Tuesday, March 27, 2018

Those Aren’t Chocolate, Easter Bunny!

A repost of an original article from March 27, 2013.

The Easter Bunny has a dirty secret. When he’s not hopping around in his pristinely white fur hiding beautifully colored eggs and decorated baskets full of treats…he’s eating his own poo. Gross!

Never trust a rabbit. Photo by the Mosman Library at Wikimedia.

But don’t judge him before you understand him. It’s not that he chooses to eat poop, but that he has to for his own health. In fact, all rabbits do.

Rabbits are herbivores, which means that they only eat plant material. Plant material is very difficult to digest, although it may not seem like it (I mean, we eat plants all the time with no problem, right?). But when it comes to digestion, it’s not what you put in your mouth and swallow that matters, but what your body can break down.

This process of breaking down food depends on digestive enzymes, a group of chemicals that break down food. Each type of digestive enzyme is specific for breaking down a particular type of food chemical. Plant material is so hard to digest because it is largely composed of cellulose, a sugar that we vertebrates don’t have an enzyme for.

Herbivorous animals that lack this enzyme have developed an alternative strategy to get the nutrients they need out of these plants – They have microbes that live in their guts and ferment the plant material. Many of these microbes, which include bacteria, protists, yeast and fungi, produce the enzyme needed to break down cellulose. But these microbes are slow-acting (which means herbivores with longer guts get more nutrients), and they are sensitive (which means herbivores with special microbe gut chambers get more nutrients).

Rabbits have a special gut chamber called a cecum (or caecum) that houses many of their gut microbes. The cecum is so important to rabbit digestion, it’s even bigger than their stomach! When a rabbit eats something, the food is broken down by chewing, swallowed, and passed on to the stomach (follow along with the diagram below). The stomach stores and sterilizes the food while breaking down some of the nutrients before it passes the food on to the small intestine. The small intestine absorbs the nutrients it can before the remaining food gets sorted at a fork in this digestive road. The fibrous food parts move on to the colon, where it is converted into little hard turd-balls. The non-fibrous parts go to the cecum, where the microbes living there work their magic, breaking down the remaining food into absorbable nutrients.

This diagram of the rabbit digestive system was posted by Sunshineconnelly at Wikimedia. Trace through it as we talk about where each digestive step happens.

The trouble is, this food has already passed the part of the digestive tract that absorbs most of these nutrients: the small intestine. Now, it has nowhere to go but out. So the cecum pushes these remaining nutrients into the colon, which turns them into cecotropes (or caecotrophes): mucus-covered, nutrient-rich, moist turds shaped like a bunch of grapes (and according to the Easter Bunny, just as delicious). And the only way rabbits can get the nutrients (and remaining microbes) out of these little nuggets is to send them through the digestive tract all over again by eating them. So that is what they do.

Eating poo sounds gross and unusual, but it is actually fairly common in the animal kingdom. So common, in fact, that there is a term for it: coprophagia. Hamsters and capybaras have similar digestive tracts to rabbits and eat their own poo for the same reasons. Other animals, like elephants, hippos, pandas, and koalas, are born without the necessary microbes to digest the food available, so the babies obtain these microbes by eating their mothers’ poo. And many coprophagous insects, like flies and dung-beetles, subsist on diets composed of the poo of large animals.

So don’t hate on the Easter Bunny for his repulsive ways. He can’t help what he is. Just appreciate him for all the chocolate eggs he brings you every Easter. Wait… Those are chocolate eggs he brought you, right?

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