Senescence in wild European badgers: the roles of parental age effects and immunosenescence

Abstract

Extensive evidence now exists for senescence (within-individual age-related declines in components of fitness) in natural animal populations. Research has now expanded to investigate the inter-generational effects of senescence (‘parental age effects’) and the underlying mechanisms that give rise to senescent declines. In this thesis I use a remarkable long-term longitudinal dataset collected from wild European badgers (Meles meles) to examine the inter-generational fitness consequences of senescence, and the role that immunological changes may play in senescence. In Chapter 2, I present evidence of a deleterious maternal age effect on offspring lifetime reproductive success (LRS) that appears to arise principally from effects on offspring survival rather than annual reproductive success. Furthermore, I find that the timing of onset of this maternal senescence in offspring performance precedes that of maternal senescence in fecundity, and thereby compounds and advances the onset of the deleterious fitness consequences of senescence for mothers. In Chapter 3, I find evidence of deleterious paternal age effects on offspring LRS, which appear to arise principally from a deleterious paternal terminal effect; a decrease in offspring LRS in the father’s terminal reproductive episode. In Chapter 4, I present rare evidence from a natural animal population of a biomedically significant form of immune system ageing termed ‘inflammageing’. My findings reveal a within-individual age-related increase in a baseline inflammatory marker; haptoglobin, and a within-individual age-related decrease in the strength of the pro-inflammatory IFN-γ cytokine response to immune stimulation; both consistent with inflammageing. My analyses also suggest that the high levels of baseline inflammation experienced in late-life may entail fitness costs. In Chapter 5, I explore the effects of population density and intra-sexual competition on immunity. I find no clear evidence of population density effects on four humoral immune parameters, but do find evidence of sex differences in immunity consistent with the expectation of costs to immunity arising from higher mean levels of investment in intra-sexual mate competition among males than females in this polygynous species. Together this work provides novel evidence of the inter-generational effects of senescence and highlights the potential for inflammageing to play a key role in age-related declines in components of fitness in natural animal populations.