Big brains or big guts: choose one

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Big brains can help an animal develop fast and flexible behavioral responses to frequent or unexpected environmental changes. But some birds simply do not need it.

A global study comparing 2,062 birds revealed that in highly variable environments, birds tend to have larger or smaller brains relative to their body size. Birds with smaller brains tend to use ecological strategies that are not accessible to big brain counterparts. Instead of relying on the gray matter to survive, these birds tend to have big bodies, to eat easily available food and to have lots of babies.

New biologist research from Washington University in St. Louis appears on August 23 in the newspaper Nature Communications.

"The fact is that there are many species that are doing rather well with small brains," said Trevor Fristoe, a former postdoctoral researcher at the University of Washington, currently at the University of Washington. University of Constance in Germany.

"What's really interesting is that we do not see any middle ground here," Fristoe said. "Resident species with intermediate brain size are almost totally absent from high-latitude environments (colder and more climatically variable) and species that do not adhere to any of the extreme strategies are forced to migrate. to more benign climates during the winter. "

"Having a big brain is usually associated with high energy demands and a slower life cycle," said Carlos Botero, assistant professor of biology of arts and sciences and co-author of the article. "Freed from these constraints, species with small brains can present traits and lifestyles never seen in larger ones.

"What we have found is that alternative ecological strategies that increase or decrease brain tissue investments are quite capable of coping with the challenges of living in high latitude environments," she said. he declared.

Because the brain is an expensive organ to develop and maintain, biologists have long sought to understand how a large brain – in all species – could have evolved.

One hypothesis is that one of the main advantages of having a big brain is that it allows a high degree of behavioral flexibility. Flexibility comes with the ability to respond to different conditions, such as large temperature variations or variations in food availability.

The so-called cognitive tampon hypothesis is not the only possible explanation for the evolution of brain size, but it is important and influential.

The relative size of the brain is a measure of the size of the brain relative to that of the body. Imagine: the brain of an ostrich is perhaps much bigger than the brain of a tit, but the ostrich's body is also. Predictably, the global distribution of the relative size of the brain of birds follows a bell curve, with most species posing squarely in the middle, and only a handful of aberrants having a relatively large or relatively small brain.

Previous studies had revealed general trends toward larger relative brain sizes at higher latitudes, where conditions are more variable and consistent with the cognitive buffer hypothesis. The new study by Fristoe and Botero is different because it examines the complete distribution of brain sizes in environments, allowing them to check whether different sizes are over or under-represented.

Excluding migrants 'contributions – birds that live in polar or temperate environments only during more favorable times of the year – the researchers found that, at high latitudes, the size of the birds' brains seemed to be bimodal. This morphological pattern means that the brains of birds are much more likely to be relatively big or relatively small compared to the size of their bodies.

What was happening here? Fristoe, born in Alaska, had some ideas.

In fact, Fristoe suggests that the Alaskan state bird, the ptarmigan, could be a good poster for small-brain species. So endearing as she is – with her stuffed chest, feathered feet and unusual scream, she's not known for her intelligence. The ptarmigan can however feed on the best twigs and willow leaves.

"In our article, we find that reduced-brain species in these environments use strategies impossible to achieve with a large brain," said Fristoe. "First, these species can persist by feeding on readily available but hard-to-digest resources, such as dormant buds, conifer needles, or even twigs.

"These foods can be found even in the winter, but they are fibrous and require a large intestine to be digested," he said. "The intestinal tissue, like the brain tissue, is energetically demanding, and the limited budgets mean that it is difficult to maintain a lot of both.

"We have also found that these species have high reproductive rates and produce many small ones each year," said Fristoe. "This would allow their populations to recover from high mortality under particularly difficult conditions.Because tender-brain species tend to invest more time in raising fewer offspring, this strategy is not accessible to them."

In other words, maybe the big brains are not all that.

"Brains do not evolve in isolation, they are part of a broader series of adaptations that help organizations succeed in their lives," Botero said. "Because of the trade-offs between different aspects of this total phenotype, we find that two different lineages can respond to selection from environmental oscillations in a completely different way.

"Given that our own species uses its brain to cope with these changes, it is not really surprising that biologists, including ourselves, have always shown a tendency to think of environmental variability as driving force of the expansion of brain size, "Botero said. . "But the interesting thing we find here is that when we take a broader view, we realize that other strategies also work – and remarkably, the alternative here is to shrink the size of the brain!"