Size matters when it comes to male humpback whale attraction to females, according to a five-year study of whale activity in the waters around West Maui.

Research by The Dolphin Institute indicates that male humpbacks prefer the company of large females, even when faced with intense competition from other males pursuing the same mating partner.

"This is the second piece of evidence that we have generated that male humpback whales are not indiscriminate about who they are escorting," said Adam Pack, vice president of The Dolphin Institute and assistant professor of psychology and biology at the University of Hawai'i-Hilo.

"The traditional view of sex roles in animals, in general, is that females are the choosy sex ... Now we know that humpback males are choosy with respect to the body size of females and are not indiscriminately mating with anybody that they can."

Pack said previous research showed that male humpbacks are more likely to pursue females without calves, apparently because most lactating females are not open to mating.

Lou Herman, president of The Dolphin Institute and a UH-Manoa professor, said the latest findings provide further clues to humpback whale mating, which has never been observed.

"The humpback's mating system is a big mystery," Herman said. "Because our views of whales are so transitory, it takes a long time to build up a story of what's going on down there. This adds to our understanding of how this mating system works."

An estimated 8,000 to 10,000 humpback whales migrate from Arctic feeding grounds to warmer Hawaiian waters in the winter and spring, assembling on shallow banks and coastal areas for breeding and calving. The whales form "competitive groups" comprising a single female and anywhere from two to 20 male escorts that may engage in high-speed charges, body blows and other aggressive tactics to fend off fellow suitors.

Adult female humpbacks generally are larger than males since they have to store enough fat during the summer feeding season to support both themselves and their calves. The largest females average 40 to 46 feet long, compared with 36 to 41 feet for males.

Researchers used videogrammetry, a technique developed by The Dolphin Institute, to measure the length of female humpbacks and their calves. The technique employs an underwater video camera and a hand-held sonar device that resembles a flashlight to calculate the distance between the camera and whales. Pack said the small sonar device emits a high frequency that does not affect whales.

The measurements revealed that larger females attracted a greater number of male escorts than smaller females, and that larger females produce larger calves.

Herman said the two findings go hand in hand, since, theoretically, larger calves have greater survival odds. "Therefore, an escort that mates with a larger female has a better chance of producing an offspring that will survive to adulthood," he said.

Larger females also are likely to be older and more experienced in calf-rearing, further enhancing the chances of a baby whale surviving the 2,500-mile migration north.

Big is beautiful for many other animal species, including fish, reptiles, land animals and even some sea lions and other marine mammals that prefer to mate with large females.

The Dolphin Institute study, published in the journal Animal Behaviour, was done between December and April from 1997 to 2002 in the 'Au'au, Kalohi and Pailolo channels around West Maui, home to one of the densest concentrations of humpback whales. Researchers gathered data from 42 competitive groups and 92 mother-calf pairs.

Other researchers involved in the study were Scott Spitz of The Dolphin Institute, Siri Hakala of NOAA's Southwest Fisheries Science Center, Mark Deakos of UH-Manoa and Elia Herman of The Dolphin Institute and Duke University.