Some birds are gifted with shockingly long lives. In captivity, crows can live into their 50s. Certain parrot species can live nearly a century. Curiously, these birds also have proportionately massive brains, which power considerable (and sometimes unnerving) intelligence. According to new research, it turns out that this longevity and brain size are closely linked: long life tends to evolve in brainy birds.
The brain is an expensive organ to keep around. Big brains suck even more energy. Our own noodles represent just 2% of our body mass, but consume roughly 20% of our calories. So evolutionary biologists have long tried to understand what makes a big brain worth such a heavy investment. One hypothesis is that big, powerful brains grant flexibility in how an animal responds to challenges, leading to an enhanced chance of survival that makes the evolution of long life more likely.
“If you don’t get eaten by predators, if you don’t starve, then [evolutionary processes] can start acting on other mechanisms that allow you to have a longer life,” said Alejandro Gonzalez-Voyer, an evolutionary biologist at the National Autonomous University of Mexico, in Mexico City.
A “cognitive buffer” from a big brain and complex behaviours could help explain why big-brained animals also tend to take longer to mature. In birds, the relationship between big brains and longevity had been noticed before by scientists, but it wasn’t clear if brain size influenced lifespan or if it was the other way around.
To try and tease this relationship apart, Gonzalez-Voyer and his colleagues compared the lifespans and brain sizes of more than 300 bird species. They included a wide range of bird families, with varied early developmental traits, like how developed the young are immediately after hatching and the age when the chicks can first fly. The team then compared brain and lifespan data using models that took into account evolutionary relationships and developmental traits.
Gonzalez-Voyer and his team found a direct link between a bird’s brain size relative to its body and its lifespan, across different developmental states upon hatching. It’s been said that the light that burns twice as bright burns half as long. But for birds, the mind that’s twice as bright lasts twice as long.
Previous studies couldn’t distinguish between direct and indirect influences of early life history traits on lifespan. But Gonzalez-Voyer’s team’s findings — published recently in Evolution — help solidify that bigger brains are behind a lifespan extension, and differences in early development just tweaks how that longer life plays out, whether it’s more time spent in the egg, or taking a longer time to make a first flight.
“[The findings] confirm that brain size can actually have a direct effect on longevity,” said Gonzalez-Voyer, noting that this could be a more broadly applicable explanation for why big brains would be favoured by evolutionary processes.
A long life could make a big, energy-draining brain worth the investment. A slow development to maturity spreads out the brain’s immense caloric cost over time, permitting a lifetime of learning innovative survival tricks, thus maximizing the brain’s benefits.
But a slower, smarter life may have its own drawbacks. Gonzalez-Voyer collaborated on research published last year showing that having a bigger brain plays a key role in how successful some mammals are in invading city environments. If these brainy beasts are cranking down their rate of development and reproduction as well, achieving longer life, they may be more vulnerable to abrupt, extreme changes that kill off a large proportion of the population.
A slow life means that “it takes longer for you to reach the population size that you had before, so it makes those species more vulnerable to extinction,” said Gonzalez-Voyer.
Going forward, Gonzalez-Voyer wants to examine how this brain-lifespan relationship plays out in different environments. The benefits provided by a big brain may be very different in an environment with little food or water resources, for example. There, being clever may not be worth the price.
“I would imagine in deserts or [environments] where the availability of food is quite limited means that there’s not much you can do to go beyond what that habitat offers,” explains Gonzalez-Voyer.
Evolution, after all, is a merciless accountant, and though it may giveth brain gains, in different conditions it may also taketh away.