As the existence of animal personalities becomes undeniable, researchers face a puzzle: how disparate personalities can coexist in a single species. Europe’s great tits are helping explain how. At long-term field sites in Germany, the United Kingdom, and the Netherlands, Niels Dingemanse, a behavioral ecologist at the Ludwig Maximilian University of Munich in Germany, and others have manipulated the number of offspring in nests and the density of nest sites. They’ve found that different conditions favor opposite personalities, thereby enabling behavioral variation to persist.
When bird populations are dense, competition for territories, mates, and food sharpens, and one might expect aggressive individuals to win out. But when Dingemanse’s postdoc, Marion Nicolaus, tracked 541 adults for 4 years, recording which survived and how many young they produced, she found the opposite was true. It seems that when birds have to compete for scarce resources, the aggressive ones often get into fights, which take a physical toll. Aggressive birds also strain to keep all their young fed, further taxing their health. Thus, compared with more docile individuals, these birds are more likely to wear themselves out and fail to survive to the next breeding year. Only when densities are low do type A birds outcompete gentler ones and thrive, Dingemanse says.
The findings parallel predictions made a decade ago about humans: that “in growing populations, competitive environments should favor shy, non-explorative, non-aggressive individuals,” Nicolaus, Dingemanse, and colleagues write in an upcoming paper in Ecology Letters.
By looking for marked fish, they found that shy individuals hadn’t simply moved out of the groups; they had vanished, most likely because they were not aggressive enough to compete for food in the group and had starved, or were too slow in reacting to predators that homed in on the school. On their own, however, the shy fish thrived, because remaining still is an effective antipredator defense. Bold fish, in contrast, became targets when isolated.
The finding suggests that personality types could play a role in evolution by helping divide a species into separate populations. Such segregation can lead to further differentiation and, eventually, to reproductive isolation. “That is often the first step in models of speciation,” Duckworth says.
Anelosimus studiosus, a small, brownish U.S. spider, lives in groups of from two to two dozen individuals and can build car-sized webs capable of snaring a small bird or mammal. Over the past decade, behavioral ecologist Jonathan Pruitt of UC Santa Barbara has determined that not only do individual spiders have personalities—bold and active or docile and inactive—but also that the mix of the two types gives each colony a distinctive “group personality.” The group personality needs to fit the demands of the local environment if the colony is to survive, he and his colleagues reported in Nature in 2014.
Thus, group selection based on different mixes of personality variants in the population. Does the same hold for humans? Continuing the speculation from above, do the cuck and ethnocentric fractions of the White population form an integrated whole, with the problem being we have too high a cuck fraction? Will a change toward more ethnocentrics (if possible) solve the problem without complete elimination of the cucks, or a “speciation” between the groups?