How to study parasites and host migration: a roadmap for empiricists

Pathogen-induced alterations in fine-scale movement behaviour predict impaired reproductive success

Pathogens play an important role in ecosystems and may impair fitness-enhancing activities such as foraging. However, the sublethal effects of pathogens on host movement behaviour and their subsequent impacts on reproductive success are poorly understood. In this study, we used high-resolution tracking to examine the movements of free-ranging European starlings (Sturnus vulgaris) associated with sublethal avian blood parasite infections. We found that naturally infected individuals displayed reduced foraging behaviour, remained closer to their breeding location, and selected lower-quality habitats. These patterns were associated with poorer body condition of adults and less favourable development for their offspring. These behavioural changes suggest physiological limitations imposed by infection, reducing parental care and reproductive output. Our results provide compelling evidence that pathogen-induced changes in fine-scale movement behaviour are linked to impaired reproductive success, further emphasizing the need for a movement ecology perspective in local host-pathogen dynamics.

Effect of resistant compartment on pathogen strategy in partially migratory populations

Migration, the recurring movement of animals between habitats, can exert pressures on the pathogens they host. Properties of host populations can determine pathogen strategy (e.g. virulence) to increase pathogen fitness. To study the effect of adding a resistant compartment on virulence evolution, we developed an SIRS model and examined the winning pathogen strategy across different rates of recovery and of immunity loss. We find that when hosts spend a relatively long time in the resistant compartment, a more virulent pathogen evolves. These results have implications in conservation of migratory animal populations afflicted by disease.

Does malaria infection increase the risk of predation-related mortality during bird migration?

The migratory culling hypothesis posits that infected individuals are less likely to survive long-distance migration due to physiological and behavioral effects, but this lacks empirical evidence. Here, we tested this hypothesis by sampling 357 passerines from 11 species during their autumn migration to wintering grounds in two different areas, i) at a stopover in southern Spain, and ii) in the Canary Islands, where they were drifted and preyed upon by Eleonora's falcons while en route to the southern Sahara. Molecular detection of infections by Plasmodium, Haemoproteus, and Leucocytozoon was conducted on bird samples. A higher prevalence of both Plasmodium and Haemoproteus was observed in birds preyed upon by falcons. While a complete understanding of the mechanistic effects of haemosporidian infections on migration performance needs experimental validation, our approach suggests that infection reduces migration success by increasing mortality due to route deviations and/or predation.

Sick without signs. Subclinical infections reduce local movements, alter habitat selection, and cause demographic shifts

In wildlife populations, parasites often go unnoticed, as infected animals appear asymptomatic. However, these infections can subtly alter behaviour. Field evidence of how these subclinical infections induce changes in movement behaviour is scarce in free-ranging animals, yet it may be crucial for zoonotic disease surveillance. We used an ultra-high-resolution tracking system (ATLAS) to monitor the movements of 60 free-ranging swallows every 8 seconds across four breeding seasons, resulting in over 1 million localizations. About 40% of these swallows were naturally infected with haemosporidian parasites. Here, we show that infected individuals had reduced foraging ranges, foraged in lower quality habitats, and faced a lowered survival probability, with an average reduction of 7.4%, albeit with some variation between species and years. This study highlights the impact of subclinical infections on movement behaviour and survival, emphasizing the importance of considering infection status in movement ecology. Our findings provide insights into individual variations in behaviour and previously unobservable local parasite transmission dynamics.

Elucidating nematode diversity and prevalence in moose across a wide latitudinal gradient using DNA metabarcoding

Parasitic nematodes are ubiquitous and can negatively impact their host by reducing fecundity or increasing mortality, yet the driver of variation in the parasite community across a wildlife host's geographic distribution remains elusive for most species. Based on an extensive collection of fecal samples (n = 264) from GPS marked moose (Alces alces), we used DNA metabarcoding to characterize the individual (sex, age class) and seasonal parasitic nematode community in relation to habitat use and migration behavior in five populations distributed across a wide latitudinal gradient (59.6°N to 70.5°N) in Norway. We detected 21 distinct nematode taxa with the six most common being Ostertagia spp., Nematodirella spp., Trichostongylus spp., T. axei, Elaphostrongylus alces, and an unclassified Strongylida. There was higher prevalence of livestock parasites in areas with larger sheep populations indicating a higher risk of spillover events. The individual level nematode richness was mostly consistent across study areas, while the number and type of nematode taxa detected at each study area varied considerably but did not follow a latitudinal gradient. While migration distance affected nematode beta-diversity across all sites, it had a positive effect on richness at only two of the five study areas suggesting population specific effects. Unexpectedly, nematode richness was higher in winter than summer when very few nematodes were detected. Here we provide the first extensive description of the parasitic nematode community of moose across a wide latitudinal range. Overall, the population-specific impact of migration on parasitism across the distribution range and variation in sympatry with other ruminants suggest local characteristics affect host-parasite relationships.

A unified evolutionary framework for understanding parasite infection and host migratory behaviour

Animal migration impacts organismal health and parasite transmission: migrants are simultaneously exposed to parasites and able to reduce infection for both individuals and populations. However, these dynamics are difficult to study; empirical studies reveal disparate results while existing theory makes assumptions that simplify natural complexity. Here, we systematically review empirical studies of migration and infection across taxa, highlighting key gaps in our understanding. Next, we develop a unified evolutionary framework incorporating different selective pressures of parasite-migration interactions while accounting for ecological complexity that goes beyond previous theory. Our framework generates diverse migration-infection patterns paralleling those seen in empirical systems, including partial and differential migration. Finally, we generate predictions about which mechanisms dominate which empirical systems to guide future studies. Our framework provides an overarching understanding of selective pressures shaping migration patterns in the context of animal health and disease, which is critical for predicting how environmental change may threaten migration.

Gaps in modelling animal migration with evolutionary game theory: infection can favour the loss of migration

Ongoing environmental changes alter how natural selection shapes animal migration. Understanding how these changes play out theoretically can be done using evolutionary game theoretic (EGT) approaches, such as looking for evolutionarily stable strategies. Here, we first describe historical patterns of how EGT models have explored different drivers of migration. We find that there are substantial gaps in both the taxa (mammals, amphibians, reptiles, insects) and mechanisms (mutualism, interspecific competition) included in past EGT models of migration. Although enemy interactions, including parasites, are increasingly considered in models of animal migration, they remain the least studied of factors for migration considered to date. Furthermore, few papers look at changes in migration in response to perturbations (e.g. climate change, new species interactions). To address this gap, we present a new EGT model to understand how infection with a novel parasite changes host migration. We find three possible outcomes when migrants encounter novel parasites: maintenance of migration (despite the added infection cost), loss of migration (evolutionary shift to residency) or population collapse, depending on the risk and cost of getting infected, and the cost currency. Our work demonstrates how emerging infection can alter animal behaviour such as migration. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

A skewed literature: Few studies evaluate the contribution of predation-risk effects to natural field patterns

A narrative in ecology is that prey modify traits to reduce predation risk, and the trait modification has costs large enough to cause ensuing demographic, trophic and ecosystem consequences, with implications for conservation, management and agriculture. But ecology has a long history of emphasising that quantifying the importance of an ecological process ultimately requires evidence linking a process to unmanipulated field patterns. We suspected that such process-linked-to-pattern (PLP) studies were poorly represented in the predation risk literature, which conflicts with the confidence often given to the importance of risk effects. We reviewed 29 years of the ecological literature which revealed that there are well over 4000 articles on risk effects. Of those, 349 studies examined risk effects on prey fitness measures or abundance (i.e., non-consumptive effects) of which only 26 were PLP studies, while 275 studies examined effects on other interacting species (i.e., trait-mediated indirect effects) of which only 35 were PLP studies. PLP studies were narrowly focused taxonomically and included only three that examined unmanipulated patterns of prey abundance. Before concluding a widespread and influential role of predation-risk effects, more attention must be given to linking the process of risk effects to unmanipulated patterns observed across diverse ecosystems.