Our big brains have long been hailed as our greatest asset. We are smart, have conquered almost every corner on Earth, manipulated our habitat to suit us, and are developing increasingly-innovative technology to make our lives easier.

Without our brains, none of these things would have been possible.

Though they are costly organs – they require a great deal of energy to power – humans are living proof that the cost can pay off, biologically speaking. We have managed to find ways to overcome the added burden it has taken to evolve ever-bigger brains. 

And yet big brains are not all they are cracked up to be. In some cases, animals like sponges do not even have a brain, but continue to be remarkably successful. Some researchers even believe that sponges once had brains but then got rid of them, perhaps because being brainless was beneficial for life anchored to rock on the seafloor.

Armed with that knowledge it may not be surprising to learn that several studies now show that carrying a big brain can be problematic for some mammals. It could even lead to their extinction.

Today many of Earth's largest mammals are at a heightened risk of extinction.

A study published in August 2016 found that about 60% of the biggest mammals on Earth are classified as threatened by the International Union for Conservation of Nature (IUCN).

Animals in sub-Saharan Africa and South East Asia are at most risk. These include rhinos, elephants, lions, tigers and gorillas.

Clearly you can't have an elephant brain in a mouse's body

The paper's lead author, William Ripple of the College of Forestry at Oregon State University, said in a statement: "The more I look at the trends facing the world's largest terrestrial mammals, the more concerned I am we could lose these animals just as science is discovering how important they are."

The report, compiled by 43 wildlife experts and published in the journal BioScience, stresses something that conservationists have long known: bigger animals are at an increased risk of going extinct.

There are several reasons for this. Large animals need more food and bigger habitats, and are often the target of illegal poaching. What's more, their habitats are vanishing as deforestation continues to increase, partly driven by the growth of farming to feed an increasing human population.

But there is a less well-known reason contributing to these large mammals' risk of extinction: their big brains.

In one sense this is obvious, because a bigger body requires a bigger brain.

Unfortunately, there are clear perpetrators who tipped this balance. Us

"We know that brain size is correlated with body size, [as] clearly you can't have an elephant brain in a mouse's body," says conservation biologist Manuela Gonzalez-Suarez of the University of Reading in the UK. "It's also rare that something very big will have a very small brain."

But Gonzalez-Suarez wondered if it went deeper than that. Might a mammal's brain size directly affect its risk of extinction? And if it does, why would a big brain be bad news?

In a study published in May 2016, Gonzalez-Suarez and colleagues studied 474 mammal species to understand the specific traits that increase a species' risk of extinction. 

They found that bigger brains had a clear downside, because they were associated with a raft of problematic traits. 

For example, big-brained infants need longer gestation periods and higher levels of parental care. This means big-brained animals take longer to reproduce, and tend to have fewer offspring at any one time. A typical gorilla mother provides almost constant care for her infant for its first three years of life, and only gives birth every three or four years.

Humans have become so dominant on Earth that other animals are experiencing devastating impacts

Longer gestation and weaning periods also mean the offspring are prone to die young. "These [traits] in turn increase risk of extinction, as populations cannot grow fast or quickly compensate for additional mortality," says Gonzalez-Suarez.

It is true that a big brain may have helped mammals – especially us – adapt to a changing environment in the past. However, the new study suggests that a large brain is not always good news, as you might think.

Gonzalez-Suarez says that a crucial tipping point has now been crossed, making large brains more trouble than they are worth. "The cost is too high."

There are clear perpetrators for this shift in the balance: people.

Animals cannot cope with the changes humans impose on the environment, says co-author Alejandro Gonzalez-Voyer of the National Autonomous University of Mexico.

Our population is growing at a pace that we may soon be unable to sustain

"They cannot respond fast enough, so the behavioural flexibility that a larger brain would have conferred and [which] was selected upon positively in the past, is not sufficient for the animals to cope with the rapid changes that are occurring now," he says.   

Ironically, it is partly because our own large brains have helped us change the world that other large-brained animals are now suffering.

This point is echoed by another study, published in February 2016, which looked at the correlation between brain and body size. Eric Abelson at Stanford University focused on 160 species, using museum skulls of 1,679 animals to assess brain size.

The animals most at risk tended to have larger brain-to-body-size ratios. These included the short-eared dog and the tiger cat, both of which are listed as "near threatened" by the IUCN.

On the face of it, humans are an exception to the rule: we have big brains and yet our population is expanding.

However, our population is growing at a pace that we may soon be unable to sustain. Moreover, despite our intelligence, we have caused our planet to warm at an alarming rate. 

"We might be too clever for our own good, [and yet] we're not clever enough to realise when we are destroying our planet," says Gonzalez-Suarez.

Gonzalez-Voyer agrees, but points out that many of us already realise that we are causing this damage. He says the problem is that the incentives to change our behaviour are too intangible for us to act on: we would be doing so largely for the benefit of future generations. "The problem is more cultural," he says.

Taken together, these reports suggest that the world could ultimately lose some of its most iconic, and big-brained, species.

We must not go quietly into this impoverished future

Gonzalez-Suarez hopes that her findings will help ecologists prioritise their limited funds, to help the animals considered most at risk of extinction.

That way, we can begin to counteract some of the damage we have caused.

"We must not go quietly into this impoverished future," the authors of the latest mass extinction report in BioScience say. "Rather, we believe it is our collective responsibility as scientists who study megafauna to act to prevent their decline."

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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