The Animal Kingdom's "Universal Code" for Monogamy Has Been Discovered

No wonder animals can't help but pair up.

There’s an amorous connection that unites a species, whether or not its members are scaled, feathered, or furred. In a study released Monday in the Proceedings of the National Academy of Sciences scientists provide evidence of a universal transcriptomic code underlying monogamy in vertebrates. Monogamy, it’s revealed, is embedded in shared molecules across various amphibians, birds, mammals, and fish — despite the fact that monogamy evolved independently in each of these clades.

This new study is unique in providing answers to the neural and molecular basis of monogamy. While scientists have long known that monogamy is scattered across the animal kingdom, what they haven’t known is how it manifests internally. Now they believe that, in some creatures, monogamy is rooted within genes.

“We found support for our hypothesis that, across vertebrates, monogamous species — that clearly evolved monogamy independently — share a gene expression signature in their fore-and midbrain,” study co-author Hans Hofmann, Ph.D. tells Inverse. “This suggests that there might be a shared ‘universal’ code of sorts, though many more monogamous and non-monogamous species will have to be examined to strengthen this point.”

zebra finches
90 percent of birds, like these zebra finches, are monogamous.

Hofmann, a professor of integrative biology at the University of Texas at Austin, and his colleagues examined the forebrain and midbrain tissues belonging to monogamous and non-monogamous reproductive-aged males of various species. The animals known for forming monogamous pairs included deer mice, passeroid songbirds, dendrobatid frogs, cichlid fishes, and voles like the ones seen in the video above.

It’s important to note that there is not one unifying definition for monogamy since the manifestation of such a social system can vary across species and individual members of a species. Here, the team used the presence of certain behavioral attributes as part of an operational definition of monogamy. A species was considered monogamous if it fit three specific criteria: a male and female form a pair bond, both parents participate in offspring care, and both parents engage in offspring defense in the face of danger. Monogamy does not mean that a male and female in a pair are exclusive to each other — the team notes that in most species, “extra-pair matings occur fairly regularly,” including the species Homo sapiens.

But these animals do form a pair bond and work together to protect a baby. Monogamous animals, the study shows, also share gene expression signatures: The team pinpointed 24 candidate genes for monogamy across the male subjects, a group that — when highly expressed — is linked to neural development, synaptic activity, learning, memory, and cognitive function. These genes were regulated in a similar fashion across the five monogamous species, and, Hofmann explains, were collectively “either more highly expressed or less expressed in the monogamous brain compared to the non-monogamous brain.”

Why these genes are connected to functions like learning and memory can only be, for now, speculated. But Hofmann suggests that forming a pair, taking care of offspring, or both, may require a change in the cognitive process underlying social behavior. It’s likely that an individual needs to be able to recognize a mate and find it rewarding to be with that mate for a pair to successfully form. Perhaps, Hofmann, says, “this may require processes known to be involved in neural and synaptic plasticity, and learning and memory.”

Humans, participating in monogamy.

Understanding the underpinnings of monogamy as a holistic concept, in turn, can help to explain why monogamy is a frequent practice in human society. Just as specific genes link monogamous birds and rodents, it’s likely that these genes are expressed in our brains as well. By learning how biological processes evolved across clades, we get to learn more about where we come from.

“This work reminds us that we humans too are the product of evolution,” Hofmann says. “This means that what we look like, how our bodies work, and yes, also our behavior, has an evolutionary history.”