Zoologger: The only males with more brain than females

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Species: an isolated population of Gasterosteus aculeatus
Habitat: Lake Mývatn, Iceland

In one of philosophy's greatest facepalm moments, the normally quite intelligent Arthur Schopenhauer wrote that "women are defective in the powers of reasoning and deliberation". If you find it hard to believe that a well-educated and original thinker could hold such a view, his essay Of Women leaves no doubt about it. Oddly enough, he never married.

However, Schopenhauer might have had a point, if only he had been a three-spined stickleback living in Lake Mývatn in Iceland. In this one population, the males have brains much larger than those of the females. They are the only species known where there is such a big disparity between the two sexes' brains.

What's surprising is that there aren't more animals like this. Species differ enormously in brain size, after all, and males and females often have different lifestyles that make different demands on their brains. Why do these few fish buck the trend?

Sticklers

Most three-spined sticklebacks live in the sea and only visit fresh water to breed, but others – like the Mývatn population – spend all their lives in fresh water. Behavioural scientists have studied them for decades because of their elaborate mating rituals.

At the start of the breeding season, the males' skin turns a bright orange-red, and their eyes go blue-green. Each male defends a patch of territory, where he builds a nest from debris like pebbles and vegetation. The males glue their building materials together with stuff called spiggin, which they make in their kidneys.

Once the nest is completed, the male installs himself in front of it and performs a zigzag dance to attract a female. When one approaches, the male leads her to the nest, and she takes a close look. If the nest is good, and the male a suitably bright red, she goes inside and lays her eggs, which the male promptly fertilises.

That done, the female clears off and leaves the male in sole charge of the eggs. They tend to get fungal infections, so he minimises the risk by waving his fins to keep water moving through the nest, and if any eggs become infected he picks them out.

Size isn't everything

Wondering if the male's complex lifestyles were reflected in their brains, Alexander Kotrschal of Uppsala University in Sweden and colleagues dissected 58 males and 61 females, and weighed their brains. On average, both sexes were 4.5 centimetres long, but the males had brains weighing 24.2 milligrams, whereas the females' weighed just 19.7 mg.

Kotrschal emphasises that the size of an animal's brain isn't everything. "It's generally assumed that larger is better," he says, because having more neurons for a given body mass should allow the brain to process more information. However, there could also be unseen differences in the numbers of connections between neurons. "The connectivity is also extremely important."

He hasn't put the males and females through intelligence tests to see whether the size difference actually translates into ability. "It's hard to infer cognitive abilities just from brain size," he says.

Nevertheless, he points out that species with larger brains in proportion to body size, like humans, do in general seem to be more intelligent than those with smaller ones. So it's possible that the male sticklebacks really are smarter than their females.

Why would that be? It could be that the males have a more challenging lifestyle: they have to build nests, perform courtship dances and then care for the eggs. The females don't help with any of this – but they do have to assess the males' dancing and nest-building, which takes quite some powers of discernment.

It could also be that the females devote a lot of energy to making eggs, leaving little to run a large brain. The female's gonads can make up 40 per cent of her body mass, and so consume lots of nutrients.

There's a precedent for that sort of effect. Similar trade-offs are seen in male bats, some of which have shrunk their brains to get bigger testes. One can only imagine what Schopenhauer would have made of that.