Thursday, January 5, 2012

Moose Seasons

Moose cow and calf in June (c) John Ashley
Moose cow and 30 lb calf in June
Each summer in Montana, the wobbly moose (Alces alces) calves of June hit the ground at 30 lbs, walking on day one. By fall, these calves will weigh 300 lbs. They’ll browse beside mom through winter and spring before becoming independent during their second summer. Moose calves that survive their first year will outgrow the abilities of most predators, other than humans.

A male moose calf will eventually grow into a 1,200-pound bull, producing annual antlers that weigh up to 75 pounds. A female calf will grow into a 700-pound cow moose and begin breeding at age two. By age four, she’ll often produce twin calves each summer.

While non-human predation has little effect on moose populations, it’s a different story when Old Man Winter arrives. By January, bulls are shedding their antlers and cows are three months pregnant. And the eight-month-old moose calves are going through the most vulnerable period they’ll ever face.

Moose cow and calf in October (c) John Ashley
Moose cow and 300 lb calf in October
During a moose’s first winter, there’s a significant correlation between calf survival and snow accumulation. Deeper snow means more difficulty reaching food, and a higher predation risk. A healthy calf will save calories and avoid starvation by following the path that his long-legged mom plows through his first winter. But a weaker calf floundering through deep snow is a likely target for large animals that hunt cooperatively during winter - like wolves.

That’s pretty straight-forward. What’s less obvious is the cumulative effect that snow depths from many previous winters can have on moose survival and twinning rates.

Research from a closed, island ecosystem (Michigan’s Isle Royal) showed a significant relationship between moose winter survival/twinning rates and the sum of snow depth for as many as seven previous winters. And as much as one-half of the variability in the moose population size was directly correlated to the sum of snow accumulations during the previous three winters.

How can winter snow depth affect a moose calf born three years later? Nutrition.

The difficulties of feeding in deep snow affect the pregnant cow’s nutrition, and this negative influence on her health can apparently accumulate for many years. Meanwhile, the cumulative effect on the female shows up each year in her calves. A relatively weaker moose mother produces fewer twins, and the calves that she does produce are more vulnerable during that critical first winter.

It should be pointed out that the same study found no strong correlation between moose survival and the numbers of wolves. Wolf predation on moose calves was secondary to the effects of winter weather on the cows, when the moose population was followed over many years.

Moose nutrition relates to the quality and quantity of the plants they eat. During spring, summer and fall, an adult moose might eat 50-60 pounds of plants each day. During winter, when plants are harder to come by, moose survive on the woody stems of several less-nutritious shrubs.

In the Gallatin Mountains of southern Montana, the most important winter moose foods are the leafless stems of western serviceberry (Amelanchier alnifolia), chokecherry (Prunus virginiana), and red dogwood (Cornus stolonifera). These woody shrubs remain important into spring, when moose add relatively more nutritious currants (Ribes spp.) and forbs to their diets. During fall, the most important moose foods are low red huckleberry (Vaccinium scoparium), subalpine fir (Abies lasiocarpa), and willow (Salix spp.).

In other parts of our state, moose rely more heavily on willow, grasses and aquatic plants – whatever it takes to survive Old Man Winter through to another Montana summer, and another round of wobbly moose calves.

Bull moose in October (c) John Ashley
1,200 pound bull moose in fall