In my occasional series, “When Plants Attack” we’ve seen some of the ways in which plants can defend themselves. So far I’ve covered the chemicals they produce to discourage other plants from growing in their space (allelopathy) and the conventional weaponry they use to guard against a physical attack. I am planning more posts to continue the series, which will include a look at the chemical defences plants have evolved to protect themselves against being eaten. But as soon as a plant evolves a defence mechanism, predators will begin to evolve or develop a way to counteract it. For example, some insects can collect poisons from the plants they’re munching on, and use them as part of their own defences. But until now it has seemed as though plant-eating mammals change their behaviour to cope with toxic plants – e.g. by changing how they forage for food, or by eating dirt (geophagy) to detox.
On Wednesday a paper published in Biology Letters put forward what the authors believe is the first evidence of large mammals evolving to combat a plant’s chemical defences. The researchers collected saliva samples from moose (Alces alces) and European reindeer (Rangifer tarandus) in Canadian zoos, whilst the animals were anesthetized to undergo necessary medical procedures. These two animals are known to feed on red fescue (Festuca rubra), a grass which occurs around the world. Red fescue uses a common defensive strategy: it forms a mutually-beneficial relationship with a fungus (Epichloë festucae), which produces toxic alkaloids.
By applying the animal saliva to grass samples, the researchers demonstrated that both moose and reindeer saliva slowed down the growth of the fungus, and so reduced the amount of toxin that was produced. Moose dribble also appeared to directly affect the levels of the toxin in European samples of the grass (the deployment of chemical defences depends on the environment in which the plant is grown), and the scientists theorize that the saliva is preventing the plant’s defence system from activating, by disrupting its signals.
So it seems that moose and reindeer aren’t just coping with the toxins produced by their diet of red fescue, but have evolved to actively combat them. “Plants have evolved defense mechanisms to protect themselves, such as thorns, bitter-tasting berries, and in the case of certain types of grass, by harbouring toxic fungus deep within them that can be dangerous or even fatal for grazing animals,” says York University’s Professor Dawn Bazely, who worked with University of Cambridge researcher Andrew Tanentzap and York University researcher Mark Vicari on the project. “We wanted to find out how moose were able to eat such large quantities of this grass without negative effects.” This interesting discovery (which will have to be verified by further studies) may seem a little esoteric, but you never know when you might need an enzyme that deactivates a toxic alkaloid (and this particular one also appears in ergot), and when you do it’s good to know that moose happen to have one handy.
You may also be fascinated to learn, as I did during the course of my research for this blog post, that whilst the common usage of “ungulates” refers to hoofed mammals (such as moose, reindeer, cattle and camels), cetaceans (whales, dolphins and porpoises) are also ungulates, sharing a common ancestor with the other species in this large group of mammals.
Tanentzap AJ, Vicari M, Bazely DR. 2014 Ungulate saliva inhibits a grass–endophyte mutualism. Biol. Lett. 10:20140460.