Lazy bees and clean hyenas: Review finds that animals vary in trait conscientiousness

Conscientiousness is a fundamental aspect of human personality, with higher levels associated with all kinds of benefits, from greater academic achievement and relationship stability to living for longer.  Yet it's the only major human personality dimension not to have been widely identified in animals, which poses an evolutionary puzzle – if animals don't show signs of conscientiousness, where did the human variety come from? But now a major review of hundreds of relevant papers, published in Psychological Bulletin, concludes that in fact, "there are many documented examples of conscientiousness behaviour in other animals". The work also suggests that there are two main branches to conscientiousness, each associated with an evolutionary drive to solve different types of problems.

Before this new research, the only clear evidence for animal conscientiousness was a study of chimpanzees and a report of aspects of conscientiousness in a group of captive orangutans. Based on this limited data, some researchers suggested that conscientiousness evolved after the last common ancestor of chimps and humans diverged from other ape species, 7 to 10 million years ago.

For their new review, Mikel Delgado and Frank Sulloway at the University of California, Berkeley, looked for indirect evidence of animal conscientiousness, searching nearly 3,000 potentially relevant papers for any of 103 trait terms related to conscientiousness in descriptions of animal behaviour consistent with how it's characterised in people (there are six facets to human conscientiousness including orderliness, dutifulness, competence and achievement striving).

The researchers identified 876 papers that provided "strong" or "possible" evidence of animal conscientiousness. For example, in searching on the trait term "achievement", they identified research showing that cows' heart rates are higher when they're learning. In searching on "lazy", they found reports of trainers' descriptions of some horses, and also work on variations in the willingness of bees to protect a hive. And in a search on "appearance" (which falls under the facet of "order"), they found a study that rated hyenas as appearing "attractive", "clean" or "scruffy".

From this, the researchers identified two categories of conscientious behaviour in animals: the first includes the traits of achievement striving and competence; the second includes order, industriousness and responsibility.

Delgado and Sulloway found that primates and other mammals are much more similar to each other than they are to other animal groups, particularly in scoring positively on the first conscientiousness category. In contrast, birds, insects and fish stood out for having positive scores on the second. Amphibians/reptiles and other invertebrates had neutral or low scores on both.

"Notwithstanding some inconsistencies, the results…closely parallel the known phylogenetic relationship of the taxa involved," they note. The noticeable exception was the placement of insects, which might have been expected to lie closer to other invertebrates than to birds and fish; the reason for this anomaly could be the relatively high number of studies of social insects, which have evolved caste systems and worker instincts.

The two categories of conscientious traits may have evolved separately to solve at least two different sets of problems, the researchers added.

The order/industriousness/responsibility category seems to relate to the building of nests or other protective structures, and colony life, and there are hints that at least some of these traits bring clear benefits – blackbirds with neat and tidy nests are better at rejecting foreign eggs, for example, and bees that remove more dead bodies from their hives gain more weight and rear more young.

The other category, focused on achievement striving and competence, relates more to intelligent behaviour generally and is important for an animal's ability to cope with the demands of living in a group. So it's possible that an increasingly complex social environment drove the development of this set of traits, Delgado and Sulloway write.

"We should be somewhat cautious in accepting these results as an actual phylogenetic model for the evolution of conscientiousness," they add. "Nevertheless, the remarkably close parallel between the known phylogenetic record…and the results of our own study…suggest that the relationship is not coincidental and that the evolution of conscientiousness has closely tracked the increasing complexity of biological evolution more generally."

Not all conscientiousness-related traits that have been identified in people could be picked up in animals. Thanks to our advanced ability to work out what others are thinking, and to use language, we seem to be unique in feeling and demonstrating "dutifulness", "conventionality", "traditionalism" and "virtue". In fact, because research on conscientiousness in people has so often focused on emotions, intentions and morality, rather than behaviours, this may explain why it hasn't been picked up in a variety of animals before, Delgado and Sulloway suggested.

One important limitation of the new review, acknowledged by the authors, is that human biases may affect the labels that researchers give to animal behaviours. Are some insects really "orderly", for example, rather than "competent" or "self-controlled"?

But the pair felt able to conclude, "We have found ample evidence that building blocks for the personality dimension of conscientiousness exist across species, from insects to primates, even if this domain of behaviour sometimes expresses itself differently in other animals than it does in humans."