Moose drool can undermine grass defenses

The slobbering of moose and reindeer as they eat may be big, wet countermeasures against the chemical defenses of grasses.

Some otherwise inviting grasses harbor live-in fungi such as Epichloë species, which can produce strong alkaloid toxins. Those compounds can make grazing animals sick enough to shy away from these grasses. Big animals, however, have a previously unappreciated way of fighting back, says ecologist Andrew Tanentzap of the University of Cambridge: They drool.

In lab tests, moose saliva dabbed onto grasses two or four times could lower the concentrations of the toxin ergovaline by 40 to 70 percent over the course of two months, Tanentzap and colleagues report July 23 in Biology Letters. Also, dripping the drool on Epichloë fungus in lab dishes slowed the spread of its thready networks, the team found.

“Big grazing animals have been studied to death,” Tanentzap says. Yet as far as he knows, his paper describes the first tests of big herbivore saliva as sabotage for the fungal defense of grasses.

Spit from various caterpillars can manipulate plant chemistry to improve the insects’ meals. But Tanentzap began wondering about spit from bigger grazers when he read a 2002 study on a different aspect of moose saliva: its power to encourage half-eaten twigs to branch as they regrow into another tender meal.

To see what big-grazer saliva might do to fungal defenses, the researchers persuaded zoos to collect drool when animals were anesthetized for medical procedures. It’s a task best left to professionals, Tanentzap says.

Slobber from moose reduced toxin concentrations in red fescue grass (Festuca rubra) — but only in plants originating from the core of the species range in Europe. Fungi in grass from the water-limited margins of the species range in southern Ontario just kept producing toxin regardless of the saliva. This discrepancy, Tanentzap says, could help researchers understand how the saliva works.

He observed that the fungus in the European fescue produces the toxin only when something eats the grass (or a researcher snips it). Some signal probably has to switch on toxin production, and the saliva may turn the switch off, Tanentzap speculates. But in fescue from the difficult conditions in southern Ontario, the fungus churns out toxin all the time, regardless of whether there has been any chewing. The fungi and their Canadian plants, which don’t respond to drool, may not have that switch, and thus no weak link for the saliva to exploit.

The saliva doesn’t act instantly, notes Stanley Faeth of the University of North Carolina at Greensboro, who has long studied fungus-plant interactions. So this gradual inhibition of toxin-making benefits animals only if they revisit old grazing patches. What he’d like to know more about, he says, is how much benefit the grazers get from their fungus-fighting saliva. He also wonders whether the battle against fungi in grasses could have been enough to drive herbivores to evolve sabotage-drooling. Alternatively, safer grass eating may just be a bonus from weaponized drool that coped with some other menace. 

Editor’s Note: This article was updated July 29, 2014, to clarify that only moose drool was used in experiments that measured toxin concentrations.