“Do Bumble Bees Play?”, 2022-10-19 ():
Ball rolling by bumble bees fulfils animal play criteria.
Ball rolling can act as an unconditioned rewarding stimulus.
Younger bees rolled more balls, with age patterns resembling mammalian juvenile play.
Males rolled balls for longer durations than females.
A variety of animals have been found to interact with and manipulate inanimate objects ‘just for fun’, that is, to play. Most clear examples of object play come from mammals and birds. However, whether insects interact with inanimate objects as a form of play has never been systematically examined.
Here, we show that rolling of wooden balls by bumble bees, Bombus terrestris, fulfils behavioral criteria for animal play and is akin to play in other animals. We found that ball rolling (1) did not contribute to immediate survival strategies, (2) was intrinsically rewarding, (3) differed from functional behavior in form, (4) was repeated but not stereotyped, and (5) was initiated under stress-free conditions.
Through the design of the experiment and with the support of behavioral observations, we excluded the possibilities that ball rolling was driven by exploration for food, clutter clearing or mating. Similar to vertebrate play, we also found age and sex differences for ball rolling by bumble bees: younger bees rolled more balls than older bees and male bees rolled individual balls for longer durations than females.
We explicitly show that ball rolling is itself a rewarding activity. After being trained to find freely movable balls in one of two differently colored chambers, bees showed a preference for the color of the chamber where they had rolled balls.
…A total of 910 ball-rolling actions by 45 bumble bees were recorded. Individual bees rolled balls 1–44× on an experimental day, and 1–117× across the whole duration of the experiment. Most bees (37⁄45) rolled balls for at least an additional day after feeding in the foraging area and 29 bees for at least 2 additional days after feeding.
Our results contribute to the question of sentience in insects and lend further support for the existence of positive affective states in these animals.
[Keywords: age difference, Bombus terrestris, cognition, conditioned place preference, invertebrate, object manipulation, play behavior, sex difference]
Video S1: An example of ball rolling by a bumble bee at 0.5× speed. The bee approaches a wooden colored ball while facing it, touches the ball with her forelegs, holds onto the ball using all of her legs, rolls the ball, detaches from and leaves the ball. The bee approaches a second ball, rolls it and detaches. (
1-s2.0-S0003347222002366-mmc1.mp4)…In a previous study it was found that bees can be trained to roll balls to gain access to a reward ( et al 2017). During the execution of this experiment, the team observed that bumble bees would often roll balls for no apparent benefit. Between experiments, balls were placed in the tunnel that connected the hive to the arena that contained food. Despite there being enough space to avoid the balls, bumble bees often seemingly unnecessarily walked over and rolled balls on their way to and from food. This observation provided the motivation for the current study, where we examined whether ball rolling fulfils the major criteria for animal play and how this behavior relates to similar object play behavior in other animals.
…The seemingly function-less ball-rolling activity is analogous to well-studied cases of solitary object play in mammals such as stone handling in macaques (Macaca spp.; et al 2016; et al 2017) and ‘rock juggling’ in various species of otters ( et al 2020; 2021). In both examples, stones are held and repeatedly manipulated and do not result in individuals gaining any immediate material reward similar to what we observed when bumble bees rolled balls.