Summary1. Initial work on oxidative stress and antioxidant defences described basic chemical and biochemical properties and processes and applied this information to issues of animal health and husbandry. Seminal experimental investigations on the damaging effects of oxidative stress and the mitigating effects of antioxidant defences were conducted primarily in domesticated organisms. 2. In recent years, ecologists have taken to studying antioxidants and oxidative stress in free-ranging organisms and have integrated principles of oxidative stress into several core evolutionary concepts, such as life-history trade-offs (e.g. survival vs. reproduction), senescence and sexual selection. This initial flurry of studies has provided major advances in our understanding of how antioxidant defences evolve and function. 3. In this overview, it is our goal to provide ecologists with an accessible summary of (i) the biochemical basis and conceptual frameworks behind oxidative stress and antioxidants, (ii) the research questions and hypotheses that are generated by incorporating antioxidants and oxidative stress into models of life-history theory, ageing, mate selection, and honest signalling, and (iii) the trends in the evidence that have emerged from initial studies in these areas. 4. Though much progress has been made on the ecological and evolutionary relevance of antioxidant and oxidative stress physiology, no consensus has emerged regarding the primacy of how oxidative stress challenges or antioxidant limits or values shape organismal life-histories. However, there are many taxonomic biases in studies to date and several ideal environmental systems that are as-of-yet untapped.

All multicellular organisms host microbial communities in and on their bodies, and these microbiomes can have major influences on host biology. Most research has focussed on the oral, skin, and gut microbiomes, whereas relatively little is known about the reproductive microbiome. Here, we review empirical evidence to show that reproductive microbiomes can have significant effects on the reproductive function and performance of males and females. We then discuss the likely repercussions of these effects for evolutionary processes related to sexual selection and sexual conflict, as well as mating systems and reproductive isolation. We argue that knowledge of the reproductive microbiome is fundamental to our understanding of the evolutionary ecology of reproductive strategies and sexual dynamics of host organisms. The Microbiome Revolution and the Reproductive MicrobiomeAnimals and plants live and evolve in a world dominated by microbes, and host a diversity of microbial communities in and on their bodies. A recent explosion in research on host-associated microbial communities (i.e., microbiota and/or microbiomes, see Glossary) is revolutionising biology. While earlier research typically considered microorganisms from a pathological perspective, it is now widely accepted that the relationship between host and microbes spans a continuum, from detrimental to beneficial. Through their influence on host health, physiology, development, and behaviour [1,2], microbiomes can be seen as an integral part of the host phenotype, and potentially also the host genome (e.g., the hologenome concept [3], but see [4]). While considerable attention has been paid to the role of oral, skin, and gut microbiomes in host ecology, evolution, and fitness, less is known about the reproductive microbiome (Box 1). This is surprising given longstanding knowledge of microbes in male and female reproductive systems (e.g., [5]), most notably in the context of sexually transmitted infections (STIs) [6], and more recent DNA-sequencing studies demonstrating the presence of dynamic microbial communities in reproductive tissues [7], especially the vagina [8,9]. Thus, the reproductive microbiome represents an outstanding challenge in the study of host-associated microbial communities.

Rapid diversification of sexual traits is frequently attributed to sexual selection, though explicit tests of this hypothesis remain limited. Spermatozoa exhibit remarkable variability in size and shape, and studies report a correlation between sperm morphology (sperm length and shape) and sperm competition risk or female reproductive tract morphology. However, whether postcopulatory processes (e.g., sperm competition and cryptic female choice) influence the speed of evolutionary diversification in sperm form is unknown. Using passerine birds, we quantified evolutionary rates of sperm length divergence among lineages (i.e., species pairs) and determined whether these rates varied with the level of sperm competition (estimated as relative testes mass). We found that relative testes mass was significantly and positively associated with more rapid phenotypic divergence in sperm midpiece and flagellum lengths, as well as total sperm length. In contrast, there was no association between relative testes mass and rates of evolutionary divergence in sperm head size, and models suggested that head length is evolutionarily constrained. Our results are the first to show an association between the strength of sperm competition and the speed of sperm evolution, and suggest that postcopulatory sexual selection promotes rapid evolutionary diversification of sperm morphology.

For sexually reproducing species, functionally competent sperm are critical to reproduction. While high atmospheric temperatures are known to influence the timing of breeding, incubation and reproductive success in birds, the effect of temperature on sperm quality remains largely unexplored. Here, we experimentally investigated the impact of ecologically relevant extreme temperatures on cloacal temperature and sperm morphology and motility in zebra finches We periodically sampled males exposed to 30°C or 40°C temperatures daily for 14 consecutive days. Following a 12-day (23°C) recovery period, birds were again exposed to heat, but under the alternate treatment (e.g. birds initially exposed to 40°C were exposed to 30°C). Elevated temperatures led to an increase in cloacal temperature and a reduction in the proportion of sperm with normal morphology; these effects were most notable under 40°C conditions, and were influenced by the duration of heat exposure and prior exposure to high temperature. Our findings highlight the potential role of temperature in determining male fertility in birds, and perhaps also in constraining the timing of avian breeding. Given the increased frequency of heatwaves in a warming world, our results suggest the need for further work on climatic influences on sperm quality and male fertility.