Effects Of Intraspecific Variation Research Articles

Inter and intraspecific variation in fish body size constrains microhabitat use in a subtropical drainage

Microhabitat characteristics are expected to influence the distribution of stream fish species at fine spatial scales (e.g., within riffle segments). Body size is probably the most important trait that constrains microhabitat occupation by fish, but the effect of intraspecific variation has been understudied. We investigated how physical microhabitat characteristics affect species and body size distribution of fish within a stream riffle segment in a coastal subtropical drainage of Brazil. Fishes were sampled by electrofishing 56 riffle plots along a 730-m long stream segment. Species composition was significantly related to four microhabitat characteristics: substrate size, flow velocity, distance to margin and depth. In addition, mean body size increased with increasing substrate size and depth of microhabitat sampling plots. However, when including species identity in linear mixed-effects models (LMM), we observed a different relationship between body size and microhabitat characteristics, but most of the variation was explained by species identity. Thus, we fitted LMMs separately for each species and found species-specific relations between intraspecific variation in body size and microhabitat characteristics. The low variation explained in the models suggests that other fine scale factors, such as biotic interactions and dispersal from adjacent habitat patches, should be incorporated in modeling microhabitat use by stream fish. Our findings suggest that body size is important by itself, but intraspecific variation in body size also constrains microhabitat use differently for each species, which may depend on other species-specific traits, such as morphology, behavior and life history.

Causes and Consequences of Intraspecific Variation in Nesting Behaviors: Insights from Blue Tits and Great Tits

Nest building is an important and yet under-studied stage of the reproductive cycle in many taxa, including birds, and whilst we have a decent understanding of interspecific variation in avian nesting behaviours, our understanding of intraspecific variation in nesting behaviours is much less developed. This is largely because an insufficient number of studies have been performed on any one species to draw robust conclusions. Fortunately, though, the amount of research on the nesting behaviours of nestbox-breeding blue tits (Cyanistes caeruleus) and great tits (Parus major) has increased dramatically in recent years and their nesting behaviours are now sufficiently well studied to offer useful insights into intraspecific variation in avian nesting behaviours. Studies show that individuals of both species select nest sites based on the presence and/or absence of conspecifics and heterospecifics and whilst neighbours were assumed to adversely affect focal individuals by competing for resources, they are now considered beneficial as they provide information about habitat quality and contribute to anti-predator defences. Nest-building females accurately gauge local weather conditions and respond to predictable variation in environmental conditions by building nests with variable amounts of cup lining material to create suitable nest microclimates for nestlings. Meanwhile, both species vary the amount of aromatic plant materials in their nests in relation to the abundance of nest-dwelling parasites and pathogens and as aromatic plant materials also play a role in sexual selection then nest materials can have multiple functions. The height of nests is negatively correlated with the local risk of predation but whilst predator avoidance favours lower nests, sexual selection favours taller nests. In fact, higher quality females build taller nests that contain more green plant material which in turn influences the amount of care provided by males. This suggests there is considerable intraspecific variation in nesting behaviours in blue and great tits, and in birds generally, and highlights that birds are adept at adaptively varying their nesting behaviours in response to multiple factors. There is, however, a paucity of evidence directly linking such variation in nest building behaviours to reproductive success or fitness more broadly and so further studies could usefully

Variance in Female Reproductive Success Differentially Impacts Effective Population Size in the Short-Nosed Fruit Bat, Cynopterus sphinx

Effective population size (N e) quantifies the effects of micro-evolutionary processes and the rate of loss of genetic diversity in a population. Several demographic and mating parameters reduce N e. Theoretical studies elucidate the impacts of various demographic and mating system parameters on N e, while empirical studies illustrate realized N e for species with differing life histories and mating systems. However, effect of intra-specific variation in mating system on effective size remains largely unexplored. In this paper we investigated the effect of promiscuous and polygynous mating on N e in two wild populations of the short-nosed fruit bat, Cynopterus sphinx. N e/N (ratio of effective population size to census size) was lower than unity in both populations, and much lower for the polygynous population compared to promiscuous population. Elasticity analyses reveal that N e/N was sensitive to deviations in the sex ratio. Variance in female reproductive success had a higher impact on N e compared to variance in male reproductive success in the promiscuous population. However, for the polygynous population, impact of variance in male reproductive success on N e was higher than that of variance in female reproductive success. Our results suggest that depending on mating system, different populations of the same species could have alternate evolutionary trajectories. The rate of loss of genetic diversity would be lower for the promiscuous population compared to the polygynous population. Our study is the first to highlight which parameters would most significantly impact population specific N e under different mating systems.

The effect of intraspecific variation and heritability on community pattern and robustness.

Intraspecific trait variation is widespread in nature, yet its effects on community dynamics are not well understood. Here we explore the consequences of intraspecific trait variation for coexistence in two- and multispecies competitive communities. For two species, the likelihood of coexistence is in general reduced by intraspecific variation, except when the species have almost equal trait means but different trait variances, such that one is a generalist and the other a specialist consumer. In multispecies communities, the only strong effect of non-heritable intraspecific variation is to reduce expected species richness. However, when intraspecific variation is heritable, allowing for the possibility of trait evolution, communities are much more resilient against environmental disturbance and exhibit far more predictable trait patterns. Our results are robust to varying model parameters and relaxing model assumptions.

How variation between individuals affects species coexistence.

Although the effects of variation between individuals within species are traditionally ignored in studies of species coexistence, the magnitude of intraspecific variation in nature is forcing ecologists to reconsider. Compelling intuitive arguments suggest that individual variation may provide a previously unrecognised route to diversity maintenance by blurring species-level competitive differences or substituting for species-level niche differences. These arguments, which are motivating a large body of empirical work, have rarely been evaluated with quantitative theory. Here we incorporate intraspecific variation into a common model of competition and identify three pathways by which this variation affects coexistence: (1) changes in competitive dynamics because of nonlinear averaging, (2) changes in species' mean interaction strengths because of variation in underlying traits (also via nonlinear averaging) and (3) effects on stochastic demography. As a consequence of the first two mechanisms, we find that intraspecific variation in competitive ability increases the dominance of superior competitors, and intraspecific niche variation reduces species-level niche differentiation, both of which make coexistence more difficult. In addition, individual variation can exacerbate the effects of demographic stochasticity, and this further destabilises coexistence. Our work provides a theoretical foundation for emerging empirical interests in the effects of intraspecific variation on species diversity.

Causes and consequences of intra-specific variation in vertebral number.

Intraspecific variation in vertebral number is taxonomically widespread. Much scientific attention has been directed towards understanding patterns of variation in vertebral number among individuals and between populations, particularly across large spatial scales and in structured environments. However, the relative role of genes, plasticity, selection, and drift as drivers of individual variation and population differentiation remains unknown for most systems. Here, we report on patterns, causes and consequences of variation in vertebral number among and within sympatric subpopulations of pike (Esox lucius). Vertebral number differed among subpopulations, and common garden experiments indicated that this reflected genetic differences. A QST-FST comparison suggested that population differences represented local adaptations driven by divergent selection. Associations with fitness traits further indicated that vertebral counts were influenced both by stabilizing and directional selection within populations. Overall, our study enhances the understanding of adaptive variation, which is critical for the maintenance of intraspecific diversity and species conservation.

Differences in phosphorus use between ancient and extant Daphnia genotypes alters algal stoichiometry and abundance

Understanding the ecological relevance of intraspecific variation and evolutionary change in traits is a central frontier in contemporary ecology. Ecological stoichiometry uses variation in the content and kinetics of elements among species to predict key ecological functions such as nutrient recycling. Although much work has focused on interspecific variation, little is known about the ecological consequences of intraspecific variation and evolutionary change in the processing of ecologically relevant elements such as phosphorus (P). We tested whether physiological evolution can alter somatic P content of Daphnia and algal stoichiometry and abundance as predicted by stoichiometric models of consumer-driven nutrient recycling. We used genotypes of Daphnia pulicaria hatched from resting eggs in lake sediments separated by several hundreds of years of evolution in the wild. Although no significant differences were observed in somatic P content of genotypes from different sediment layers, radiotracer (33P) assays in 2 P–supply conditions (HiP and LoP) revealed considerable differences in P incorporation, particularly under HiP conditions. Further, algae co-occurring with the modern, P-inefficient Daphnia genotypes contained more 33P and exhibited faster population growth compared to counterparts co-occurring with ancient, P-efficient genotypes, with differences more prominent under HiP conditions. This study highlights the potential for intraspecific variation in P-use physiology in affecting trophic interactions via consumer-driven nutrient recycling. Melding ecological stoichiometry and resurrection ecology should elucidate the evolutionary sources and ecological consequences of such variation.

Emergence of a novel prey life history promotes contemporary sympatric diversification in a top predator

Intraspecific phenotypic variation can strongly impact community and ecosystem dynamics. Effects of intraspecific variation in keystone species have been shown to propagate down through the food web by altering the adaptive landscape for other species and creating a cascade of ecological and evolutionary change. However, similar bottom-up eco-evolutionary effects are poorly described. Here we show that life history diversification in a keystone prey species, the alewife (Alosa pseudoharengus), propagates up through the food web to promote phenotypic diversification in its native top predator, the chain pickerel (Esox niger), on contemporary timescales. The landlocking of alewife by human dam construction has repeatedly created a stable open water prey resource, novel to coastal lakes, that has promoted the parallel emergence of a habitat polymorphism in chain pickerel. Understanding how strong interactions propagate through food webs to influence diversification across multiple trophic levels is critical to understand eco-evolutionary interactions in complex natural ecosystems.

Timescales alter the inferred strength and temporal consistency of intraspecific diet specialization.

Many populations consist of individuals that differ substantially in their diets. Quantification of the magnitude and temporal consistency of such intraspecific diet variation is needed to understand its importance, but the extent to which different approaches for doing so reflect instantaneous vs. time-aggregated measures of individual diets may bias inferences. We used direct observations of sea otter individuals (Enhydra lutris nereis) to assess how: (1) the timescale of sampling, (2) under-sampling, and (3) the incidence- vs. frequency-based consideration of prey species affect the inferred strength and consistency of intraspecific diet variation. Analyses of feeding observations aggregated over hourly to annual intervals revealed a substantial bias associated with time aggregation that decreases the inferred magnitude of specialization and increases the inferred consistency of individuals' diets. Time aggregation also made estimates of specialization more sensitive to the consideration of prey frequency, which decreased estimates relative to the use of prey incidence; time aggregation did not affect the extent to which under-sampling contributed to its overestimation. Our analyses demonstrate the importance of studying intraspecific diet variation with an explicit consideration of time and thereby suggest guidelines for future empirical efforts. Failure to consider time will likely produce inconsistent predictions regarding the effects of intraspecific variation on predator-prey interactions.

Effects of intraspecific variation in rice resistance to aboveground herbivore, brown planthopper, and rice root nematodes on plant yield, labile pools of plant, and rhizosphere soil

In agricultural ecosystems, most crop cultivars are bred for pest resistance with the aim of resisting aboveground herbivores, but it is unclear if this resistance also affects abundance, composition, and activity of underground pests and content of soil labile pools. A fully factorial greenhouse experiment was carried out, using four rice cultivars with distinct resistance to shoot herbivore brown planthopper, Nilaparvata lugens, with and without rice root nematodes, Hirschmanniella spp., to test two hypotheses: (1) rice cultivars resistant to brown planthopper are more susceptible to rice root nematodes and (2) rice root nematodes of resistant cultivars cause greater modification of labile pools of rhizosphere soil than those of susceptible cultivars by inducing more leakage of plant metabolites. Nematode abundance, soluble sugar and soluble amino acid contents of plant, and microbial biomass C and N, dissolved organic C and N, and soluble sugar and amino acid contents of rhizosphere soil were analyzed. The abundance of rice root nematodes substantially increased under all rice cultivars following root nematode inoculation. Rice cultivar resistance to brown planthopper did not affect the numbers of rice root nematodes but decreased negative effects of nematodes on root growth, shoot soluble sugar and amino acid contents, and labile pools of rhizosphere soil, probably implying that integrated complex mechanisms other than only resource allocation were involved in rice resistance to aboveground and belowground herbivores. Our study will contribute to a better understanding of aboveground–belowground interactions and to the development of integrated pest managements by considering belowground ecosystem responses in crop systems.

Habitat adaptations in the felid forearm

Extant cats inhabit different kinds of habitat, for example open landscapes, forests, and rocky mountainous areas. In this study, the radius and ulna of extant felids were investigated to identify ecomorphological adaptations to different habitats. Simple scatter plots and multivariate analyses (factor analysis, discriminant function analysis) revealed two distinct clusters of cats preferring open habitats to cats preferring closed habitats (i.e., forests). We also applied our approach to an existing, large database of extant cats to determine the effect of intraspecific variation on the forearm bones. According to our dataset, cat species adapted to closed habitats have relatively shorter but thicker forearm bones whereas those with longer and slimmer forearms are adapted to open habitats. Cats that inhabit mountainous habitats show no distinct differences in adaptation and plot in one of the other two habitat categories. Overall, the radius measurements are sufficient to distinguish open and forest habitats, but with inclusion of the ulna measurements, the results are more confident. The presented method of using just four measurements of the forearm bones has much potential for application to fossil taxa. We tested the approach for fossil taxa from the Miocene to Pleistocene with the result that most fossil felids were adapted to closed habitats but with some species being adapted to more open intermediate habitats.

Hemiclonal analysis of interacting phenotypes in male and female Drosophila melanogaster.

BackgroundIdentifying the sources of variation in mating interactions between males and females is important because this variation influences the strength and/or the direction of sexual selection that populations experience. While the origins and effects of variation in male attractiveness and ornamentation have received much scrutiny, the causes and consequences of intraspecific variation in females have been relatively overlooked. We used cytogenetic cloning techniques developed for Drosophila melanogaster to create “hemiclonal” males and females with whom we directly observed sexual interaction between individuals of different known genetic backgrounds and measured subsequent reproductive outcomes. Using this approach, we were able to quantify the genetic contribution of each mate to the observed phenotypic variation in biologically important traits including mating speed, copulation duration, and subsequent offspring production, as well as measure the magnitude and direction of intersexual genetic correlation between female choosiness and male attractiveness.ResultsWe found significant additive genetic variation contributing to mating speed that can be attributed to male genetic identity, female genetic identity, but not their interaction. Furthermore we found that phenotypic variation in copulation duration had a significant male-associated genetic component. Female genetic identity and the interaction between male and female genetic identity accounted for a substantial amount of the observed phenotypic variation in egg size. Although previous research predicts a trade-off between egg size and fecundity, this was not evident in our results. We found a strong negative genetic correlation between female choosiness and male attractiveness, a result that suggests a potentially important role for sexually antagonistic alleles in sexual selection processes in our population.ConclusionThese results further our understanding of sexual selection because they identify that genetic identity plays a significant role in phenotypic variation in female behaviour and fecundity. This variation may be potentially due to ongoing sexual conflict found between the sexes for interacting phenotypes. Our unexpected observation of a negative correlation between female choosiness and male attractiveness highlights the need for more explicit theoretical models of genetic covariance to investigate the coevolution of female choosiness and male attractiveness.

Phenology of hibernation and reproduction in ground squirrels: integration of environmental cues with endogenous programming

AbstractMammalian hibernation is characterized by prolonged dormancy consisting of pronounced depression of metabolism and body temperature. Though hibernation occurs in at least seven mammalian orders and several families of the order Rodentia, the ecology and physiology of hibernation in rodents has been most extensively studied in the family Sciuridae, particularly in the so‐called ground squirrels, that is, the tribe Marmotini. Early studies of these rodents demonstrated the important role of an endogenous circannual clock in the persistence of annual timing and phasing of key seasonal events, including weight gain, hibernation and reproduction. Here, we review the causes and consequences of intraspecific variation in the timing of hibernation and reproduction in these sciurids and examine the physiological mechanisms that contribute to phenotypic plasticity in seasonal timing. Although the duration, annual phasing and predictability of seasonal change in environmental conditions likely promoted the evolution of endogeneity, precision and brevity of breeding seasons in the annual cycles of sciurids, substantial intraspecific variation in hibernation and reproductive phenology exists along latitudinal and altitudinal clines, as well as among locally varying environmental micro‐conditions. We suggest that much of this variation is a function of plasticity in the physiological mechanisms controlling annual cycles. While studies of captive animals have been instrumental in establishing the role of an endogenous rhythm, a greater emphasis on experimental field manipulations is needed to better understand the function, causes and consequences of phenological shifts in wild populations. Ultimately, the capacity of hibernators to adjust their timing in response to changing environmental conditions, either through phenotypic plasticity or microevolution, will be an important determinant of the resilience of sciurid species to changing climate.

Clonal growth is enhanced in the absence of a mating morph: a comparative study of fertile stylar polymorphic and sterile monomorphic populations of Nymphoides montana (Menyanthaceae)

Many aquatic species with stylar polymorphisms have the capacity for clonal and sexual reproduction and are sensitive to the balance of the two reproductive modes when there are a limited number of mating morphs within a population. This study asked how the clonal and sexual reproductive modes perform in populations that contain only a single morph and where fitness gain through sexual reproduction is rare. In clonal aquatic Nymphoides montana, polymorphic populations normally contain two mating morphs in equal frequencies. Populations are sexually fertile and appear to be maintained by pollen transfer between the two partners. However, in a monomorphic population of N. montana where mating opportunities are unavailable, female and male function is impaired and clonality maintains the population. Here, the consequences of intraspecific variation in sexuality were explored between monomorphic and polymorphic N. montana populations in eastern Australia. Comparative measurements of male and female fertility, total dry mass and genotypic diversity using ISSR markers were made between populations with variable sexuality. Very few seeds were produced in the monomorphic population under natural and glasshouse conditions due to dysfunctional pollen and ovules. Stigma-anther separation was minimal in the monomorphic population, which may be a consequence of the relaxed selective pressures that regulate the maintenance of sexual function. However, clonal reproduction was favoured at the expense of sexual reproduction in the monomorphic population; this may facilitate the establishment of sterility throughout the population via resource reallocation or pleiotropic effects. The ISSR results showed that the monomorphic population was one large, single genotype, unlike the multi-genotypic fertile polymorphic populations. Evolutionary loss of sex in a clonal population in which a mating morph is absent was evident; under these conditions clonal growth may assure reproduction and expand the population via spreading stolons.

Explaining intraspecific diversity in plant secondary metabolites in an ecological context

Plant secondary metabolites (PSMs) are ubiquitous in plants and play many ecological roles. Each compound can vary in presence and/or quantity, and the composition of the mixture of chemicals can vary, such that chemodiversity can be partitioned within and among individuals. Plant ontogeny and environmental and genetic variation are recognized as sources of chemical variation, but recent advances in understanding the molecular basis of variation may allow the future deployment of isogenic mutants to test the specific adaptive function of variation in PSMs. An important consequence of high intraspecific variation is the capacity to evolve rapidly. It is becoming increasingly clear that trait variance linked to both macro- and micro-environmental variation can also evolve and may respond more strongly to selection than mean trait values. This research, which is in its infancy in plants, highlights what could be a missing piece of the picture of PSM evolution. PSM polymorphisms are probably maintained by multiple selective forces acting across many spatial and temporal scales, but convincing examples that recognize the diversity of plant population structures are rare. We describe how diversity can be inherently beneficial for plants and suggest fruitful avenues for future research to untangle the causes and consequences of intraspecific variation.

Effects of intraspecific variation in reproductive traits, pectoral fin use and burst swimming on metabolic rates and swimming performance: a study on the Trinidadian guppy (Poecilia reticulataPeters)

There is considerable intraspecific variation in metabolic rates and locomotor performance in aquatic ectothermic vertebrates; however, the mechanistic basis remains poorly understood. Using pregnant Trinidadian guppies (Poecilia reticulata), a live-bearing teleost, we examined the effects of reproductive traits, pectoral fin use and burst-assisted swimming on swimming metabolic rate, standard metabolic rate (O2std) and prolonged swimming performance (Ucrit). Reproductive traits included reproductive allocation and pregnancy stage, the former defined as the mass of the reproductive tissues divided by the total body mass. Results showed that the metabolic rate increased curvilinearly with swimming speed. The slope of the relationship was used as an index of swimming cost. There was no evidence that reproductive traits correlated with swimming cost, O2std or Ucrit. In contrast, data revealed strong effects of pectoral fin use on swimming cost and Ucrit. Poecilia reticulata employed body-caudal fin (BCF) swimming at all tested swimming speeds; however, fish with a high simultaneous use of the pectoral fins exhibited increased swimming cost and decreased Ucrit. These data indicated that combining BCF swimming and pectoral fin movement over a wide speed range, presumably to support swimming stability and control, is an inefficient swimming behaviour. Finally, transition to burst-assisted swimming was associated with an increase in aerobic metabolic rate. Our study highlights factors other than swimming speed that affect swimming cost and suggests that intraspecific diversity in biomechanical performance, such as pectoral fin use, is an important source of variation in both locomotor cost and maximal performance.

Effects of intraspecific variation in white cabbage (Brassica oleracea var. capitata) on soil organisms

Intraspecific variation in plants can affect soil organisms. However, little is known about whether the magnitude of the effect depends on the degree of interaction with the roots. We analyzed effects of plant intraspecific variation on root herbivores and other soil organisms that interact directly with living plant roots, as well as on decomposer organisms that interact more indirectly with roots. We used four different white cabbage (Brassica oleracea var. capitata) cultivars exhibiting a high degree of intraspecific variation in root glucosinolate profiles. Intraspecific variation affected root-feeding nematodes, whereas decomposer organisms such as earthworms and Collembola were not affected. Root-feeding nematodes were most abundant in one of the cultivars, Badger Shipper, which lacked the glucosinolate gluconasturtiin. The effect of the intraspecific variation in glucosinolate composition may have been restricted to root-feeding nematodes due to the rapid degradation of glucosinolates and their breakdown products in the soil. Additionally, the low biomass of root-feeding nematodes, relative to other soil organisms, limits the possibility to affect higher trophic level organisms. Our results show that variation in root chemistry predominantly affects belowground herbivores and that these effects do not extend into the soil food web.

Intraspecific variation of testis size and sperm length in the yellow-pine chipmunk ( Tamias amoenus ): implications for sperm competition and reproductive success

Comparative analyses have found that relative testis size is a strong predictor of the prevalence of sperm competition for many taxa, including mammals, yet underlying this pattern is the assumption that intraspecific variation in testis size is related to individual fitness. Because intraspecific variation in ejaculate investment underlies interspecific patterns, it is critical to understand the causes and consequences of intraspecific variation in ejaculate investment. We examined relationships between ejaculate investment (testis size and sperm length) and reproductive success, body size and condition in the yellow-pine chipmunk (Tamias amoenus), a small ground squirrel in which sperm competition occurs. We examined genetic estimates of male reproductive success from a wild population of yellow-pine chipmunks and determined that males with large testes had higher annual reproductive success than males with small testes. This result provides empirical support for the numerous comparative studies that indicate testis size is associated with the intensity of sperm competition. In addition, males in good condition had relatively larger testes than males in poor condition, but there was no evidence of sperm length being dependent on condition. Finally, contrary to many predictions, males that invested more in sperm production (relatively heavy testes) produced shorter sperm, not longer sperm, than males that invested less.

Effects of body size and mass on running speed of male yellow-pine chipmunks (Tamias amoenus)

Male-biased sexual size dimorphism in mammals is usually attributed to the success of large males in intrasexual combat for mates. However, mating success may be determined by contests that are not combative. In the mating chase of the yellow-pine chipmunk (Tamias amoenus), a mammalian species with female-biased sexual size dimorphism, fast males may have an advantage in acquiring matings with estrous females. However, the effects of intraspecific variation in body size on running speed are not obvious; heavy individuals may run more slowly than light individuals because excess mass can be a hindrance to locomotion, but individuals that are structurally large may run faster than small individuals because of longer stride length. We examined the effects of both body mass and structural size on running speed in male yellow-pine chipmunks using manipulated runs in which male chipmunks were chased over a known distance. Structurally large male chipmunks had faster running speeds than small males, potentially giving large males an advantage when chasing estrous females. However, small male chipmunks are known to be aggressively dominant over large males. This leads to a potential trade-off in male body size between two behavioural components of mating success (running speed and dominance) that may constrain the evolution of male body size, ultimately leading to female-biased sexual size dimorphism.

A condition dependent link between testosterone and disease resistance in the house finch.

Testosterone has recently been proposed as a link between male quality and health and the expression of sexual traits. We investigated the relationship between testosterone and measures of the individual condition and health of males in a natural population of house finches (Carpodacus mexicanus). We also conducted a captive experiment in order to test for the effects of testosterone on resistance to coccidia, which is a common parasite of house finches. Free-living males in better condition had higher testosterone levels and lower corticosterone levels than free-living males in poor condition. In our captive experiment, increased testosterone accelerated the rate of coccidial infection as compared with sham-implanted or gonadectomized males. Although the differences were not significant, free-living males infected with coccidia had lower levels of testosterone and higher levels of corticosterone than males that were not infected. Thus, experimentally elevating testosterone levels in captive males resulted in a higher percentage of infected males, while free-living males with coccidial infection had low testosterone levels. This apparent discrepancy between captive and free-living males in the association of testosterone and disease may be explained by the condition dependence of testosterone. These results suggest that the testosterone-dependent sexual traits reliably indicate male overall condition and health and, thus, females could benefit from assessing potential mates based on these traits.