Coevolution of relative brain size and life expectancy in parrots (2022) Smeele et al., Proceedings of the Royal Society Biological Sciences, https://doi.org/10.1098/rspb.2021.2397
Figuring out what exactly drives a species’ lifespan has proved more of a puzzle than it might at first seem. Sure, we can look at a single species and provide a few reasons for why it might live as long as it does, but finding predictable patterns relating different factors to life expectancy (let’s say longevity from here) is a little complex.
Take brain capacity for instance. There are three mostlymain theories (which are all somewhat linked) as to how brain capacity affects longevity. The cognitive buffer hypothesis suggests that the ability to solve puzzles granted by a larger brain enables a species to survive situations that other species couldn’t, giving them a longer lifespan. The expensive brain hypothesis suggests that a brain takes up more energy, therefore slowing growth and extending longevity. And the delayed benefits hypothesis suggests that a larger brain capacity allows for more skilled food-finding techniques, resulting in higher diet quality, less adult deaths, and most importantly, the ability for a longer learning period from their parents, resulting in more skill transfer.
Parrots are very smart creatures, almost on the same level as primates when it comes to relative brain size. Today’s authors wanted to test for links between brain capacity and longevity in parrots, and see if their findings lined up with any of the three hypotheses.
What They Did
The team drew their longevity data from Species 360, an organisation which collects information from conservation bodies worldwide. They used life expectancy as their measure of longevity, and compared it to relative brain size, as well as other features like body mass, latitudinal range and diet, which have been shown to affect longevity before.
The authors also tested a few other models which included measurements of developmental time and parental investment to see if either of these had an impact. Either being important could shed light on whether or not the expensive brain or delayed benefits hypothesis play a part in development.
Did you Know: Parrots As Invaders
Their bright colours and intelligence make parrots an inherently charismatic species, one we often sympathise with when we hear of their threatened status and degraded ecosystems. But some species of parrot are biting back, with rose-ringed parakeets (pictured below) now a damaging invasive species in much of Europe. A warming climate and rising numbers will likely only see their range expand.
What They Found
As suspected, larger parrot species tended to have longer lives. But larger relative brains also led to longer lives, though it wasn’t as large a contributor as body size was. The other parameters, including those related to diet, developmental time and parental investment, didn’t have a meaningful effect on parrot longevity in these models.
One added tidbit – the Cacatua, a genus which includes the sulphur-crested cockatoo (pictured above) were the longest lived birds, with the Large Fig Parrot of South East Asia coming in last, with a life expectancy of under two years.
Testing hypotheses in science is made easier by the fact that often they’re mutually exclusive, and concluding that research supports one hypothesis is often a direct result of rejecting another. Yet the researchers today were testing three hypotheses that were certainly not mutually exclusive, which really muddies the waters, and makes teasing the effects apart a little difficult.
The fact that diet and developmental factors had no effect here is interesting, as at least the delayed benefits hypothesis suggests that better diet may lead to longer lives. The expensive brain hypothesis also suggests that increased brain capacity contributes to a longer life by extending development time, so it’s odd that development time had no effect on longevity.
Ultimately the research here doesn’t disprove any of the theories, and perhaps shows most proof for the cognitive buffer hypothesis, suggesting that increased problem-solving abilities can contribute to longer lifespans. Since longer-lived species are often more likely to be threatened, their increased intelligence could be used as a conservation tool, seeing as we humans are often more enamoured with more intelligent species.
Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology who loves parrots almost enough to wish they would stop messing about and just invade Norway. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.