Parenting is costly and because the relationship between the mother and embryos is not mutualistic, mother–offspring conflicts may exist whenever resource are scarce. However, intergenerational trade‐offs and conflicts resulting from limited access to water, a vital and depreciable resource, remain largely overlooked. In this study, we examined the physiological, reproductive and life‐history responses to water restriction in the European Common Lizard (Zootoca vivipara). We hypothesized that, under water‐limited conditions, pregnant females experience both short‐term and long‐term physiological impacts (dehydration and stress) underlying an allocation trade‐off for water between mothers and offspring. Water restriction led to a decrease in body mass, and an increase in plasma osmolality (dehydration) and corticosterone concentration in both males and females. The extent of the dehydration was positively correlated with fecundity in females. This suggests a trade‐off between maternal water balance and allocation of water to developing embryos during reproduction. Water restriction had no immediate effect on reproductive output or offspring morphology at birth. Yet, water restriction in pregnant females enhanced their reproductive effort the following year but reduced the early life growth and annual survival of their second‐year offspring. These delayed fitness responses to water restriction in offspring and mothers suggest that water can trigger intergenerational conflicts as demonstrated for energy. Although the mediation of this conflict remains to be clarified, we hypothesized that it represents a selective force that influences reproductive strategies. A plain language summary is available for this article.

Animals use a variety of strategies to avoid acute dehydration and death. Yet, how chronic exposure to sub-lethal dehydration may entail physiological and fitness costs remains elusive. In this study, we experimentally tested if water restriction causes increased oxidative stress (OS) and telomere length (TL) shortening, two well-described mediators of environment–fitness relationships. We exposed 100 yearling female and male common lizards (Zootoca vivipara) either to a 51-day period of water restriction or to water ad libitum, followed by 45 days in common garden outdoor conditions. We measured the kinetic changes in OS and TL and found that water-restricted males had enhanced antioxidant defences and decreased oxidative damage at day 36, whereas females did not immediately respond. A month and a half after water restriction, both sexes experienced a drop in antioxidant capacity but only males exhibited significant TL shortening. In the following 3 years, we found that lizards with longer initial TL and those who maintained stronger antioxidant defences experienced higher longevity, irrespective of sex and water restriction. Together, these results unravelled sex-specific responses to water restriction, with potential applications in better understanding the physiological costs of increasing summer droughts as a result of global climate change.

When effective immune defenses against parasites are costly and resources limited, individuals are expected to alter their investment in immunity in response to the risk of infection. As an ecological factor that can affect both food abundance and parasite exposure, host density can play an important role in host immunity and host–parasite interactions. High levels of intraspecific competition for food and social stress at high host density may diminish immune defenses and increase host susceptibility to parasites. At the same time, for contagious and environmentally transmitted parasites, parasite exposure often increases with host density, whereas in mobile parasites that actively search for hosts, parasite exposure can decrease with host density due to the “encounter‐dilution effect.” To unravel these multiple and potentially opposing effects of host density on immunity, we manipulated density of the common lizard Zootoca vivipara and measured local inflammation in response to PHA injection and levels of infestation by the tick Ixodes ricinus, a mobile ectoparasite for which we expected an encounter‐dilution effect to occur. Local inflammation strongly decreased with lizard density in adults, but not in yearlings. Tick infestation (abundance and prevalence) was negatively correlated with lizard density in both age classes. Using path analyses, we found independent, direct negative density feedbacks on immunity and parasite exposure in adults, supporting the hypothesis of energy constraints and/or physiological stress acting on immunity at high density. In contrast, for yearlings, the best path model showed that density diluted exposure to parasites, which themselves down‐regulated immune defenses in lizards. These results highlight the importance of investigating the pathways among host density, host immunity, and parasite infestation, while accounting for relevant individual traits such as age.