If a male lion is to sire cubs, he must leave his home pride and take over a new one. Because this means displacing the resident male(s) in the new pride and defending it against future rivals, he will likely team up with one or more partners in a coalition to improve his chances.
In the Gir Forest of western India, males form coalitions in surprising ways, thanks to the influence of local demographic patterns, according to research led by Stotra Chakrabarti, a postdoctoral research associate in Associate Professor Joseph Bump’s lab in the U of M’s College of Food, Agricultural and Natural Resource Sciences (CFANS); and Yadvendradev V. Jhala, dean of the Wildlife Institute of India (WII). Study coauthors were Bump and WII faculty member Vishnupriya Kolipakam. Published in the journal Scientific Reports, the study breaks new ground in showing how demographics—e.g., a scarcity of kin—can push animals to follow strategies less than ideal for maximizing reproductive success as a group.
The researchers studied male fitness, defined as the potential for passing on his genes to offspring. Fitness can be gained directly by siring cubs, and also indirectly by helping brothers or cousins pass on genes they have in common. The scientists predicted that larger coalitions would have higher proportions of related males because while the dominant male(s) skew the mating opportunities in their favor, subordinates gain indirect fitness points if their loss results in brothers’ or cousins’ gain.
The researchers also predicted that unrelated males would team up only when mating opportunities between partners were closest to equally shared. Because it’s easiest to “sneak in” matings behind the back of a single dominant partner, pairs seem the optimal size for coalitions of unrelated males.
Fitness vs. prevalence
The scientists collected genetic samples from 23 adult male Gir lions in 10 coalitions ranging from one to four individuals to determine how closely partners were related. They sorted out dominance rankings from long-term behavioral observations on disparities in mating events and food sharing. Factoring in this and other data, they calculated fitness of individuals and groups. Ideally, the coalition size that yields the best cumulative fitness—the sum of fitness of individual lions in a group—should be optimal.
As predicted, lower-ranking males in large coalitions (three or four males) “were typically related to the dominant males, causing their resultant fitness to be higher than that of single males,” says Bump. This indicated the crucial role of indirect fitness benefits in forming large coalitions of (related) males. Within pairs, both individuals—even a subordinate unrelated to the dominant partner—scored higher fitness than single males. In most pairs (71%) the partners were unrelated.
In all coalitions, partners behaved similarly during territorial fights, indicating that close cooperation within coalitions wasn’t based on kinship.
“A 1995 study from the U of M by Jon Grinnell, Craig Packer and Anne Pusey had also found similar responses between coalition partners of male Serengeti lions when presented with threats,” Chakrabarti says. “This shows that coalitionary support remains typically similar across lion systems, with subtle variations owing to resources.” Packer is a Distinguished McKnight University Professor in the College of Biological Sciences; Grinnell and Pusey have left the University.
While individual fitness scores were clearly higher in both pairs and larger coalitions than in single males, larger coalitions had the highest cumulative fitness. This suggests that large coalitions with related partners would be ideal for Gir lions.
But in observations of 37 established coalitions, only 13% had more than two members. The majority (68%) were pairs, and 19% were loners.
This surprising result occurs because in the Gir Forest, most male lions leave their home prides alone and have few kin to join forces with. They’re much more likely to meet and bond with an unrelated male or go solo. Also, large coalitions in the Gir Forest, unlike their counterparts in East Africa, are limited by a scarcity of large prey that could sustain bigger groups. Thus, in the Gir Forest, two-male coalitions are the best compromise for males seeking mates.
“It is only with individual identification and long-term monitoring that such trends in coalition formation could be established, and we are thankful to the lions for letting us follow them day and night, and to our dedicated team of field assistants who helped us follow them,” says Chakrabarti.
Read a longer version of this story on the CFANS site.