Contrasting effects of large density changes on relative testes size in fluctuating populations of sympatric vole species

Across species, there is usually a positive relationship between sperm competition level and male reproductive effort on ejaculates, typically measured using relative testes size (RTS). Within populations, demographic and ecological processes may drastically alter the level of sperm competition and thus, potentially affect the evolution of testes size. Here, we use longitudinal records (across 38 years) from wild sympatric Fennoscandian populations of five species of voles to investigate whether RTS responds to natural fluctuations in population density, i.e. variation in sperm competition risk. We show that for some species RTS increases with density. However, our results also show that this relationship can be reversed in populations with large-scale between-year differences in density. Multiple mechanisms are suggested to explain the negative RTS–density relationship, including testes size response to density-dependent species interactions, an evolutionary response to sperm competition levels that is lagged when density fluctuations are over a certain threshold, or differing investment in pre- and post-copulatory competition at different densities. The results emphasize that our understanding of sperm competition in fluctuating environments is still very limited

Life-history genotype explains variation in migration activity in Atlantic salmon (Salmo salar)

Peer reviewe

Polyandry and its effect on male and female fitness

Miksi metsämyyränaaraalla on useampia rakastajia, on mielenkiintoinen kysymys evoluutioekologiassa. Yksi koiras riittäisi hyvin hedelmöittämään kaikki naaraan munasolut, ja useamman koiraan kanssa jatkuvat lemmenleikit altistavat naaraan niin pedon hampaille kuin tautiriskille. Useamman koiraan kanssa parittelu näyttää kuitenkin olevan enemmän sääntö kuin poikkeus.Ines Klemmen väitöskirjatutkimuksen aiheena oli selvittää, mitä hyötyä naaraalle on useamman koiraan kanssa parittelusta. Klemme osoitti tutkimuksessaan, että mitä useamman koiraan kanssa metsämyyränaaras parittelee yhden lisääntymiskerran aikana, sitä parempi on sen lisääntymistulos. Metsämyyrän poikuekoko on 5-6, ja naaras saattaa synnyttää 4-5 poikuetta kesässä.Naaras tuli yksiselitteisesti useammin tiineeksi paritellessaan useamman kerran. Sillä, paritteliko naaras yhden vai useamman rakastajan kanssa, ei ollut merkitystä. Hedelmöityminen ei siis riipu kumppanien vaan stimulaation määrästä. Useamman koiraan kanssa paritelleet myyränaaraat saivat kuitenkin selkeästi enemmän lapsenlapsia. Tämä uusi ja yllättävä tulos voidaan selittää ainoastaan geneettisillä eduilla, jotka naaras on saanut useamman koiraan perintöaineksen kautta. Todennäköisin mekanismi havaitun tuloksen taustalla on spermakilpailu: useamman koiraan siittiöiden kilpajuoksussa paremmat voittavat, ja syntyvät pojat jatkavat tätä superlinjaa seuraavassa sukupolvessa.Ines Klemmen väitöskirja kuuluu evoluutioekologian osa-alueeseen, jossa tutkitaan tiettyjen käyttäytymispiirteiden syntyä. Metsämyyrä on tällaiseen tutkimukseen erinomainen mallilaji. Tuloksia voidaan ja niitä tuleekin tarkastella lisääntymisjärjestelmien syntymisen näkökulmasta yleensä. Uusien DNA-analyysien valossa näyttää siltä, että niin koiraat kuin naaraatkin parittelevat nisäkäsmaailmassa yleisesti useamman partnerin kanssa eli sekasopuisesti. Tutkijat kuitenkin varoittavat oman lajimme seksuaalikäyttäytymistä koskevista suoraviivaisista tulkinnoista.Polyandry, i.e., mating with multiple males within one reproductive event, is currently regarded as one of the most significant female reproductive behaviours. Its adaptive functions and its consequences on male reproductive success, however, are still not identified in most species. In this thesis I have studied causes and consequences of polyandry in a small mammal species, the bank vole (Clethrionomys glareolus), focusing on two questions. First, are dominance-related traits in males related to reproductive success in polyandrous species and second, which benefits do females gain from mating with multiple males. I conducted five experiments, two under laboratory conditions and three under semi-natural conditions in large outdoor enclosures. I found evidence that two dominance-related traits, male body size and scent marking behaviour, are related to male reproductive success. However, this was only the case under certain environmental and social conditions, suggesting that the strength of sexual selection on these traits is variable. I was further able to show that bank vole females gain direct and genetic fitness benefits from multiple mating, suggesting that polyandry is a beneficial strategy for females. The results of this study may help to understand the evolution of polyandry and its implications on male and female reproductive success, which is of general importance for the study of sexual selection

Shoaling with infected conspecifics does not improve resistance to trematode infection

Group‐living animals can gain protection against parasitic infections through social contacts with previously infected conspecifics (social immunization). Recent research suggests that such protective effects can be induced through visual or chemical cues released by infected individuals, resulting in anticipatory immune upregulation among group members. Here, we study cue‐induced social resistance in rainbow trout Oncorhynchus mykiss exposed to a trematode parasite, the eye‐fluke Diplostomum pseudospathaceum. We established groups of naïve individuals (receivers) that were paired with previously infected individuals (donors) at different ratios of donors to receivers and at different time points since donor exposure to capture varying concentrations of the anticipated cues. While the pre‐infection elevated resistance among the donors, there was no evidence of social transfer of resistance, regardless of the ratio of donors and receivers in a group or the time since the pre‐infection. The results suggest that resistance through social signalling may be system‐specific and requires further study into the generality of the phenomenon as well as the nature of the cues involved.peerReviewe

Data from: Learned parasite avoidance is driven by host personality and resistance to infection in a fish–trematode interaction

Cognitive abilities related to the assessment of risk improve survival. While earlier studies have examined the ability of animals to learn to avoid predators, learned parasite avoidance has received little interest. In a series of behavioural trials with the trematode parasite Diplostomum pseudospathaceum, we asked whether sea trout (Salmo trutta trutta) hosts show associative learning in the context of parasitism and if so, whether learning capacity is related to the likelihood of infection mediated through host personality and resistance. We show that animals are capable of learning to avoid visual cues associated with the presence of parasites. However, avoidance behaviour ceased after the likely activation of host resistance following consecutive exposures during learning, suggesting that resistance to infection outweighs avoidance. Further, we found a positive relationship between learning ability and boldness, suggesting a compensation of risky lifestyles through increased investment in cognitive abilities. By contrast, an increased risk of infection due to low resistance was not balanced by learning ability. Instead, these traits were positively related, which may be explained by inherent physiological qualities controlling both traits. Overall, the results demonstrate that parasitism, in addition to other biological interactions such as predation, is an important selective factor in the evolution of animal cognition

Data from: Parasite-mediated changes in host traits alter food web dynamics

<p><span>Parasites commonly alter the phenotype of their host, thereby influencing competitive and consumer-resource interactions. This could trigger a cascade effect on the dynamics of biological communities, but the role of parasites in ecosystem processes is poorly understood. In this study, we investigate how parasite-induced trait modifications shape the dynamics of a complex lake food web using an allometric trophic network model (ATN). We simulated infections of stage-structured fish host populations via increased maintenance costs and predation risk. Our results show that host trait modifications can significantly impact host demography, with stage-specific biomass declines up to 60%. However, less severely affected host stages buffered these effects and sustained the population. Importantly, host biomass decline altered the dynamics of species interactions and these effects cascaded through the entire community, with biomass changes observed at all trophic levels. Our findings emphasize the importance of incorporating both indirect parasite effects and host life history in ecological network studies for more realistic simulations of community dynamics.</span></p><p>Funding provided by: Academy of Finland<br>Crossref Funder Registry ID: https://ror.org/05k73zm37<br>Award Number: 317495</p><p>Funding provided by: Natural Sciences and Engineering Research Council<br>Crossref Funder Registry ID: https://ror.org/01h531d29<br>Award Number: </p><p>Funding provided by: European Research Council<br>Crossref Funder Registry ID: https://ror.org/0472cxd90<br>Award Number: 770884</p&gt

Data from: Parasite-mediated changes in host traits alter food web dynamics

<p><span>Parasites commonly alter the phenotype of their host, thereby influencing competitive and consumer-resource interactions. This could trigger a cascade effect on the dynamics of biological communities, but the role of parasites in ecosystem processes is poorly understood. In this study, we investigate how parasite-induced trait modifications shape the dynamics of a complex lake food web using an allometric trophic network model (ATN). We simulated infections of stage-structured fish host populations via increased maintenance costs and predation risk. Our results show that host trait modifications can significantly impact host demography, with stage-specific biomass declines up to 60%. However, less severely affected host stages buffered these effects and sustained the population. Importantly, host biomass decline altered the dynamics of species interactions and these effects cascaded through the entire community, with biomass changes observed at all trophic levels. Our findings emphasize the importance of incorporating both indirect parasite effects and host life history in ecological network studies for more realistic simulations of community dynamics.</span></p><p>Funding provided by: Academy of Finland<br>Crossref Funder Registry ID: https://ror.org/05k73zm37<br>Award Number: 317495</p><p>Funding provided by: Natural Sciences and Engineering Research Council<br>Crossref Funder Registry ID: https://ror.org/01h531d29<br>Award Number: </p><p>Funding provided by: European Research Council<br>Crossref Funder Registry ID: https://ror.org/0472cxd90<br>Award Number: 770884</p&gt

Within-host interactions shape virulence-related traits of trematode genotypes

Within‐host interactions between co‐infecting parasites can significantly influence the evolution of key parasite traits, such as virulence (pathogenicity of infection). The type of interaction is expected to predict the direction of selection, with antagonistic interactions favouring more virulent genotypes and synergistic interactions less virulent genotypes. Recently, it has been suggested that virulence can further be affected by the genetic identity of co‐infecting partners (G × G interactions), complicating predictions on disease dynamics. Here, we used a natural host–parasite system including a fish host and a trematode parasite to study the effects of G × G interactions on infection virulence. We exposed rainbow trout (Oncorhynchus mykiss) either to single genotypes or to mixtures of two genotypes of the eye fluke Diplostomum pseudospathaceum and estimated parasite infectivity (linearly related to pathogenicity of infection, measured as coverage of eye cataracts) and relative cataract coverage (controlled for infectivity). We found that both traits were associated with complex G × G interactions, including both increases and decreases from single infection to co‐infection, depending on the genotype combination. In particular, combinations where both genotypes had low average infectivity and relative cataract coverage in single infections benefited from co‐infection, while the pattern was opposite for genotypes with higher performance. Together, our results show that infection outcomes vary considerably between single and co‐infections and with the genetic identity of the co‐infecting parasites. This can result in variation in parasite fitness and consequently impact evolutionary dynamics of host–parasite interactions.peerReviewe

Negative associations between parasite avoidance, resistance and tolerance predict host health in salmonid fish populations

Genetic variation in defence against parasite infections is fundamental for host–parasite evolution. The overall level of defence of a host individual or population includes mechanisms that reduce parasite exposure (avoidance), establishment (resistance) or pathogenicity (tolerance). However, how these traits operate and evolve in concert is not well understood. Here, we investigated genetic variation in and associations between avoidance, resistance and tolerance in a natural host–parasite system. Replicated populations of Atlantic salmon (Salmo salar) and sea trout (an anadromous form of brown trout, Salmo trutta) were raised under common garden conditions and infected with the eye fluke Diplostomum pseudospathaceum. We demonstrate significant genetic variation in the defence traits across host populations and negative associations between the traits, with the most resistant populations showing the weakest avoidance and the lowest infection tolerance. These results are suggestive of trade-offs between different components of defence and possibly underlie the genetic variation in defence traits observed in the wild. Because the three defence mechanisms affect host–parasite evolution in profoundly different ways, we emphasize the importance of studying these traits in concert.peerReviewe

dataset for Klemme & Karvonen 2016

Complete dataset for study by Klemme & Karvonen, including fish morphometric data, results from repeated personality tests and from repeated avoidance learning tests as well as parasite numbers acquired before the behavioural tests, during the behavioural tests and during a resistance test