Is there a sicker sex? Dose relationships modify male–female differences in infection prevalence

Abstract

Throughout the animal kingdom, there are striking differences in the propensity of one sex or the other to become infected. However, precisely when we should expect males or females to be the sicker sex remains unclear. A major barrier to answering this question is that very few studies have considered how the susceptibility of males and females changes across the full range of pathogen doses encountered in nature. Without quantifying this ‘dose–susceptibility’ relationship, we have likely underestimated the scope for sex differences to arise. Here, we use the Daphnia magnia – Pasteuria ramosa system to reveal that sex differences in susceptibility are entirely dose-dependent, with pathogens having a higher probability of successfully establishing an infection in mature males at low doses, but mature females at high doses. The scope for male–female differences to emerge is therefore much greater than previously appreciated—extending to sex differences in the upper limits to infection success, per-propagule infectivity risks and density-dependent pathogen behaviour. Applying this expanded scope across the animal kingdom will help us understand when and why a sicker sex emerges, and the implications for diseases in nature—where sex ratios, age structure and pathogen densities vary drastically.

Acclimation to warmer temperatures can protect host populations from both further heat stress and the potential invasion of pathogens

“…Due to differences in early survival, handling errors, and male individuals set up unintentionally, sample sizes for the different treatment combinations and disease traits varied between 17 and 26. All exposed individuals were included in the analysis of host traits in order to capture the changes in a host's phenotype that can manifest even when infection isn't ultimately "successful" for the pathogen in producing mature transmission spores (Butterworth et al, 2024;Hall et al, 2024).…”