Smarter mouse lemurs survive longer

In Kirindy Forest, Madagascar, primate researchers caught 198 wild gray mouse lemurs, and subjected them to an array of cognitive tests and personality quizzes.

The researchers from the Leibniz Institute for Primate Research in Germany wanted to know whether the lemurs’ cognitive abilities or personality might play a role in their survival. Their findings are published in Science Advances.

Body mass (the heavier the better) was already established as a predictor of lemur survival. 

The new research, conducted between 2015 and 2019, shows gray mouse lemur (Microcebus murinus) survival is also predicted by their cognitive performance and exploratory behaviours.

The researchers caught and tested lemurs, keeping them in temporary captivity over three nights to perform a range of cognitive and personality tests. 

The lemurs completed up to four intelligence tests in random order for problem solving, spatial memory, inhibitory control and causal understanding. 

They also undertook two personality tests measuring each primate’s exploratory behaviour and neophilia, or interest in new objects.

“Mouse lemurs were only tested when they voluntarily entered and explored the experimental platform”, the study explains, noting that the animals were lured with a stick covered with banana. 

The problem-solving puzzle box had six compartments, each containing small pieces of banana. Mouse lemurs were timed on how long it took them to slide each of the six lids open and extract the rewards. 

A spatial-memory maze extended in four directions, with only one of the maze ends containing banana. To make things challenging, the researchers placed banana peel on top of all four ends, meaning the lemurs couldn’t rely on their sense of smell.

To test inhibitory control, a food reward (again, banana) was placed in the centre of a clear cylinder. To reach the reward, the animal had to ‘detour’, approaching the food prize indirectly via one of the cylinder ends.

Finally, a string pulling task tested causal understanding. Lemurs had to pull the cable tie to release a banana snack.

Personality tests included observing the mouse lemur exploring in an ‘open field’ up to 80cm wide. The other, examining the mouse lemur’s level of interest in novel objects, provided in the form of a plastic Snoopy or toy car.

Cognitive ability was scored as an average for each animal, provided it had completed at least three of the four tasks.

“Because all six tests could not be performed in a single session, individuals had to be recaptured, resulting in a total of 194 individuals who performed both personality tests, of which 130 performed four, 17 individuals three, 32 individuals two, and 15 individuals one cognitive test,” the paper explains.

Researchers also weighed and recorded each mouse lemur’s sex and age. Age was able to be determined as the mouse lemur population had been regularly monitored on a monthly basis as part of a long term study since 1995.

Mouse lemur survival was then recorded over time. 

Animals were considered dead if not recaptured within 161 days. The study makes this assumption, “because mouse lemurs exhibit ‘trap-happiness’ and enter traps regularly”. 

The findings – that lemur survival is also linked to cognitive performance and exploratory behaviour – are consistent with studies in humans showing cognitive function and body mass index are predictors of mortality risk, the paper says.

Of the four cognitive abilities, problem-solving and inhibitory control were most likely to predict survival.