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The sharing of vigilance and alarms by golden-mantled ground squirrels and yellow-bellied marmots is not unique

Golden-mantled ground squirrels, Spermophilus lateralis, are familiar to many people for a variety of reasons. They occur throughout the Rocky Mountains from New Mexico to Alberta and from British Columbia to California. Locally, they are abundant in habitats from piñon-juniper woodlands through stands of ponderosa pine, lodgepole pine, limber pine and aspen and they occur less frequently in spruce-fir forests and the alpine tundra. They are commonly seen in campgrounds and parks where they boldly beg for food. A less appreciated aspect of these squirrels is that their alarm calls not only advertise danger to other squirrels, but also to other species.

Golden-mantled ground squirrels live in a world full of predators--red-tailed hawks, northern goshawks, foxes, coyotes, bobcats, striped skunks, weasels, and several species of snakes are known to prey on them. In response to predators diving out of the sky and striking and pursuing from every direction, ground squirrels evolved alarm calls to warn relatives about impending danger. The call is a trill starting with a high primary note followed by about nine lower secondary notes plus ultrasonic elements. Although this call varies among the subspecies, alarm calls are distinctly different among squirrel species. Alarm calls also differ distinctly among species of chipmunks, ground squirrels, pika and marmots.

Golden-mantled ground squirrels sound an alarm call that warns not only squirrels but marmots as well. Photo by Jeff Mitton.

A golden-mantled ground squirrel cannot give an alarm call with impunity, for sounding a loud, shrill noise may warn relatives but result in the death of the caller. Furthermore, predators employ stealth to obtain the prey that they need--sometimes they strike successfully before they are detected. Ground squirrels need all the help they can get to protect their family and population. 

If two prey species lived sympatrically (intermixed or adjacent) and they had one or more predators in common, each could benefit by heeding alarms and evasive activities in the other. Naturalists had noticed that this was the case for golden-mantled ground squirrels and yellow-bellied marmots, Marmota flaviventris. But naturalist's observations, while valuable, are not sufficient to convince some colleagues, who demand experimental data. 

Playback experiments were conducted by Walter McKee Shriner at the White Mountain Research Station to test the hypothesis that squirrel alarm calls were heeded by marmots, and vice versa.  Alarm calls were recorded from yellow-bellied marmots and from golden-mantled ground squirrels and then played back to both species to determine if they responded to heterospecific (different species) alarm calls differently from conspecific (same species calls).

The naturalists were right--both squirrels and marmots reacted the same way to conspecific and heterospecific alarm calls. Responses included immediate vigilance and dashes to the safety of a burrow. Whether a coyote was detected first by a squirrel or a marmot, the first alarm call was heeded by both species. They were sharing information, for a coyote or a weasel was seen as a deadly threat by both squirrels and marmots.

The sharing of vigilance and alarms by golden-mantled ground squirrels and yellow-bellied marmots is not unique. Sympatric species that have predators in common can only benefit by knowing when the other species has detected to a stalking predator. Simply put, the greater the number of vigilant individuals, the greater the probability of detecting predators. And if a heterospecific screams an alarm, no need for a redundant call that might focus an attack on the altruist that sounded the alarm. 

Interspecific responses to alarm calls were subsequently demonstrated for collared pika, Ochotona collaris, hoary marmots, Marmota caligata, and Arctic ground squirrels, Spermophilus parryii, in the Yukon. Although pika increased vigilance in response to the calls of the marmots and squirrels, they responded more to calls from pikas. But clearly collared pikas listen to sentries in three species--pika, marmots and squirrels.

When the alarm calls of yellow-bellied marmots were played to mule deer, Odocoileus hemionus, at the Rocky Mountain Biological Station in Gothic, Colorado, the mule deer became vigilant. So, the species that are listening to alarm calls in the mountains of Colorado include yellow-bellied marmots, golden-mantled ground squirrels, American pika and mule deer. A similar group of species listens to one another in the Yukon. How far does this communication network extend? Are chipmunks, snowshoe hairs, cottontails, deer mice and beaver also listening and responding to the alarm calls of marmots, ground squirrels, and pika?

When ground squirrels need to transport food to their burrow, they fill up their cheek pouches, leaving their legs free for climbing and running. Photo by Jeff Mitton.