Discovery Files

Beauty Is in the Genes of the Beholder

New research on American pronghorns explains why not everything is about looks

When humans select mates, what's on the outside is important. Other factors may come into play but there's no denying the role of physical attraction.

Animal behavior research once held that physical ornamentation is what females of a species look for, when selecting a suitable mate.

With the American pronghorn, however, this is not the case. According to animal behaviorist John Byers--a professor in the department of biological sciences at the University of Idaho and a researcher who has studied this population for more than 20 years--female pronghorns are looking for good genes when they select mates. Furthermore, females can detect males that have a genetic advantage, regardless of the males' appearance. So, one could say that beauty is in the genes of the beholder.

Byers has studied the American pronghorn population on the National Bison Range in western Montana since 1981, and he says he already knows "the complete pedigree of the population through three generations."

With support from the National Science Foundation (NSF), Byers discovered that pronghorn females can choose good genes regardless of male appearance.

"The male characteristics that are favored by females are running speed, endurance, agility, and tactical spatial sense," Byers wrote in his book, "American Pronghorn: Social Adaptations and the Ghosts of Predators Past." Clearly, these are not visual attributes.

Visually speaking, both sexes of American pronghorn vary very little. They have horns but horn length among males is not that different.

Each year, most pronghorn females sample potential mates over a two-week period, then mate once. All females older than one year give birth to two fawns each spring.

So How Do Females Select Their Potential Mates?

According to Byers, before and during the period of sexual excitement in the female pronghorns, males suitable for breeding occupy fixed locations. The males collect females that come near and defend the females from other males. The female groups are called harems.

"When a female is about two weeks out from estrus--the period of maximum sexual receptivity--she begins to move at an accelerating rate, from one harem to another. Harem males try to prevent exits from their harems, but in an escalated escape attempt, the female always wins," Byers explains. If the male is not able to repel the other males that approach the harem, then the females leave. "While traveling from one harem to another, females commonly attract groups of 'floater' males, and this provides a new tactical challenge for the new male whose harem she joins," Byers notes. So, as the female moves among males at an increasing rate and the mating period approaches, the females test the males' vigor.

Byers' team used a two-prong approach in their study of American pronghorns. In addition to studying the animals' behavior, they obtained genetic markers for all pronghorn individuals in the wildlife refuge and assigned paternity to the offspring. The female mate sampling creates a small group of "fathers" that each year sire more than one-half of all young.

The researchers discovered that the offspring from that genetic pool are stronger and more likely to survive to weaning. Also, the fawns tended to grow faster and suckle less from their mothers.

Byers says he became interested in this particular population when he "was looking for an ungulate (hoofed mammal) that had an intermediate level of sociality" to compare with the results he had on the collared peccary. The wild, hog-like animal found in the southwestern United States as well as in Central and South America, is "the most social ungulate known," Byers adds.

Byers is now planning to focus on inbreeding reduction and inbreeding avoidance. Pointing out that the American pronghorn recently went through a population clog, he says that in the next few years, when choosing a mate, females "will often be faced with the choice of mating with a genetically superior male, versus avoiding mating with that male because he is a close relative."