A Fresh Look at the Straying Ways of the Female Chimp
By NATALIE ANGIER
Consider it a serious case of chimp change.
They are not about to win any awards for being chaste homebodies, but female chimpanzees, it seems, are satisfied with chasing after the bodies back home.
In 1997, a startling paper appeared in the journal Nature, reporting that, contrary to the expectations and observations of most primatologists, female chimpanzees were sneaking off and mating furtively with males outside their social set.
Dr. Pascal Gagneux and Dr. David S. Woodruff of the University of California at San Diego, and Dr. Christoph Boesch, then at the University of Basel in Switzerland, announced that their DNA analysis of a community of 52 chimpanzees, living in the Tai forest of Ivory Coast, indicated that more than half the infants they tested had been sired by males outside the community.
The scientists did not know who the mystery fathers were, but they proposed that the females, despite having been observed copulating vigorously with the boys at home, were still dissatisfied enough to seek extragroup mating opportunities. The chimpanzee social unit, apparently, was not equivalent to the chimpanzee reproductive unit.
Moreover, the study suggested that the male chimpanzees' tireless striving for high rank was largely in vain, for alpha-hood appeared to confer very little benefit on males when it came to the basic currency of natural selection: reproductive success. The females seemed to dash off right at the moment of their peak fertility.
The researchers expanded on the study and its implications in a lengthy paper that appeared in the journal Animal Behaviour in 1998.
The unexpected results were widely reported (an article appeared in The Times on May 27, 1997) and soon found their way into books and mainstream lore. "I meet people who don't know much about chimps, but who say to me, `Oh, yes, I read that half of infant chimps are not fathered in the group,' " said Dr. Anne E. Pusey, a behavior ecologist at the University of Minnesota.
Now Dr. Pusey and her co-workers have published a study that completely contradicts the earlier findings. Reporting their results of a DNA study of a community of 39 chimpanzees living in Gombe National Park in Tanzania, the scientists said they could comfortably assign paternity for all 14 offspring they sampled to males within the group.
And far from being an apparent waste of effort, among chimpanzees it pays for a male to seek power: five of the progeny were fathered by alpha males, and another two by high- ranking males. In addition, when the scientists reanalyzed the Tai data using their statistical methods, they were able to match the infants to likely fathers in the Tai troop.
Dr. Pusey reports the results in the current issue of the journal Molecular Ecology, together with a colleague, Dr. Julie E. Constable, Dr. Mary V. Ashley of the University of Illinois at Chicago and Dr. Jane Goodall of the Jane Goodall Institute in Silver Spring, Md. Dr. Goodall has studied the Gombe chimpanzees since the 1960's.
Dr. Gagneux and Dr. Boesch said that while they disagreed with some of the technical methodologies of the new report, as well as with several of the paternity assignments proposed for the Tai progeny, they concurred that their initial analysis was seriously flawed. "I unfortunately have to agree with them that there seem to be serious problems with my genotyping results," Dr. Gagneux said. "I was not being conservative enough in scoring the genotypes. This is obviously extremely embarrassing to me."
He added that a new study under way by Dr. Boesch, now at the Max- Planck Institute, together with Dr. Linda Vigilant, an originator of the "Out of Africa" theory of human evolution, had also found errors in the original data. And in checking the paternity of infants born more recently in the Tai group, Dr. Gagneux said, Dr. Vigilant has been able to match most to males in the community.
If extragroup paternity occurs, Dr. Gagneux said, "it's probably pretty rare."
The latest debacle underscores the difficulty of performing DNA analysis on animals in the field, since they cannot readily be herded up for blood collections. Instead, researchers have struggled to devise effective noninvasive sampling methods. Dr. Gagneux relied on DNA extracted from shed hairs, which contain extremely small quantities of genetic material.
More recently, researchers have succeeded in extracting comparatively larger amounts of DNA from animal feces. When Dr. Constable reported her luck with purifying DNA from bear stools, Dr. Pusey went to her and asked, Can you also do this with chimpanzees? "I have a gift for getting things to work with problem samples," Dr. Constable said.
For the new project, workers at Gombe who were familiar with all the chimpanzees in the community used sterile sticks to collect stool samples as soon as they were deposited, and sent the samples back to Dr. Constable in vials of ethanol. Dr. Constable then extracted the DNA from the stools and amplified them, using a technique called polymerase chain reaction, or P.C.R., the same method that Dr. Gagneux had applied to amplifying the hair DNA.
Dr. Gagneux and Dr. Constable suggest similar reasons, apart from the great difficulty of working with shed hair, for why Dr. Gagneux concluded that half the infants at Tai could not have been fathered by resident males. Dr. Gagneux performed about three P.C.R. reactions for each stage of his analysis, while Dr. Constable might do double or triple that amount.
By hammering away at multiple runs of her P.C.R. reactions, Dr. Constable was better able than Dr. Gagneux to distinguish between so-called heterozygous and homozygous alleles — that is, she could tell when the infant had two different copies of a gene, rather than two identical copies of that gene, a discrepancy that might implicate rather than exclude a known male as the possible sire.
In addition, where Dr. Gagneux had eyeballed the X-ray photos of his DNA samples and scored them according to his judgment, Dr. Constable had used an automatic sequencer and thus obtained more sheerly quantitative results. Dr. Constable also relied on a statistical modeling program called Cervus that she said was particularly suited to analysis of paternity in cases where DNA samples were small and error rates were likely to be high.
Primatologists who study chimpanzees expressed relief that the new results confirmed their years of painstaking observational studies, in which they had almost never seen females mating outside the home territory.
Within the group, the females were famously promiscuous around estrus, usually mating multiple times with multiple males and blithely defying efforts by the alpha male or other high-ranker to guard them and keep other males at bay.
It was because the females mated so wantonly in the group that Dr. Goodall years ago began questioning the benefit of being an alpha male. Could the striving for power among male chimpanzees be some sort of holdover from a previous era, she wondered, when an alpha king could manage to monopolize the fertile females? Researchers rank animals in a hierarchy by observing who threatens whom with a dominance charge, and who submits with a pant and a grunt.
The puzzle of the male status dance sharpened with the publication of the Nature paper. And though the results seemed to defy field research, many people accepted them, Dr. Pusey said. "There was precedence from DNA studies of birds," she said.
"It was a real surprise to people to find that among monogamous birds, where you have the males around feeding the babies, that half of the babies turn out not to be the father's." For some researchers, she added, "it was pleasing to find a similar thing in mammals."
Dr. Pusey was not among the amused. "She was always skeptical of the Tai results," Dr. Constable said, "and she was absolutely thrilled when I came back and said, Guess what I found." What the researchers found was that power pays, but not absolutely. As the admittedly limited sample indicates, alpha males have some advantages in fertility hits. They may not be able to prevent females from sleeping around — locally — but they are clearly effective studs, perhaps because they can sense when a female is at her peak fertility and monopolize her then.
But other male mating strategies beyond autocracy work. Half the infants at Gombe were sired by low- or mid-level males, who may try to befriend females rather than bat them around alpha-style. Nor are females necessarily impressed by male swagger. The most successful mother in Gombe, the legendary Fifi, who has given birth to eight offspring and seen all but one survive to adulthood, is a queen with scant taste for kings. Of her four offspring sampled in the new study, only one was sired by an alpha male. "In chimpanzees, as in most mammals, internal fertilization gives females last word," Dr. Boesch said. "A female chimpanzee may not have the muscular strength to enforce her way, but when it comes to the subtle mechanisms that determine which sperm are going to fertilize her eggs, the female is in charge."
Maintained by Francis F. Steen, Communication Studies, University of California Los Angeles