Mammal brain size and fertility is determined by the availability of caregivers

Researchers at the University of Zurich have investigated how the energy inputs of various caregivers may affect brain size and fertility among mammals. The study revealed that more paternal care is linked to a larger brain size in the offspring, while additional care and support for offspring outside of the biological parents is linked to higher fertility in the mothers.

The investigation was focused on the data comparison of 478 different species, including lions, mice, meerkats, monkeys, and apes.

“Both reproduction and brain tissue are energetically very expensive, and one way for females to reduce their cost is by distributing that cost over other individuals by sharing the burden of care,” explained study co-author Dr. Sandra Heldstab.

“Unlike previous studies, we distinguished between paternal and alloparental care because we expected there to be a difference between how reliable they are and in the effect they may have on brain size and fertility.”

The research was based on the expensive brain hypothesis, which predicts that the brain will only evolve to become larger if the additional energy that is made available to a female through help with raising the offspring is predictable and constant.

“Paternal care is both reliable and stable; therefore we’d expect it to be associated with brain size,” said Dr. Heldstab. “Additional care from individuals who are not the offspring’s parents often fluctuates as they adjust their caring effort depending on both food availability and their own reproductive needs.”

“This unpredictable type of care doesn’t provide enough stable energy to affect brain size, but our findings suggest that the additional energy it does provide is associated with a significant increase in fertility, as females readily respond through litter size adjustments to variable amounts of energy inputs.”

According to the researchers, the relatively high reproductive output and extremely large brain size of humans may be explained by multi-family cooperative parenting, which involves stable and reliable care by both parents and non-parents.

The study is published in the journal Behavioral Ecology and Sociobiology.

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By Chrissy Sexton, Earth.com Staff Writer

Image Credit: Shutterstock/Ricardo Reitmeyer