„Temperature-induced aggregation of young honeybees:
Individual behaviour vs. collective behaviour“
Collective behaviour is a term that describes the behaviour of a meta-organism like a swarm similar to the behaviour of an individual. It is determined by the behaviour of interacting individuals, but yields capabilities that none of the individuals can achieve on its own. This emergent phenomenon is known for many species of social organisms with a wide spectrum of organizational complexity, from simple slime mould to sophisticated animals like mammals or birds. One of the best studied species that exhibit swarm intelligent behaviour are honeybees.
Honeybees (Apis mellifera ssp.) are eusocial animals that live in colonies of up to some 10,000 individuals. The population consists of a queen and a small number of males, which together constitute the fertile stock of the colony and a large number of sterile workers. The latter exhibit a pronounced division of labour among age dependent casts (age polyethism). The homeostasis of the hive and, as a consequence, the survival of the colony is solely dependent upon these worker casts.
The honeybees‘ aggregation behaviour is a prominent example for collective behaviour. They form aggregations (clusters) at different occasions. A reduced set of worker bees survive the winters in cold and temperate climates by forming a tightly packed winter cluster in the hive which allows them to conserve body temperature and to protect their queen, which remains encapsulated at the center of the cluster. Reproductive swarms, which consist of a queen and a large portion of a spawning colony’s worker bees, repeatedly form large clusters while searching for a new nesting site. Aggregations regularly occur in the hive, especially among freshly emerged worker bees in the brood nest. Honeybees are able to precisely perceive temperature with the help of their antennae and they have an age dependent preference for specific temperatures. The young bees in the brood nest have a preference for approximately 36 °C (96.8 °F). This preference has been determined in the 1950s by Herbert Heran, who exposed honeybees of different casts to a simple and steep linear thermal gradient and evaluated the positions they preferred. In these experiments, the bees quickly moved to the position of their preferred temperature and stayed there. Five decades after Heran’s experiments, we oberved honeybees in a more complex, two-dimensional thermal gradient and found that, in contrast to Heran’s report, only larger groups of bees, but not single bees or small groups, were consistently able to find an area of preferred temperature. This stunning observation lead us to initiate this project, in which we determined the individual behavioural background of the bees‘ swarm-intelligent temperature-induced aggregation behaviour, which in turn allowed us to develop algorithms to facilitate swarm intelligent behaviour in groups of robots.
Brood nest of a honeybee observation hive. In the center of the brood nest comb are the cells with open (Bo) and capped (Bv) brood. The brood is surrounded by pollen (Po) and honey (Ho). The bees regulate the temparature in the center of the brood nest in a very narrow range from approx. 34 °C to 36 °C (93,2 °F to 96.8 °F). This stable thermal environment is crucial for the proper development of the larvae and freshly emerged bees. Picture © Robert Brodschneider