Barbara Tschirren

1 Introduction Ageing, the decrease in the rate of reproduction and survival with increasing age, is mediated by a complex process of molecular, cellular, and physiological damage accumulation that leads to a loss of function, ultimately causing death (Fontana et al. 2010; Kirkwood and Austad 2000). The rate of ageing varies among taxa (Reinke et al. 2022; Riddle et al. 2024), with large species typically living longer than smaller ones (de Magalhaes et al. 2007; Ricklefs 2010).

1 Introduction Across host–parasite systems, hosts display marked inter-individual heterogeneity in their contribution to onward transmission of disease (Lloyd-Smith et al. 2005; Lloyd et al. 2020; VanderWaal and Ezenwa 2016; Woolhouse et al. 1997). Transmission heterogeneity is generalised into the 80:20 rule, where 80% of infections stem from 20% of hosts (Lloyd-Smith et al. 2005; Stein 2011; Sun et al. 2021).

1 INTRODUCTION A reduced fitness of offspring resulting from matings between relatives (i.e. inbreeding depression) is ubiquitous in both wild and domesticated species (Crnokrak & Roff, 1999; Keller, 2002; Leroy, 2014). Inbreeding depression's persistence can be explained by at least two genetic mechanisms: overdominance and partial dominance (Charlesworth & Charlesworth, 1987; Wright, 1977).