Female Lions Unique Among Social Carnivores
What: Female lions truly 'equal opportunity breeders'
When: Embargoed by Science until 1 p.m. CDT Thursday, July 26
Who: Craig Packer, ecology, evolution and behavior dept., (612) 625-5729
Contacts: Deane Morrison, University News Service, (612) 624-2346
(07/27/2001) —When it comes to breeding, the queen of beasts takes a downright democratic approach. In a study led by University of Minnesota lion researcher Craig Packer, female lions exhibited no trace of any hierarchy in which certain animals were more likely than others to produce cubs. Such egalitarian breeding sets female lions apart from the usual pattern of reproductive "despots" that thrive in small, kin-based carnivore groups like the canid (dog) family, meerkats, mongooses, hyenas and even male lions. The study will be published in the July 27 issue of Science.
"In some years, only one or two females manage to raise a litter of cubs, but over time, allthe females have the opportunity to breed," said Packer, a professor of ecology, evolution and behavior. " Female lions are equal opportunity breeders." Coauthors with Packer are Anne Pusey, also a professor of ecology, evolution and behavior,and Lynn Eberly, an assistant professor of biostatistics in the university's School of Public Health.
In many animal species, females experience "despotism," in which one or a few individuals garner all the reproductive opportunities. Bees and other social insects have a single queen and numerous workers. Many bird groups have a single breeding female and many "helpers." In carnivores like canids, mongooses and meerkats, a dominant reproductive female is attended by subordinates. In hyena clans, many females breed, but the top female turns out offspring at the fastest clip, thanks to better access to resources. Earlier work by Pusey and then-graduate student Jennifer Williams used statistical analysis to reveal a hierarchy of reproductive success in female chimpanzees. Previous work by Packer and Dennis Gilbert and Steve O'Brien of the National Cancer Institute revealed a more definite hierarchy among male lions.
Female lions were not obviously like any of these; they were social carnivores, but seemed to lack the despotic structure typified by wolf packs and male lions. They hunted together, raised their cubs together and never seemed to interference with one another's reproduction. If subtle reproductive hierarchies existed in female chimps, however, the possibility remained that such a pattern could also hold for lions.
To settle the question, Packer and his colleagues compared the lifetime reproductive variation in females from 31 prides of Tanzania's Serengeti National Park and Ngorongoro Crater to variation in lifetime reproduction in simulated prides. The simulated prides had the same reproductive rates and demography as the real prides, but births were randomly allocated to females. The researchers found that in every pride, some females had more offspring than others, but the pattern in real prides was no more skewed--evidence of subtle despotism--than the simulated prides, where reproduction was random.
Bolstering the female lion's claim to egalitarianism, the researchers analyzed patterns of competition between pairs of lions at a kill. When an adult male competed for a spot on the carcass with another lion, the outcome had relatively little to do with who got to the spot first; the smart money was always on the bigger (male) animal. When adult females competed, however, the outcome depended much more on which lion got to the choice spot first, even if the first arrival was a cub. In other words, females respected the "owner's rights" of other lions.
The female lion's democratic inclinations are quite unusual because females in a pride have close kinship ties--a favorable environment for despotism to flourish. In general, individuals tolerate despotism and the loss of reproductive rights if they are related to the breeders. "They at least get nieces and nephews out of it," Packer said. Despotism can also thrive when individuals find an advantage to living in a group, as female lions do. By keeping their cubs together and defending them against invading males, they have a much greater chance of reproductive success.
The key to the lion's nondespotic behavior is probably that females can't control each other's reproduction, said Packer. Lions go off by themselves to give birth, bringing their young into the social life of the prides several weeks after parturition. This, plus the nasty wounds a lion can inflict on other lions, makes it hard for one to attack another's newborns. Also, while females care for the cubs as a group, they participate only if they have cubs of their own. Therefore, if a dominant female were to kill a subordinate's cubs, she would lose the services of that subordinate to the group, which included her own cubs. A female that "dumped" her cubs with her pridemates and tried to have a quick second litter would lose the older cubs, who would be at a strong disadvantage when trying to nurse from any lion but their mother. And, since females tend to have litters in synchrony, a second litter brought to the group would be smaller and less able to compete with the older cubs.
This combination of factors make female lions not just the queen of beasts, but likely one of a kind, said Packer.
"The female lion is one of nature's few true democrats," he said