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Why did some species, such as humans and dol­phins, evolve large brains rel­a­tive to the size of their bod­ies? Why did oth­ers, such as blue whales and hip­pos, evolve to have brains that, com­pared to their bod­ies, are rel­a­tively puny?

It has long been thought that species with brains that are large rel­a­tive to their body are more intel­li­gent. Despite decades of research, the idea that rel­a­tive brain size pre­dicts cog­ni­tive abil­i­ties remains highly con­tro­ver­sial, because there is still lit­tle exper­i­men­tal evi­dence to sup­port it. How­ever, a paper pub­lished online before print on 25 Jan­u­ary in the jour­nal Pro­ceed­ings of the National Acad­emy of Sci­ences describes a mas­sive exper­i­ment that sup­ports the theory.

Sarah Benson-​Amram, an assis­tant pro­fes­sor in the Depart­ment of Zool­ogy and Phys­i­ol­ogy at the Uni­ver­sity of Wyoming, is the lead author on this new paper, titled “Brain size pre­dicts problem-​solving abil­ity in mam­malian car­ni­vores.” It shows that car­ni­vore species with larger brains, rel­a­tive to their body size, are bet­ter at solv­ing a novel problem-​solving task.

The study
The authors trav­elled around the Con­tigu­ous United States to nine dif­fer­ent zoos and pre­sented 140 ani­mals from 39 dif­fer­ent mam­malian car­ni­vore species with a novel problem-​solving task. The study included polar bears, Arc­tic foxes, tigers, river otters, wolves, spot­ted hye­nas and some rare, exotic species such as bin­tur­ongs, snow leop­ards and wolver­ines. Each ani­mal was given 30 min­utes to try to extract food from a closed metal box. To access the food, an ani­mal had to slide a bolt latch, which would allow a door to open. The box was baited with the study animal’s favourite food, so red pan­das received bam­boo and snow leop­ards got steak.

The results
The main result is that species with larger brains rel­a­tive to their body size were more suc­cess­ful than species with rel­a­tively smaller brains.

“This study offers a rare look at prob­lem solv­ing in car­ni­vores, and the results pro­vide impor­tant sup­port for the claim that brain size reflects an animal’s problem-​solving abil­i­ties — and enhance our under­stand­ing of why larger brains evolved in some species,” Benson-​Amram says.

A video sum­mary of the paper with footage of some of the zoo ani­mals included in the study try­ing to extract food from a closed metal box:

Inter­est­ingly, larger ani­mals were less suc­cess­ful over­all than smaller-​bodied ani­mals. The paper also reports that man­ual dex­ter­ity did not affect problem-​solving success.

What about the social brain hypoth­e­sis?
In addi­tion to exam­in­ing the influ­ence of brain size on problem-​solving abil­i­ties, the authors also asked whether species that live in larger aver­age group sizes are more suc­cess­ful prob­lem solvers.

Kay E Holekamp, another co-​author, explains, “A hypoth­e­sis that has gar­nered much sup­port in pri­mate stud­ies is ‘the social brain hypoth­e­sis,’ which pro­poses that larger brains evolved to deal with chal­lenges in the social domain. This hypoth­e­sis posits that intel­li­gence evolved to enable ani­mals to antic­i­pate, respond to and, per­haps, even manip­u­late the actions of oth­ers in their social groups. If the social brain hypoth­e­sis is cor­rect, then we would expect that species that live in larger social groups would be more intel­li­gent. How­ever, we did not find any sup­port for the social brain hypoth­e­sis in this study. There was no indi­ca­tion that social group size influ­enced problem-​solving abilities.”

(Source: Uni­ver­sity of Wyoming news release, 25.01.2016)