The following is an excerpt from The Three-Minute Outdoorsman Returns by Robert M. Zink (October 2018). Zink is a conservation biologist and animal ecologist in the School of Natural Resources at the University of Nebraska–Lincoln. He is the author of The Three‑Minute Outdoorsman: Wild Science from Magnetic Deer to Mumbling Carp (Minnesota, 2014) and a frequent contributor to Outdoor News.

21. Moose Drool

We humans have long since lost sight of many things that were important to our survival during our caveman/woman days. We also probably don’t appreciate the many things that go into the survival of wild animals. Here I take a look at costs to animals of what they eat.

That might seem fairly simple. Eat what you like, whenever. Go to the refrigerator, pick out what you want, and if it’s not there, put it on the shopping list. Not so for wild animals. How animals eat and what they eat perpetually co-evolve: prey (even plants) evolve better ways to escape being eaten, and their predators (or herbivores) evolve counter-measures. Food choices have been affected by Darwinian natural selection just like all other aspects of an organism’s anatomy and physiology. Animals that make better choices in what they eat, or learn how to eat more efficiently, leave more offspring.

You might think I’m referring to healthy versus junk food, organic versus non-organic, or good fats versus bad fats. What we humans have lost sight of is that finding food, subduing it, and eating it, constitute costs to an animal that must be offset by the value of eating the food. Hunting burns calories, physically subduing prey burns calories, making prey edible burns calories, and chewing is a cost. Last but not least, digesting and excreting are costs! In economic terms, animals should not eat things that will put their energy balance in the red.

For example, imagine a bird that fancies fat, juicy, energy-rich butterflies. It takes aerobatic skill to catch them (cost); then, because the wings are inedible, they have to be removed (cost) before the best part, the abdomen, can be eaten (cost). Although the butterfly might be a veritable banquet, there might be more net energy gain by finding a bunch of clumped beetle larvae and eating many of them because the costs of catching and “preparing” a butterfly have been eliminated.

What I have described is standard logic on how natural selection has shaped animal foraging choices. But the interplay between predator and prey, or plant and herbivore, can be much more complex and subtle than we think.

For example, assume, for argument, that you’re a grass plant. It is a huge bummer when a moose or caribou eats the long leaves that you’ve invested so much energy and time in making under the hot sun. You hope that you’ll get overlooked among a sea of other individuals. But many herbivores are numerous and can mow a huge area from which a plant cannot escape, leading to the evolution of various plant defenses, like thorns, deadly seeds, flowers, leaves, or berries.

Grasses do it their way. Many grasses make a deal with an “endophytic fungus.” The deal is a two-way street: the grass provides “habitat” and nutrients for the fungus, and the fungus synthesizes compounds that are toxic to herbivores like moose. It is a cost to the grass to host the fungus, but this cost is low compared to becoming moose chow. The fungus also has a stake, as it doesn’t want the grass plant to be eaten either.

No matter how big and bad you are, a mouthful of foul-tasting food humbles all. Maybe you can get used to bad-tasting food (think lutefisk, the fish equivalent of the Christmas fruitcake, only way more pungent), but some herbivores like moose might short-circuit the plant- fungus mutual defense system. Such a strategy was just discovered in moose and caribou by Andrew J. Tanentzap and colleagues; their work was published in the journal Biology Letters, and it was focused on drool.

If you’ve watched moose and caribou, you know they drool. I always thought that they were just big, dumb animals that didn’t know any better. There is more to drool, however, than meets the eye. The researchers wondered if something in moose or caribou saliva might neutralize the noxious substances produced by the fungus in the grass. This notion might seem far-fetched, but vampire bats and some biting insects (e.g., mosquitoes) inject an anticoagulant into their prey. Spiders inject an “enzyme soup,” which causes the prey to be digested from the inside out, and the spider then sucks it out through a straw-like mouthpart. Is something similar working in moose and caribou?

This might seem easy to study. But you can’t just follow moose around, watch where they drool, and come back later and see if it had any effect. You have to experiment.