carcasses, including those of other hippos. In a new paper published in the journal Mammal Review, Dudley and his fellow researchers argue that these incidents aren’t as unusual as they seem or isolated to a few animals or populations. They say there’s a pattern of carnivorous behavior in hippo populations across the animal’s entire range—and that behavior has consequences for hippos. 

Evolution has outfitted hippos and other big herbivores for a plant-based diet, and their guts and the microbes that live within them are adapted for fermenting and digesting lots of plant material. That doesn’t mean these herbivores animals can’t add meat to their menu, though. Many can and do. Antelope, deer, and cattle have been known to feed on carrion, birds’ eggs, birds, small mammals, and fish. What might hold most of these animals back from more frequent carnivory, Dudley suggests, isn’t their digestive physiology, but “biomechanical limitations” in securing and ingesting meat. In other words, they aren’t built for taking down prey or biting into flesh. The hippo is another story. 


“Due to its large body size and unusual mouth and dental configurations, the hippo may represent an extreme case in which the predation and scavenging of large mammals by an ungulate species is not constrained by biomechanical factors,” Dudley and his team write. Not only can hippos kill and eat other big animals more easily than other herbivores, the researchers say, the fact that they’re territorial and highly aggressive may facilitate carnivory, putting them in situations where they kill other animals and can get themselves something to eat.

And eat they do. Since Dudley made the first scientific record of carnivory in hippos in 1996, other cases of hippo carnivory and even cannibalism have also been documented. Dudley lists instances where wild hippos have fed on impalas, elephants, kudus, wildebeest, zebras, and other hippos that they either killed themselves or were killed by other predators. Events like these have been seen both during times when carnivory may be a last resort (e.g. droughts when food is scarce), and when it was merely a convenient opportunity, like a mass drowning of wildebeest crossing a river. There are also reports of captive hippos in zoos killing and eating their neighbors, including tapirs, wallabies, flamingoes and pygmy hippos.

“Our scientific records, coupled with those of other investigators and observers, demonstrate that the phenomenon of carnivory by hippos is not restricted to particular individuals or local populations but is an inherent characteristic of the behavioral ecology of hippos,” the team writes.


If that’s the case, then why did it take so long for anyone to figure it out? Part of the blame can fall on conflicting schedules. Hippos are mostly active at night, which means their meals, meat or otherwise, usually go unseen by humans. Their carnivorous ways, Dudley thinks, have simply been overlooked. 

They may also explain why hippos are so susceptible to anthrax and experience higher mortality rates during outbreaks, the researchers say. Hippos, they think, are doubly exposed to the disease because they ingest and inhale bacterial spores on plants and in the soil like other herbivores, and also consume them when feeding on contaminated carcasses. Cannibalism during outbreaks exacerbates the problem. 

That carnivory might make these outbreaks worse in hippo populations has implications for controlling the disease and protecting both animals and humans. During anthrax outbreaks among wildlife, many human illnesses occur because of contaminated “bush meat.” During a 2011 outbreak in Zambia, for example, 511 human cases of anthrax and at least five deaths were linked to people handling and consuming meat from infected hippos. Burying or burning suspected infected animal carcasses is a standard practice during anthrax outbreaks, and the researchers think that this may be especially effective in hippo habitats because it takes infected meat off the menu for both humans and hippos. 

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Big Questions
Why Do Dogs Howl at Sirens?
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A dog's behavior can often prove confusing to their human colleagues. We know they like to eat their own poop, but puzzle at their motivations. We're surprised when dogs give a ladybug the same greeting as a home intruder.

Topping the list of eccentric canine behavior: Why do dogs howl at sirens? Is there some genetic predisposition to responding to a high-pitched alarm from passing ambulances or police vehicles?

As it turns out, the reason dogs howl at sirens is because of their ancestry—namely, the wolf. When members of a pack are fractured and spread out, their companions will howl to provide a way of locating them. Think of it as nature’s GPS: By howling, dogs are able to communicate their respective locations to one another, even across long distances.

Since dogs really don’t know what a cop car is supposed to sound like, they’ll often interpret a siren as an animal’s howl. It’s also possible that dogs consider sirens to be a sign that something is abnormal in their environment, and that they want you, the pack leader, to be aware of it.

Contrary to belief, a dog is rarely howling because the noise hurts their delicate ears. If that were the case, some experts say, then they would display other behaviors, like running and hiding.

The more a dog hears and responds to a siren, the more they might be compelled to continue the behavior. That’s because dogs who howl and then notice the sound drifting away might begin to associate their vocalizing with the disappearance of the noise. In the future, they’ll probably recall that they “drove” the interloper away with their warbling and repeat the process.

While howling is usually harmless, sometimes it can be a sign that your pet is feeling separation anxiety from an owner or that they’re feeling unwell. If howling persists even without a screaming siren within earshot, you might consider taking them in for a check-up.

If you’ve wondered why dogs howl at sirens, now you know. It’s simply a way of signaling their location and not because it pains them. Owners, on the other hand, might feel differently.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at

5 Ways to Keep Your Dog Calm on the Fourth of July

The Fourth of July can be rough for dogs. Fireworks displays light up their senses with unfamiliar noises, flashes, and smells, and parties flood their homes with strange guests who may invade the rooms they usually have as private retreats. And when distressed dogs escape, howl, or thrash around the house, Independence Day can quickly become a nightmare for their owners, too. To minimize Fido's stress this holiday, we spoke to some dog experts to discover the best ways to keep your canine calm on the Fourth of July.


Anthony Newman, the dog whisperer who runs New York City's Calm Energy Dog Training, says that exercise is a great way to help your dog let off some nervous energy. “Whenever Fido is going to be neglected for an extended period of time, or around any stressful stimuli, it always helps to tire him out just before—and even during the night if you can,” Newman says. “As the saying goes, a tired dog is a good dog! He’ll be calmer, happier, and more peaceful.”


Dr. Stephanie Liff, head veterinarian at Pure Paws Veterinary Care, says the best place to keep your pet during a fireworks show is inside and away from the windows. “If the pet is very scared, an escape-proof crate or a sound-insulated room, such as an internal bathroom, may help the pet to feel more secure,” Liff tells us. “If you cannot keep your pet inside, make sure that the pet is prevented from escape (monitor all exits and tell guests to monitor your pet).”


While your dog may feel more secure in a room away from all the noise, Newman points out that keeping your dog isolated in another room for too long can be stressful for your pet. “Release his curiosity and let him in on the fun, to run around and play with both two-legged as well as four-legged guests,” Newman says. “Then back to his obedient room, bed, car, crate, or spot. Rinse and repeat as needed throughout the night."


According to Newman, the best way to keep your dog calm during the chaos of July 4th is to stay in charge. “If your dog winces, shivers, and runs away at loud noises, the last thing he wants is to feel like nobody else is looking out for him,” Newman says. Don’t let your dog run rampant around the house or follow him around trying to soothe him. Instead, Newman says it's important to “take control by attaching a super-light leash that you can grab and lead him whenever you need.”


In extreme cases of nervousness, Liff says that you should talk to your vet about medication to sedate your dog.


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