I don’t remember the exact circumstances that brought this one up, but out of the blue my son asked the following question: “Do kangaroos eat meat?”
“Uhm…” I temporized, racking my brains. I know a few odd facts about kangaroos – for example, their legs have evolved so that once they start hopping they barely have to use any energy to continue hopping in a straight line at a steady pace, and females have two uteruses – but I suddenly realized that I have [i]no idea[/i] what they eat. “I don’t know,” I admit. “I think they’re herbivores, though.”
“No, they don’t eat meat,” my wife adds. “They do eat bugs, though.”
“Bugs are meat!” my son retorts.
Well played, son. Well played.
What do kangaroos eat?
This looks pretty straight forward. Kangaroos are herbivores. Most of them are “grazing herbivores”, meaning that they eat kind of like cattle – grass, leaves from shrubs and low-hanging trees, fungus if they find it, that sort of thing. They crop the plant matter with their sharp front teeth and grind it with their molars before swallowing. As the molars wear down, new molars grow at the back of the mouth and push forward, shoving out the worn front molars.
Also like cattle, kangaroos have two stomach chambers and chew their cud. The plant matter they swallow is held in the front chamber, where it is fermented by bacteria and fungi until it is regurgitated and chewed and swallowed again. Unlike cattle, though, they don’t fart. Cows fart because their stomach bacteria produces methane as they ferment their cud. Kangaroo stomach bacteria, however, produce hydrogen that is changed into acetate which is used for additional energy.
That seems pretty straight forward
It did, didn’t it? But then I saw this:
Wait. What the heck was that?
That was a kangaroo eating a bird on a beach.
But… but… wait! You said they were herbivores!
Yeah, that’s what I said. And it makes sense to say that, because they’re adapted to a plant-eating diet. And we all learn from an early age that animals fit neatly into one of three categories: herbivores (who eat plants), carnivores (who eat meat), and omnivores (who eat meat and plants). But it turns out that nature is messy and doesn’t really care at all about the neat little categories human beings come up with to explain things.
The animals we call “herbivores” are best thought of as “animals that are adapted for, and primarily consume, a plant-based diet”. Let’s start with the easy proof of that: do you really think that a cow would spit out a worm or a mouse if it picked it up while tucking in to a bale of hay? The answer is, it might. Or it might keep chewing. Cows have to eat a lot, after all.
But it goes beyond simple things like “oops, I ate a mouse”. Deer will actively prey on bird nests, consuming the eggs and even the hatchlings within. They’ll also rummage through animal remains, both to eat partially digested foods in the remains and to eat the meat left behind – going as far as eating human remains. Deer aren’t the only predatory “herbivores”, either. Cows have been observed eating chickens and giraffes have been observed eating animal bones. Hippos have been observed hunting, killing, and eating other animals.
In a less bloody example of “eating outside your assigned category”, alligators and crocodiles will eat fruit and vegetables. And anyone who’s been around cats or dogs has seen them eat grass and houseplants.
So, uhm, why?
Sadly, I couldn’t find any specific explanation for this behavior. I assume that it happens for the same reason any animal eats anything at all: either it tastes good, or there’s something in the meat the animal needs, or both. Some nutrients may just be easier to get from animals – calcium, for instance, is easier to obtain in large quantities if you eat a bone. And a chicken has a lot more protein than the same weight of grass.
So, yes. Kangaroos do eat meat. So do a lot of other animals you wouldn’t expect to be eating meat. But they’re not predators, not really. Primarily herbivorous animals just aren’t adapted for a meat-heavy diet, or to hunt down and kill other animals.
Except for hippos. They’ll probably just kill you for fun.