Elks on the horns of a dilemma

For male American elks, shedding their antlers each year is a process fraught with risk and opportunity – and achieving a balance between the two is likely to have played a key role in the evolution of the species.

That’s the conclusion reached following a long-term study conducted by a team of scientists led by Matthew Metz from the University of Montana in the US.

Metz and his colleagues spent 13 years observing interactions between elks (Cervus canadensis) and wild wolves that live in the Yellowstone National Park. Of particular interest was the ways in which the predators and prey interacted in relation to the antlers of the latter.

The elks’ magnificent antlers are what biologists call “sexually selected weapons”. Their primary role involves male-to-male contests over access to fertile females during the mating season. The physics are pretty basic: all other matters being equal, bigger horns lead to more mates.

The hitch, though, comes after the rutting ends. Sometime thereafter, the antlers are shed – “cast” in the jargon – which means the elk lose their secondary, but very valuable, function: defence.

As soon as the antlers are lost, and during at least the early part of the subsequent two-to-three-month regrowth period, the animals become vulnerable to wolf attack.

It’s a devilish trade-off. Animals that cast the earliest enjoy maximum regrowth time before the next mating season, resulting in bigger weapons and thus greater mating success – but only if they survive long enough to enjoy it.

Metz and colleagues report “that male elk that cast their antlers early are preferentially hunted and killed by wolves”.

In one sense, this was a surprising result. “Classic expectations”, the scientists say, are that wolves generally select prey animals that are in relatively poor nutritional condition, but early-casters conserve energy through not having to meet the metabolic demands of big weapons and thus tend to be in better condition after the mating season than peers who retain antlers for longer.

Fitter they may be, however, but also defenceless. The research revealed that groups of male elk that contained at least one antler-less member were 10 times more likely to be attacked by wolves compared to groups in which all members retained their weapons.

They also found that younger males – those not likely to breed in the upcoming mating season – tended to retain their antlers for the longest period. Older males, on the other hand, which are more likely to have reproductive success, tended to cast early and run the risk of becoming wolf food.

The study, they note, demonstrates the “evolutionary tension between the benefits and costs of an extreme, sexually selected weapon”.

“Through identifying this trade-off, our study reveals the largely unexplored importance of secondary functions of sexually selected structures,” they conclude.

The research is published in the journal Nature Ecology and Evolution.

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