Bulls are from Mars; Cows are from Venus

In late spring, cow moose find a secluded spot to give birth to calves, alone. Meanwhile, bull moose are found nowhere near the cows they impregnated. In calving season and for most of the year, bulls and cows don't mingle.

The apparent indifference between the sexes intrigues Terry Bowyer, a biologist at the University of Alaska Fairbanks Institute of Arctic Biology. Though male and female moose always seem to find one another during mating season, for the rest of the year they have little to do with one another. Their separation is so extreme Bowyer thinks cows and bulls should be managed as different species.

Moose are not alone in this aloofness toward the opposite sex. Most ruminants (hoofed creatures that chew cuds, such as caribou, antelope, deer and sheep) only prefer the company of mates when seized by the urge to reproduce.

Bowyer, who has studied moose in Denali National Park for almost a decade, has spent many hours watching the behavior of bull moose in rut. A single bull may have as many as 40 cows in one harem, but there its responsibility ends.

"The males contribute nothing to their young but sperm," Bowyer said.

After mating season, the bulls and cows diverge. In springtime, cows go to extreme lengths to avoid males. Because researchers have shown that 80 percent of moose calves are killed by grizzly bears or wolves in the first 20 days of life, having the father around would seem to be an advantage for a moose calf. Not so, said Bowyer. A bull moose, with small, velveted antlers in spring, could do little more to defend a calf from a grizzly than could a cow. Also, grizzlies may zero in on calves by first locating an adult moose.

Apparently, a bull moose's odds of passing on its genes are better if it copulates with a collection of females than if it was to have one offspring and hang around to father that calf. A moose's desire to pass on its genes might be part of the sexual separation puzzle, but Bowyer thinks a few pieces are still missing.

Bowyer searched for ruminants who might not fit the segregation pattern and he found them in the forests of Africa. Members of two species of small antelope have just one mate their entire lives. Forest antelope are different from whitetail deer, moose, and other ruminants in that male and female forest antelope are approximately the same size, while a large bull moose outweighs its mates by more than 500 pounds.

The larger the bull moose, the better it is able to joust competitors during the rut; the larger its harem of cows becomes, the more of its genetic material is passed on when calves are born the following spring. Sneaky, smaller bulls may be able to breed when the dominant bull isn't watching, but it's a rarity, Bowyer said.

Because natural selection favors big males that can withstand the rigors of the rut and a large body is less important in females, Bowyer thinks something relating to body size is the key to why bulls and cows don't hang out together. He has made and thrown away several hypotheses--that cows and bulls take different-sized bites; that one has a bigger stomach than the other--but he thinks the answer is out there, and it may not be complicated. Maybe cows and bulls prefer differences in food as subtle as eating male or female parts of willow tree. Whatever the answer, Bowyer feels compelled to find it.

"If there's one thing I'm going to do before I die," he said, "I'm going to understand this."