Biology subjects

Bal, X.

Publications and source records attributed to Bal, X..

7 recordsLinked to original sources

bioRxiv2026

No evidence sex-specific senescence varies with resource competition in Soay sheep

Demographic senescence, the age-related decline in survival and reproduction, is widespread. Older individuals, and particularly males in polygynous systems, are expected to be disproportionately vulnerable to environmental stressors due to physiological decline and high reproductive costs. However, studies testing if harsh conditions exacerbate senescence, and whether this effect is sex-dependent, remain limited in wild populations. Using 40 years of data from wild Soay sheep (Ovis aries) on St Kilda, Scotland, we tested whether increased resource competition (indexed by population density) accelerates demographic senescence, and if late-life environmental sensitivity is male-biased. High densities consistently suppressed overall adult performance, making individuals less likely to survive, breed, or rear high-quality offspring under more intense competition. Although survival and reproductive traits exhibited clear senescence, increased population density did not accelerate late-life declines in any trait. Rates of decline with age were unaffected by density and did not differ between the sexes. Our results challenge the assumption that older individuals and males are inherently more vulnerable to harsh conditions, suggesting physiological or behavioural buffering may maintain late-life resilience against resource limitation in some systems.

evolutionary biology↗

bioRxiv2025

Plasma insulin-like growth-factor 1 (IGF-1) concentrations predict early life-history traits in a wild mammal

O_LIThe hormone insulin-like growth factor 1 (IGF-1) is a key player in the insulin/IGF-1 signaling (IIS) pathway. Extensive biogerontological research demonstrates that this evolutionarily conserved nutrient-sensing pathway plays a causal role in the regulation of growth, reproduction and longevity under laboratory conditions. However, its potential role as a mediator of adaptive life-history variation in highly variable natural environments remains unclear. C_LIO_LIWe measured IGF-1 concentrations in blood samples from approximately four-month-old wild Soay sheep lambs (n=669), collected over nine summers. We tested whether IGF-1 (i) was positively correlated with proxies of resource availability, (ii) was associated with morphological traits measured concurrently, and (iii) predicted subsequent fitness-related traits. C_LIO_LIPlasma IGF-1 concentrations were higher in males compared to females, and positively correlated with measures of resource availability in both sexes. IGF-1 was lower in years of high population density when per capita food availability was reduced; in twin lambs who have fewer available resources compared to singletons; and in lambs born to young and old mothers, who have poor maternal provisioning compared to mothers of intermediate age. C_LIO_LIHigher IGF-1 levels in summer were correlated with higher body mass, faster post-natal somatic growth and increased skeletal size, measured at the same time. These associations were independent of our proxies of resource availability. C_LIO_LILambs with higher summer IGF-1 were more likely to survive their first winter and reproduce the following spring. The association between IGF-1 and reproduction was independent of our resource availability proxies, whereas the association with first-winter survival was not. The association between summer IGF-1 and reproduction was mediated by positive associations with summer body mass. C_LIO_LIOur study reveals population-level phenotypic plasticity in circulating IGF-1, also finding IGF-1 to be positively associated with key morphological traits and positively predict fitness traits in early life. These findings highlight IGF-1 as a candidate physiological mechanism underpinning plastic responses to variation in food availability and influencing life-history traits in a wild mammal. C_LI

ecology↗

bioRxiv2025

Environment dependent benefits of sociality in Soay sheep

An individuals social connections have strong effects on fitness. Despite this, there is pronounced among-individual variation in social behaviour. This variation may be maintained if different types of social connections have environment-dependent fitness benefits, but this has rarely been tested. We applied network analysis to 37 years of social association data in Soay sheep to test this hypothesis. Our results show that both relationship quantity (having many association partners) and relationship quality (having strong associations) are linked with survival. Crucially, the relative importance of relationship quantity and quality depends on overwinter weather, with harsh conditions favouring quantity over quality, likely due to thermoregulatory benefits. Relationship quality, meanwhile, was most beneficial under benign weather conditions, possibly due to enhanced familiarity with conspecifics. These findings advance our understanding of social benefits in fission-fusion societies and show that changing environmental conditions may be an important mechanism maintaining variation in social behaviour in natural populations.

animal behavior and cognition↗

bioRxiv2024

Sex-specific effects of early-life adversity on adult fitness in a wild mammal

Early-life adversity influences adult fitness across a range of vertebrates. In polygynous systems with intense intrasexual competition, males may be more sensitive to conditions experienced during growth and development. However, the relative importance of different aspects of the early environment and how their effects differ between the sexes remain poorly understood. Here, we used a long-term study of wild Soay sheep to characterise the early-life environment in terms of weather, infection, resource competition and maternal investment, and test the hypothesis that males are more vulnerable to early adversity than females. Birth weight positively predicted lifetime breeding success in both sexes, suggesting a classic silver spoon effect, though the effects were stronger in males. Males experiencing high population densities in their first year had lower lifetime breeding success suggesting lasting negative consequences of nutritional stress, but there was no association in females. In contrast, challenging weather in the first winter of life appeared to act as a selective filter, with males surviving these harsh conditions having higher adult fitness. Our findings further evidence the important long-term fitness consequences of early-life adversity in wild vertebrates, highlighting that different aspects of the early environment may shape later fitness in different and sex-specific ways.

evolutionary biology↗

bioRxiv2023

Age specific impacts of vegetation functional traits on gastro-intestinal nematode parasite burdens in a large herbivore

O_LIGastro-intestinal nematode (GIN) parasites play an important role in the ecological dynamics of many animal populations. Recent studies suggest fine-scale spatial variation in GIN infection dynamics are important in wildlife systems, but the environmental drivers underlying this variation remain poorly understood. C_LIO_LIWe used data from over two decades of GIN parasite egg counts, host space use, and spatial vegetation data from a long-term study of Soay sheep on St Kilda to test how spatial autocorrelation and vegetation in an individuals home range predict parasite burden across three age groups. We developed a novel approach to quantify the plant functional traits present in a home range to describe the quality of vegetation present. C_LIO_LIEffects of space and vegetation varied between age classes. In immature lambs, strongyle parasite faecal egg counts (FEC) were spatially structured, being highest in the north and south of our study area. Independent of host body weight and spatial autocorrelation, plant functional traits predicted parasite egg counts. Higher egg counts were associated with more digestible and preferred plant functional traits, suggesting the association could be driven by host density and habitat preference. C_LIO_LIIn contrast, we found no evidence that parasite FEC were related to plant functional traits in the host home range in yearlings or adult sheep. Adult FEC were spatially structured, with highest burdens in the north-east of our study area, while yearling FEC showed no evidence of spatial structuring. C_LIO_LIOur findings support the importance of fine-scale environmental variation for wildlife disease ecology and provides new evidence that such effects may vary across demographic groups within a population. Parasite burdens in immature individuals appear more readily influenced by fine-scale spatial variation in the environment, highlighting the importance of such heterogeneity for our understanding of wildlife epidemiology and health. C_LI

ecology↗

bioRxiv2021

Functionally distinct T-helper cell phenotypes predict resistance to different types of parasites in a wild mammal

O_LIThe adaptive immune system is critical to an effective, long-lasting ability to respond to infection in vertebrates and T-helper (Th) cells play a key role in orchestrating the adaptive immune response. Laboratory studies show that functionally distinct Th responses provide protection against different kinds of parasites (i.e., Th1 responses against microparasites and Th2 against macroparasites). C_LIO_LINatural populations must deal with challenges from a wide range of infectious agents and co-infection with different types of parasite is the norm, so different Th responses are likely to play an important and dynamic role in maintaining host health and fitness. However, the relationship between T helper immune phenotypes and infection with different types of parasites remains poorly understood in wild animals. C_LIO_LIIn this study, we characterised variation in functionally distinct Th responses (Th1, Th2, Th17 and regulatory responses) in a wild population of Soay sheep using flow cytometry to detect Th-subset specific transcription factors, and ex vivo lymphocyte stimulation to quantify release of Th-associated cytokines. We specifically tested the prediction that raised Th1 and Th2 responses should predict reduced apicomplexan (coccidian) and helminth (nematode) parasite burdens, respectively. C_LIO_LICell counts of different Th subsets measured by flow cytometry did not vary with age or sex. However, all measures of Th-associated ex vivo cytokine production increased with age, and Th17- and regulatory Th-associated cytokine production increased more rapidly with age in males than females. C_LIO_LIIndependent of age and sex, Th2-associated immune measures negatively predicted gastro-intestinal strongyle nematode faecal egg count, while production of the Th1-associated cytokine IFN-{gamma} negatively predicted coccidian faecal oocyst count. C_LIO_LIOur results provide important support from outside the laboratory that Th1 and Th2 responses confer resistance to different kinds of parasites (micro- and macro-parasites, respectively). They also add to mounting evidence from wild populations that Th1/Th2 trade-offs often observed in controlled laboratory experiments may not readily translate to more complex natural systems. C_LIO_LIOur study illustrates that harnessing more specific reagents and tools from laboratory immunology has the potential to illuminate our understanding of epidemiology and host-parasite co-evolution in the wild. C_LI

ecology↗

bioRxiv2021

Seasonal dynamics and repeatability of gastrointestinal parasite burdens in wild Soay sheep

Seasonality is a ubiquitous feature of wildlife disease ecology, but is determined by a complex interplay of environmental, parasitological and host factors. Gastrointestinal parasites often exhibit strong seasonal dynamics in wild vertebrate populations due to, for example, environmental influences on free-living or vectored life stages, and variation in the physiological and immune status of hosts across their annual cycle. At the same time, wild populations are typically infected with multiple parasites. The seasonal dynamics of co-infecting parasites may differ depending on age and reproductive status, and associations among parasites may be driven by short-term within-individual changes or longer-term interactions that are consistent among hosts. Here, we used faecal samples and egg counts collected repeatedly from individually marked and monitored wild Soay sheep that were part of a long-term study to investigate seasonal dynamics of six gastrointestinal parasite groups (strongyle nematodes, coccidian protozoa, Capillaria, Strongyloides, Nematodirus, and Moniezia). Prevalence and abundance generally tended to be higher spring and summer, and burdens were higher in lambs than adults. Within the highly prevalent strongyle nematode group, we found differences in seasonality of egg counts depending on adult reproductive status. Reproductive ewes had increased counts in spring around the time of birth followed by a drop in abundance in summer, while barren ewes showed little evidence of seasonality. Males showed a sustained rise in egg counts through spring and summer, and sex differences were only strongly apparent in summer. In contrast, in similarly prevalent coccidia we found a peak in faecal oocyst counts in spring but no differences in seasonality among males, barren and pregnant ewes. Using multivariate mixed-effects models, we went on to show that both strongyle and coccidia counts are moderately repeatable across seasons among individuals. We further show that apparent positive correlation between strongyle and coccidia counts was driven by short-term within-individual changes in both parasite burdens rather than long-term among-individual covariation. Overall, our results demonstrate that seasonality varies across demographic and parasite groups and highlight the value of investigating fluctuating susceptibility and exposure over time for understanding epidemiology of a population.

ecology↗