Host in reserve: The role of common shrews (Sorex araneus) as a supplementary source of tick hosts in small mammal communities influenced by rodent population cycles.

Rodents often act as important hosts for ticks and as pathogen reservoirs. At northern latitudes, rodents often undergo multi-annual population cycles, and the periodic absence of certain hosts may inhibit the survival and recruitment of ticks. We investigated the potential role of common shrews (Sorex araneus) to serve as a supplementary host source to immature life stages (larvae and nymphs) of a generalist tick Ixodes ricinus and a small mammal specialist tick I. trianguliceps, during decreasing abundances of bank voles (Myodes glareolus). We used generalized mixed models to test whether ticks would have a propensity to parasitize a certain host species dependent on host population size and host population composition across two high-latitude gradients in southern Norway, by comparing tick burdens on trapped animals. Host population size was defined as the total number of captured animals and host population composition as the proportion of voles to shrews. We found that a larger proportion of voles in the host population favored the parasitism of voles by I. ricinus larvae (estimate = -1.923, p = .039) but not by nymphs (estimate = -0.307, p = .772). I. trianguliceps larvae did not show a lower propensity to parasitize voles, regardless of host population composition (estimate = 0.875, p = .180), while nymphs parasitized shrews significantly more as vole abundance increased (estimate = 2.106, p = .002). These results indicate that common shrews may have the potential to act as a replacement host during periods of low rodent availability, but long-term observations encompassing complete rodent cycles may determine whether shrews are able to maintain tick range expansion despite low rodent availability.

High-elevational occurrence of two tick species, Ixodes ricinus and I. trianguliceps, at their northern distribution range.

BACKGROUND: During the last decades a northward and upward range shift has been observed among many organisms across different taxa. In the northern hemisphere, ticks have been observed to have increased their latitudinal and altitudinal range limit. However, the elevational expansion at its northern distribution range remains largely unstudied. In this study we investigated the altitudinal distribution of the exophilic Ixodes ricinus and endophilic I. trianguliceps on two mountain slopes in Norway by assessing larval infestation rates on bank voles (Myodes glareolus). METHODS: During 2017 and 2018, 1325 bank voles were captured during the spring, summer and autumn at ten trapping stations ranging from 100 m to 1000 m.a.s.l. in two study areas in southern Norway. We used generalized logistic regression models to estimate the prevalence of infestation of both tick species along gradients of altitude, considering study area, collection year and season, temperature, humidity and altitude interactions as extrinsic variables, and host body mass and sex as intrinsic predictor variables. RESULTS: We found that both I. ricinus and I. trianguliceps infested bank voles at altitudes up to 1000 m.a.s.l., which is a substantial increase in altitude compared to previous findings for I. ricinus in this region. The infestation rates declined more rapidly with increasing altitude for I. ricinus compared to I. trianguliceps, indicating that the endophilic ecology of I. trianguliceps may provide shelter from limiting factors tied to altitude. Seasonal effects limited the occurrence of I. ricinus during autumn, but I. trianguliceps was found to infest rodents at all altitudes during all seasons of both years. CONCLUSIONS: This study provides new insights into the altitudinal distribution of two tick species at their northern distribution range, one with the potential to transmit zoonotic pathogens to both humans and livestock. With warming temperatures predicted to increase, and especially so in the northern regions, the risk of tick-borne infections is likely to become a concern at increasingly higher altitudes in the future.

Do bank voles (Myodes glareolus) trapped in live and lethal traps show differences in tick burden?

In studies assessing tick abundance, the use of live traps to capture and euthanize rodent hosts is a commonly used method to determine their burden. However, captive animals can experience debilitating or fatal capture stress as a result prior to collection. An alternative method is the use of lethal traps, but this can potentially lead to tick drop-off between the time of capture and collection. In this study, in order to determine whether subjecting animals to capture stress is inevitable, we tested the difference in sheep tick (Ixodes ricinus) larval burdens between bank voles (Myodes glareolus) captured alive and euthanized, and lethally trapped bank voles. During 2017 and 2018, 1318 bank voles were captured using live (Ugglan Special no. 2) and lethal (Rapp2 Mousetrap) traps during two consecutive years over three seasons in two locations in Norway. Voles captured alive would remain captive until euthanized, while lethally trapped voles were killed instantly upon capture. Log-linear models, accounting for overdispersion, were used to determine whether trap type was influencing observed tick burden. Bank voles captured in lethal traps carried 5.7% more larvae compared to euthanized voles captured in live traps, but this difference was not significant (p = 0.420). Males were overall captured 2.7 times more frequently than females, and the sex ratio was equal in both trap types. This study shows that the use of lethal traps to determine tick burden of rodents is sufficiently reliable, without having to subject animals to potentially lethal stress, hereby reducing some ethical concerns of animal suffering and the results thereof, without compromising accuracy. Lethal trapping is also often more economical and practical, further favoring this collection method.

Island biogeography theory explains the genetic diversity of a fragmented rock ptarmigan (Lagopus muta) population.

The island biogeography theory is one of the major theories in ecology, and its applicability to natural systems is well documented. The core model of the theory, the equilibrium model of island biogeography, predicts that species diversity on an island is positively related to the size of the island, but negatively related by the island's distance to the mainland. In recent years, ecologists have begun to apply this model when investigating genetic diversity, arguing that genetic and species diversity might be influenced by similar ecological processes. However, most studies have focused on oceanic islands, but knowledge on how the theory applies to islands located on the mainland (e.g., mountain islands, forest islands) is scarce. In this study, we examined how the size and degree of isolation of mountain islands would affect the genetic diversity of an alpine bird, the rock ptarmigan (Lagopus muta). Within our study area, we defined the largest contiguous mountain area as the mainland, while smaller mountains surrounding the mainland were defined as islands. We found that the observed heterozygosity (H o) was significantly higher, and the inbreeding coefficient (F is) significantly lower, on the mainland compared to islands. There was a positive significant relationship between the unbiased expected heterozygosity (H n.b.) and island size (log km2), but a negative significant relationship between H o and the cost distance to the mainland. Our results are consistent with the equilibrium model of island biogeography and show that the model is well suited for investigating genetic diversity among islands, but also on islands located on the mainland.

Development and evaluation of 16 new microsatellite loci for the rock ptarmigan (Lagopus muta) and cross-species amplification for the willow grouse (L. lagopus).

OBJECTIVE: The genetic markers designed for this study can facilitate future genetic studies on the rock ptarmigan (Lagopus muta). To our knowledge no microsatellite markers have ever been developed specifically for this species before. These new microsatellite markers will be useful for population genetics studies and for future conservation projects. RESULTS: Using Next Generation Sequencing 6252 potential microsatellite sequences were found. Sixteen nonpalindromic tetranucleotide microsatellites and their respective primers were selected. The markers were tested on both the rock ptarmigan and the willow grouse (L. lagopus). The number of alleles varied between 2 and 18 for the rock ptarmigan, and between 3 and 13 for the willow grouse. Expected heterozygosity was in the range 0.1244-0.8692 and 0.1358-0.8722 for the rock ptarmigan and the willow grouse, respectively.

Dispersal of male ortolan buntings away from areas with low female density and a severely male-biased sex ratio.

Dispersal is expected to enhance individual fitness, and individuals should thus disperse from areas with poor conditions to areas with more favourable conditions. Few studies have compared conditions before and after dispersal of the same individuals, and in birds little is known about the effects of sex ratio and female density on male dispersal decisions. In this study we examined various fitness-related parameters that adult male ortolan buntings, Emberiza hortulana, might use as cues in their decisions to disperse and settle. The study population has a strongly male-biased sex ratio. Using pairwise comparisons of pre- and post-dispersal conditions, we found that males moved from areas with low female density and a severely male-biased sex ratio to areas with higher female density and a less male-biased sex ratio. Male density and male age structure did not affect male dispersal and settlement. The sex ratio of the pre-dispersal sites was below the population average, but post-dispersal sites were not significantly better than the population average. This suggests that dispersal was triggered by poor conditions, whereas settlement may have been unrelated to the conditions at the new site. In the year of dispersal, males that undertook dispersal were less successful at acquiring a female than males that remained faithful to the site, but in subsequent years there was no difference. We suggest that dispersal might be adaptive under more natural conditions with a more balanced sex ratio. These results show that male ortolan buntings abandon areas with low female density and a severely male-biased sex ratio, and thereby end up in better places, although probably not better than the population average.