Cooperation and cheating in sub-social spiders: negative frequency dependent selection and sex allocation

• Applicants: Professorin Marie Herberstein, Ph.D.; Professorin Dr. Jutta M. Schneider
• Subject Area: Sensory and Behavioural Biology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558031607

The evolution of cooperative behaviour is puzzling, particularly when it is associated with costs to the altruists, which are not returned by the recipient. Cooperation is nevertheless widespread among animals and many theoretical models have been developed to explain its persistence. One proposed mechanism is that defectors, cheaters or scroungers are self-limited by negative frequency dependent selection (NFDS) to a proportion where costs for co-operators are still low. In spite of the convincing logic of the concept few studies have shown NFDS experimentally. Here we build upon previous work with the subsocial crabspider Australomisidia ergandros in which we have discovered that some individuals in a family group consistently hunted and shared prey (producers), while others benefited from these hunting efforts (scroungers). Interestingly, scroungers were most frequently females while males produced, suggesting sex-related behavioural types. The system offers an ideal opportunity to experimentally manipulate producer/scrounger ratios and directly measure the fitness consequences of this variation under otherwise standardised conditions. Our first objective is to experimentally address hypotheses derived from NFDS and sex allocation theory, namely that family fitness will rise with increasing numbers of producers (or males). Our second objective is to develop theoretical models that explain our findings. Thirdly, we will manipulate the condition of mothers to test whether mothers can influence the sex and producer ratio in their broods to maximize growth and survival of her offspring. Finally, we will manipulate sex ratio and producer/scrounger ratio in natural nests, return them to the field, where they will be closely monitored to measure fitness outcomes. To achieve our goals, we will collect a large number of leaf nests with females and offspring in Australia, transport them to Hamburg and test all individuals for their behavioural type (producer or scrounger) using an established assay. Then we will assemble groups of marked siblings at different producer:scrounger proportions and carefully monitor their interactions using automatic tracking methods and establish fitness outcomes. Tissue samples from each individual will allow us to molecularly determine the sex of each juvenile using a novel sexing technique. We anticipate that our project will provide novel insights into mechanisms that maintain cooperation and how this might connect to sex ratio in brood-caring species.

• DFG Programme: Research Grants