From birds and bees to frogs, wolves and ants, it’s an enduring source of wonder that animals can count. This is not just a fancy trick – its importance for survival has been highlighted by a review in the journal Trends in Ecology and Evolution.
They may not be able to perform calculus, but animals’ adaptive ability to grasp quantities – “numerical competence” – gives them a survival benefit for crucial life skills including hunting, finding a mate, reproducing, and even getting home.
The pervasiveness of this phenomenon – often discovered accidentally as a by-product of other research – is what prompted Andreas Nieder, from the University of Tübingen, Germany, to turn the spotlight on its evolutionary purpose.
“Different groups of animals obviously developed this trait independently from other lineages and that strongly indicates that it has to be of adaptive value,” he says.
For honeybees (Apis mellifera), industrious critters that have shown a remarkable capacity for maths, remembering the number of landmarks they pass when searching for food helps them find their way back to the hive.
“The last common ancestor between honeybees and us primates lived about 600 million years ago,” says Nieder. “But still, they evolved numerical competence that, in many respects, is comparable to vertebrae numerical competence.”
Numerical competence, not necessarily precise, is defined as “the ability to estimate and process the number of objects and events”.
When frogs (Bombina orientalis) were given a choice between two meals, they couldn’t distinguish between those that had three or four larvae but would reliably choose the meal with six over three morsels.
This ability to choose the greater quantity of food is seen across the animal tree, from tortoises, dolphins and salamanders to monkeys, spiders, birds and molluscs.
Fish also display tendencies to assess quantity; for instance, a three-spine stickleback (Gasterosteus aculeatus) can assess how many of its fellow species are travelling in different directions and choose the biggest group for the most reliable decision on the best way to go.
To seek shelter from predators, mosquitofish (Gambusia holbrooki) can discriminate between different sized shoals to join the larger one.
Elks (Cervus elaphus) use a similar, but more refined, strategy to avoid wolves (Canis lupus) either by gathering in small herds that wolves rarely encounter, or in large herds to minimise the risk of becoming a victim if a wolf is on the prowl.
Wolves, for their part, hunt more successfully depending on how many are in their pack relative to the size of the prey – for elks and moose they only need about six to eight but for bison they need more like nine to 13 co-prowlers.
Animals can communicate in quantities as well.
Black-capped chickadees (Poecile atricapillus), for instance, increase the number of “dees” in their “chick-a-dee” warning call to communicate greater danger from a predator – two “dees” are used for a relatively harmless great grey owl while up to five are emitted to warn of a more nimble small pygmy owl.
Frogs communicate with their calls to attract females. A horny male tungara frog (Physalaemus pustulosus) will call out with a “whine” then one or more “chucks”. Competitors will add from one to six more chucks to outdo them, as females go for the more complex calls.
Even mealworm beetles (Tenebrio molitor) have intense competition – these promiscuous males can sniff out how many females are around and go for the largest number to boost their chances. Then the time they spend keeping other males away depends on how many rivals they had come across.
Nieder put these and many more examples of numerical competence into a comprehensive framework, arguing for a more direct analysis and greater understanding of its adaptive value.