Baby got (hump) back. Two male humpback whales compete over a potential mate.

The Dolphin Institute, NOAA Permit # 1071-1770-00

Big = Sexy in Whale Society

Hollywood may favor the skinny look, but in the ocean, larger ladies are the stars. That's because male humpback whales seek out hulking females as mates, fresh research shows. This gives new insight into the mysterious mating habits of this creature and may aid in whale conservation.

Despite decades of observation, no one has witnessed humpback whales mating. What researchers do know is that during breeding season, which spans winter and spring, up to 20 males jostle to get close to a single female. The competition can get vicious, says Adam Pack, a marine mammal biologist and psychologist at the University of Hawaii, Hilo. The whales will blow bubbles to warn each other and body slam or strike rivals, he says, occasionally drawing blood. Some females attract more suitors than others, and researchers have wondered what makes these particular ladies worth the fuss.

Pack and colleagues thought a female's size might convey her sex appeal. Humpbacks fast during the breeding season, so longer females--and hence, says Pack, those with more fat reserves because they have a larger area in which to store fat--are probably better suited to carry and nurse the young. To test this hunch, Pack's team observed 42 competitive groups--those with one female surrounded by varying numbers of males--in the waters around Hawaii from 1997 to 2002. Divers videotaped the whales underwater and used sonar to calculate their length.

As expected, longer and therefore larger females attracted more male escorts, the researchers report in the March issue of Animal Behaviour. An additional one-half meter in length attracted about four more males, Pack says. The researchers also measured mother-calf pairs and found that each meter increase in the mother's length translated into an extra one-third meter in her calf, when controlling for the calf's age. The calves have about a 20% mortality rate, so mating with bigger females could increase a male's chance of reproductive success by siring larger, hardier offspring, Pack says.

Although not surprising, the findings do provide an important confirmation of what draws more males to a particular female , says Mason Weinrich, executive director and chief scientist of the Whale Center of New England in Gloucester, Massachusetts: "It's a nice step forward in understanding the mating system of humpbacks." But important questions remain, he says. For example, male humpback whales sometimes sing as they move in and out of competitive groups, but researchers don't know what role the songs play in mating.

Pack agrees that much work remains before scientists understand the mating system. But even smaller breakthroughs can be significant for whale conservation by showing what the animals need to reproduce, he says. "We've got to know what's important to them and how their society functions before we can begin to make legislative decisions about what's important to protect and what isn't."