From the Wallowa Mountains of Oregon to the Canadian Rockies, mountain goats and bighorn sheep – North America's most iconic alpine ungulates – overlap here and there in their lofty and often spellbinding abodes.

Image: Florent Déry

Though they both favour tundra and parkland pastures within easy reach of predator-proof escape cliffs or talus, it's unusual to see goats (woolly, snow-white, spike-headed) and bighorns (short-haired, brownish-grey and curl-horned) actually mingling.

A long-running goat study on Caw Ridge in the Rocky Mountain foothills of Alberta, however, has documented the two species (not at all closely related) crossing paths a bit more frequently in recent years. Goats and sheep mostly segregate on different parts of the ridge complex, but their overlapping summer range sometimes sees them grazing and resting in close proximity.

And every once in a while, the researchers record direct interactions. "Maybe five or ten times per summer for 100 days of observation," reports Frédéric Dulude-de Broin, a biologist with the Caw Ridge Mountain Goat Research Project.

Image: F. Dulude-de Broin
Image: F. Dulude-de Broin

The pecking order, as it turns out, is pretty clear-cut: mountain goats reliably dominate bighorns. Female goats, aka nannies, are the ones most often seen sharing space with sheep – and sometimes muscling them around – in the Caw Ridge study area; billies here tend to spend the summer fattening up for the rut in an area isolated from bighorns and other goats alike.

As Dulude-de Broin notes, the mountaintop hierarchy isn't especially surprising. "[Mountain goat nannies] have higher rates of intraspecific aggression than any other female ungulate for which it has been measured," he says. And their daggered horns are much more formidable than a bighorn's blunter headgear. (Caw Ridge nannies have been seen ramming wolves in defence of their young.)

"[Nannies] can severely injure conspecifics and predators, so they could also be a serious threat to sheep," he adds. "If I was a bighorn facing a nanny, I would get out of the way!"

(Incidentally, the same advice applies to mountaineers of the two-legged category: goats can be aggressive towards people, and a billy killed a hiker in the Olympic Mountains of Washington in 2010.)

More: Angry mountain goat confronts a hiker on the snowy slopes

Dulude-de Broin says he's never seen a nanny engage with a big, "full-curl" bighorn ram – as with mature billies, the older rams tend to utilise different geographies on the Caw Ridge summer range – but he notes the goats have been observed asserting themselves over younger male sheep.

Nannies may be on the belligerent side among their own kind and in dealings with bighorns, but Dulude-de Broin has noticed they exercise that belligerence differently depending on its object.

He and his colleague Florent Déry both emphasise that goats often appear somewhat on edge when sheep are nearby. "They don't seem as willing to bed and appear more vigilant if sheep are amongst them," says Dulude-de Broin. "It is as if they are not used to interacting with sheep so they are more careful."

Goats approach other goats directly, whereas there's a bit more of a ritualised production involved when they confront bighorns. "In most interactions that I witnessed, they walked slowly towards the sheep in an aggressive manner (arching their back to appear bigger or displaying horns) and then waited for the sheep to flee," Dulude-de Broin explains.

He speculates the difference lies in the fact that goats are well acquainted with one another's social rankings, whereas they may feel compelled to more energetically show the less-familiar bighorns just who's boss. "I saw one of the most subordinate nannies of Caw Ridge easily displacing sheep," Dulude-de Broin recalls, "so even if they are careful [around bighorns] they are clearly more dominant."

Such interspecies interactions are an example of the interesting observations that accrue over years of fieldwork by dedicated (and hardy) biologists in a long-term study such as the Caw Ridge Mountain Goat Research Project, which has been monitoring Caw Ridge goats since 1988.

Of greater interest to Dulude-de Broin and his fellow researchers than the odd nanny/bighorn faceoff? The general decline in the ridge's mountain-goat population: 164 in 2008, it's now at just 28 animals.



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