The tragedy of the commons: unsustainable population structure of Iberian red deer in hunting estates

Comparative Analysis of Microsatellite and SNP Markers for Genetic Management of Red Deer

The analysis of population genetic structure and individual multilocus heterozygosity are crucial for wildlife management and conservation. Microsatellite markers have traditionally been used to assess these genetic parameters. However, single-nucleotide polymorphisms (SNPs) are becoming increasingly popular. Our goal here was to determine to what extent SNPs can provide better insights than microsatellites into the overall genetic status and population genetic processes in the species. To this end, we genotyped 210 red deer (Cervus elaphus) in the Spanish wild population with both 11 microsatellites and 31,712 SNPs. We compared parameters related to population genetic structure and individual multilocus heterozygosity obtained with both types of markers. Our results showed correlations between parameters measured using both microsatellites and SNPs, particularly those related to the level of genetic diversity and genetic differentiation. However, we found notably lower precision of microsatellites in measuring the distribution of genetic diversity among individuals. We conclude that microsatellites can be used to monitor the overall genetic status and detect broad patterns in red deer populations. Nevertheless, the greater precision of SNPs in inferring genetic structure and multilocus heterozygosity leads us to encourage scientists and wildlife managers to prioritize their use whenever possible.

Is the Intrasexual Competition in Male Red Deer Reflected in the Ratio of Stable Isotopes of Carbon and Nitrogen in Faeces?

Isotopic analysis of carbon and nitrogen in faeces is a reliable methodology for studying ecology in wildlife. Here, we tested this technique to detect variations in carbon and nitrogen isotopic ratios (δ¹³C and δ¹⁵N) in two different intrasexual competition scenarios of male Iberian red deer (Cervus elaphus hispanicus) using faeces of individuals collected during hunting actions in South-eastern Spain. The carbon isotopic ratio (δ¹³C) was not found to be significant, likely due to similar diet composition in all individuals. However, the nitrogen isotopic ratio (δ¹⁵N) was found to be lower in populations where sexual competition between males during the rut was higher compared to low-competition populations. Therefore, this study suggests a different use of proteins by an individual male red deer depending on the sexually competitive context in which he lives. Although further research is needed, these results show the potential of isotopic analysis as a tool for studying individual and populational variations in the level of intrasexual competition, with implications in evolutionary ecology and population management.

Use of Wild Boar (Sus scrofa) as a Sustainable Alternative in Pork Production

Pork production involves several sustainability issues. The recent increase in the natural wild boar population and the possibilities of its breeding to produce meat and for sport hunting have revived attention on this wild species. The most important factors that could account for its expansion and niche invasion are briefly summarized with the scientific opinion on management strategies. The information available to date on the quantitative, nutritional, and sensory characteristics of wild boar meat is reviewed to highlight its potential, if properly managed, as a sustainable option in meat production. This review reports on the opportunity of using wild boar meat in processed products and the need for research on processing qualities and acceptability for different final products. Above all, this review suggests that wild boar can be considered a sustainable alternative to meet the animal protein demand, as it can be established in marginal areas where it is already adapted to the environment, representing an interesting addition to traditional zootechnics.

Exploring the potential links between gut microbiota composition and natural populations management in wild boar (Sus scrofa)

We surveyed wild boar (Sus scrofa) populations using 16S rRNA gene analysis of the gut microbiota in fresh faeces taken from 88 animals hunted in 16 hunting estates. The wild boar is a very convenient model system to explore how environmental factors including game management, food availability, disease prevalence, and behaviour may affect different biological components of wild individuals with potential implications in management and conservation. We tested the hypotheses that diet (according to stable carbon isotopes analyses), gender (i.e., animal behaviour studying males and females), and both health (analyses of serum samples to detect exposure to several diseases) and form statutes (i.e., thoracic circumference in adults) are reflected in changes in the intestinal microbiota. We focused on a gut functional biomarker index combining Oscillospiraceae and Ruminococcaceae vs. Enterobacteriaceae. We found that gender and the estate (population) were explanatory variables (c.a. 28% of the variance), albeit a high degree of overlapping among individuals was observed. The individuals with higher abundance of Enterobacteriaceae showed a gut microbiota with low diversity, mostly in males. Significant statistical differences for thoracic circumference were not found between males and females. Interestingly, the thoracic circumference was significantly and inversely related to the relative abundance of Enterobacteriaceae in males. Overall, we found that diet, gender, and form status were major factors that could be related to the composition and diversity of the gut microbiota. A high variability was observed in the biomarker index for populations with natural diet (rich in C3 plants). Although, we noticed a marginally significant negative trend between the index (higher abundance of Enterobacteriaceae) and the continuous feeding of C4 plants (i.e., supplementary maize) in the diet of males. This result suggests that continuous artificial feeding in hunting estates could be one of the factors negatively influencing the gut microbiota and the form status of wild boars that deserves further investigations.

Complementary roles of wild boar and red deer to animal tuberculosis maintenance in multi-host communities

The contribution of wildlife species to pathogen maintenance in multi-host communities has seldom been quantified. To assess the relative contribution of the main wildlife hosts of animal tuberculosis (TB) to its maintenance, we estimated the basic reproduction number (R₀) of Mycobacterium tuberculosis complex in wild boar and red deer at 29 sites in the Iberian Peninsula. Host abundance and true TB prevalence were estimated for each species at each site by sampling from distributions incorporating the uncertainty in the proportion of the population harvested each year, sensitivity, and specificity of the diagnostic methods, while excretion of mycobacteria was estimated using site-occupancy models. The distributions of these parameters were then used to estimate, at each site, the R_{0,wild boar} (range 0.1 - 55.9, average 8.7, standard deviation 11.8), and the R_{0,red deer} (0.1 - 18.9, 2.2, 3.9). Animal TB is maintained in epidemiological scenarios ranging from any single species acting as a maintenance host (the wild boar in 18 sites and the red deer in 5), to facultative multi-host disease (6 sites). The prevalence of TB in the red deer is likely an important driver of the epidemiology in multi-host communities. The wild boar was the main maintenance host of TB in most of the study sites and could have an epidemiological role linking the wildlife multi-host community and livestock.

Stable carbon and nitrogen isotope values in hair reveal management differences and hidden practices in wild boar populations

The analysis of stable isotopes in different tissues has been widely used to obtain information on the ecology and nutritional patterns of wildlife. The isotope ratios of the stable isotopes of carbon and nitrogen (δ¹³C and δ¹⁵N) analysed in different tissues are directly related to the animal's diet and, to some extent, to the environment where the individual has growth. Specifically, this type of analysis in hair samples has become relevant as it provides information on the quality and long-term composition of the diet that produced the isotope accumulation during the tissue growth. We took samples of wild boar (Sus scrofa) hair from 7 different populations in the south-west of Spain (Mediterranean habitats), in the 2018/2019 hunting season. The main objectives of this study were (i) to investigate the use of hair stable isotopes to reveal differences in composition and quality of the diet of wild boar within the same population or between populations, and (ii) to use hair isotopes as a tool to uncover hidden management practices that may occur in hunting areas associated with the use of supplementary feeding or even captive breeding and release. Each animal had a hair (long 10 cm) analysed in duplicate, previously cut into parts of equal size (from the oldest part of the hair to the most recent part), that were analysed separately. We found differences in δ¹³C and δ ¹⁵N between hair parts and populations, which can be related to management actions at different times during the hair growth. Moreover, the use of corn, a type of plant not occurring naturally in the study area, can be documented with the isotope analysis to prove unauthorized supplementary feeding or captive origin of wild boar in hunting areas.

Red deer reveal spatial risks of Crimean-Congo haemorrhagic fever virus infection

Crimean-Congo haemorrhagic fever virus (CCHFV) continues to cause new human cases in Iberia while its spatial distribution and ecological determinants remain unknown. The virus remains active in a silent tick-animal cycle to which animals contribute maintaining the tick populations and the virus itself. Wild ungulates, in particular red deer, are essential hosts for Hyalomma ticks in Iberia, which are the principal competent vector of CCHFV. Red deer could be an excellent model to understand the ecological determinants of CCHFV as well as to predict infection risks for humans because it is large, gregarious, abundant and the principal host for Hyalomma lusitanicum. We designed a cross-sectional study, analysed the presence of CCHFV antibodies in 1444 deer from 82 populations, and statistically modelled exposure risk with host and environmental predictors. The best-fitted statistical model was projected for peninsular Spain to map infection risks. Fifty out of 82 deer populations were seropositive, with individual population prevalence as high as 88%. The highest prevalence of exposure to CCHFV occurred in the southwest of the Iberian Peninsula. Climate and ungulate abundance were the most influential predictors of the risk of exposure to the virus. The highest risk regions were those where H. lusitanicum is most abundant. Eight of the nine primary human cases occurred in or bordering these regions, demonstrating that the model predicts human infection risk accurately. A recent human case of CCHF occurred in northwestern Spain, a region that the model predicted as low risk, pointing out that it needs improvement to capture all determinants of the CCHFV infection risk. In this study, we have been able to identify the main ecological determinants of CCHFV, and we have also managed to create an accurate model to assess the risk of CCHFV infection.

Host Genetic Diversity and Infectious Diseases. Focus on Wild Boar, Red Deer and Tuberculosis

Host genetic diversity tends to limit disease spread in nature and buffers populations against epidemics. Genetic diversity in wildlife is expected to receive increasing attention in contexts related to disease transmission and human health. Ungulates such as wild boar (Sus scrofa) and red deer (Cervus elaphus) are important zoonotic hosts that can be precursors to disease emergence and spread in humans. Tuberculosis is a zoonotic disease with relevant consequences and can present high prevalence in wild boar and red deer populations. Here, we review studies on the genetic diversity of ungulates and determine to what extent these studies consider its importance on the spread of disease. This assessment also focused on wild boar, red deer, and tuberculosis. We found a disconnection between studies treating genetic diversity and those dealing with infectious diseases. Contrarily, genetic diversity studies in ungulates are mainly concerned with conservation. Despite the existing disconnection between studies on genetic diversity and studies on disease emergence and spread, the knowledge gathered in each discipline can be applied to the other. The bidirectional applications are illustrated in wild boar and red deer populations from Spain, where TB is an important threat for wildlife, livestock, and humans.

The dark ventral patch: A bimodal flexible trait related to male competition in red deer

Sexual signals play a central role in male-male competition in polygynous species. In red deer (Cervus elaphus), male’s ventral area become dark during the rutting season due to urine spraying behaviour and retains many chemical compounds potentially revealing individual features. Here we investigate the variation in size of this trait, exploring its relationship with age and male competitive features such as antlers or body size, as well as populational level of intrasexual competition for mates. We found that the size of the dark ventral patch followed a clearly bimodal distribution, i.e. males mostly expressed the full-size trait or just developed a very small one. For these two groups of males according to trait expression, the relationships of trait size with age and antler size differed. Populational level of intrasexual competition appeared to affect the relationship between antler size and the probability of a fully developed ventral patch. These results indicate that the trait encodes information on body size, antler size, age and populational level of mate competition, thus suggesting a role in signalling male’s competitive features and willingness to allocate reproductive effort within a particular season.

Social environment modulates investment in sex trait versus lifespan: red deer produce bigger antlers when facing more rivalry

Theory predicts that the plastic expression of sex-traits should be modulated not only by their production costs but also by the benefits derived from the presence of rivals and mates, yet there is a paucity of evidence for an adaptive response of sex-trait expression to social environment. We studied antler size, a costly and plastic sex trait, and tooth wear, a trait related to food intake and longevity, in over 4,000 male Iberian red deer (Cervus elaphus hispanicus) from 56 wild populations characterized by two contrasting management practices that affect male age structure and adult sex-ratio. As a consequence, these populations exhibit high and low levels of male-male competition for mating opportunities. We hypothesized that males under conditions of low intra-sexual competition would develop smaller antlers, after controlling for body size and age, than males under conditions of high intra-sexual competition, thus reducing energy demands (i.e. reducing intake and food comminution), and as a consequence, leading to less tooth wear and a concomitant longer potential lifespan. Our results supported these predictions. To reject possible uncontrolled factors that may have occurred in the wild populations, we carried out an experimental design on red deer in captivity, placing males in separate plots with females or with rival males during the period of antler growth. Males living with rivals grew larger antlers than males living in a female environment, which corroborates the results found in the wild populations. As far as we know, these results show, for the first time, the modulation of a sexual trait and its costs on longevity conditional upon the level of intra-sexual competition.

Immune challenge of mating effort: steroid hormone profile, dark ventral patch and parasite burden in relation to intrasexual competition in male Iberian red deer

Testosterone secretion may regulate the reproductive effort and the development of sexual traits, but it may also involve costs at the immunological and metabolic levels. However, the evidence for this trade-off in wild populations is scarce. Cortisol also plays an important role in mediating the reproductive and immune functions. In this study, we analyzed whether the endoparasite burden relates to hormonal levels (fecal testosterone and cortisol metabolites) and/or morphological sexual traits (size of the dark ventral patch, a trait that indicates reproductive effort in males) in male Iberian red deer. For this purpose, we sampled male red deer harvested during hunting actions in 2 types of populations in south western Spain that differed in structure, affecting the level of male-male competition for mates. We used coprological analyses to estimate the parasite burden mainly of gastrointestinal and bronchopulmonary nematodes and of protozoa, and assessed testosterone and cortisol metabolite levels from fecal pellets. We found a positive relationship of host parasitation with both testosterone levels and the size of the dark ventral patch, but these relationships depended on the intensity of male-male competition in the population, being only found under the high-competition scenario. These results are discussed under the hypothesis of the testosterone immunocompetence handicap, suggesting a cost at the immunological level, and, therefore, higher susceptibility to parasite infection in males that make a greater reproductive effort. However, this effect seems to be modulated by the social environment (male-male competition) that might lead to different optima in testosterone production and sexual trait development.

Quantification of the Animal Tuberculosis Multi-Host Community Offers Insights for Control

Animal tuberculosis (TB) is a multi-host zoonotic disease whose prevalence in cattle herds in Europe has been increasing, despite a huge investment in eradication. The composition of the host community is a fundamental driver of pathogen transmission, and yet this has not been formally quantified for animal TB in Europe. We quantified multi-host communities of animal TB, using stochastic models to estimate the number of infected domestic and wild hosts in three regions: officially TB-free Central-Western Europe, and two largely TB-endemic regions, the Iberian Peninsula and Britain and Ireland. We show that the estimated number of infected animals in the three regions was 290,059-1,605,612 and the numbers of infected non-bovine domestic and wild hosts always exceeded those of infected cattle, with ratios ranging from 3.3 (1.3-19.6):1 in Britain and Ireland to 84.3 (20.5-864):1 in the Iberian Peninsula. Our results illustrate for the first time the extent to which animal TB systems in some regions of Europe are dominated by non-bovine domestic and wild species. These findings highlight the need to adapt current strategies for effective future control of the disease.

How climate change and wildlife management affect population structure in wild boars

Global climate change affects many species and contributes to the exceptional population growth of wild boar populations and thus to increasing human-wildlife conflicts. To investigate the impact of climate change on wild boar populations we extended existing models on population dynamics. We included for the first time different juvenile conditions to account for long-lasting effects of juvenile body mass on adult body mass and reproductive success. Our analysis shows that incorporating phenotypes, like body mass differences within age classes, has strong effects on projected population growth rates, population structures and the relative importance of certain vital rates. Our models indicated that an increase in winter temperatures and food availability will cause a decrease in mean body mass and litter size within Central European wild boar populations. We further analysed different hunting regimes to identify their effects on the population structure as well as their efficiency in limiting population growth. While targeting juveniles had the lowest effect on population structure, such strategies are, however, rather ineffective. In contrast, culling predominantly yearlings seems very effective. Despite being equally effective, only focusing on adults will not result in a reduction of population size due to their low proportion within populations.

Testosterone and the dark ventral patch of male red deer: the role of the social environment

The expression of male sexual traits, which is stimulated by testosterone, entails significant costs for individuals. Consequently, natural selection is expected to favour the modulation of sexual trait development according to the balance between its costs and benefits. The proportion of rivals in a population may affect this balance by increasing or decreasing the reproductive benefits associated with the development of sex traits. Here, we explore the relationship between testosterone level and sex trait size under two populational conditions of mate competition: fenced (i.e. high male-male competition; all male age groups are present) and unfenced (i.e. low competition; most males present are juveniles). Our model species is the Iberian red deer (Cervus elaphus hispanicus), and the sex trait is the dark ventral patch that males exhibit during the rutting season. Our results showed that the positive relationship between testosterone levels and the size of the dark ventral patch depends on the environmental level of male-male competition. Only in populations where the operational sex ratio was high (i.e. high proportion of rival males), individuals with high levels of testosterone developed the sex trait. Conversely, when mate competition was low, there was no significant relationship between testosterone level and trait size. This result reinforces the idea that the effect of testosterone in promoting the development of sex traits may be mediated by the intensity of mate competition in the population, as well as the role of sexual selection in the evolution of the dark ventral patch in red deer.

The intensity of male-male competition may affect chemical scent constituents in the dark ventral patch of male Iberian red deer

During the mating season, Iberian red deer males (Cervus elaphus hispanicus) present a large visible dark ventral area in their abdomen. This characteristic dark-haired area is formed by the impregnation of the hair with sprayed urine and gland secretions and contains volatile compounds that can be used in intraspecific communication. Here, we used gas chromatography-mass spectrometry (GC-MS) to describe the lipophilic chemicals from the dark ventral patch of males from different populations with different levels of intrasexual competition. Amongst all the compounds found, m-cresol, benzoic acid, cholesterol and 4-hydroxy-benzenopropanoic acid were the most abundant. The proportions of these compounds varied with age as well as with the level of intra-sexual competition, independently of age. In particular, red deer males experiencing higher intra-sexual competition had lower proportions of aromatic compounds (especially m-cresol) but higher proportions of carboxylic acids on their dark bellies. Males in a high male-male competition situation, invest in volatile compounds that can reveal their age, dominance status and condition, and that, in addition, enhance this signal. On the contrary, males from low intra-sexual competition populations have chemical profiles more characteristic of young individuals. This research shows a first glance of how secretion of volatile compounds of male deer can be modulated due to the intensity of male-male competition in the population.