The parents of mammals with larger brains tend to share parenting duties, according to a new study, with both mom and dad involved in raising offspring. Scientists also found mammals with greater reproductive success tend to have help from non-biological parents, or alloparents.

Researchers analyzed data on the parenting behaviors, brain size and fertility of 478 mammal species, including lions, mice, meerkats, monkeys and apes. The analysis showed different types of parental support aids different evolutionary advances.

"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," Sandra Heldstab, an anthropologist at the University of Zurich, said in a news release. "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 new research, published this week in the journal Behavioral Ecology and Sociobiology, is based on the "expensive brain hypothesis," which posits increases in brain size requires new energy sources for females to be stable and reliable.

Because male parenting support is likely to be more stable and reliable than alloparental help, which tends to fluctuate, it makes sense that helpful dads would be correlated with increased brain size.

"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," Heldstab said. "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."

With both large brains and high reproductive output, humans are an outlier. Researchers suggest the impressive combination is likely made possibly by "multi-family cooperative parenting." Early in the evolutionary history of humans, moms benefited from stable and reliable help provided by both parents and alloparents.

Of course, parental help isn't the only way moms can find new sources of energy for evolutionary advances. Changes in diet, like the addition of freshwater crabs or other aqueous food sources, can also provide the added energy necessary to evolve larger brains.

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