Developmental temperature alters the thermal sensitivity of courtship activity and signal-preference relationships, but not mating rates

Mating behaviors are sensitive to novel or stressful thermal conditions, particularly for ectothermic organisms. An organism's sensitivity to temperature, which may manifest in altered mating outcomes, can be shaped in part by temperatures experienced during development. Here, we tested how developmental temperature shapes the expression of adult mating-related behaviors across different ambient conditions, with a focus on courtship behavior, mating rates, and mating signals and preferences. To do so, we reared treehoppers under two temperature regimes and then tested the expression of male and female mating behaviors across a range of ambient temperatures. We found that developmental temperature affects the thermal sensitivity of courtship behavior and mating signals for males. However, developmental temperature did not affect the thermal sensitivity of courtship or mate preferences in females. This sex-specific plasticity did not alter the likelihood of mating across ambient temperatures, but it did disrupt how closely mating signals and preferences matched each other at higher ambient temperatures. As a result, developmental temperature could alter sexual selection through signal-preference de-coupling. We further discuss how adult age may drive sex-specific results, and the potential for mismatches between developmental and mating thermal environments under future climate change predictions.

Experimental evidence for stronger impacts of larval but not adult rearing temperature on female fertility and lifespan in a seed beetle

Temperature impacts behaviour, physiology and life-history of many life forms. In many ectotherms, phenotypic plasticity within reproductive traits could act as a buffer allowing adaptation to continued global warming within biological limits. But there could be costs involved, potentially affecting adult reproductive performance and population growth. Empirical data on the expression of reproductive plasticity when different life stages are exposed is still lacking. Plasticity in key components of fitness (e.g., reproduction) can impose life-history trade-offs. Ectotherms are sensitive to temperature variation and the resulting thermal stress is known to impact reproduction. So far, research on reproductive plasticity to temperature variation in this species has focused on males. Here, I explore how rearing temperature impacted female reproduction and lifespan in the bruchid beetle Callosobruchus maculatus by exposing them to four constant temperatures (17 °C, 25 °C, 27 °C and 33 °C) during larval or adult stages. In these experiments, larval rearing cohorts (exposed to 17 °C, 25 °C, 27 °C and 33 °C, from egg to adulthood) were tested in a common garden setting at 27 °C and adult rearing cohorts, after having developed entirely at 27 °C, were exposed to four constant rearing temperatures (17 °C, 25 °C, 27 °C and 33 °C). I found stage-specific plasticity in all the traits measured here: fecundity, egg morphological dimensions (length and width), lifespan and egg hatching success (female fertility). Under different larval rearing conditions, fecundity and fertility was drastically reduced (by 51% and 42%) at 17 °C compared to controls (27 °C). Female lifespan was longest at 17 °C across both larval and adult rearing: by 36% and 55% compared to controls. Collectively, these results indicate that larval rearing temperature had greater reproductive impacts. Integrating both larval and adult rearing effects, I present evidence that female fertility is more sensitive during larval development compared to adult rearing temperature in this system.

Male and female developmental temperature modulate post-copulatory interactions in a beetle

Sexual selection theory has proven to be fundamental to our understanding of the male-female (sperm-egg) interactions that characterise fertilisation. However, sexual selection does not operate in a void and abiotic environmental factors have been shown to modulate the outcome of pre-copularory sexual interactions. Environmental modulation of post-copulatory interactions are particularly likely because the form and function of primary reproductive traits appears to be acutely sensitive to temperature stress. Here we report the effects of developmental temperature on female reproductive architecture and the interaction between male and female developmental temperature on the outcome of sperm competition in the bruchid beetle Callosobruchus maculatus. When females were reared at developmental temperatures above and below typical temperatures the bursa copulatrix (site of spermatophore deposition) were smaller and, were either shorter and broader (high temperatures) or longer and thinner (low temperatures) than those reared at intermediate temperatures. Males and females reared at low developmental temperatures were less likely to mate than those reared at higher temperatures. Where copulation occurred, females reared at the highest temperature copulated for longest, whilst males reared at the lowest temperature spent longer in copula. Male developmental temperature had a significant impact on the outcome of sperm competition: males reared at 17 °C were largely unsuccessful in sperm competition against control (27 °C) males, although some of the variation in the outcome of sperm competition was a product of the interaction between male and female developmental temperature. Our results demonstrate that male-female interactions that characterise pre- and post-copulatory outcomes are sensitive to developmental temperature and that plasticity in cryptic female preferences could lead to heterogeneous selection on the male reproductive phenotype.

Temperature impacts all behavioral interactions during insect and arachnid reproduction

Temperature shapes the processes and outcomes of behaviors that occur throughout the progression of insect and arachnid mating interactions and reproduction. Here, we highlight how temperature impacts precopulatory activity levels, competition among rivals, communication with potential mates, and the relative costs and benefits of mating. We review how both the prevailing temperature conditions during reproductive activity and the temperatures experienced early in life influence mating-related behavior. To effectively predict the consequences of global warming for insect and arachnid mating behavior, we advocate for future work that universally integrates a function-valued approach to measuring thermal sensitivity. A function-valued approach will be especially useful for understanding how fine-scale temperature variation shapes current and future selection on mating interactions.

Temperature-induced developmental plasticity in Plodia interpunctella: Reproductive behaviour and sperm length

In both plants and animals, male gametogenesis is particularly sensitive to heat stress, to the extent that a single hot or cold day can compromise crop productivity or population persistence. In animals, heat stress during development can impact a male's ability to secure copulations and/or his post-copulatory fertility. Despite such observations, relatively few studies have examined the consequences of developmental temperature on the reproductive behaviour and physiology of individuals. Here, we report for the first time the effects of developmental temperature on the phenotypic expression of both apyrene and eupyrene sperm and the copulatory behaviour of the Indian meal moth, Plodia interpunctella. We show that the length of both apyrene and eupyrene sperm decreases with increasing developmental temperature and that males are less likely to engage in copulation when reared at the highest and lowest temperatures. Where copulation occurred, the duration of copula decreased as male developmental temperature increased. We argue that identification of the mechanisms and consequences of reproductive failure in animals facing heat stress will help understand how wild and domesticated populations will respond to global climate change. We also contend that such studies will help elucidate long-standing evolutionary questions around the maintenance of genetic variation in traits highly relevant to fitness and the role of phenotypic plasticity in driving the evolution of novel traits.