Bumble bee string-pulling skill spreads between colonies under open diffusion conditions

Socially transmitted behavioural traits can, if they persist in a group of animals over time, give rise to locally adapted phenotypes that can enhance survival. This capacity is widespread through the animal kingdom, and forms the foundation of cultural inheritance. While social learning is well documented among insects, and particularly in social insects such as bumble bees, the extent to which such behaviours can spread beyond initial kin groups and persist over time remains largely unknown. String pulling is a non-natural foraging behaviour where bees must manipulate a string to extract an out-of-reach artificial flower and collect a reward, and has previously been shown to spread via social learning. However, this was demonstrated only in highly controlled paired dyad settings, where interactions between bees were strictly limited. Here, we show that string pulling can spread both within and between bumble bee colonies, and persist over time, under previously untested open diffusion conditions. These are of greater ecological validity compared with classical paired dyad paradigms, and involve the seeding of a manually trained demonstrator into a group of naïve conspecifics. From this single point of origin, string-pulling behaviour spread rapidly within original, ‘primary’ colonies. Once the behaviour was established in the primary colonies, ‘secondary’ colonies were introduced, and string pulling was also acquired by these new foragers. Furthermore, string pulling was acquired through individual trial-and-error learning by a small number of bees in control colonies, which lacked trained demonstrators. These results confirm and build upon previous findings in bumble bees, and contribute to a growing body of evidence suggesting that social learning enables animals to establish local behavioural adaptations in the absence of the computational power provided by large brains.