Fri. Mar 31st, 2023
What do you think they use all those neurons for?

What do you think they use all those neurons for?

Birds are smart. They use tools, engage in social learning, plan for the future, and do all sorts of other things that were once thought to be exclusive to primates. But hundreds of millions of years of evolution separate mammals and birds, and structurally their brains look very different. In addition, there is the whole format. If you look at a bird’s head, it’s clear that there isn’t much room for mental hardware. So how do the birds do with smaller brains?

While other studies have addressed many of the structural differences, a new one was published this week PNAS shows that size doesn’t matter to some extent. The authors show that birds put neurons in their brains at densities well above the densities in mammalian brains, placing some relatively compact avian brains in the same realm as that of primates when it comes to total cell count.

And the funny thing is, we probably should have known this was the case.

If you look at a typical bird brain without knowing much about brains, you will be particularly impressed by its size (or lack thereof). Some of the heaviest brains in birds are found in the macaws, weighing less than 25 grams. The raven, a large bird with a well-earned reputation for intelligence, has a brain that typically weighs around 15 grams. That’s in the same neighborhood as a rabbit.

However, if you know your way around some neuroanatomy, other things will stand out. Many of the structures we associate with higher cognition in mammals (and especially in primates) are either not clearly present or look quite different in birds, suggesting that bird cognition must be radically different from mammalian cognition.

But once we identified the proteins that act as key regulators of mammalian brain development, we discovered that the same proteins are all present in birds as well. By following their expression as the brain develops, we’ve been able to determine that some brain structures that look physically different in birds and mammals actually have the same developmental history and express the same set of genes when they mature. Finally, manipulating the activity of these genes affects the brains of birds and mammals in similar ways.

So all the same basics seem to be present in both birds and mammals, which leaves the question of raw horsepower. Mammal brains are just so much bigger that it seems inevitable that they can get more done.

But size isn’t everything. Neural abilities seem to be based on the number of neurons present and the number of connections they can make. Can birds simply cram more neurons into the same amount of physical space and get more done with a smaller brain?

We expected the answer to be yes. It turns out that flying animals tend to shrink the size of their genomes compared to their non-flying relatives. This applies to both bats and birds. A consequence of this smaller genome is that the cells carrying these genomes also become smaller. That tendency has been used to argue that the group of dinosaurs that evolved into birds had already experienced a shrinking genome millions of years earlier.

But we might as well have applied that logic to neurons. If bird cells are smaller, more cells can be squeezed into the same volume. Under those circumstances, a small brain wouldn’t be as much of a risk as it might seem. But logic only gets you so far, so a team of researchers set out to count all the neurons in the brains of a range of birds, mainly songbirds, corvids and parrots.

Small songbirds, weighing only 4.5 grams, have really small brains. Their brains can weigh as little as a third of a gram and contain only about 100 million neurons. But the heavier birds can have brains weighing more than a dozen grams and containing more than 2 billion neurons. On average, birds have twice as many neurons per unit mass as mammals. So a bird called the goldcrest, which Wikipedia introduces as “a very small songbird,” weighs just over 10 percent of your average mouse, but has more than double the number of neurons.

The largest parrot brain, on the other hand, weighs 20 grams, even though the body weight of parrots is comparable to that of the heaviest songbirds. The parrot brain also has over 3 billion neurons. In fact, when it comes to the largest corvids and parrots, the authors write that “their total number of neurons is comparable to that of small apes or much larger ungulates.”

Those cells also have an interesting distribution: As more cells are added, they preferentially add to a brain region called the pallium, which in humans handles things like spatial reasoning, language and memory. As a result, this area has an impressive number of cells. Ravens and keas (a type of parrot that do live in the fjords) have more neurons in the pallium than a capuchin monkey. A macaw has more than a rhesus monkey.

As with size and weight, there is no simple relationship between the number of neurons in a brain and its capabilities. But the work certainly presents an argument that we shouldn’t assume that birds’ thought processes should be limited by the size of their brains. And the authors even suggest there may be some benefits; with more neurons packed closer together, signals typically don’t have to travel that far to reach their destination. Birds can therefore process information slightly faster than mammals.

PNAS2016. DOI: 10.1073/pnas.1517131113 (About DOIs).

By akfire1

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