Sex ratio and sexual conflict in a collective action problem

The maintenance of cooperation is difficult whenever collective action problems are vulnerable to freeriding (reaping the benefits without contributing to the maintenance of the good). We identify a novel factor that can make a system tolerate an extent of freeriding. If a population consists of discrete types with demographically distinct roles, such that the success of one type does not imply it can spread to replace other types in the population, then collective goods may persist in the presence of free-riders because they are necessarily kept in a minority role. Biased sex ratios (e.g. in haplodiploids) create conditions where individuals of one sex are a minority. We show that this can make the less common sex contribute less to a public good in a setting where the relevant life-history stage — larval group defence against predators — does not feature any current breeding opportunities that might lead to confounding reasons behind sex-specific behaviour. We test our model with haplodiploid pine sawfly larvae, showing that female larvae are the main contributors to building the antipredator defence against predators.Significance statementIndividuals in groups can cooperate to achieve something together, but with an evolutionary difficulty: if benefits of cooperation are shared equally among all, freeriders get the same benefit as others while paying less for it. We propose a novel reason why freeriding does not automatically spread until the collectively beneficial outcome is destroyed: sometimes groups consist of individuals of distinct categories, limiting freerider spread. If, for example, there are always fewer males than females, then even if every male becomes a freerider, the whole group still survives simply because not everyone can be male. Pine sawfly larvae defend against predators by regurgitating sticky fluids, but females contribute more to this common defence, and we show this example fits our model.

Mother knows best: maternal egg-clutch size predicts larval group size in pine sawflies (genus Neodiprion)

Evolutionary conflicts are pervasive in nature and have the potential to drive antagonistic coevolution of conflict-related traits. However, when such conflicts are weak or idiosyncratic, phenotypic signatures of coevolutionary arms races may be absent. Here, we ask whether variation in group-living traits among pine-sawfly species in the genus Neodiprion is consistent with a history of parent-offspring conflict. To address this question, we compile data on adult female clutch size, larval aggregation behavior, and larval group size for a monophyletic group of 19 eastern North American Neodiprion species from field observations, laboratory assays, and published descriptions. We then evaluate the extent to which each trait exhibits phylogenetic signal and, based on these results, examine correlations between group-size traits both with and without phylogenetic correction. Although female oviposition behavior and larval grouping behavior varies among species and variation in these traits is decoupled from phylogeny, we find no evidence of antagonistic coevolution between these traits. Furthermore, while larvae are physically capable of dispersal, female clutch size is a strong predictor of larval colony size, indicating that larvae do not substantially alter initial group size after hatching. Thus, although theoretical work demonstrates the potential for parent-offspring conflict over group size in animals that lack parental care, our data suggest that this type of conflict is not likely to be a long-term driver of phenotypic evolution.

Spatial variability of planktonic invertebrate larvae in the Canary Islands area

In October 1991, invertebrate larvae abundances were analysed to study the influence of the disturbance of the Canary Current flow by the Canary Islands archipelago on the variability of larval distribution. Two transects and two time-series stations located to the north (non-perturbed zone) and the south (perturbed zone) of the Canary Islands were sampled. Oceanographical data showed a highly stratified water column and zonally uniform salinity and temperature seaward of the African upwelling in the non-perturbed zone, while the perturbed zone presented strong turbulence in the form of mesoscale eddies. Invertebrate larval abundances were lower for most taxa studied in the non-perturbed zone and northern time-series station. Significant differences (P < 0.001) of invertebrate larval abundance between the two zones sampled were found. Decapod larvae were the most abundant larval group in both zones. Stations located in eddy structures presented the highest values of larval densities. Specifically, the larvae collected at Station 18, located in the core of an anticyclonic eddy, represented 60±18% of total larvae collected in the south transect. Finally, our results suggest that eddies, mainly anticyclonic eddies, act as a strong larval retention zone south of the islands, and that there is a local northward transport from the Canary Islands.

Rapid induction of immune density-dependent prophylaxis in adult social insects

The innate immune system provides defence against parasites and pathogens. This defence comes at a cost, suggesting that immune function should exhibit plasticity in response to variation in environmental threats. Density-dependent prophylaxis (DDP) has been demonstrated mostly in phase-polyphenic insects, where larval group size determines levels of immune function in either adults or later larval instars. Social insects exhibit extreme sociality, but DDP has been suggested to be absent from these ecologically dominant taxa. Here we show that adult bumble-bee workers ( Bombus terrestris ) exhibit rapid plasticity in their immune function in response to social context. These results suggest that DDP does not depend upon larval conditions, and is likely to be a widespread and labile response to rapidly changing conditions in adult insect populations. This has obvious ramifications for experimental analysis of immune function in insects, and serious implications for our understanding of the epidemiology and impact of pathogens and parasites in spatially structured adult insect populations.

Oviposition Behaviour of the Jack-pine Sawfly, Neodiprion americanus banksianae Roh., as Indicated by an Analysis of Egg Clusters

This study was undertaken as part of a broader investigation of the behaviour of the jack-pine sawfly, Neodiprion americanus banksianae Roh. The greater part of the investigation was directed towards an understanding of larval group-feeding behaviour, but in order to clarify the situation in which this insect begins larval life, large numbers of egg clusters were analysed with special attention paid to the positional relations of egg-bearing needles. Such analysis has permitted certain inferences on the oviposition behaviour of this insect and this indirect information is of value in view of the difficulties experienced in efforts to induce this species to oviposit under laboratory conditions for direct observation. Fortunately, egg clusters obtained from the field are well adapted to descriptive measurement and analysis, for in order to lay its full complement of eggs the adult female sawfly must select as many as 20 to 30 needles, thus leaving behind it a readily measured record of its egg-laying behaviour.