“Female-biased sex ratios increase colony survival and reproductive output in the spider Anelosimus studiosus”
James L. L. Lichtenstein, Ambika Kamath, Sarah E. Bengston, Leticia Avilés, and Jonathan N. Pruitt (Nov 2018)
Female-biased sex ratios increase social spider colony survival, generating group selection on female-biased sex ratios
Animals across the world tend to have 1 : 1 sex ratios, because whichever sex is rarer, it will always be advantageous to produce more of it. However, a few animals have highly female-skewed ratios, including social spiders, inbred spiders that work together to capture prey. In a new paper appearing in The American Naturalist, James L. L. Lichtenstein and colleagues evaluate one advantage to producing many daughters: Extremely inbred social groups with many females can grow more quickly, helping groups survive and reproduce. The authors tested this by assembling colonies of the semi-social spider Anelosimus studiosus and then putting colonies in the wild with manipulated sizes and sex ratios in areas where spider colonies thrive and places where they struggle. The researchers thought that groups with female-biased sex ratios would be more likely to survive and produce egg cases, especially for small colonies and colonies in areas where spiders struggle to survive. After five months, the scientists returned to these colonies to see whether they were alive and how many egg cases they produced.
As expected, female-biased colonies were more likely to survive and produced more egg cases. These advantages were stronger when spider colonies were small and were in tough areas. This means that colonies full of female spiders were able to flourish in harrowing conditions by doing more helping capture prey and maintain webs more than male spiders, producing offspring who can work, or both. This advantage may explain the evolution of female-biased ratios in this very interesting yet very specific case.
Negative frequency-dependent selection acting on the sexes is hypothesized to drive populations towards a balanced sex ratio. However, numerous examples of female-biased sex ratios pepper the arthropods. Theoretical examinations have proposed that female-biased populations or groups can have higher chances of surviving and propagating, which may be advantageous. We evaluate this hypothesis in the semi-social spider, Anelosimus studiosus, by creating artificial colonies of varying sex ratios and sizes and observing colony performance at sites with high vs. low group extinction rates. We also tested whether colony extinction rates and sex ratios were correlated across 25 collection sites spanning 10º latitude. We found that colonies with female-biased sex ratios produced more egg cases and were more likely to survive the duration of a field season, suggesting that female-biased sex ratios confer both survival and reproductive advantages upon colonies. The effect of sex ratio on colony survival and reproductive output was strongest for small colonies in high extinction areas. Moreover, we found that female-biased sex ratios correlated with greater extinction rates across 25 sites, indicating that female-biased sex ratios may have evolved at some sites in response to high extinction rates. These findings suggest that selection favoring groups with female-biased sex ratios may operate in A. studiosus, shedding light on some of factors that may drive the evolution of biased sex ratios.