Sex-biased parental care and sexual size dimorphism in a provisioning arthropod

The diverse selection pressures driving the evolution of sexual size dimorphism (SSD) have long been debated. While the balance between fecundity selection and sexual selection has received much attention, explanations based on sex-specific ecology have proven harder to test. In ectotherms, females are typically larger than males, and this is frequently thought to be because size constrains female fecundity more than it constrains male mating success. However, SSD could additionally reflect maternal care strategies. Under this hypothesis, females are relatively larger where reproduction requires greater maximum maternal effort – for example where mothers transport heavy provisions to nests. To test this hypothesis we focussed on digger wasps (Hymenoptera: Ammophilini), a relatively homogeneous group in which only females provision offspring. In some species, a single large prey item, up to 10 times the mother’s weight, must be carried to each burrow on foot; other species provide many small prey, each flown individually to the nest. We found more pronounced female-biased SSD in species where females carry single, heavy prey. More generally, SSD was negatively correlated with numbers of prey provided per offspring. Females provisioning multiple small items had longer wings and thoraxes, probably because smaller prey are carried in flight. Despite much theorising, few empirical studies have tested how sex-biased parental care can affect SSD. Our study reveals that such costs can be associated with the evolution of dimorphism, and this should be investigated in other clades where parental care costs differ between sexes and species

Group Size and Direct Fitness in Social Queues

We explore the effects of group size on the direct reproductive success of subordinate helpers in eusocial animals where only a single, dominant individual reproduces at one time. Helpers can reproduce directly if they inherit dominance, but when dominance is age based, an individual born into a larger group has a longer wait to inherit. We show that this disincentive to help can potentially be offset by increased productivity, increased life span, and insurance-based benefits for helpers if they survive to inherit dominance in larger groups. We analyze a field experiment in which group size was manipulated in the hover wasp Liostenogaster flavolineata. Productivity increased linearly with group size, larger groups were less likely to fail, and dominants in larger groups may have lived longer. Combined with the probability of inheriting dominance, these effects led overall to a negative correlation between group size and expected direct fitness, mainly because group size decreased during our study period, so that helpers could not expect to inherit as large a group as they started queuing in. Our analysis suggests that the relationship between group size and productivity plays a central role in determining the fitness consequences of helping

Parental care and sexual size dimorphism in wasps and bees

Sexual size dimorphism, in which one sex is larger than the other, occurs when body size has differential effects on the fitness of males and females. Mammals and birds usually have male-biased size dimorphism, probably because of strong sexual competition among males. Invertebrates usually have female-biased size dimorphism, perhaps because their inflexible exoskeletons limit ovary size, leading to a strong correlation between female body size and fecundity. In this paper, we test whether an additional factor, the type of parental care provided, affects the degree of sexual size dimorphism. Among wasps and bees, there is a contrast between provisioning taxa, in which females must gather and transport heavy loads of provisions to nests they have constructed, and non-provisioning taxa, in which females lay eggs but do not construct nests or transport provisions. Males have no role in parental care in either case. An analysis of British wasps and bees shows that provisioning taxa have significantly more female-biased size dimorphism than non-provisioning taxa. This is true for simple cross-species comparisons and after controlling for phylogeny. Our data imply that the demands of carrying provision loads are at least part of the explanation for this pattern. Thus, sexual size dimorphism is greatest in pompilid wasps, which carry the heaviest prey items. Bees, which transport minute pollen grains, exhibit the least dimorphism. We also find that cavity-nesting species, in which nest construction costs may be minimized, exhibit reduced dimorphism, but this was not significant after controlling for phylogeny

Insurance-based advantages for subordinate co-foundresses in a temperate paper wasp

Recent explanations for the evolution of eusociality, focusing more on costs and benefits than relatedness, are largely untested. We validate one such model by showing that helpers in foundress groups of the paper wasp Polistes dominulus benefit from an insurance¿based mechanism known as Assured Fitness Returns (AFRs). Experimental helper removals left remaining group members with more offspring than they would normally rear. Reduced groups succeeded in preserving the dead helpers' investment by rearing these extra offspring, even when helper removals occurred long before worker emergence. While helpers clearly gained from AFRs, offspring of lone foundresses failed after foundress death, so that AFRs represent a true advantage for helpers. Smaller, less valuable offspring were probably sacrificed to feed larger offspring, but reduced groups did not preferentially attract joiners or increase their foraging effort to compensate for their smaller workforce. We failed to detect a second insurance¿based advantage, Survivorship Insurance, in which larger groups are less likely to fail than smaller groups. We suggest that through their use of small offspring as a food store to cope with temporary shortages, wasps may be less susceptible than vertebrates to offspring failure following the death of group members

Ecological constraints on independent nesting in facultatively eusocial hover wasps

Recent field experiments suggest that cooperative breeding in vertebrates can be driven by a shortage of breeding territories. We did analogous experiments on facultatively eusocial hover wasps (Stenogastrinae: Liostenogaster flavolineata). We provided nesting opportunities by removing residents from 39 nests within a large aggregation (1995), and by glueing 20 nests obtained from a distant site into a second aggregation (1996). We prevented nest-less floaters from competing for these opportunities in 1995 but not in 1996. In both years, helpers in unmanipulated groups were given opportunities to nest independently without having to incur nest-building costs and with a reduced wait before potential helpers emerged. Helpers visited the nests we provided, but adopted only a small proportion (5% of 111 vacancies created in 1995). Others were adopted by floaters, but a significant proportion of nests were never adopted (nine out of 20 in 1995, seven out of 20 in 1996). Helpers that visited nests did not originate from particular kinds of social group. Nests containing older brood were more likely to be adopted, and adopting females rarely destroyed older brood. A general feature of social insect but not vertebrate life-histories, the long period of offspring dependency relative to the short life expectancy of adult carers, may be a key factor constraining independent nesting

Comparison of CA-125 and standard definitions of progression of ovarian cancer in the intergroup trial of cisplatin and paclitaxel versus cisplatin and cyclophosphamide.

PURPOSE: A definition for progression of ovarian cancer has been proposed based on either a confirmed doubling of CA-125 levels from the upper limit of normal or from the nadir level if levels are persistently elevated. Retrospectively, we determined whether the use of this CA-125 definition in a randomized trial would have shown the same magnitude of difference between the treatment arms as was shown when the standard progression definition was used. PATIENTS AND METHODS: A retrospective analysis was performed on 680 patients in the Taxol Intergroup Trial with advanced epithelial ovarian carcinoma, of whom 628 were assessable according to CA-125. The date of progression according to clinical or radiologic criteria was compared with the date of progression according to CA-125. RESULTS: Of the 628 patients assessable for both definitions, 556 clinical or radiologic progressions were determined compared with 389 according to the CA-125 definition. There was a highly significant difference in the hazard of progression between the paclitaxel and cisplatin arm (TP) compared with the cyclophosphamide and cisplatin arm (CP) when either standard or CA-125 criteria were used to define progression (standard, P = .002; CA-125, P = .011). The hazard ratio of TP/CP over time was similar when comparing the different methods of defining progression. CONCLUSION: The results of this analysis show that the magnitude of the therapeutic benefit was similar whether CA-125 or standard criteria were used to define progression.Clinical Trial, Phase IIIComparative StudyJournal ArticleRandomized Controlled Trialinfo:eu-repo/semantics/publishe

Colony genetic structure in a facultatively eusocial hover wasp

The degree of genetic heterogeneity among the individuals in an animal society depends on the society's genetic structure. Genetic heterogeneity, in turn, means that group members will differ in their reproductive objectives and conflicts over reproduction may arise. The resolution of these conflicts may be reflected in the way that reproduction is partitioned between potential reproductives. We used 5 microsatellite loci to investigate genetic structure and reproductive skew in 17 nests of the Malaysian hover wasp, Parischnogaster alternata. Parischnogaster alternata colonies are small (1¿10 females), and all adult colony members are capable of mating and producing offspring. We found that colonies tended to consist of closely related individuals and that at any one time the production of both female and male offspring was nearly always monopolized by a single dominant female, despite considerable variation between nests in parameters predicted to affect skew. Subordinate females that remained in their natal colonies obtained indirect fitness benefits by helping to raise offspring to which they were related. Subordinate females also appeared to be positioned within an age-based queue for inheritance of the dominant egg-laying position. We suggest that the high skew in P. alternata may result from strong ecological constraints on solitary nesting, high relatedness, and a relatively high probability that subordinates will eventually inherit the position of dominance

Insurance-based advantage to helpers in a tropical hover wasp

The origin and maintenance of eusociality is a central problem in evolutionary biology1, 2. Eusocial groups contain individuals that forfeit their own reproduction in order to help others reproduce. In facultatively eusocial taxa, offspring can choose whether to found new nests or become helpers in their natal groups. In many facultatively eusocial insects, offspring need continuous care during development, but adult carers have life expectancies shorter than the developmental period3, 4, 5, 6, 7. When a lone foundress dies, her partly reared brood are usually doomed. Here, we show that helpers in a tropical hover wasp (Liostenogaster flavolineata) have an insurance-based advantage over lone foundresses because after a helper dies, most of the brood that she has partly reared will be brought to maturity by surviving nest-mates. After some of the helpers are experimentally removed from a multi-female nest, the reduced group is left with more brood than it would normally rear. We found that larger, more valuable extra brood were reared through to maturity, but not smaller, less valuable brood. Smaller brood may be sacrificed to feed larger brood, and reduced groups probably benefited from increased short-term helper recruitment. Rearing extra brood did not increase adult mortality or brood development time