Dance of the sexes: a lemur needs some unusual traits to survive in Madagascar's unpredictable environment

Natural History, June, 2005 by Sharon T. Pochron, Patricia C. Wright

The group of lemurs, known as Milne-Edwards's sifakas, was small--an adult male, an adult female, and two large offspring. With only four animals, distinguishing them should have been easy. "That's the male," said Georges Rakotonirina, pointing. Rakotonirina was the lead field technician, a native of Madagascar who had been studying the sifakas with one of us (Wright) since 1986. "And that's the female." The novice among us (Pochron), new to the study in 2000, stared at the dark forms up in the tree and blinked. They all looked the same.

"Look," said Rakotonirina. "They're eating vahia-banikondro."

"What?" Pochron thought to herself. "How can he tell from down here what they're eating? And can I possibly learn to pronounce and spell ... whatever it is?" Hearing chattering in the forest canopy, Pochron then asked aloud, "What bird is that?"

Rakotonirina laughed. "That's the sifaka," he said. "It means he wants to stop fighting." Pochron knew then and there she had some catching up to do, notwithstanding her previous experience studying baboons in Tanzania. But like Wright and many others whose first encounter with lemurs was life-changing, she was hooked.

The lemurs of Madagascar are the surviving members of a lineage that has been genetically isolated from the rest of the primate family for at least 65 million years. The island became separated from the African mainland 160 million years ago, and from the Indian landmass 80 million years ago. The ancestors of lemurs probably colonized the island by rafting there on drifting vegetation. Until relatively recently, lemurs lived in a separate world. Meanwhile, primates elsewhere evolved into monkeys, apes, and humans.

That ancient genetic split is surely one reason lemurs often boast such unusual traits, compared with humanity's closer primate relatives. For example, dwarf lemurs store up fat in their tails and then draw on it while hibernating; in contrast, no monkey or ape hibernates. Members of one lemur family, the indriids, maintain an upright, kangaroolike posture as they leap from one tree trunk and cling to another; monkeys, however, are quadrupedal, like squirrels. All lemurs have toothcombs--a set of teeth ideally shaped for grooming; monkey and ape teeth are shaped for biting and chewing.

Especially surprising to evolutionary biologists, in most groups of lemurs, females are dominant over males. In some lemur species female dominance becomes manifest only in conflicts over food; in other species it emerges in all social settings. Yet in monkeys and apes--indeed, in mammals generally--female dominance is rare. What has led to such an unusual social characteristic among lemurs, with its far-reaching implications?

Female mammals that do dominate males are usually well equipped physically to do so. Female spotted hyenas are often bigger than males. Female reindeer rule over males during the short season when males have shed their antlers prior to growing new ones and the females have not yet shed theirs. Female golden hamsters call the shots when they are fatter than males.

But female lemurs are not usually larger than males, nor do they have any special weapons for enforcing dominance, such as bigger teeth. Members of the two sexes are virtually monomorphic, or similar, when it comes to physical strength. How do females manage to get their way without the brawn to back up a threat? We and our colleagues do not yet have a definitive answer to that question, but after eighteen years studying one indriid species, we have some inklings.

The center of our universe is the Milne-Edwards's sifaka (Propithecus edwardsi). Until recently it was considered a subspecies of the diademed sifaka, but geneticists have now determined that it is a separate species. Weighing in at about thirteen pounds and looking like something out of the Muppet studio, the animal lives throughout Ranomafana National Park, a 170-square-mile emerald forest set in cloud-covered mountains, and in adjacent regions [see map on preceding, page]. It has orange eyes and woolly, water-resistant fur (a useful trait in a rainforest), which is colored dark brown to black except for two large, white patches on the animal's back. The females have a lemony, maple-syrupy smell; the males, which have more glands for scent marking, smell muskier.

Active by day, Milne-Edwards's sifakas prefer to hang out some forty feet up in the trees, and they travel, as do other indriids, by leaping from one tree trunk to the next. Adults are mainly leaf eaters, but they also rely heavily on fruits and seeds.

Females and males do not often come into conflict, but when they do, the females win about 95 percent of the time. Apparently males are letting females win such altercations. What are they giving up by submitting? The answer may be calories. Adult females, for instance, appear to eat more seeds than adult males do. The difference is most pronounced during the mating season. Seeds are generally high in fat, and storing up fat is good preparation for a female on her way to becoming reproductively active. When you see males and females fighting, you will probably find tempting seeds nearby.