The Effect of Group Size on the Interplay between Dominance and Reproduction in Bombus terrestris

Social insects provide good model systems for testing trade-offs in decision-making because of their marked reproductive skew and the dilemma workers face when to reproduce. Attaining reproductive skew requires energy investment in aggression or fertility signaling, creating a trade-off between reproduction and dominance. This may be density-dependent because the cost of achieving dominance may be higher in larger groups. We investigated the effect of group-size in B. terrestris queenless workers on two major reproduction-dominance correlates: between-worker aggression, and pheromone production, aiming at mimicking decision-making during the transition of worker behavior from cooperation and sterility to aggressive reproductive competition in whole colonies. Despite the competition, reproductive division of labor in colonies can be maintained even during this phase through the production of a sterility signal by sterile workers that has an appeasement effect on dominant nestmates. Worker-worker aggression, ovary activation, and production of sterility-appeasement signals may therefore constitute components of a trade-off affecting worker reproduction decisions. By constructing queenless groups of different size and measuring how this affected the parameters above, we found that in all groups aggression was not evenly distributed with the α-worker performing most of the aggressive acts. Moreover, aggression by the α-worker increased proportionally with group-size. However, while in small groups the α-worker monopolized reproduction, in larger groups several workers shared reproduction, creating two worker groups: reproductives and helpers. It appears that despite the increase of aggression, this was evidently not sufficient for the α-worker to monopolize reproduction. If we compare the α-worker to the queen in full-sized colonies it can be hypothesized that worker reproduction in B. terrestris colonies starts due to a gradual increase in the worker population and the queen's inability to physically inhibit worker oviposition. This may shift the trade-off between cost and benefit of worker reproduction and trigger the competition phase

The percentage of workers that lost their Dufour's gland esters in queenless groups and colonies.

<p>Workers with less than 1% esters per total secretion were defined as workers that had lost their ester-sterility signal. The workers were 5-day-old and were kept in queenless groups of 3, 5 and 10. Workers' age in colonies was distributed normally (for more details see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0018238#pone.0018238-Amsalem1" target="_blank">[24]</a>). The numbers in brackets denote the numbers of workers per colony and the size of the sample (number of groups or colonies). All the workers in each group were dissected, therefore the n describes the number of Dufour glands as well. Letters above the columns denote statistical differences at p<0.05. Data are presented as mean ± SE.</p

Ovarian development and amount of esters in Dufour's gland of α- and β-workers in 3 different group-sizes.

<p>The experiments were performed in 5-day-old workers that were kept in queenless groups of 3, 5 and 10 (12 groups for each group-size). Data are presented as mean ± SE for the α-workers and β-workers in each group. Different letters denote statistical differences using two-way ANOVA test.</p

Aggression level per worker in α-workers and β-workers in 5-day-old workers.

<p>Workers were kept in queenless groups of 3, 5 and 10 (12 groups for each group-size). Each group was observed for a total of 120 minutes. Data are presented as mean ± SE for the α-workers and β-workers in each group. Different letters denote statistical differences using two-way ANOVA test.</p

Percentage of aggression exhibited by the α-worker towards the other females in the group.

<p>Aggression level includes cumulative aggression (attack and darting) during 5 days. Humming was excluded because of inability to determine directionality with certainty. Each group was observed for 120 minutes. Workers were kept in queenless groups of 3, 5 and 10 (12 groups for each group-size). Workers are presented in accordance with the amount of aggression each has received from the α-worker (e.g. 2′ female received the highest amount of aggression).</p

Reproductive competition in the bumble-bee Bombus terrestris: do workers advertise sterility?

Reproductive competition in social insects is generally mediated through specific fertility pheromones. By analysing Dufour's gland secretion in queens and workers of Bombus terrestris under varying social conditions, we demonstrate here that the volatile constituents of the secretion exhibit a context-dependent composition. The secretion of egg-laying queens is composed of a series of aliphatic hydrocarbons (alkanes and alkenes), while that of sterile workers contains in addition octyl esters, dominated by octyl hexadecanoate and octyl oleate. These esters disappear in workers with developed ovaries, whether queenright (QR) or queenless (QL), rendering their secretion queen-like. This constitutes an unusual case in which the sterile caste, rather than the fertile one, possesses extra components. Individually isolated (socially deprived) workers developed ovaries successfully, but failed to oviposit, and still possessed the octyl esters. Thus, whereas social interactions are not needed in order to develop ovaries, they appear essential for oviposition and compositional changes in Dufour's gland secretion (ester disappearance). The apparent link between high ester levels and an inability to lay eggs lends credence to the hypothesis that these esters signal functional sterility. We hypothesize that by producing a sterility-specific secretion, workers signal that ‘I am out of the competition’, and therefore are not attacked, either by the queen or by the reproductive workers. This enables proper colony function and brood care, in particular sexual brood, even under the chaotic conditions of the competition phase

Data from: Chemical communication is not sufficient to explain reproductive inhibition in the bumblebee Bombus impatiens

Reproductive division of labour is a hallmark of eusociality, but disentangling the underlying proximate mechanisms can be challenging. In bumblebees, workers isolated from the queen can activate their ovaries and lay haploid, male eggs. We investigated if volatile, contact, visual or behavioural cues produced by the queen or brood mediate reproductive dominance in Bombus impatiens. Exposure to queen-produced volatiles, brood-produced volatiles and direct contact with pupae did not reduce worker ovary activation; only direct contact with the queen could reduce ovary activation. We evaluated behaviour, physiology and gene expression patterns in workers that were reared in chambers with all stages of brood and a free queen, caged queen (where workers could contact the queen, but the queen was unable to initiate interactions) or no queen. Workers housed with a caged queen or no queen fully activated their ovaries, whereas ovary activation in workers housed with a free queen was completely inhibited. The caged queen marginally reduced worker aggression and expression of an aggression-associated gene relative to queenless workers. Thus, queen-initiated behavioural interactions appear necessary to establish reproductive dominance. Queen-produced chemical cues may function secondarily in a context-specific manner to augment behavioural cues, as reliable or honest signal