Heterocephalus glaber, better known as the naked mole-rat, deviates from the typical understanding of a “normal” mammalian lifestyle, in more ways than one. Naked mole-rats live in tightly knit colonies underground that more so resemble ant hills than mammal burrows. Each colony functions with one ruling queen, usually one breeding male, and dozens of subordinate non-breeders who forage, burrow, and defend. Therefore, for most naked mole-rats, reproduction is off-limits.

In 2007, behavioural neuroscientist, and current director of the Social Neuroscience lab at the University of Toronto Mississauga, Dr. Melissa Holmes, and her colleagues published a groundbreaking study looking into this lack of subordinate reproduction in naked mole-rats. Their work, entitled “Social Control of Brain Morphology in a Eusocial Mammal”, uncovered a characteristic foundational in driving the direction for naked mole-rat research for years to come: their brains are not shaped by differences in sex but shaped by social status.

The team looked at the brains of both breeders and subordinate non-breeders, examining regions linked to reproductive and social behaviours. In most mammals, these areas would show clear sexual dimorphism. However, in the brains of subordinate naked mole-rats, these sex-based brain differences were absent, also correlating with the relative absence of behavioural differences between male and female non-breeders.

Holmes’ study further found that breeders, showing increased cell density, possessed larger, more developed brain regions associated with reproduction and social behaviour. On the contrary, subordinates were shown to have small, uniform brain regions across sexes. In other words, it seemed that it didn’t matter whether you were a male or female, rather it mattered more whether you were a queen/breeder or a subordinate.

This finding continues to fascinate researchers today. When asked about it, Joaquin, a graduate student in the Holmes lab, recalled his surprise when first joining the lab, stating that “(adult) naked mole-rats not showing sexual dimorphism is very rare, especially in mammals… everyone calls them weird because of it.” These mammals are an “exception” to what we know, as highlighted by the young researcher, and they challenge decades of assumptions on how sex differences develop in the brain.

The mechanism behind the subordinate naked mole-rats’ reproductive suppression is a mystery still being investigated to this day. When asked what keeps their brains reproductively neutral, Joaquin explained that it was tied to the queen’s dominance in the colony, but the exact mechanism remains “a debate”. He continues, saying that “[some think] it’s physical stress or shoving”, however, there might also be “specific signals” that inhibit reproduction coming from the queen to her subordinates. Current research suggests that the queen’s physical aggression against subordinates causes an increase in glucocorticoid levels, effectively suppressing their reproductive capabilities. At the same time, the removal of the queen causes chaos, instability, and death within the colony, emphasizing the importance of her presence. Whatever the exact mechanism, the result is clear: the social environment exerts biological control.

While it may seem niche, Holmes’ work has broader implications for neuroscience and behaviour, human or otherwise, by suggesting that social context can overpower biological sex in terms of brain development. “It’s a good way to research puberty,” Joaquin says, because “[they] can transition into a reproductive animal at any time because of the social implications, their social environment.” Suggesting that this flexibility makes them an ideal model to study how biological development and environmental pressures intersect. For humans, naked mole-rats might echo how our own environmental pressures like chronic stress, social instability, or relationship dynamics, affect our brain development as well. For example, a 2024 study by Thanaraju et al., found that higher socioeconomic status across various life stages was linked to better adult brain health, seen in increased cortical thickness and grey matter volume of the brain.

Dr. Holmes’ 2007 study was only the beginning. Almost two decades later, researchers have access to newer, more sophisticated techniques to study the brain. When asked how he would do the study differently today, Joaquin reflected on his Principal Investigator’s (Dr. Holmes’) study, “If I remember correctly… Dr. Holmes chose very specific brain regions because they were candidates for social behaviours. But obviously social behaviours encompass a wide net of brain regions, brain areas, as well as connections.”

He explained that the Holmes lab has expanded from classic lab techniques, like immunohistochemistry, to now include molecular and genetic techniques. Nowadays, “we can look at DNA, RNA expression, epigenetics”, Joaquin states, “we can look at different types of brain regions, we can even inhibit brain regions or specifically target one brain region, as well as target specific genes that may be responsible for activating or inhibiting these regions”, suggesting that it is now possible to examine the naked mole-rat’s sexual dimorphism (or rather lack thereof) at the molecular and genetic level.

Although the study is from 2007, it continues to be foundational for the field of behavioural neuroscience. Today, the Holmes lab continues exploring the various quirks and oddities of the naked mole-rat at a neuroscientific level. As it remains, Dr. Holmes’ discovery reminds us that even the smallest, strangest creatures can challenge our assumptions. In the end, naked mole-rats remind us that biology does not operate in isolation, rather it also takes its cues from the social world.