How Is Being Brainy Correlated To Longevity In Parrots?

Parrots apparently evolved larger brain sizes so they could solve problems that might otherwise have killed them, and their braininess ended up promoting their long-term survival

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Why are parrots so long-lived? It’s an obvious question that is not well understood by the scientific community despite the fact that parrots have lived in close association with people for thousands of years.

We do know that larger animals generally live longer than smaller ones, but other factors, such as diet, latitude and sociality also affect longevity. But even the largest parrots are much smaller than people, yet macaws and cockatoos live as long as most people. According to new research, it turns out that longevity and brain size are closely linked: brainy birds tend to evolve long lifespans. But why?

There are three main hypotheses that may explain this relationship between a large brain and a long life. They are (Figure 1):

1. the Cognitive Buffer Hypothesis proposes that the greater intelligence that is typically associated with a larger brain serves to reduce the likelihood of premature death (ref). Thus, brainy individuals were more likely to survive to adulthood and to breed, thereby passing on their genes for a large brain.
2. the Expensive Brain Hypothesis suggests that an increased lifespan is an indirect result of a prolonged juvenile stage, allowing for a greater investment in building expensive brain tissues, accompanied by increased parental investment in offspring (ref).
3. the Delayed Benefits Hypothesis argues that a higher quality, skill-based diet lowers adult mortality rates and thus supports a longer juvenile period that facilitates learning from adults how to obtain a high quality diet. This extended development period allows for greater investment in brain growth and this further supports and promotes learning (ref).

These hypotheses are complicated and are not mutually exclusive, so more than one could be true for parrots. To test these hypotheses, an international team of researchers, led by behavioral ecologist, Simeon Smeele, a PhD student studying vocal and social complexity in parrots at The Max Planck Institutes, used statistical models to examine zoo records for 133,818 individual parrots of 244 species.

As expected, their study found a strong and positive correlation between relative brain size and life expectancy in parrots. Mr Smeele and his collaborators then statistically tested the data to determine which of the three hypotheses may show the strongest influence over brain size.

“Our results best supported a direct relationship between larger brains and longer life expectancy, as predicted under the Cognitive Buffer Hypothesis. It should be noted that this result is also consistent with the Delayed Benefits Hypothesis”, the authors concluded in their paper (ref).

Was this a surprise?

“I would have thought that the expensive brain hypothesis played a bigger role”, Mr Smeele replied in email. “In other words, I would have thought that it was because parrots have to raise one or two chicks (because of the large brain) that they lived longer. But it seems parrots are more like other birds than like primates in this respect.”

Could parrots’ long juvenile period, or “childhood”, also influence longevity and brain size? Possibly.

“I believe the big missing link here is sociality and late development”, Mr Smeele explained in email. “We hope to publish a second paper where we look at the influence of social learning (especially during the period between fledging and first reproduction). Some species have really long juvenile periods, and it would be interesting to know if this links to social learning and increased longevity.”

The Delayed Benefits Hypothesis proposes that diet could enhance longevity.

“As far as we could detect, diet did not have much of an influence on life expectancy”, Mr Smeele replied in email.

Does occasional meat-eating, as seen in kea, Nestor notabilis, play a role in brain size? Or perhaps feeding on sugar-rich nectar, as seen in many of the lories (Loriinae), may play a role in supporting an energetically demanding brain?

“We did not test the effect on brain size directly, so it could well be that it has an effect on brain size”, Mr Smeele ruminated in email. “We would then expect high protein content to increase brain size, but this is especially important for the diets parents feed chicks. This is of course speculative, since we didn’t have any data on chick diet.”

Brain size is often used as a crude indicator of cognitive capacity, but brain organization also plays a large role. For example, previous research has established that parrots and corvids have almost twice as many densely packed neurons per square inch as do mammals with similar brain sizes, and further, these birds’ neuron numbers essentially resemble those of higher primates (ref). So their larger and more densely-packed brains appear to allow parrots to demonstrate more cognitive complexity.

“Yes, we believe it allows a more flexible response”, Mr Smeele explained in email. “So if an individual encounters a problem, it can innovate a solution, which can then potentially spread via social learning. This could be especially important if an important food source runs very low due to drought or something similar.”

Because I know my readers want to know the answer to this question, I must ask you: according to your findings, which parrot species is the brainiest?

“Anodorhynchus hyacinthinus [hyacinthine macaw] was the species with the largest absolute brain [size]”, Mr Smeele replied in email. This is not surprising because hyacinthine macaws are the largest living parrot species, so of course their brain should be the largest amongst the parrots. (Keep in mind that the flightless kākāpō is the heaviest living parrot species.)

“Relative to body mass it was Probosciger aterrimus [palm cockatoo] with the largest brain”, Mr Smeele added in email.

So basically, the relationship between a big brain and longevity is an investment strategy. But the big question remains: why is investing in a big brain worth the cost?

Source:

Simeon Q. Smeele, Dalia A. Conde, Annette Baudisch, Simon Bruslund, Andrew Iwaniuk, Johanna Staerk, Timothy F. Wright, Anna M. Young, Mary Brooke McElreath and Lucy Aplin (2022). Coevolution of relative brain size and life expectancy in parrots, Proceedings of the Royal Society B | doi:10.1098/rspb.2021.2397

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