features that help finding a mate may lead to smaller brains

A longstanding question in evolutionary biology is how sexual selection influences how entire genomes develop. Sexual selection is where individuals with certain traits have higher reproductive success, leading to the spread of those traits throughout a species.

A study by me and my colleagues at the Milner Centre for Evolution has uncovered a significant link between the difference in body size between males and females – known as sexual size dimorphism (SSD) – and genetic changes in mammals. These findings provide new insights into how sexual selection shapes the structure and function of the genome.

Sexual selection is a powerful evolutionary force that influences reproductive traits. It typically acts through mate choice (intersexual selection) and competition among individuals of the same sex (intrasexual selection). Over time, these constant pressures shape genome architecture, driving rapid evolution in genes associated with reproductive success.

This may affect the voice, body size, plumage or other feature of a species over time. In fact, such pressures may be behind a rise in height in male humans compared with females.

Recent work highlights how sexual selection contributes to changes in the genetic blueprint (genome) and genes actively used (transcriptome).

Many sexually dimorphic traits arise through sex-specific differences in gene expression. This allows a single shared genome to produce distinct male and female types.

Males and females differing in body size is a common outcome of sexual selection. Some examples are the southern elephant seal (Mirounga leonina), domestic ferret (Mustela putorius furo) and northern fur seal (Callorhinus ursinus), where males are more than 250% heavier than females. In contrast, species such as the natal long-fingered bat (Miniopterus natalensis), humans and wombats (vombatus ursinus) show lower SSD, with males weighting less than 50% more than females.

A large difference often correlates with intense male-male competition, leading to the evolution of traits that enhance reproductive success, such as tall stature. However, while the impact of this difference on physical traits is well documented, its influence on genome evolution has remained largely unexplored.

Sense of smell versus brain size

We analysed groups of related genes called gene families across 124 mammalian species. Our study provides compelling evidence that SSD is associated with major shifts in the sizes of such families.

Specifically, species with high SSD have an expansion of gene families linked to sense of smell. At the same time, their gene families related to brain development tend to contract.

This suggests that in species with strong male competition, investment in traits that aid in reproductive success, such as olfactory cues for mate recognition, is prioritised over cognitive development.

Conversely, species with low SSD show an expansion of brain-related gene families. This pattern suggests that in these mammals, natural selection may favour cognitive abilities and complex social behaviours rather than traits driven by sexual competition.

Sexual conflict, where selection acts in opposing directions in males and females, plays an important role in genome evolution. This may involve males evolving brighter colours and outstanding features, as seen in peacocks (Pavo cristatus) and guppies (Poecilia reticulata). While these traits enhance male success by attracting females, they might also increase the risk of being spotted by predators.

Many sex differences arise due to selection acting differently on shared genetic material, creating evolutionary tension. This can lead to sex-biased gene expression, allowing genes to function differently in males and females. This is the case for genes controlling bright colouration in guppies, for example.

Studies have suggested that genes under strong sexual selection tend to evolve rapidly, particularly those associated with male reproductive traits, such as body size or colour. Additionally, genomic features, such as the duplication of genes, can help the evolution of sex-specific traits, helping to alleviate conflicts between the sexes.

Our findings support these ideas by demonstrating that SSD influences gene family evolution, shaping molecular pathways critical for sexual and cognitive development.

Evolutionary give and take

Sexual selection does not act in isolation. It interacts with other evolutionary forces, such as natural selection and ecological pressures, to shape diversity. For example, larger body size in males may confer advantages in physical competition. But it can also increase metabolic demands and the risk of being caught by predators.

Similarly, large brains and complex social structures may be favoured in species where cognitive abilities play a role in reproductive success, such as humans. But this comes at the cost of slower development and greater energy expenditure.

This interplay between sexual selection and other evolutionary pressures highlights the complexity of genome evolution. Traits that provide reproductive advantages may not always align with those that enhance survival. This leads to give-and-take situations that shape species diversity over time.

By examining the genetic underpinnings of SSD, our study provides new perspectives on how these situations play out at the molecular level. Our findings ultimately refine our understanding of how sexual selection influences genome evolution among mammals.

Future research should explore in depth how these genomic changes influence behaviour and cognitive abilities in different species. These findings will open exciting new avenues for research, helping to answer fundamental questions about how evolution shapes biodiversity at the genetic level.