Seasonal variations in temperature, rainfall and resource availability are ubiquitous and can exert strong pressures on population dynamics. Infectious diseases provide some of the best-studied examples of the role of seasonality in shaping population fluctuations. In this paper, we review examples from human and wildlife disease systems to illustrate the challenges inherent in understanding the mechanisms and impacts of seasonal environmental drivers. Empirical evidence points to several biologically distinct mechanisms by which seasonality can impact host-pathogen interactions, including seasonal changes in host social behaviour and contact rates, variation in encounters with infective stages in the environment, annual pulses of host births and deaths and changes in host immune defences. Mathematical models and field observations show that the strength and mechanisms of seasonality can alter the spread and persistence of infectious diseases, and that population-level responses can range from simple annual cycles to more complex multiyear fluctuations. From an applied perspective, understanding the timing and causes of seasonality offers important insights into how parasite-host systems operate, how and when parasite control measures should be applied, and how disease risks will respond to anthropogenic climate change and altered patterns of seasonality. Finally, by focusing on well-studied examples of infectious diseases, we hope to highlight general insights that are relevant to other ecological interactions.

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Seasonality and the dynamics of infectious diseases.

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Complex Population Dynamics: A Theoretical/Empirical Synthesis

Summary
1Classic evolutionary models interpret ageing as a cost of reproduction, but evolutionary research has thus far largely neglected the conceptual links between the evolution of ageing and a key mode of selection on male and female reproductive strategies – sexual selection and sexual conflict.
2We synthesize ideas and evidence linking sex and ageing, and make the case that a focus on this fascinating problem will ultimately lead to a more complete understanding of both the evolution of ageing and the evolution of sexual strategies.
3The primary and secondary differentiation of male and female reproductive strategies is expected to produce sex-specific optima for traits that affect longevity and ageing rate, often favouring a ‘live fast, die young’ strategy in males, relative to females, although numerous exceptions to this pattern are observed and sex-differences in ageing rate, in particular, remain poorly understood.
4Conversely, environmental factors that influence life expectancy or ageing rate can thereby determine the magnitude or even sign of sexual selection.
5Sexual conflict is expected to displace the sexes from their sex-specific life-history optima through sexually antagonistic interactions, as well as sex-specific selection on loci expressed in both sexes.
6Despite the availability of interesting and testable hypotheses linking sexual selection and ageing, relevant empirical studies are remarkably sparse, and the complex relation between sex, mortality rate and ageing remains poorly understood.

/pdf/sexual-selection-sexual-conflict-and-the-evolution-of-ageing-2gdlx1wuzp.pdf

Sexual selection, sexual conflict and the evolution of ageing and life span

Suppressive effects of androgens on the immune system.

Summary
1. Ecological immunology and disease ecology are two relatively young disciplines that apply ecological approaches and principles to traditionally non-ecological fields. In both cases, an ecological perspective has allowed new insights to emerge by focusing attention on variation over space and time, and by emphasizing the role of the environment in shaping individual responses and the outcome of host-pathogen interactions. Here we review the growing conceptual interface between these two rapidly evolving fields.

2. Areas of synergy between ecological immunology and disease ecology aim to translate variation in within-host processes (e.g. immunity) into between-host dynamics (e.g. parasite transmission). Emerging areas of synergy include potential immune mechanisms that underlie host heterogeneity in disease susceptibility, teasing apart the effects of environmental factors such as seasonality and climate on host susceptibility and pathogen dynamics, and predicting the outcome of co-infection by functionally distinct groups of parasites that elicit different immune responses.

3. In some cases, practical limitations have constrained the merging of ideas in ecological immunology and disease ecology. We discuss several logistical challenges, including dissecting the relative roles of host exposure and susceptibility, establishing links between measures of immunity and pathogen resistance in wild populations, and incorporating relevant immune variation into prevailing disease ecology modeling frameworks.

4. Future work at the interface of these two fields should advance understanding of life-history theory, host-pathogen dynamics, and physiological ecology, and will also contribute to targeted approaches for wildlife health and zoonotic disease prevention.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-2435.2010.01753.x

Disease ecology meets ecological immunology: understanding the links between organismal immunity and infection dynamics in natural populations

The expression of sexual ornaments has been suggested to reliably indicate individual quality, such as the ability to cope with parasites and diseases. The Immunocompetence Handicap Hypothesis (IHH) states that testosterone-dependent ornaments honestly signal such quality because of physiological costs associated with testosterone, such as impaired immune function. We tested predictions of the IHH both correlatively and experimentally in red grouse Lagopus lagopus scoticus. Male grouse exhibit supra-orbital red combs whose size is testosterone-dependent. We found that comb size was not correlated to infection intensity by two parasites (coccidia and the nematode Trichostrongylus tenuis), but it was significantly positively correlated with condition and T-cell-mediated immunity (the ability to mount a primary inflammatory response). We manipulated testosterone by means of implants and re-caught males after a month to investigate the effects on comb size, condition, immunity, and parasite load. Males implanted with testosterone had increased comb size, lost more condition, and had lower T-cell-mediated immunity than control males. Increased testosterone also resulted in a significant increase in coccidia infection intensity but had no effect on T. tenuis burden. The results are consistent with predictions of the IHH and suggest that comb size honestly indicates immunocompetence and males’ ability to cope with certain parasites. Females could thus benefit from choosing mates based on the expression of this sexual trait. Key words: Immunocompetence Handicap Hypothesis, parasite, red grouse, sexual ornament, T-cell-mediated immunity, testosterone. [Behav Ecol 15:930–937 (2004)]

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Testosterone, immunocompetence, and honest sexual signaling in male red grouse

The reliability of different egg counting methods for estimating the intensity of Trichostrongylus tenuis infections in red grouse, Lagopus lagopus scoticus, was investigated in the autumn, when grouse may harbour high parasite intensities. Possible limitations to the use of these methods were also examined. Faecal egg counts were found to accurately estimate T. tenuis worm intensities, at least up to an observed maximum of c. 8000 worms. Two egg counting methods (smear and McMaster) gave consistent results, although the exact relationship with worm intensity differed according to the method used. Faecal egg counts significantly decreased with increasing length of sample storage time, but egg counts were reliable for estimating worm intensity for three weeks. The concentration of eggs in the caecum was also found to reliably estimate worm intensity. However, egg counts from frozen gut samples cannot be used to estimate worm intensities. These results conclude that, despite some limitations, faecal and caecum egg counts provide useful and reliable ways of measuring T. tenuis intensities in red grouse.

/pdf/faecal-egg-counts-provide-a-reliable-measure-of-k8ufrpl3ox.pdf

Faecal egg counts provide a reliable measure of Trichostrongylus tenuis intensities in free-living red grouse Lagopus lagopus scoticus.

Field studies of mechanisms involved in population regulation have tended to focus on the roles of either intrinsic or extrinsic factors, but these are rarely mutually exclusive and their interactions can be crucial in determining dynamics. Experiments on red grouse Lagopus lagopus scoticus have shown that population instability can be caused both by the effects of a parasitic nematode, Trichostrongylus tenuis, on host production or by changes in testosterone influencing aggressive behaviour and recruitment. We experimentally tested for an interaction between testosterone and T. tenuis in free-living male grouse. A total of 123 grouse were caught in autumn, treated with an anthelmintic to remove parasites, and then given either testosterone or empty, control, implants. After one month grouse were re-infected with a standard dose of parasites. We show that males with increased testosterone levels had greater parasite intensities than controls after one year. We discuss possible physiological and behavioural mechanisms linking testosterone and increased parasite intensity, and the implications for our understanding of complex, unstable population dynamics.

/pdf/interactions-between-intrinsic-and-extrinsic-mechanisms-in-a-3w4m9mckil.pdf

Linzi J Seivwright

Papers

Testosterone, immunocompetence, and honest sexual signaling in male red grouse

Faecal egg counts provide a reliable measure of Trichostrongylus tenuis intensities in free-living red grouse Lagopus lagopus scoticus.

Interactions between intrinsic and extrinsic mechanisms in a cyclic species: testosterone increases parasite infection in red grouse.