Photo credit: Craig Packer
Cats have a well-deserved reputation for independence. From tabbies to tigers, they hold individual territories. But lions are a glaring exception.
Lions’ habit of living socially in prides has long been viewed as an adaptation to life on the African savanna, where grass dominates the landscape and resources like game, water and shade trees tend to be found in clumps.
Now, work led by Anna Mosser, a teaching assistant professor in the College of Biological Sciences, indicates that it was a combination of the savanna landscape and certain behaviors that drove the evolution of lions’ social living. The work is published in Behavioral Ecology.
Ruling out suspects
Solving the mystery of why lions should be so social has long occupied Craig Packer, a Distinguished McKnight University Professor of ecology, under whom Mosser worked as a graduate student. Over the last few decades, he and his graduate students have tested hypotheses one by one, focusing on female lions, who form the nucleus of any pride. An early idea was that groups had better success at hunting.
“Craig and a graduate student found that females could eat fine alone, taking wildebeest and anything smaller,” says Mosser. “But hunting success dropped in groups of about two to four females, then rose as groups got bigger.” The dip put to rest the hypothesis that groups necessarily had better hunting success than individuals.
Next, Packer and colleagues asked whether lions grouped together primarily to defend their cubs.
“It’s a factor–they do pool their cubs in crèches to protect them from hyenas, leopards and male lions trying to take over a pride,” says Mosser. “But even cubless females stick together. [Because groups of three have less hunting success] three females with no cubs should split and hunt alone.” But they don’t.
Location, location, location
When Mosser began her research, she studied the “real estate value” of various lion territories, terming the high-value locations “hot spots.” The best territories were near the confluences of streams, where water was plentiful and dense vegetation offered shade, a place to hide the cubs and cover for stalking.
“We tested this by looking where kills occurred near confluences. Many happened inside the ‘V’ where streams come together,” she says.
Was territorial defense the reason for grouping?
It appeared to be; larger prides had better territories. And when a pride improved its territory—by moving closer to a confluence—from one year to the next, it also gained adult female members. Conversely, a decline in territory quality was linked to decreases in pride size.
“It wasn’t just that prides were growing to fit the quality of their territory—they were also jostling for access to hot spots, with the outcome dependent upon pride size,” Mosser explains.
In her latest report, Mosser, with colleagues Packer and Margaret Kosmala, used real data on the African savanna landscape and lion behavior to simulate how the landscape drove lions with the right genetic capacity to adopt a social way of life.
And the answer is … synergy
Simulations began with lions living solitary lives and not leaving their territories to offspring. But they could experience a genetic mutation that would allow social behaviors: defending territory cooperatively, expanding their territory, and bequeathing territory to daughters.
The researchers varied the quality of the landscape. High-value landscapes (hot spots) could support more animals total; this led to high density and more competition for resources. They also varied landscape heterogeneity—the clumpiness of resource distribution. Then they watched to see how those factors changed the proportion of animals exhibiting one or more of the three social behaviors.
“We found that as the landscape value and heterogeneity rose, so did the proportion of social animals,” says Mosser. … “[A] combination of landscape value, landscape heterogeneity and multiple behaviors are necessary to explain the rise of group territoriality in African lions.”
The picture that emerges is of a heterogeneous landscape where some locations are worth fighting for. Lions equipped to behave socially form groups to win and defend the best territories, expand them, and pass them on to female offspring.
“What surprised me most about the sociality model was the fact that it was such a clear example of an ‘emergent property’—three things had to line up for sociality to evolve,” says Packer.
Mosser adds that a pattern of emergence is “when the whole is more than the sum of its parts. Here, when you have landscape heterogeneity and quality, plus the right behaviors, sociality emerges.”
Why not leopards?
If so, why aren’t leopards, which share the savanna with lions, also social? Studies by Packer and others indicate that leopards have to put up with “interference competition” from the larger, stronger lions.
“Leopards don’t have access to hot spots the way lions do,” says Mosser. “At the water’s edge, they have to avoid lions.” Thus, any leopards that banded together and tried to occupy the same hot spots as lions would soon be dispersed.
Lions would probably have been in the same predicament when the even larger saber-toothed cats roamed the earth. The species could not have evolved sociality “until they could dominate the landscape in the same manner as in our model,” the researchers say.
And those fierce saber-tooths may themselves have been social.
“Their remains show healed wounds,” says Mosser. “Thus, they must have had a group hunting for them.” Likewise, she says, lions share food with injured pride mates.
“The same reasons for being social might explain its evolution in other species, such as wolves, European badgers, and Australian magpies,” Mosser says. “I’d also argue that even if a species was initially social for other reasons, if it exhibits group territorial defense, that will be a strong pressure to maintain group ties.”
The recent uproar over the killing of a lion in Zimbabwe raises the question of whether lion hunting can be compatible with preservation of the species. Craig Packer tackles the issue in his new book, Lions in the Balance (University of Chicago Press 2015).
He has said that killing male lions age 6 or older won’t torpedo conservation efforts because by that age almost all males have had their chance to reproduce and protect their cubs. He has found several means to tell a lion’s age, such as by the amount of spotting on its nose, but suggestions that lions be identified as over six years old before pulling the trigger have met with stiff resistance from governments that rely on hunting income.
“Without hunting, there would be less tolerance for lions,” Packer says. “If there is a well-regulated hunting industry, it’s one of best ways to preserve wildlife in Africa.” Unfortunately, “well—regulated” doesn’t seem to apply very often.
Lions in the Balance is expected to arrive in bookstores in September.