The moose, and the two parasites that weigh less than a gram

The Algonquian word we borrowed means eater of twigs. That is what a moose does, and almost everything else about it follows from being very large, very cold-adapted, and a herbivore that eats the growing tips off trees.

Dewlaps are longest on younger bulls, and can freeze and partly break off in very cold weather. Ed Sharron.

Antlers, and the thing nobody can explain

A full rack spans six feet and weighs 60 pounds or more. Its job is legible: size advertises health to females, and the antlers are what bulls spar with.

The dewlap — the flap of skin and hair under the chin — is not legible at all. Theories run from heat regulation to mating ritual. It is larger on males and longest on bulls of three to five years, which suggests it signals health the way antlers do. It may also carry scent: a bull rubs its chin on a cow and leaves urine and saliva behind.

Moose, voles, and why beaver meadows stay open

This is the best thing in the moose's ecological file, and it takes four species to tell.

1. Beavers fell aspens for the branches and twigs.
2. Moose browse the re-sprouting aspen so hard it cannot recover, which frees the spruce and fir underneath to grow.
3. A shaded wall of conifer closes in around the pond. With the aspen gone, the beavers leave, the dam fails, and the pond drains into a meadow of sedges, irises and grasses.

And then the meadow simply stays a meadow, for decades. Spruce and fir seedlings appear and die within a couple of years.

The reason is in the soil. Conifers depend on mycorrhizae — a fungal network woven into their roots, taking carbohydrate from the tree and giving back nitrogen and phosphorus. The pond smothered it, so when the water goes there is nothing for a seedling to partner with.

Which raises the real question: why does the fungus not come back?

A University of Minnesota study points at two voles. The forest-dwelling red-backed vole eats the fruiting bodies of these fungi and spreads the spores in its droppings — and it stays out of beaver meadows, because the meadow vole lives there and the two are violently intolerant of each other. So the spores never arrive, and the meadow persists.

Beaver meadows can persist for decades after a pond drains. Ed Sharron.

Built for an ice age that is ending

The modern moose evolved perhaps 500,000 years ago, in the middle of the last glacial epoch. Great body mass, long legs, thick insulating fur — all of it is equipment for deep snow and hard cold, and all of it is a liability in heat.

Populations along the southern edge of the range are falling hard:

Heat stress is part of it. Moose density is part of it. The rest is two parasites, and neither weighs a gram.

Brainworm, which arrived with the deer

White-tailed deer and moose did not historically share ground: short legs and thin fur make deer unsuited to the snow moose thrive in. Warmer winters have changed that, and the deer have come north — with brainworm.

Deer carry it almost harmlessly, even as it burrows into their brains. It spreads through their droppings into land snails and slugs, which a browsing moose swallows by accident. Inside a moose the larvae head for the brain and spinal cord, looking for the interior landmarks of a deer, and wreck the nervous system on the way.

The resulting disease — cerebrospinal nematodiasis — presents as weakness, fearlessness, loss of coordination, circling, deafness, failing sight and paralysis. It usually kills.

Winter ticks, in numbers that do not sound real

Warmer winters have let winter tick populations boom, and regrowth after logging in Maine and northern New Hampshire has produced exactly the dense moose habitat that makes finding a host easy.

Moose groom poorly compared with deer. Some rub the ticks off and leave themselves bare patches and exposed to the cold. Biologists have found moose carrying 40,000 ticks. 75,000. Up to 90,000.

Each adult female takes about a millilitre of blood. Ninety thousand of them is almost 24 gallons. Moose die of anaemia, and cows carrying that load are less likely to carry a pregnancy to term.

In spring the ticks drop off to breed; by autumn the larvae are questing for a host. Deep snow or a long cold snap kills them before they find one — and Maine now has about two weeks less winter than it did a century ago.

Two things being tried

Fewer moose, on purpose. Several north-eastern states are allocating hunting permits to thin the densest herds. Biologists are aware of how killing moose to save moose sounds; the research says lower host density — a sort of enforced social distancing — cuts tick abundance.

Fungus against the tick. With US Fish and Wildlife Service support, a University of Vermont project is testing naturally occurring fungal spores from New England soils, spread as oil, granules or powder. On contact the fungus penetrates the tick and poisons it.

Where this belongs

• Acadia National Park

Sources

Written from the National Park Service's Species Spotlight — Moose, a work of the United States government in the public domain. Photographs by Ed Sharron.