Inside the Hive: the Science Behind our Beloved Honey Bees’ Evolutionary Behaviours

Eusociality is a social behaviour observed across the Arthropoda and Chordata phylums that are characterised by reproductive division of labour, cooperative brood care and the overlap of generations. This complex and highly networked system has evolved over temporal and spatial scales to yield each individual within a colony a specific role to perform. In some species such as the beloved honeybees (Apis genus), helpers working underneath the reproductive queen never get to reproduce themselves, yet they care for new generations of young in the hive instead.

It would seem as if this organisation goes against the drive for life that the majority of organisms on the planet experience — to pass on their own successful genes to offspring; however, strong selection pressures have ensured that this phenomenon has become deeply rooted into some species’ systems.

Honey bees have long been the focus of immense research efforts, so we now understand the intricate web of life inside the hive.

The research done on this species begs to answer the question as to why cooperation should exist in a world dominated by intense competition for the survival of the fittest [1].

Reproductive division of labour

A reproductive division of labour is one of the key elements in defining eusocial behaviour. If we start with a small-scale example, division of labour can be seen at the cell level where it is basal. Asymmetric cleavage during meiosis yields ‘germ and soma’ cell distinction — somatic cells serve only somatic function in the animal’s body via mitosis, while germline cells produce the reproductive gametes. Despite both of these types of cells performing independent roles across time and space, their performance leads to the successful functioning of an entire individual as a whole. This divisive mechanism where somatic and germline cells stayed together in the body after dividing was evidently successful enough to become established at the cellular level, and evolution has since propelled it further up the biological hierarchy — into species level.

As seen by the cells, multiple replicating entities remaining together after division forms a greater replicating system. At this higher species level, the division of labour is represented as multicellular organisation, of which stems eusocial behaviour.

In the case of our beloved honey bees, the multiple replicating entities can be seen as the bees themselves, the colony is the matter which they remain together in, and hence the hive becomes the entire replicating system.

The division of two castes inside the honey bee hive is fundamental to the significant success of the species. The morphologically distinct queen is responsible for colony founding, dispersal, and egg-laying while the workers perform tasks such as colony defence, nursing, and foraging in order to maintain the colony, yet do not reproduce themselves [2]. In order to keep this system successful in its operation and avoid the establishment of any individuals developing ‘cheating’ methods that enable them to reproduce, workers are morphologically constrained by a lack of functioning organs for sexual reproduction [3].

Today, eusociality takes the form of adult offspring remaining in the colony to help their mother reproduce; instead of doing so themselves [4].

Cooperative brood care:

Workers in the Apis genus who exhibit distinct morphological differences from their queen are restricted to only gaining indirect fitness by helping to rear related offspring. The colony’s inclusive fitness is therefore a function of their reproductive output, with total offspring production depending on the quality of the queen and her mate, and of the cooperation of the workers. Much of the colony’s social life therefore revolves around brood care [4].

The fitness benefits that honey bees inside the colony receive from cooperative brood care means that they continue to work in this system, despite seemingly going against the typical drive for reproduction as most species, including ourselves, experience. The division of labour employed by this species means that their own individual fitness is enhanced as it allows more efficient conversion of resources into reproductive capacity [7, pp. 368 - 373].

Overlap of generations as a causal factor for this behaviour

The high degree of social complexity observed in these colonies can be explained by the degree of close genetic relatedness from several overlapping generations. The social behaviour observed in honey bees is facilitated by a unique system of genetics known as haplodiploidy; a system in which females develop from fertilised diploid eggs, and males from unfertilised haploid eggs. The consequence of this is that the male passes on his entire genome to his offspring, while the queen passes on 50% of hers, meaning that offspring are 75% related to one another. This genetic system creates an irregular genetic asymmetry in which full sisters are more closely related to each other than a mother is to her own daughters [5].

Figure 1. Insect sociality among a range of species, ranging from solitary insects (left) to completely eusocial (right). Indirect fitness becomes increasingly important throughout complexity as it is reflectant of the entire colony (Vijendravarma et al., 2017).

As a result, the dynamicity of the whole colony changes due to increased levels of relatedness. Essentially, the fitness of an individual bee is based on the combined effects that its actions have on other individuals, weighted by their relatedness to that individual.

Thus selection acts to maximise inclusive fitness of the entire colony, albeit through a trade off between expending energy into a bee’s own reproduction or investing in helping its relatives. The overall purpose of this behaviour is to increase the abundance of beneficial alleles present in the colony, which is directly beneficial for all individuals due to their high degree of genetic relatedness. This cooperative behaviour is known as altruism, and can evolve between related individuals over time and space. In altruism, a gene directs aid at other individuals who are likely to bear the same gene to itself despite the reduced offspring of its bearer [1]. From an evolutionary standpoint, we can understand that honey bee workers who rear their siblings are able to achieve maximal inclusive fitness when compared to individuals who reproduce themselves [5].

Using Hamilton’s rule, we can understand how evolution selected for the loss of reproductive organs in worker bees, and instead favoured one reproductive queen. This rule is a theorem that acts as a foundation to predict whether social behaviour evolves under combinations of relatedness, cost, and benefit [6].

Hamilton's rule gives an equation to show when an organism should sacrifice their own reproduction in order to help relatives; given as rB > C, where r is the degree of relatedness between two individuals, B is the benefit to the recipient of the behaviour, and C is the cost of the behaviour to the individual giving the aid. C and B can be viewed as lifetime changes in the direct fitness [1].

Whether an organism should make this sacrifice or not depends on the value that is denoted by r. A gene for social behaviour is favoured by selective pressures if the sum of rB and C exceeds zero [1].

Honey bees have become one of the most successful insects on Earth due to their immense range span and establishment; and evidently their unique genetic system greatly contributes to this success. The honey bee colony arose through major evolutionary transitions that were dependent on cooperating entities finding a situation of inclusive fitness that kept them together for their own fitness benefit [1].

Today, these insects provide us with valuable resources and ecosystem services such as being key pollinators of our flora all over the planet. Perhaps next time you see a honey bee, think about the details of its hidden genes and how remarkable these are, as they allow for their widespread success and hence, ours too.