Male grizzly bears basically have two courting styles: find a female and guard her from other males; or, find one that is mating, chase the male away and take over. Those are the conclusions of "Do Big Mean Studs Get All the Action?" and "Why Are Deadbeat Dads Often Abusive?" two chapter titles Lance Craighead employed in his doctoral thesis on grizzly bears. Craighead, son of bear scientist Frank Craighead Jr., also showed with DNA testing that more than one male can sire a litter of cubs, reports the Billings Gazette. The tests were part of a six-year pedigree study of a bear population in northwest Alaska that he says is "virtually undisturbed and not hunted. It's probably been there since the bears first came to the New World." The study, which identified 256 bears and 53 different family groups, showed that 44 percent of the males successfully bred and that breeding success was not age-related. The oldest male in the study, 32, was still fathering cubs last year. But aggression seems to play a major part in successful breeding. When males kill the cubs of other sires, the mother goes back into estrus. Aggression is also important between competing males. Craighead says one male had a softball-size chunk taken out of his back, "and he was the winner."