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Masters of the mountains

Mountain goats thrive in the alpine of Southeast, but what are keys to their success?

Posted: April 14, 2011 - 3:47pm
A mountain goat overlooks Berners Bay. ADF&G research has documented distinct goat populations east and west of Berners Bay, and north and south of the Katzehin River, with little movement of animals between populations.  Photo by Kevin White
Photo by Kevin White
A mountain goat overlooks Berners Bay. ADF&G research has documented distinct goat populations east and west of Berners Bay, and north and south of the Katzehin River, with little movement of animals between populations.

One day in late March, I wandered toward the Mendenhall Glacier in search of mountain goats. March is often a good time to see them near Nugget Falls or across the lake on the big rock peninsula at the foot of Mount McGinnis — or even on the lake itself! In winter, mountain goats leave their summer range in the alpine and venture down into the forested zones, where they can find food, albeit sometimes of poor quality, under the trees.

I previously tried several times to find them near the glacier, but had been “skunked.” I was beginning to think that perhaps the rangers at the visitor center had locked up the goats somewhere (along with the beavers that are never visible in the beaver cam when I look for them).

Finally, on this day, I got lucky. There, just beyond Nugget Falls, was the elusive white beast. But wait — this one had eight legs! It was a nanny, with a nearly year-old kid walking right next to its mom. The kid moved on the far side of her, so its presence could only be detected by those supernumerary legs. I was too far away to tell if the nanny was pregnant with this year’s kid, which would be born later in the spring.

Female mountain goats in Southeast don’t reproduce until they are four or five years old, according to ADF&G research. Then, if conditions are good, they may produce a kid every year for several years. If conditions are poor, they may skip a year. However, the probability of survival in Southeast decreases steadily after age four or five, and drops rather quickly after age eight or so.

Snow can create big problems for mountain goats, especially as they get older. Deep snow makes much of their plant food inaccessible and also makes travel energetically expensive, unless the snow is well compacted and can support their weight. Fresh snow on unconsolidated snow pack can lead to avalanches, a significant source of death for goats. I once found an entire skeleton of a fairly young goat (its teeth were not very worn) at the base of a long avalanche chute. In general, mountain goat survival in Southeast decreases as snowfall increases. Late winter and early spring are seasons of lowest survival, when the cumulative effects of winter difficulties take effect. Old animals and males are the hardest hit.

You might think that summer is totally benign, in terms of weather. But mountain goats can’t handle really warm weather. When temperatures rise, they tend to seek north-facing slopes, or shady spots behind rocks, or higher elevations, or at least a nice cool snow pack for resting. Average temperatures greater than about 48 degrees F in July and August are “hot” even for young goats. The average temperature is correlated with the number of days that reached over 60 degrees F. Older goats may suffer heat stress at even lower temperatures. Recent ADF&G research in Southeast suggests that warm summers are often followed by increased winter mortality, especially for older goats.

Why should warm summers create difficulties? One possibility is that high summer temperatures melt the snow rapidly, so that the ground vegetation all matures in a relatively short time. In contrast, a cool summer would melt back the snow pack more gradually, so that the emerging vegetation is exposed over a longer period of time. Some evidence suggests that young, emerging plants have higher nutritional value than mature ones. Then, cool summers would offer high quality forage over a longer period of time as opposed to warm summers. Furthermore, higher temperatures may more directly cause lowered forage quality because the plants grow faster and contain more indigestible fiber. Cool summers also reduce heat stress and may allow goats to forage more efficiently and perhaps more often (if they don’t have to seek shade as frequently). Thus, cool summers might allow goats to go into winter in better condition than after warm summers.

Mountain goats, like some other alpine specialists, tend to develop localized populations that are genetically different from each other. For example, ADF&G research has documented distinct goat populations east and west of Berners Bay, and north and south of the Katzehin River, with little movement of animals between populations. Relative isolation of populations often allows the development of local adaptations specific to each population, if environmental and demographic conditions are different, but it remains to be seen to what extent this is true for mountain goats.

Detailed genetic analyses by Canadian researchers has shown that mountain goats probably survived the last major glaciation in at least two refugia. Fossil evidence had previously supported the idea of a southern refugium, perhaps in southern British Columbia or thereabouts. But the more recent molecular data show that another refugium probably existed in northern BC or Southeast Alaska; goats from northern areas tend to be different genetically from those of the southern areas. In the thousands of years since the Pleistocene glacier began to recede, some movement of goats must have occurred, because some populations now consist of both northern and southern genetic types. Similar historical differentiation of northern and southern population is known for other mammals as well, including ermine, red fox, and mountain sheep.

 

• Mary F. Willson is a retired professor of ecology.

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