Which animal has the fastest digestive system?

The elephant shrew. © Anthony Bannister/Gallo Images/Corbis

How fast does it take for food to travel through the digestive system?

First of all, let's reign in the question a bit and consider only the vertebrate world. Because that's where most of the research seems to focus.

Secondly, let's define what we're talking about: gut transit rate. A specific species' rate will vary depending on environment and diet, as well as whether you're talking liquid or solid waste. So we'll be dealing with general numbers here.

For humans, total digestion time takes anywhere between 24 and 72 hours. According to the Mayo clinic, you're looking at a 53-hour average.

As far as animals go, let's turn to "Comparative Physiology of the Vertebrate Digestive System" by C. Edward Stevens, Ian D. Hume. Here's a quick, comparative reference guide of gut transit times for animals covered in the text:

Cedar waxwing: 0.7 hours Rufous hummingbird: 0.8 hours Elephant shrew: 3.4 hours Ring-necked pheasant: 5 hours Norwegian rat: 13 hours Ostrich: 48 hours Red-eared slider turtle: 58-64 hours Ringtail possum: 112 hours Ringneck snake: 15-35 hours Spectacled caiman: 162 hours Green iguana: 207-386 hours

© Nick Saunders/All Canada Photos/Corbis

As you can see, reptiles tend to have the slowest digestion time while birds boast the fastest. Among mammals, the small carnivores win the digestion race and large herbivores finish last.

It's not surprising to see a shrew and a hummingbird at the top of that list as both species are notoriously ravenous to support their super-charged metabolisms. But what are we to make of the Ceder Waxwing (pictured right)? Powered flight requires a high metabolism, but why is this berry-eating flyer beating the hyper-charged hummer?

Here's a little info on the waxwing's gut transit time from a 1991 study published in The American Naturalist:

"Waxwings separated pulp from seeds and defecated seeds well in advance of pulp, thereby allowing both an increase in consumption and, presumably, a greater assimilation of nutrients in the pulp. We propose that rapid seed processing has influenced pulp composition and seed-packaging traits."

The hummingbird may boast the higher metabolism rate, but the waxwing has the ability to expedite the gut transit of indigestible seeds.

I primarily tackled this question in "Monster of the Week: The 'Feast' Creatures," but I figured the question deserved specific focus here given the sparsity of answers out there on the various question sites.

About the Author: Robert Lamb spent his childhood reading books and staring into the woods — first in Newfoundland, Canada and then in rural Tennessee. There was also a long stretch in which he was terrified of alien abduction. He earned a degree in creative writing. He taught high school and then attended journalism school. He wrote for the smallest of small-town newspapers before finally becoming a full-time science writer and podcaster. He’s currently a senior writer at HowStuffWorks and has co-hosted the science podcast Stuff to Blow Your Mind since its inception in 2010. In his spare time, he enjoys traveling with his wife Bonnie, discussing dinosaurs with his son Bastian and crafting the occasional work of fiction.