Why Don’t More Animals Eat Wood?

Hi, this is Julián from MinuteEarth. Make sure you stick around to the very end
of the video to see a fun new thing we’re trying! On average, wood contains around 10 times
more calories per kilogram than fruit or leaves. Yet while animals across the animal kingdom
eat almost every part of trees: leaves, fruits, flowers, seeds, heck even their internal fluids,
the wood is almost exclusively eaten by a few insects. What’s keeping all those other animals from
dining on, well, the dining table? While wood is made up of the same nutritious
carbohydrate called cellulose that makes up other tasty tree parts, most of the cellulose
in wood is bound together by a complex molecule called lignin. This makes wood super strong on both the molecular
level and the macro level, so… wood is hard to eat because it’s literally hard. Some animals, like beavers, have the right
masticating machinery, like sharp teeth and strong jaws, to break wood down on the macro-level,
which does release about a third of the yummy cellulose, but the rest is not digested and
gets pooped out still tangled up in the lignin. Insects like termites can go whole hog on
the whole log, because they can “chew” the wood with their serrated mandibles AND
also have the right gut machinery – AKA microbes – to break the lignin down on the molecular
level. Because lignin evolved in part to help plants
defend themselves against attackers – mainly independently-living microbes – lignin has mostly stymied microbes’
attempts to break it down. But life finds a way … and a few million
years after the evolution of lignin, a few microbes finally untangled the mystery. We still don’t understand all the details,
but some microbes seem to produce enzymes that use oxygen to break the tight bonds in
lignin molecules, setting the cellulose free … to be eaten. Which is actually how wood decomposes. In order for termites and a few other insects
to digest wood, they had to turn those independently-living lignin-breaking microbes into gut-living lignin-breaking
microbes. We aren’t sure why more animals haven’t
been able to put these microbes to work for them; perhaps it’s because their guts don’t
have the right chemical composition or because other biota already living there are hostile
towards newcomers. Or maybe termites are just early adopters
and millions more years of evolution will lead to more diversity of wood-eating animals. Who knows maybe one day woodchucks will chuck
wood … into their mouths. This video was sponsored by the Okinawa Institute
of Science and Technology Graduate University, an international graduate school devoted to
the advancement of science, education and innovation in Japan and throughout the world. OIST offers a fully-funded PhD program and
research internship opportunities that attract talented young scientists from around the
world to work with researchers like Professor Tom Bourguignon, whose Evolutionary
Genomics lab investigates how insects managed to conquer the world both in terms of animal
biomass and biodiversity. The lab’s research on termite gut microbe
DNA actually informed this video, as they’ve been able to map how the wood-digesting microbes
actually evolved, discovering that some microbes only exist within termites. To learn more about OIST – or maybe even apply
– visit admissions.oist.jp. Still here? Sweet. So, we’re trying something new: a cartoon
caption contest! Winners (like this one) will be featured at
the end of upcoming MinuteEarth videos. Plus they’ll get this super-limited edition
not-sold-in-stores MinuteEarth tote bag. Nice! The next cartoon is already up, patrons at
any tier can play at Patreon.com/MinuteEarth.


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