Unrelated helpers in a primitively eusocial wasp: is helping tailored towards direct fitness?

Abstract

The paper wasp Polistes dominulus is unique among the social insects in that nearly one-third of co-foundresses are completely unrelated to the dominant individual whose offspring they help to rear and yet reproductive skew is high. These unrelated subordinates stand to gain direct fitness through nest inheritance, raising the question of whether their behaviour is adaptively tailored towards maximizing inheritance prospects. Unusually, in this species, a wealth of theory and empirical data allows us to predict how unrelated subordinates should behave. Based on these predictions, here we compare helping in subordinates that are unrelated or related to the dominant wasp across an extensive range of field-based behavioural contexts. We find no differences in foraging effort, defense behaviour, aggression or inheritance rank between unrelated helpers and their related counterparts. Our study provides no evidence, across a number of behavioural scenarios, that the behaviour of unrelated subordinates is adaptively modified to promote direct fitness interests.

Figure 1. Sequence of experiments.

Black/grey sections represent one day of no censusing. Observations continued until the rank of each wasp on the nest was known (pausing for bad weather).

Figure 2. Relatedness in the study population and sample.

a) Distribution of pairwise nest-mate relatedness across whole population, based on a sample of 4396 cofoundress pair (broad line). Further lines represent pairwise relatedness from simulated populations comprising 4396 pairs of sisters, cousins, and unrelated wasps b) Distribution of relatedness to the dominant wasp for subordinates classed as sisters of the dominant, cousins of the dominant, and non-relatives, on nests used for behavioural observations. Categories overlap because allocations are based on pairwise likelihoods, which depend on the population allele frequencies, and not absolute cut-off values. For example, an individual that is related to the dominant by less than 0.1 might be found to be more likely a cousin than unrelated, if the particular alleles that the two individuals do share are rare in the population.

Figure 3. Inheritance rank in relation to a subordinate's relatedness to the dominant wasp.

Medians, interquartile range and max/min values are indicated.

Figure 4. Foraging effort of sisters, cousins and non-relatives of the dominant wasp.

Foraging effort is estimated based on proportion of time spent away from the nest.

Figure 5. Aggressive responses to a conspecific usurper by sisters, cousins and non-relatives of the dominant.

Figure 6. Aggression levels within founding groups.

Mean aggression rates on nests where co-foundresses were all sisters, cousins and sisters, or contained at least one wasp that was not related to the rest of the group are shown.