Size Doesn’t Matter for Mammals With More Complex Brains, According to New Study

The University of Bath study on 124 mammals shows that significant size differences between sexes correlate with larger olfactory gene families and smaller brain development gene families. This indicates different evolutionary investments based on sexual size dimorphism, influencing behaviors and mating strategies. Future research will focus on the role of testes size in genome evolution.

New research led by the University of Bath in the UK suggests that mammals with more advanced brain development tend to exhibit smaller size differences between males and females within the species.

In many mammal species, males are often larger than females (or vice versa), a characteristic known as sexual size dimorphism (SSD). For instance, male elephant seals are about three times the size of females. In contrast, dolphins exhibit no size difference between sexes. Humans fall somewhere in between, with the average male being larger than the average female, though there is considerable overlap across the population.

To understand how this trait is associated with genome evolution, scientists from the Milner Centre for Evolution at the University of Bath in the UK looked at similarities between the genomes of 124 species of mammals.

They grouped the genes into families of similar functions and measured the size of these gene families. They found that those species with a big difference in size between the sexes had bigger gene families linked to olfactory functions (sense of smell) and smaller gene families associated with brain development.

Therefore, this could also mean that those species with very little difference in sizes between males and females (termed monomorphic) had bigger gene families associated with brain development.

Implications of Gene Family Size

Publishing in Nature Communications, the authors suggest that in species with a large SSD, traits such as the sense of smell could be important for identifying mates and territories.

In contrast, mammals with a smaller SSD are potentially investing in their brain development and tend to have more complex social structures. This means they compete for mates using other means than simply using size to select who to breed with.

Dr Benjamin Padilla-Morales, from the Milner Centre for Evolution at the University of Bath led the research. He said: “We were surprised to see such a strong statistical link between a large SSD and expanded gene families for olfactory function. Even more interestingly, the gene families under contraction were linked with brain development.

“This could mean that those species with a small SSD have bigger gene families associated with brain function and tend to show more complex behaviors such as biparental care and monogamous breeding systems.

“It shows that while size in some species is an important sexual selection pressure for evolution, for others it doesn’t matter so much.

“It makes us ask the question how traits like SSD are shaping the evolution of our brains and genomes.”

In future work, the researchers want to investigate how testes size impacts the evolution of mammals’ genomes.

Reference: “Sexual size dimorphism in mammals is associated with changes in the size of gene families related to brain development” by Benjamin Padilla-Morales, Alin P. Acuña-Alonzo, Huseyin Kilili, Atahualpa Castillo-Morales, Karina Díaz-Barba, Kathryn H. Maher, Laurie Fabian, Evangelos Mourkas, Tamás Székely, Martin-Alejandro Serrano-Meneses, Diego Cortez, Sergio Ancona and Araxi O. Urrutia, 24 July 2024, Nature Communications.
DOI: 10.1038/s41467-024-50386-x