” Mini Moose Evolve on Isle Royale.” ©2011 by American Institute of Biological Sciences..
Rows of moose skulls, moose antlers, and club-size moose metatarsal bones fill a clearing behind the weathered wooden cottage where biologist Rolf Peterson has spent the last 40 summers, on Isle Royale, Michigan. Hauled each year from the spruce bogs and fir forests on the Lake Superior island, the display is part of the world’s largest collection of moose bones.
The bones tell many tales—of periodontal disease and arthritis, of lean years and flush years, of two bulls that crossed antlers in a duel and died that way, racks locked. Most famously, they tell a long-running tale of predator and prey, of how wolf numbers have affected moose numbers, and vice versa. Now Peterson, John Vucetich, and their colleagues have extracted a new evolutionary tale from the bones: Living on the island downsized the moose.
The phenomenon has been found elsewhere: Mini hippos and elephants once resided on Mediterranean islands, and a hobbit-like human ancestor lived on an island in what is now Indonesia. Limited resources can account for this island rule, and that seems to have been the case for Isle Royale’s moose, says Peterson, at least for the first half of their history.
That history dates back to the early 1900s, when a few Canadian moose probably swam 20 miles of Lake Superior to the 210-square-mile island of predator-free boreal forest. They displaced the caribou, gorged on bog plants in summer and balsam fir in winter, and exploded in number, reaching several thousand in the 1920s. Their free rein ended in 1949, when a curious pair of Canadian wolves managed the same journey across a frozen lake and claimed the new territory. Ten years later, when Allen of Purdue University began studying the predator-prey interaction, the wolves were up to 20 and the moose down to 538. The two population numbers have seesawed ever since.
Peterson, who joined the project as a graduate student in 1970 and later moved to Michigan Technological University in Houghton, was teaching dissection using some moose metatarsal bones that he had collected from mainland Michigan. (The metatarsus in a moose makes up the hind leg and serves as a proxy for body size.) Peterson noticed that their size was “consistently above the average for Isle Royale moose. Then when researchers in Minnesota started collaring moose and making collections from dead animals, I saw an opportunity to collect enough moose bones to do a respectable comparison.”
Researchers measured the length of more than 1000 metatarsal bones collected on Isle Royale and found that the mean was significantly shorter than that of bones collected from nearby mainland moose, by about 9 millimeters for females and double that for males. The difference with moose bones collected from mainland Sweden and Alaska was even greater; Isle Royale moose, it seems, may be among the smallest in the world.
Shripad Tuljapurkar, a Stanford University evolutionary biologist who has worked on the evolution of phenotypic traits, calls the paper “an insightful historical analysis that provides valuable detail about evolution in a large mammalian species and marches with the general island rule.” The detail, he notes, should help in creating analytical models of size change for species with island and mainland populations.
Although the island’s limited resources have downsized the moose, ongoing research suggests that another evolutionary process has been countering that force: Wolf predation seems to be selecting for larger moose. Smaller moose are more likely to end up as wolf fare, and that preference shows up in the bone collection: The longer the metatarsus was, the older the moose was, which makes it more likely that the moose had had more offspring. And since most of the metatarsus develops in utero and is fully grown by the time the moose is one to two years old, “the pattern of increasing bone length with increasing age can’t have a physiological explanation,” says Vucetich.
That leaves the role of the wolves, which creates another evolutionary tale. “Darwin was keen that predators shape the lives of their prey,” says Vucetich. What is distinctive, he notes, is the wolves’ effect on the body size of a large, long-lived vertebrate and during a relatively short period. “Even though it’s 50 years, it’s very brief in evolutionary time.”
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