Make sure you have access to food and avoid being snatched by a predator – these are two ingredients for living a long life in the animal kingdom. Having a big brain might also help, because big-brained animals tend to be smarter and are able to solve life-threatening problems that might otherwise cause an early death. It is therefore logical to assume that, over generations of being able to outsmart external causes of mortality such as predation, animals with bigger brains will evolve to age more slowly.

However, there is a contradictory theory lurking behind this seemingly logical relationship. Brain tissue is the most energetically expensive tissue in the body. Growing a big brain means that animals have to divert energy and resources away from other bodily processes such as growth, reproduction and general tissue repair and maintenance. So, according to this theory, it would be logical to assume that animals with bigger brains should actually age faster, as they are unable to maintain and repair their bodies over time.

To disentangle these competing ideas, Alexander Kotrschal, Alberto Corral-Lopez and Niclas Kolm, from Stockholm University, Sweden, studied ageing in a unique line of artificially selected Trinidadian guppies. These fish were selected over four generations to be either large- or small-brained, resulting in a whopping 12% difference in relative brain size by the fourth generation. The researchers raised the guppies from birth in a safe environment where they always had access to food and there was no threat from predation. This gentle rearing environment therefore removed any external causes of mortality. The guppies were even kept in individual tanks to avoid any competition or aggression among the fish, but they could still see each other so they would not get lonely.

The team monitored the guppies over their entire lifespans to reveal that the large-brained fish had a 22% shorter lifespan than their small-brained counterparts. Large-brained fish lived for an average of 2.9 years, while small-brained fish lived for 3.5 years. This study certainly took up some time for the researchers, because the longest-lived small-brained fish was over 5 years old when it died. Their findings support the idea that larger brains are costly and there is a trade-off with ageing. The big-brained guppies in this study were likely re-allocating their bodily resources to brain growth. There is already some support for this notion, because the team had previously found that these large-brained guppies also have smaller digestive tracts and poorer immune systems.

Kortrschal's experiment is one of the first to experimentally show how ageing is affected by brain size. While big brains appear to be quite bad for longevity (a cost of 0.6 years – almost a quarter of their lifespan – for large-brained guppies!), the team also highlights that their findings are restricted to a benign and protected environment. More work is now needed to understand how brain size and ageing interact with external causes of mortality, such as predation or starvation, which can also affect animal survival in the wild.

Kotrschal
,
A.
,
Corral-Lopez
,
A.
and
Kolm
,
N.
(
2019
).
Large brains, short life: selection on brain size impacts intrinsic lifespan
.
Biol. Lett.
15
,
e20190137
.