Sperm competition and brain size evolution in mammals

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Neuroscience

Background:

• The expensive tissue hypothesis proposes a trade-off between energetically costly organs, like the brain.
• The expensive sexual tissue hypothesis specifically suggests larger testes constrain brain size due to sperm competition.

Purpose of the Study:

• To test the evolutionary trade-off between brain and testis mass across diverse mammalian groups.
• To determine if sperm competition, indicated by mating systems, influences brain size evolution.

Main Methods:

• Comparative analysis of brain and testis mass across various mammalian orders (rodents, ungulates, primates, carnivores).
• Statistical examination of the relationship between brain mass and mating systems to infer sperm competition levels.
• Focused analysis on echolocating bats to investigate specific group dynamics.

Main Results:

• No general negative evolutionary trade-off between brain and testis mass was found in mammals.
• Brain mass did not significantly vary with mating systems across most mammalian orders.
• A negative relationship was observed in echolocating bats, but mating systems appeared to be a stronger predictor of brain size in this group.

Conclusions:

• The expensive sexual tissue hypothesis explains little to none of the variation in mammalian brain size.
• Broader evolutionary frameworks are needed to understand the energetic costs and constraints of brain evolution.