Hólar University College

Lake size is associated with ecological conditions that can affect the microbiome of hosts, such as diet and habitat characteristics. However, whether host-microbe interactions are affected by lake size remains poorly understood. We characterized the gut microbiota of 191 wild threespine stickleback fish ( Gasterosteus aculeatus ) from six Alaskan lakes using 16S rRNA gene sequencing. We tested how gut microbiota varied across lakes with different surface areas. We then examined whether host ecotype and host traits, including sex, body mass, gravidity, and Schistocephalus solidus ( S. solidus ) infection were associated with gut microbial composition across lakes differing in surface area. Fish from intermediate-sized lakes harbored significantly more diverse microbiota than those from small and large lakes, independent of stickleback ecotype. We examined other factors commonly associated with fish microbiota. Model-predicted marginal effects from the mass and lake surface area interaction analysis indicated that the association between fish mass and microbial alpha diversity was strongly negative in the smallest lakes, weakest in intermediate-sized lakes, and strongly positive in the largest lakes. Differential abundance analysis revealed lake size-dependent associations between body mass and individual taxa. Sex and S. solidus infection were significantly associated with gut microbial composition, whereas fibrosis and gravidity had minimal effects. Together, these findings suggest that host-microbe interactions are not fixed within a species but depend on environmental conditions. By linking environmental variation with individual traits, our findings highlight how gut microbiota diversity varies in natural ecosystems. Understanding these patterns is important for predicting how gut microbiota respond to environmental change and for disentangling the effects of habitat and host biology on gut microbial communities. These insights improve our ability to understand how environmental differences shape animal health and ecosystem function across natural populations.

Abstract Knowledge about whether weight carrying capacity in horses is affected by training is lacking. The purpose of the study was to evaluate a 7-week, 3 times/week, ridden vs unridden training program on the physiological response to a ridden incremental weight carrying exercise test (WET) in 14 Icelandic horses (aged 13 ± 3 years). During training sessions (∼30 min), horses were exercised in pairs, one ridden (n = 7) and one unridden (n = 7), in all gaits. The pairs performed the WET simultaneously, before and after the training period, in tölt (5.2 ± 0.3 m/s), carrying approximately 20%, 25%, 30% and 35% of their own body weight (BW) at each step (650 m), respectively. Respiratory rate (RR) was measured after the last step and 5 min of recovery. Blood samples were collected after each step and 5 min of recovery for analysis of plasma lactate concentration. An additional blood sample was collected 24 h post-exercise for analysis of aspartate aminotransferase (AST) and creatine kinase (CK). The weight carried as a percentage of each horse’s BW at a plasma lactate concentration of 4 mmol/l was estimated and did not differ between training groups or occasions ( ). There was an interaction effect between group and occasion ( ), where plasma lactate concentration tended to be higher after training in the unridden group and lower in the ridden group compared with before the training period ( ). No effect of training group or occasion was found for RR, or concentration of AST and CK ( ). The results provide no clear evidence that ridden training is beneficial for weight carrying capacity in horses, although the effect on plasma lactate concentration warrant further investigation of the potential effects of ridden training.

Abstract This research note discusses two small-scale preliminary studies on regenerative tourism conducted in the capital and rural countryside, respectively (Reykjavík and Hvammstangi, Iceland) in September 2025. Workshops were used to engage tourism practitioners and enterprises in discussions around current practices, challenges, and emerging pathways towards regenerative approaches. Participants highlighted that while sustainability has become an established discourse, regenerative tourism requires a paradigm shift that goes further than sustainability, actively contributing to the flourishing of ecosystems, communities, and cultures. The workshops are positioned as two micro-level case studies. The aim was to understand how regenerative principles are contextualized within a local tourism setting. We used action setting and discussions focusing on dialogical learning, where diverse stakeholders reflected on their roles within tourism and their connections within their communities to envision a new tourism future. Iceland, with its deep reliance on nature-based tourism and strong local identities, emerges as a particularly relevant context to explore how regenerative thinking might be mobilised beyond conceptual debate and into practical application. Our research note argues that such participatory encounters can provide empirical insights into the current understanding of regenerative tourism and possible inspiration for transformative action that strengthens the regenerative capacity of tourism systems.

Climate change is causing rapid temperature increases that drive adaptive changes. Predicting these outcomes remains challenging as the extremes of warming have not yet been experienced by natural populations. Contemporary populations inhabiting geothermally-warmed environments can provide valuable insights, including phenotypic and genomic responses involved in adaptation to warming. Here, we examine independent instances of divergence between geothermal and ambient ecotypes of threespine stickleback. While we identify similar patterns of phenotypic divergence in appetite, fat storage, and fat loss (during starvation), genomic divergence of loci is non-parallel. However, at a functional level, the mapk signalling pathway differs across ecotype pairs and expression of mapk genes differs between ecotypes, are thermally plastic, and influenced by hybridization. We also identify an inversion on chromosome XXI, previously attributed to freshwater colonization, diverging in parallel between ecotypes. Candidate genes within the inversion are associated with metabolic traits, including appetite regulation and fat storage in correspondence with our phenotypic findings. Overall, polygenic adaptation conferring functionally similar outcomes and structural variation are key genomic mechanisms facilitating adaptation to warmed environments. Functional molecular pathways should be prioritized in conservation, as precise allelic changes are difficult to predict and potentially less relevant to thermal adaptation than previously thought.

Growth and reproduction in insects are strongly influenced by dietary protein and carbohydrate intake, yet parasitism may alter how hosts acquire and allocate these resources. Hairworms (Nematomorpha) develop within arthropod hosts and consume host-derived nutrients, but the effects on host nutrition and development remain poorly understood. We tested the effects of Paragordius varius (Leidy, 1851) parasitism on diet choice, growth, and reproduction in experimentally infected female house crickets, Acheta domesticus (Linnaeus, 1758). Crickets were offered complementary high-protein and high-carbohydrate diets, and we quantified food intake, nutrient preference, and host performance metrics. Infected crickets consumed similar protein-to-carbohydrate ratios as controls but ingested 14% less total food. Control crickets exhibited higher standard growth rates (SGR), whereas SGR decreased with increasing parasite intensity in infected individuals. Infected crickets produced no eggs and had significantly reduced lipid content, ovipositor growth, and fecal output relative to controls. Nutritional indices on the basis of wet mass indicated no difference in the conversion of intake to body mass; however, indices on the basis of parasite-free dry mass revealed that control crickets converted food intake 3.7 times more efficiently than infected crickets. These results indicate that hairworm parasitism reduces host growth and reproductive output and decreases the efficiency of energy assimilation, likely due to direct or indirect resource competition with developing parasites.

Fertility is an important but often cryptic and intrinsic characteristic of domesticated animals. Predicting reproductive potential is of great importance for the industry but assessment through indirect proxies is laborious and often impractical. Among other biological factors, genetic effects are expected to play a crucial role in shaping male and female fertility. In cases where heritable components are strong, polygenic merit could be a valuable tool for decision-making in breeding schemes. Here we estimate sex-specific variance components affecting fertilization success by analyzing outcomes of over 3000 controlled mating events in an Arctic charr breeding nucleus from Iceland. Furthermore, a machine learning framework using relationships-to-founders vectors as input and a two-tower neural network architecture is proposed and tested for prediction of fertilization success. Both approaches seem to capture a meaningful biological signal and offer alternative tools for ranking, selecting or even allocating matings between breeding candidates.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12711-026-01070-9.

Abstract Individual variation in foraging and social behavior provides insights into how animals exploit and share local resources and habitats. In this study, we examined the foraging mode and social behavior for 55 Arctic charr (fork length = 5–17 cm) during summer in four freshwater lava cave ponds in the subarctic Lake Mývatn area in NE-Iceland. As expected for lentic waters, cave-living Arctic charr were highly mobile when searching for prey (mean proportion of mobile search = 0.748; SD = 0.251). Foraging attempts were mostly directed towards the water surface (mean proportion = 0.730; SD = 0.358), and rarely toward prey on the bottom (mean = 0.164; SD = 0.291) and in the water column (mean = 0.106; SD = 0.225). Although most fish foraged primarily on surface prey, with small subsidies from the other two locations, some individual fish fed almost exclusively on either benthos or prey in the water column. Aggression among fish suggests that size-related dominance hierarchies occur within the lava caves. Specifically, larger fish typically chased other fish more often than smaller fish did, whereas smaller fish were more likely to be chased. These findings add to our understanding of behaviors that may contribute to the survival and local adaptation of these unique populations of cave-living Arctic charr in North Iceland. The study also gives insights into the variable natural history of aquatic cave animals and expands the limited knowledge that exists on foraging mode and social behavior in salmonids in lentic waters.