It's simple: brains need fuel to function. But when there isn't enough fuel to go around, does the brain get priority access to resources? Researchers turned to toads to find out.

The expensive tissue hypothesis was proposed to explain how it is possible for animals to have large brains without a corresponding increase in their basal metabolic rates, or calorie requirements. The hypothesis says that there are trade-offs between brain size and the size of other body tissues. And yes, according to this hypothesis, the brains of large-brained organisms receive a disproportionate amount of energy for development, effectively decreasing the overall size of other expensive tissues. 

Such trade-offs have been suggested as an evolutionary method for facing periods of environmental uncertainty or disruption. A larger brain provides greater cognitive capacity and enhanced ability to address challenges, increasing chances of survival. However, not all challenges are best addressed with the brain.

Following studies that have found trade-offs between the brain and the gut, and between the brain and reproductive and cognitive performance, scientists from China West Normal University investigated a specific type of brain trade-off, the fat-brain trade-off, in 38 species of anurans (frogs and toads). 

Anurans' body sizes vary seasonally based on food availability in their environments. The fat-brain trade-off explores whether it is better for an animal to invest in brain size (cognitive safeguard) or body fat storage (physiological safeguard) to face the challenges of environmental disruption. 

The researchers compared the anurans' brain sizes to their abilities to store body fat. They hypothesized that the better a species was at storing fat, the smaller their brain size would be, because more of their energy is presumably going into developing the physiological defense against periods of scarce resources. 

They found a negative correlation between relative brain size and body fat storage in the frogs and toads, indicating that the physiological safeguard outweighed the cognitive one. This result reflects similar findings from studies of mammalian and bird species suggesting that differential brain investment may be an evolutionary response employed by vertebrates to deal with periods of environmental uncertainty.