When actor Michael Douglas announced his engagement to the much younger actress Catherine Zeta-Jones, the gap in their ages barely raised eyebrows. The "May-December" romantic scenario has become a classic cliche that rarely elicits notice from social critics.

But evolutionary biologists have long wondered if these relationships have unconscious advantages for both parties. The theory is that older men want access to young, fertile women so they can pass on their genes. For women, a union with an older man gives her access to better resources for raising a child. Chris Beck, a lecturer in biology, is using computer simulations to find some answers to mate-choice.

"People want to test various theories to explain why mate choice has evolved in different species," said Beck. "There's a good bit of empirical research suggesting that females prefer older males. The common assumption is that females prefer these males because older males have better genes."

A male who is able to survive to an older age must have good genes for escaping predators and acquiring resources, Beck said, but he is challenging the simplicity of conventional mate-choice models that do not allow for the possibility of changing female tastes.

It has been demonstrated that female mollies (an aquarium fish) have distinct preferences in males. Beck wants to see if a female's preference remains constant throughout her life. Also, he wants know whether these females prefer older males.

To get some answers, he simulates thousands of years of evolution with computer programs. In these programs, each female selects a male and mates with him if she prefers his age class. She carries certain "preference genes" that will determine this preference. Over thousands of generations, certain preference genes become more common, just as genes become more common in real life. The advantage of this system is that computer evolution condenses thousands of years into 24 hours of supercomputer time.

Beck has noticed a pattern in these computer simulations. When the fish have low juvenile mortality with many surviving to old age, females do prefer older males. But when few fish make it to old age, females are just as likely to prefer young males, as there aren't enough older males for mating. But what do computer programs and theory have to do with live organisms?

"One can inform the other," Beck said. "Evolutionary biologists agree that we must develop theory and then test the theoretical predictions with empirical studies."

To complement these theoretical studies, Beck will be testing the computer results using live mollies. Female mollies will be placed in a tank with males of different ages placed in glass chambers. The female's preference for a particular male is noted when she spends a large portion of time in front of his chamber. Beck will also track how her choice might change with age. "Does female age affect her selection criteria?" Beck asked. "After her first mating, she could become more choosy."

The relevance of this research may not seem apparent, but ever since Darwin biologists have known that mate choice can drive the evolution of certain traits.

"In nature," Beck said, "you see apparently disadvantageous features—such as a peacock tail, which obviously makes the peacocks prone to predation. Darwin's suggestion was that even if a long tail had a disadvantage in survival, it had a benefit in reproduction."

Back to Emory Report Oct. 9, 2000