Drop two adult rats of the same sex into a cage, and it’s a near certainty that the bigger rat, even if only slightly bigger, will dominate from the first minutes. But what happens if you take several freshly weaned rats, all of equal size and from good homes, and put them together? A hierarchy nonetheless emerges, according to a new experiment, but the determining factors remain a mystery. These factors—in good news for humans at the low end of the social ladder—may be mutable.

Darlene Francis, a psychologist at the University of California, Berkeley, placed 80 newly weaned rats in cages of four, with cage mates matched for size, activity level and early life environment. To Francis’s amazement, it took weeks—until the rats were well past puberty—for a social hierarchy to evolve (as indicated by which mouse got first dibs at food and water, among other measures). Perhaps more surprising was that the hierarchies were not determined by the differences in weight, activity or size that had developed among the maturing quartets—or by anything else Francis could identify.

This mystery was unexpected and intriguing. “Social rank is a huge deal, because in both rats and people, how you do in life depends more on social place than almost any other individual difference you can measure. This study suggests that social status is determined by something quite subtle.”

For Francis, who studies the effects of early experience on animals’ physical and cognitive fitness, this experiment was a way to see what differences might arise among rats from similar upbringings. She took particular care to match the foursomes not only for size, weight and activity level but also for level of maternal care. Work by Francis and others over the past decade has shown that rat mothers tend to be either highly nurturing or barely nurturing—an 8 or a 2, as it were—and that more highly nurtured rats go through life more confidently and competently. These factors raise their social standing, which breeds more confidence and better performance, and so on, in a happy loop. Rats from low-nurturing mothers, meanwhile, tend to be tentative, which lowers their social rank, which makes them more tentative.

For rats as for people, social rank and the individual’s response to it have tremendous consequences. The top rats in Francis’s study, for instance, performed far better than the second-, third- and fourth-ranked rats at cognitive tests such as finding hidden treats. They also acted more confidently and were less stressed (as indicated by stress hormone levels in their blood) when exposed to unfamiliar environments and other challenges. The lower-ranking rats solved puzzles more slowly. And even in their home cages, Francis says, they “looked like animals being tested in a novel environment. Not even home felt safe to them.”

A dispiriting development, perhaps, after the weeks of early equality. Yet Francis sees in this study the outlines of levers by which such decrements might be reversed. The social hierarchy’s slow development, along with its indepen­dence from size and activity level, suggests to her that whatever factors dictated the social gradient involved subtle and highly malleable gene-environment loops: traits that emerge in reaction to experience and then, in turn, help to shape further experience.

“Whatever these determining factors are,” Francis says, “they’re very plastic. If we can find where that plasticity lies, then perhaps we can help those animals improve their lot, even if they’re near the bottom. You think about people, the same possibilities exist there. If you can identify what drives movements in social hierarchies and find ways to tweak those factors, you can reduce the huge penalty people pay for low social standing.”

Note: This article was originally printed with the title, "Hierarchies among Equals".

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