It might seem like bigger brains are better for survival because they often lead to greater intelligence, but carrying around all that grey matter isn't easy. In fact, bigger-brained mammals are far more likely to face endangerment and extinction than their smaller-brained peers, a new study has found.

First off, it's important to note that, despite what you might believe about humans, our brains are not the biggest in the world. On average, our brains weigh about 1.3 kg (3 pounds), which is about the same size as a dolphin's. Though this is quite large compared to, say, a dog's brain, it's nothing compared to the 7.7-kg (17-pound) brain of a sperm whale.

This means that intelligence and brain size are not directly related in many ways. Sure, sperm whales are smarter than other creatures, but we're smarter than them with smaller brains. Therefore, brain size, though a factor, doesn't scale with intellect.

So, what's the problem with having a big brain? Basically, the physical size of it. According to Eric Abelson from Stanford University, big brains require more fuel to keep them going, so he hypothesised that big-brained mammals probably have a higher risk of becoming endangered than small-brained ones since it's harder find enough fuel and food to support them. 

Abelson and his team tested this hypothesis by examining skull and body sizes of over 1,650 different animals from 160 species in the Americas. He then cross-referenced this brain size data with the International Union for Conservation of Nature's (IUCN) 'Red List', which details rates of extinction for certain species.

They found that there was a very clear connection to animals on the Red List and those with larger brains. This connection was even stronger in North American mammals that had large brains compared to their body size. The team suggests that a large dietary dependence - which can make finding food a lot harder - is partly to blame for a high risk of extinction.

"For the past 40 million years, carnivore species with larger relative brain sizes were less likely to become extinct, but in mammalian species alive today, we find the opposite trend," Abelson told Discovery News.

"There is no cognitive solution for the problems faced by non-human mammals living in a forest that is being bulldozed over, or for those that are in a highly polluted stream," he added. "A larger brain can even be a liability in some cases."

The study comes at a time when most people are still talking about how the world is currently in the throes of its sixth mass extinction. While there are many factors besides brain size that contribute to a particular species' chances of survival, the researchers hope that their results will help us to pinpoint at-risk populations and take steps to conserve them before it's too late. 

The study has been published in Proceedings of the Royal Society B