Is variation in female aggressiveness across Drosophila species associated with reproductive potential?

Aggression is a key determinant of fitness in many species, mediating access to mates, food and breeding sites. Variation in intrasexual aggression across species is likely to be driven by variation in resource availability and distribution. While males primarily compete over access to mates, females are likely to compete over resources to maximize offspring quantity and/or quality, such as food or breeding sites. To date, however, most studies have focused on male aggression, and we know little about drivers of female aggression across species. To investigate potential reproductive drivers of female aggression, we tested the relationship between three reproductive traits and aggression in eight Drosophila species. Using machine learning classifiers developed for Drosophila melanogaster, we quantified aggressive behaviours displayed in the presence of yeast for mated and unmated females. We found that female aggression was correlated with ovariole number across species, suggesting that females who lay more eggs are more aggressive. A need for resources for egg production or oviposition sites may therefore be drivers of female aggression, though other potential hypotheses are discussed.

The relative effects of pace of life-history and habitat characteristics on the evolution of sexual ornaments: A comparative assessment

Selection may favor greater investment into sexual ornaments when opportunities for future reproduction are limited, for example, under high adult mortality. However, predation, a key driver of mortality, typically selects against elaborate sexual ornaments. Here, we examine the evolution of sexual ornaments in killifishes, which have marked contrasts in life-history strategy among species and inhabit environments that differ in accessibility to aquatic predators. We first assessed if the size of sexual ornaments (unpaired fins) influenced swimming performance. Second, we investigated whether the evolution of larger ornamental fins is driven primarily by the pace of life-history (investment into current vs. future reproduction) or habitat type (a proxy for predation risk). We found that larger fins negatively affected swimming performance. Further, males from species inhabiting ephemeral habitats, with lower predation risk, had larger fins and greater sexual dimorphism in fin size, compared to males from more accessible permanent habitats. We show that enlarged ornamental fins, which impair locomotion, evolve more frequently in environments that are less accessible to predators, without clear associations to life-history strategy. Our results provide a rare link between the evolution of sexual ornaments, effects on locomotion performance, and natural selection on ornament size potentially through habitat differences in predation risk.

Fast life-histories are associated with larger brain size in killifishes

The high energetic demands associated with the vertebrate brain are proposed to result in a trade-off between the pace of life-history and relative brain size. However, because both life-history and brain size also have a strong relationship with body size, any associations between the pace of life-history and relative brain size may be confounded by coevolution with body size. Studies on systems where contrasts in the pace of life-history occur without concordant contrasts in body size could therefore add to our understanding of the potential coevolution between relative brain size and life-history. Using one such system - 21 species of killifish - we employed a common garden design across two ontogenetic stages to investigate the association between relative brain size and the pace of life-history. Contrary to predictions, we found that relative brain size was larger in adult fast-living killifishes, compared to slow-living species. Although we found no differences in relative brain size between juvenile killifishes. Our results suggest that fast- and slow-living killifishes do not exhibit the predicted trade-off between brain size and life-history. Instead, fast and slow-living killifishes could differ in the ontogenetic timing of somatic versus neural growth or inhabit environments that differ considerably in cognitive demands.

Sex ratios deviate across killifish species without clear links to life history

Sex ratios can differ from an expected equal proportion of males and females, carrying substantial implications for our understanding of how mating systems evolve. Typically, macro-evolutionary studies have been conducted without assessing how deviations from an equal sex ratio could be explained by sex-biased mortality or dispersal. Our understanding of sex ratio evolution independent of these confounds, in addition to any putative links between skewed sex ratios and other factors (e.g. life history), therefore remains largely unexplored. Here, we conducted an exploratory study investigating differences in sex ratios across closely related species while controlling for extrinsic mortality. We also tested two factors, non-overlapping/overlapping generations and the social environment, which have both been hypothesised to affect sex ratios. Specifically, we raised 15 species of killifish, which have either overlapping or discrete generations, under both solitary and social treatments. We found substantial divergences in sex ratios across closely related species, which exhibited both male and female biases. In conjunction with a low phylogenetic signal, our results suggest that sex ratios can evolve rapidly in this group. However, we found no evidence that overlapping generations or the social environment affected sex biases, suggesting that other factors drive the rapid evolution of sex ratios in killifishes.

Strong species differences in life history do not predict oxidative stress physiology or sensitivity to an environmental oxidant

Species typically align along a fast-slow life-history continuum, yet it is not clear to what extent oxidative stress physiology can be integrated with this continuum to form a 'pace-of-life syndrome', especially so in invertebrates. This is important, given the assumed role of oxidative stress in mediating life-history trade-offs, and the prediction that species with a faster pace should be more vulnerable to oxidative stress. We tested whether a species' life-history pace, here represented by its growth rate, can predict species-level differentiation in physiology and sensitivity to oxidative stress. Therefore, we exposed four species of Ischnura damselflies that strongly align along a fast-slow life-history continuum to different levels of ultraviolet (UV) radiation. We measured an extended set of physiological traits linked to the pace-of-life: standard metabolic rate, oxidative stress physiology (antioxidant enzymes and oxidative damage) and defence/condition traits (investment in immune function, energy storage and structural defence). Despite strong species differences in growth rate and physiology, growth rate did not predict species-level differentiation in physiology. Hence there was no support for the integration of metabolic rate, oxidative stress physiology or defence/condition traits into a species-level syndrome. Ultraviolet exposure affected nearly all traits: it reduced growth rate and increased metabolic rate, affected all oxidative stress physiology traits and increased the two defence traits (immune function, and melanin content). Nevertheless, the pace-of-life based on growth rate did not predict sensitivity to UV. Instead, the observed pattern of investment in structural UV defence (melanin) might have reduced the need for enzymatic antioxidant defence, this way potentially decoupling the covariation between the life-history pace and oxidative stress physiology. The absence of an integrated axis of life-history and physiological variation indicates no major constraints for the evolution of these traits among the studied damselfly species. Our study highlights that ecological differences between species may decouple covariation between species' life-history pace and their physiology, as well as their sensitivity to environmental stressors.