Intensity of male‐male competition predicts morph diversity in a color polymorphic lizard

Sexual selection is one of the main processes involved in the emergence and maintenance of heritable color polymorphisms in a variety of taxa. Here, we test whether the intensity of sexual selection, estimated from population sex ratio, predicts morph diversity in Podarcis muralis, a color polymorphic lizard with discrete white, yellow, orange, white‐orange, and yellow‐orange male and female phenotypes (i.e., morphs). In a sample of 116 Pyrenean populations and 5421 lizards, sex ratios (m/f) vary from 0.29 to 2.5, with the number of morphs for each sex ranging from 2 to 5. Male‐biased sex ratios are associated with increased morph diversity as measured with Shannon's diversity index. The main factor accounting for this relationship is male morph richness (i.e., the number of morphs). In contrast, female morph diversity is not related to sex ratio. These results suggest a relationship between the intensity of male intrasexual competition and male morph diversity. While other selective forces may interact with sexual selection in maintaining the color polymorphisms in P. muralis, this evidence suggests a complex evolutionary scenario possibly involving frequency‐dependent selection of alternative reproductive tactics and/or complex balancing selection.     

Healthier or bigger? Trade‐off mediating male dimorphism in the black scavenger fly Sepsis thoracica (Diptera: Sepsidae)

1. Life history trade‐offs emerge when limited resources are allocated to multiple functions of an organism. Under highly competitive conditions trade‐offs can result in alternative phenotypes that differ morphologically and physiologically. Such is the case in insect species that grow under high densities, where competition for resources but also the risk of disease contagion is high, prompting important adjustments in immune response and melanic cuticular pigmentation, with consequent sacrifices in other fitness‐related traits. 2. In the present study, the potential trade‐offs between total‐ and active phenoloxidase (PO), body size and body pigmentation in Sepsis thoracica black scavenger flies that show alternative male morphs differing in cuticular pigmentation, and body size were evaluated. 3. As expected, small/dark (obsidian) males showed higher total‐PO activity than larger/orange (amber) males. A negative relationship was found between total‐PO activity and body size in females and obsidian but not amber males, suggesting that growth and immunity are more costly for the former. In contrast, density did not affect PO activity, as predicted by the density‐dependent prophylaxis hypothesis, which had not been tested in dipterans before. However, rearing density did affect the body size negatively in females and amber but not obsidian males, showing that male morph is largely determined by condition‐dependent plasticity rather than genes. 4. This study provides good evidence that trade‐offs between different life‐history traits can result in alternative resource allocation strategies, even within one species. These strategies can produce strikingly different alternative phenotypes, evincing that there is not only one optimal solution to address fitness optimisation.     

Ecological polymorphism in Arctic charr

Arctic charr, Salvelinus alpinus (L.), commonly exhibits two coexisting morphotypes, dwarf and normal charr, which are characterized by differences in adult body size and colouration. We tested whether or not the morphotypic differences were genetically determined in rearing experiments with offspring of the two morphs and of their crosses. The experiments suggest that this ecological polymorphism in Arctic charr is largely environmentally determined. When reared under similar conditions, offspring of each of the two morphs differed little in size at the same age, and they had the same early developmental rate and maturation pattern. Moreover, the presence of parr marks along the flanks of the fish, one characteristic of dwarf charr, depended on body size and not on parental morph. Genetic differences between the morphs were suggested for growth rate during the second year of life, and for jaw morphology. Comparisons between charr life histories in captivity and in the wild suggest that ecological polymorphism in Arctic charr is chiefly a result of variation in growth conditions between different habitats.     

Evolution of sexual size dimorphism in grouse and allies (Aves: Phasianidae) in relation to mating competition,fecundity demands and resource division

Sexual size dimorphism (SSD) is often assumed to be driven by three major selective processes: (1) sexual selection influencing male size and thus mating success, (2) fecundity selection acting on females and (3) inter‐sexual resource division favouring different size in males and females to reduce competition for resources. Sexual selection should be particularly strong in species that exhibit lek polygyny, since male mating success is highly skewed in such species. We investigated whether these three selective processes are related to SSD evolution in grouse and allies (Phasianidae). Male‐biased SSD increased with body size (Rensch’s rule) and lekking species exhibited more male‐biased SSD than nonlekking ones. Directional phylogenetic analyses indicated that lekking evolved before SSD, but conclusions were highly dependent on the body size traits and chosen model values. There was no relationship between SSD and male display agility, nor did resource division influence SSD. Although clutch mass increased with female body size it was not related to the degree of SSD. Taken together, the results are most consistent with the hypothesis that lekking behaviour led to the evolution of male‐biased SSD in Phasianidae.     

Temporal relationship between genetic and warning signal variation in the aposematic wood tiger moth (Parasemia plantaginis)

Many plants and animals advertise unpalatability through warning signals in the form of colour and shape. Variation in warning signals within local populations is not expected because they are subject to directional selection. However, mounting evidence of warning signal variation within local populations suggests that other selective forces may be acting. Moreover, different selective pressures may act on the individual components of a warning signal. At present, we have a limited understanding about how multiple selection processes operate simultaneously on warning signal components, and even less about their temporal and spatial dynamics. Here, we examined temporal variation of several wing warning signal components (colour, UV‐reflectance, signal size and pattern) of two co‐occurring colour morphs of the aposematic wood tiger moth (Parasemia plantaginis). Sampling was carried out in four geographical regions over three consecutive years. We also evaluated each morph's temporal genetic structure by analysing mitochondrial sequence data and nuclear microsatellite markers. Our results revealed temporal differences between the morphs for most signal components measured. Moreover, variation occurred differently in the fore‐ and hindwings. We found no differences in the genetic structure between the morphs within years and regions, suggesting single local populations. However, local genetic structure fluctuated temporally. Negative correlations were found between variation produced by neutrally evolving genetic markers and those of the different signal components, indicating a non‐neutral evolution for most warning signal components. Taken together, our results suggest that differential selection on warning signal components and fluctuating population structure can be one explanation for the maintenance of warning signal variation in this aposematic species.     

Assembly rules of ground-foraging ant assemblages are contingent on disturbance, habitat and spatial scale

Aim A major endeavour of community ecology is documenting non‐random patterns in the composition and body size of coexisting species, and inferring the processes, or assembly rules, that may have given rise to the observed patterns. Such assembly rules include species sorting resulting from interspecific competition, aggregation at patchily distributed resources, and co‐evolutionary dynamics. However, for any given taxon, relatively little is known about how these patterns and processes change through time and vary with habitat type, disturbance history, and spatial scale. Here, we tested for non‐random patterns of species co‐occurrence and body size in assemblages of ground‐foraging ants and asked whether those patterns varied with habitat type, disturbance history, and spatial scale. Location Burned and unburned forests and fens in the Siskiyou Mountains of southern Oregon and northern California, USA. Methods We describe ground‐foraging ant assemblages sampled over two years in two discrete habitat types, namely Darlingtonia fens and upland forests. Half of these sites had been subject to a large‐scale, discrete disturbance – a major fire – in the year prior to our first sample. We used null model analyses to compare observed species co‐occurrence patterns and body‐size distributions in these assemblages with randomly generated assemblages unstructured by competition both within (i.e. at a local spatial scale) and among (i.e. at a regional scale) sites. Results At local spatial scales, species co‐occurrence patterns and body‐size ratios did not differ from randomness. At regional scales, co‐occurrence patterns were random or aggregated, and there was evidence for constant body‐size ratios of forest ants. Although these patterns varied between habitats and years, they did not differ between burned and unburned sites. Main conclusions Our results suggest that the operation of assembly rules depends on spatial scale and habitat type, but that it was not affected by disturbance history from fire.     

Shaping intraspecific variation: Development,ecology and the evolution of morphology and life history variation in tiger salamanders

The tiger salamander,Ambystoma tigrinum, is a geographically widespread, morphologically variable, polytipic species. It is among the most variable species of salamanders in morphology and life history with two larval morphs (typical and cannibal) and three adult morphs (metamorphosed, typical branchiate, cannibal branchiate) that vary in frequency between subspecies and between populations within subspecies. We report morphometric evidence suggesting that branchiate cannibals arose through intraspecific change in the onset or timing of development resulting in the wider head and hypertrophied tooth-bearing skull bones characteristic of this phenotype. We also quantified bilateral symmetry of gill raker counts and abnormalities, then evaluated fluctuating asymmetry as a measure of the developmental stability of each morph. There was a significant interaction between fluctuating asymmetry of developmental abnormalities in cannibals and typicals and the locality where they were collected, suggesting that relative stability of each phenotype could vary among populations. While altered timing of developmental events appears to have a role in the evolution and maintenance of morphs, novel phenotypes persist only under favorable ecological conditions. Predictability of the aquatic habitat, genetic variation, kinship, body size, intraspecific competition and predation all affect expression and survival of the morphs inA. tigrinum. This taxon provides an excellent model for understanding the diversity and complexity of developmental and ecological variables controlling the evolution and maintenance of novel phenotypes.     

CAN PARALLEL DIVERSIFICATION OCCUR IN SYMPATRY? REPEATED PATTERNS OF BODY‐SIZE EVOLUTION IN COEXISTING CLADES OF NORTH AMERICAN SALAMANDERS

A classic paradigm in evolutionary biology is that geographically isolated clades inhabiting similar selective regimes will diversify to create similar sets of phenotypes in different locations (e.g., similar stickleback species in different lakes, similar Anolis ecomorphs on different islands). Such parallel radiations are not generally expected to occur in sympatry because the available niche space would be filled by whichever clade is diversified first. Here, we document a very different pattern, the parallel evolution of similar body-size morphs in three sympatric clades of plethodontid salamanders ( Desmognathus, Plethodon, Spelerpinae) in eastern North America. Using a comprehensive, time-calibrated phylogeny of North American plethodontids from nuclear and mitochondrial DNA sequences, we show that these three clades have undergone replicated patterns of evolution in body size and that this parallel diversification occurred in broad-scale sympatry. At the local scale, we find that coexisting species from these clades are more similar in body size than expected under a null model in which species are randomly assembled into communities. These patterns are particularly surprising in that competition is known to be important in driving phenotypic diversification and limiting local coexistence of similar-sized species within these clades. Although parallel diversification of sympatric clades may seem counterintuitive, we discuss several ecological and evolutionary factors that may allow the phenomenon to occur.     

Performance capacity, fighting tactics and the evolution of life-stage male morphs in the green anole lizard (Anolis carolinensis)

The evolution of alternative male phenotypes is probably driven by male-male competition for access to reproductive females, but few studies have examined whether whole-organism performance capacities differ between male morphs, and if so whether any such differences affect fighting ability. We show how ontogenetic changes in performance and morphology have given rise to two distinct life-stage male morphs exhibiting different fighting tactics within the green anole lizard (Anolis carolinensis). Field studies show a bimodal distribution of adult males within a single population: larger 'heavyweight' males have relatively large heads and high bite forces for their size, whereas smaller 'lightweight' males have smaller heads and lower bite forces. In staged fights between size-matched heavyweight males, males with greater biting ability won more frequently, whereas in lightweight fights, males with greater jumping velocity and acceleration won more often. Because growth in reptiles is indeterminate, and the anole males examined are sexually mature, we propose that the heavyweight morph arose through selection against males with small heads and poor bite forces at the lightweight-heavyweight size transition. Our findings imply that one may not be able to predict male fighting success (and hence potential mating success) by examining aspects of male 'quality' at only one life stage.     

Foraging information affects the nature of competitive interactions

If animals can learn environmental cues, how might their use of information affect competitive interactions between species? I used shared‐preference isoleg theory to generate four different predictions regarding possible ways information could affect density‐dependent habitat selection. To test these predictions, I conducted field experiments on the foraging behavior of three coexisting species of hummingbirds. I studied black‐chinned (Archilochus alexandri), blue‐throated (Lampornis clemenciae), and magnificent (Eugenes fulgens) hummingbirds at the Southwestern Research Station in the Chiricahua Mts. of Southeastern Arizona, USA. Blue‐throated hummingbirds behaviorally dominate the other two species. I found that birds foraging with complete information (via learned color cues) avoid some of the negative effects from competition. Birds that foraged with complete information remained highly selective on rich feeders even with high competitor densities. When birds suffered an information deficit, however, the two subordinate species shifted their foraging preferences. The dominant species did not shift preference. Each subordinate's shift reflects its unique place in this competitively structured guild. With high competitor densities, black‐chinneds shifted from selective to opportunistic foraging when they also suffered from an information deficit. Thus, an information deficit caused black‐chinneds to cross over their first isoleg. Rather than reduce foraging efficiency, the magnificents shortened their foraging time. I use the magnificents’ response to competition and an information deficit to explore the possibility that this guild of hummingbirds is centrifugally organized. My results require us to consider the information‐gathering (e.g., learning) abilities of individual decision‐makers when we evaluate density‐dependent habitat selection. Differences in the level of usable information can alter the way coexisting species respond to competitive interactions that potentially structure communities.     

Consequences of sex‐selective harvesting and harvest refuges in experimental meta‐populations

Harvesting for food or sport is often non‐random with respect to demographic state, such as size or life stage. The population‐level consequences of such selective harvesting depend upon which states are harvested and how those states contribute to population dynamics. We focused on a form of selective harvesting that has not previously been investigated in an experimental context: sex‐selective harvesting, a common feature of exploited, dioecious populations. Using simple metapopulations (two patches connect by dispersal) of sexually dimorphic Bruchid beetles in the laboratory, we contrasted the effects of female‐selective, male‐selective, and non‐selective harvesting over six generation of population dynamics. We also tested the ability of a harvest refuge (one patch of the metapopulation free from harvesting) to mitigate the effects of harvesting, and whether refuge effects interacted with sex selectivity. Sex‐selective harvesting significantly perturbed operational sex ratios and harvest refuges dampened these perturbations. Metapopulations assigned to male‐selective and non‐selective treatments were able to fully compensate for harvesting, such that their dynamics did not differ from non‐harvested controls. Only female‐selective harvesting led to significant reductions in population size and this effect was completely offset by dispersal from a harvest refuge. A two‐sex model confirmed that population dynamics are more sensitive to female vs. male harvesting, but suggested that higher levels of male harvest than included in our experiment would cause population decline. We discuss the roles of density‐dependent competition and frequency‐dependent sexual processes in the population response to sex‐selective harvesting.     

Size‐selective fishing affects sex ratios and the opportunity for sexual selection in Alaskan sockeye salmon Oncorhynchus nerka

Selective exploitation can cause adverse ecological and evolutionary changes in wild populations and also affect sex ratios but few studies have empirically documented skewed sex ratios in exploited fishes (other than species with extreme sexual size dimorphism, SSD). To investigate the possibility of sex‐selective fishing on Alaskan sockeye salmon Oncorhynchus nerka, we assessed sex ratios in fish at two spatial scales: within each of five fishing districts and among 13 breeding populations in one of these districts. We predicted that populations’ sex ratios would vary based on the average size of fish and SSD because size affects vulnerability to fishing. At the larger scale, we found a small but significant bias in fish returning to four of the five fishing districts (average = 52% females), and in four of the five districts males were caught at significantly higher rates than females. At the finer scale there was marked variation in sex ratio on the breeding grounds, ranging from 36% to 47% males. Populations with fish of intermediate sizes experienced the greatest sex ratio biases; the greater vulnerability of males than females to fishing resulted from a combination of larger SSD and different harvest rates between the sexes associated with the fishery size‐selectivity curve shape. Skewed sex ratios may change competition and behavior on the breeding grounds, relaxing selection on male traits associated with mate choice by females or intra‐sexual competition and altering demographic and evolutionary pressures on the fish. Assessment of the size selectivity of fishing gear and the population's SSD can help to illuminate if and how exploitation can affect sex ratios. Future studies examining size‐selective fishing should also evaluate the consequences for sex ratios, as this might help explain changes in harvested population structure and sustainability.     

Morph‐dependent form of asymmetry in mandibles of the stag beetle Prosopocoilus inclinatus (Coleoptera: Lucanidae)

Abstract 1. The form of asymmetry in bilateral organs usually follows the same pattern within single populations. However, some exceptions may occur when a population consists of different phenotypes that are from different ontogenic backgrounds and under different selective pressures. We investigated the asymmetric patterns of mandibles of larvae, females, and males in the stag beetle Prosopocoilus inclinatus. 2. Larval mandibles exhibited directional asymmetry both in length and cross direction, whereas female mandibles showed directional asymmetry in cross direction. These asymmetric structures might be more effective in cutting wood fibres. 3. For the relation of male mandible length to body size, a model with a switch point showed a better fit to the data than a convex curve model. This shows that the males are dimorphic with two distinct morphs. 4. The form of asymmetry in male mandible length differed between the morphs. The smaller males exhibited left‐biased directional asymmetry in common with larvae, whereas the larger males exhibited fluctuating asymmetry. 5. This is a novel finding of a morph‐dependent asymmetry. The morph‐dependent asymmetry in males may be as a result of different selection on each morph or a developmental constraint from larval mandibles to adult ones.     

Fitness trade-offs and the maintenance of alternative male morphs in the bulb mite (Rhizoglyphus robini)

Alternative reproductive phenotypes (ARPs) occur across a wide range of taxa. Most ARPs are conditionally expressed in response to a cue, for example body size, that reliably correlates with the status of the environment: individuals below the (body size) threshold then develop into one morph, and individuals above the threshold develop into the alternative morph. The environmental threshold model provides a theoretical framework to understand the evolution and maintenance of such ARPs, yet no study has examined the underlying fitness functions that are necessary to realize this. Here, we empirically examined fitness functions for the two male morphs of the bulb mite (Rhizoglyphus robini). Fitness functions were derived in relation to male size for solitary males and in relation to female size under competition. In both cases, the fitness functions of the two morphs intersected, and the resulting fitness trade-offs may play a role in the maintenance of this male dimorphism. We furthermore found that competition was strongest between males of the same morph, suggesting that fitness trade-off in relation to male size may persist under competition. Our results are a first step towards unravelling fitness functions of ARPs that are environmentally cued threshold traits, which is essential for understanding their maintenance and in explaining the response to selection against alternative morphs.     

Mouth dimorphism in scale‐eating cichlid fish from Lake Tanganyika advances individual fitness

Random asymmetry, that is the coexistence of left‐ and right‐sided (or ‐handed) individuals within a population, is a particular case of natural variation; what triggers and maintains such dimorphisms remains unknown in most cases. Here, we report a field‐based cage experiment in the scale‐eating Tanganyikan cichlid Perissodus microlepis, which occurs in two morphs in nature: left‐skewed and right‐skewed individuals with respect to mouth orientation. Using underwater cages stocked with scale‐eaters and natural prey fish, we first confirm that, under semi‐natural conditions, left‐skewed scale‐eaters preferentially attack the right flank of their prey, whereas right‐skewed individuals feed predominantly from the left side. We then demonstrate that scale‐eaters have a higher probability for successful attacks when kept in dimorphic experimental populations (left‐ and right‐skewed morphs together) as compared to monomorphic populations (left‐ or right‐skewed morphs), most likely because prey fishes fail to accustom to strikes from both sides. The significantly increased probability for attacks appears to be the selective agent responsible for the evolution and maintenance of mouth dimorphism in P. microlepis, lending further support to the hypothesis that negative frequency‐dependent selection is the stabilizing force balancing the mouth dimorphism at quasi‐equal ratios in scale‐eating cichlids.     

Character displacement and the evolution of niche complementarity in a model biofilm community

Colonization of vacant environments may catalyze adaptive diversification and be followed by competition within the nascent community. How these interactions ultimately stabilize and affect productivity are central problems in evolutionary ecology. Diversity can emerge by character displacement, in which selection favors phenotypes that exploit an alternative resource and reduce competition, or by facilitation, in which organisms change the environment and enable different genotypes or species to become established. We previously developed a model of long‐term experimental evolution in which bacteria attach to a plastic bead, form a biofilm, and disperse to a new bead. Here, we focus on the evolution of coexisting mutants within a population of Burkholderia cenocepacia and how their interactions affected productivity. Adaptive mutants initially competed for space, but later competition declined, consistent with character displacement and the predicted effects of the evolved mutations. The community reached a stable equilibrium as each ecotype evolved to inhabit distinct, complementary regions of the biofilm. Interactions among ecotypes ultimately became facilitative and enhanced mixed productivity. Observing the succession of genotypes within niches illuminated changing selective forces within the community, including a fundamental role for genotypes producing small colony variants that underpin chronic infections caused by B. cenocepacia.     

Sperm competition generates evolution of increased paternal investment in a sex role‐reversed seed beetle

When males provide females with resources at mating, they can become the limiting sex in reproduction, in extreme cases leading to the reversal of typical courtship roles. The evolution of male provisioning is thought to be driven by male reproductive competition and selection for female fecundity enhancement. We used experimental evolution under male‐ or female‐biased sex ratios and limited or unlimited food regimes to investigate the relative roles of these routes to male provisioning in a sex role‐reversed beetle, Megabruchidius tonkineus, where males provide females with nutritious ejaculates. Males evolving under male‐biased sex ratios transferred larger ejaculates than did males from female‐biased populations, demonstrating a sizeable role for reproductive competition in the evolution of male provisioning. Although larger ejaculates elevated female lifetime offspring production, we found little evidence of selection for larger ejaculates via fecundity enhancement: males evolving under resource‐limited and unlimited conditions did not differ in mean ejaculate size. Resource limitation did, however, affect the evolution of conditional ejaculate allocation. Our results suggest that the resource provisioning that underpins sex role reversal in this system is the result of male–male reproductive competition rather than of direct selection for males to enhance female fecundity.     

Testing multiple assembly rule models in avian communities on islands of an inundated lake,Zhejiang Province,China

Aim A fundamental question in community ecology is whether general assembly rules determine the structure of natural communities. Although many types of assembly rules have been described, including Diamond’s assembly rules, constant body‐size ratios, favoured states, and nestedness, few studies have tested multiple assembly rule models simultaneously. Therefore, little is known about the relative importance of potential underlying factors such as interspecific competition, inter‐guild competition, selective extinction and habitat nestedness in structuring community composition. Here, we test the above four assembly rule models and examine the causal basis for the observed patterns using bird data collected on islands of an inundated lake. Location Thousand Island Lake, China. Methods  We collected data on presence–absence matrices, body size and functional groups for bird assemblages on 42 islands from 2007 to 2009. To test the above four assembly rule models, we used null model analyses to compare observed species co‐occurrence patterns, body‐size distributions and functional group distributions with randomly generated assemblages. To ensure that the results were not biased by the inclusion of species with extremely different ecologies, we conducted separate analyses for the entire assemblage and for various subset matrices classified according to foraging guilds. Results The bird assemblages did not support predictions by several competitively structured assembly rule models, including Diamond’s assembly rules, constant body‐size ratios, and favoured states. In contrast, bird assemblages were highly significantly nested and were apparently shaped by extinction processes mediated through area effects and habitat nestedness. The nestedness of bird assemblages was not a result of passive sampling or selective colonization. These results were very consistent, regardless of whether the entire assemblage or the subset matrices were analysed. Main conclusions Our results suggest that bird assemblages were shaped by extinction processes mediated through area effects and habitat nestedness, rather than by interspecific or inter‐guild competition. From a conservation point of view, our results indicate that we should protect both the largest islands with the most species‐rich communities and habitat‐rich islands in order to maximize the number of species preserved.     

Size‐Dependent Male Mate Preference and its Association with Size‐Assortative Mating in a Mangrove Snail,Littoraria ardouiniana

In many animals, body size plays a crucial role in mating success in the context of competition and preference for mates. Increasing evidence has shown that male mate preference can be size‐dependent and, therefore, an important driver of size‐assortative mating. To test this theory, mate choice experiments were performed during the three consecutive stages of mating behaviour, namely trail following, shell mounting and copulation, in the dioecious mangrove snail, Littoraria ardouiniana. These experiments identified two possible forms of size‐dependent male mate preference which could contribute to the formation of size‐assortative mating in these snails. Firstly, whereas small males were unselective, large males were selective and preferred to follow mucus trails laid by large females. Alternatively, the results can also be interpreted as all males were selective and adopted a mating strategy of selecting females similar to, or larger than, their own sizes. Both small and large males also copulated for longer with large than with small females, and this was more pronounced in large males. When two males encountered a female, they engaged in physical aggression, with the larger male excluding the smaller male from copulating with the female. This study, therefore, demonstrated that size‐dependent male mate preference may, along with male–male competition, play an important role in driving size‐assortative mating in these mangrove snails, and this may also be the case in other species that exhibit male mate choice.     

Male dimorphism in three sympatric species of Onthophagus (Coleoptera: Scarabaeidae)

Onthophagus trituber, O. taurinus, and O. proletarius, three sympatric dung beetle species native to southern Taiwan, exhibit normally distributed body size (pronotum width) but non-normally distributed horn length due to the presence of horned and hornless morphs. The scaling relationships between horn length and body size were established by using sigmoidal and segmented linear estimation, and the horned/hornless ratio in each of the species was estimated. The ratios estimated by sigmoidal curves showed that all three species were biased toward the hornless morph, whereas the ratios from segmented lines showed that only one species (O. taurinus) was biased toward the hornless morph. Nevertheless, the results of the two methods of estimation were concordant in ranking the horned morph proportions among the three species; O. proletarius had the largest proportion of horned morphs, whereas O. taurinus had the smallest, suggesting that the fewer horned morph in O. taurinus likely resulted from both intra- and inter-specific competition.