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Host sex is an important source of heterogeneity in the severity of epidemics. Pinpointing the mechanisms causing this heterogeneity can be difficult because differences in behaviour among sexes (e.g. greater territorial aggression in males) can bias exposure risk, obfuscating the role of immune function, which can lead to differences in pathology, in driving differential susceptibility between sexes. Thus, sex‐biased transmission driven by differences in immune function independent of behaviour is poorly understood, especially in non‐mammalian systems.

Here we examine the previously unexplored potential for male‐biased pathology to affect transmission using an avian host–pathogen system. We employ a sex‐dependent multistate transmission model parameterized with isolated, individual‐based experimental exposures of domestic canaries and experimental transmission data of house finches.

The experiment revealed that male birds have shorter incubation periods, longer recovery periods, higher pathogen burdens and greater disease pathology than females. Our model revealed that male‐biased pathology led to epidemic size rapidly increasing with the proportion of male birds, with a nearly 10‐fold increase in total epidemic size from an all‐female to an all‐male simulation.

Our results demonstrate that female‐biased resistance, independent of male behaviour, can drive sex‐dependent transmission in wildlife, indicating that sex‐based differences in immune function, not just differences in exposure risk, can shape epidemic dynamics.

 

While infection and perceived infection risk can influence social and reproductive behavior in several taxa, relatively little is known about how infection specifically affects pair bond behaviors. Some pair bond maintenance behaviors may be costly to maintain during infection, and infection could promote avoidance behaviors within an established pair. Many species exhibiting pair bonds are part of larger social groups, and behavioral shifts in established pairs can result in altered extra-pair contact rates that could also shape disease transmission. Using captive zebra finches (Taeniopygia guttata), we examined how an immune challenge with lipopolysaccharide (LPS) influences activity, social behavior, and pair bond maintenance behaviors in established pairs and their healthy neighbors. We observed shifts in individual and pair maintenance behaviors in both immune-challenged pairs and healthy pairs exposed to a social cue of infection (sick conspecifics). Specifically, LPS-challenged birds decreased activity and social interaction attempts relative to control birds, consistent with LPS-induced sickness behavior. LPS-challenged birds also increased the frequency of clumping (perching together in bodily contact) between individuals within a pair. Healthy birds exposed to immune-challenged conspecifics decreased flight activity and increased self-preening, behaviors which could function to limit infection risk. Exploring how both infection and the perceived risk of infection shape behaviors within and among paired individuals will increase our understanding of the role of social behaviors in shaping disease dynamics.

 

Abstract A major driver of wildlife responses to climate change will include non-genomic effects, like those mediated through parental behavior and physiology (i.e., parental effects). Parental effects can influence lifetime reproductive success and survival, and thus population-level processes. However, the extent to which parental effects will contribute to population persistence or declines in response to climate change is not well understood. These effects may be substantial for species that exhibit extensive parental care behaviors, like birds. Environmental temperature is important in shaping avian incubation behavior, and these factors interact to determine the thermal conditions embryos are exposed to during development, and subsequently avian phenotypes and secondary sex ratios. In this article, we argue that incubation behavior may be an important mediator of avian responses to climate change, we compare incubation strategies of two species adapted to different thermal environments nesting in extreme heat, and we present a simple model that estimates changes in egg temperature based on these incubation patterns and predicted increases in maximum daily air temperature. We demonstrate that the predicted increase in air temperature by 2100 in the central USA will increase temperatures that eggs experience during afternoon off-bouts and the proportion of nests exposed to lethal temperatures. To better understand how species and local adaptations and behavioral-plasticity of incubation behavior will contribute to population responses to climate change comparisons are needed across more avian populations, species, and thermal landscapes. 

Elucidating factors that limit the number of offspring produced is fundamental to understanding life‐history evolution. Here, we examine the hypothesis that parental ability to maintain an optimal physical developmental environment for all offspring constrains clutch size via effects on offspring quality.

Experimental laboratory studies of birds have shown that a <1°C difference in average incubation temperature has diverse effects on fitness‐related post‐hatching offspring phenotypes. Thus, the inability of parents to maintain optimal incubation temperatures could constrain clutch sizes.

A fundamental question that has not been sufficiently addressed is whether larger clutch sizes lead towithinnest variation in egg temperature that is large enough to produce offspring with different phenotypes within a brood. This could lead to differential survival among offspring, and could create a trade‐off between offspring number and quality.

We manipulated clutch size in nests of free‐living wood ducks and measured incubation temperature among and within clutches using multiple temperature loggers.

As clutch size increased, average incubation temperatures were lower and more variable, and eggs took longer to hatch. Notably, the range inaverageincubation temperature among eggswithinnests increased with clutch size and exceeded 1°C in large clutches. Clutch size did not affect hatch success.

In conjunction with our companion laboratory studies that used artificial incubation to document the effects of temperature variation on fitness‐related traits in this species, our work suggests that suboptimal incubation temperatures could be a factor that limits clutch size through diminishing returns on post‐hatch offspring quality.

A freeplain language summarycan be found within the Supporting Information of this article.