An experimental approach to altering mating tactics in male horseshoe crabs (Limulus polyphemus)

Alternative reproductive tactics are often correlated with phenotype, density, environment, or social context. Male horseshoe crabs(Limulus polyphemus) have two mating tactics that are associatedwith phenotype. Males in good condition arrive at the nestingbeach and spawn while attached to females, whereas those inpoorer condition come ashore unattached and crowd around thenesting couples as satellites, fertilizing eggs through sperm competition. The correlation between mating tactic and phenotypemay be due to males choosing tactics based on condition, orit may be that males that have not found a female choose tocome ashore as satellites. To distinguish between these twopossibilities, I conducted an experiment on male horseshoe crabsin the field at Seahorse Key on the northern Gulf coast ofFlorida. I prevented males from attaching to females by placingsmall plastic bags over the claws they use to attach. The resultsshowed that males in poor condition came ashore as satellites,whereas males in good condition remained at sea. This meansthat mating tactics are cued by information about the male'scondition and not about whether he found a female. The evolutionof phenotype-correlated mating tactics can be represented bya model in which the fitness of each tactic changes with conditionand fitness curves cross. I hypothesize that male horseshoecrabs in good condition have higher fitness when attached and that males in poorer condition to better when unattached.     

Predator-prey relationships and the evolution of colour polymorphism: a comparative analysis in diurnal raptors

Genetically based variation in coloration occurs in populations of many organisms belonging to various taxa, including birds, mammals, frogs, molluscs, insects and plants. Colour polymorphism has evolved in raptors more often than in any other group of birds, suggesting that predator–prey relationships was a driving evolutionary force. Individuals displaying a new invading colour morph may enjoy an initial foraging advantage because prey have difficulties in learning the colour of a rare morph (apostatic selection), or because morphs provide alternative foraging benefits allowing differently coloured individuals to exploit distinct food niches (disruptive selection). Plumage polymorphism should therefore have evolved in species that prey upon animals having the physiological ability to distinguish between differently coloured predators but also to flee once a predator has been detected. From this assumption, we can predict that closely related polymorphic and monomorphic species prey upon different animals. They may also differ in morphology, because foraging upon different prey may require different foraging modes, and in turn different morphological structures. We tested these two predictions in a comparative study of raptors. As expected, polymorphic and monomorphic species had a different diet, and there was a difference in wing length between polymorphic and monomorphic species within two genera ( Buteo and Accipiter ). Across all raptors for which phylogenetic relationships are known, polymorphic species preyed more often upon mammals than did monomorphic ones. These two types of raptor did not differ in the frequency of birds, insects and reptiles in their diets. We discuss these results in the light of the hypothesis that predator–prey relationships played a role in the evolution of colour polymorphism. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 565–578.     

Colour variation and alternative reproductive strategies in females of the common lizard Lacerta vivipara

Within-sex colour variation is a widespread phenomenon in animals that often plays a role in social selection. In males, colour variation is typically associated with the existence of alternative reproductive strategies. Despite ecological conditions theoretically favourable to the emergence of such alternative strategies in females, the social significance of colour variation in females has less commonly been addressed, relative to the attention given to male strategies. In a population of the common lizard, females display three classes of ventral colouration: pale yellow, orange and mixed. These ventral colours are stable through individual's life and maternally heritable. Females of different ventral colourations displayed different responses of clutch size, clutch hatching success and clutch sex-ratio to several individual and environmental parameters. Such reaction patterns might reflect alternative reproductive strategies in females. Spatial heterogeneity and presence of density- and frequency-dependent feedbacks in the environment could allow for the emergence of such alternative strategies in this population and the maintenance of colour variation in females.     

Colour-associated mating success in a polymorphic Ladybird Beetle, Harmonia axyridis

1. Body size is often an important character in mating success, but has been only infrequently mentioned in regard to colour polymorphism. In this study, mating success was investigated in a colour polymorphic Ladybird Beetle, Harmonia axyridis , with reference both to colour morph and to body size.
2. In the non-melanic males the mating individuals were significantly larger than solitary individuals, while in melanic males there was no significant difference.
3. The mating pattern was close to random mating with respect to colour morph and there was no significant deviation.
4. The results suggest both body size and colour morph affect the male mating success and males of different body size obtain mating advantage according to the colour morph. Colour polymorphism in this species is controlled by alleles on a single locus. Thus, the alleles on that locus significantly influence the effect of selection on the quantitative character.     

Adaptive significance of permanent female mimicry in a bird of prey

Permanent female mimicry, in which adult males express a female phenotype, is known only from two bird species. A likely benefit of female mimicry is reduced intrasexual competition, allowing female-like males to access breeding resources while avoiding costly fights with typical territorial males. We tested this hypothesis in a population of marsh harriers Circus aeruginosus in which approximately 40 per cent of sexually mature males exhibit a permanent, i.e. lifelong, female plumage phenotype. Using simulated territorial intrusions, we measured aggressive responses of breeding males towards conspecific decoys of females, female-like males and typical males. We show that aggressive responses varied with both the type of decoys and the type of defending male. Typical males were aggressive towards typical male decoys more than they were towards female-like male decoys; female-like male decoys were attacked at a rate similar to that of female decoys. By contrast, female-like males tolerated male decoys (both typical and female-like) and directed their aggression towards female decoys. Thus, agonistic responses were intrasexual in typical males but intersexual in female-like males, indicating that the latter not only look like females but also behave like them when defending breeding resources. When intrasexual aggression is high, permanent female mimicry is arguably adaptive and could be seen as a permanent 'non-aggression pact' with other males.     

Multivariate phenotypes and the potential for alternative phenotypic optima in wall lizard (Podarcis muralis) ventral colour morphs

A major goal in evolutionary biology is to determine how phenotypic variation arises and is maintained in natural populations. Recent studies examining the morphological, physiological and behavioural differences among discrete colour morphotypes (morphs) have revealed several mechanisms that maintain discrete variation within populations, including frequency‐dependence, density‐dependence and correlational selection. For example, trade‐offs over resource allocation to morphological, physiological and behavioural traits can drive correlational selection for morph‐specific phenotypic optima. Here, we describe a ventral colour polymorphism in the wall lizard (Podarcis muralis) and test the hypothesis that morphs differ along multivariate axes defined by trade‐offs in morphological, physiological, and immunological traits. We show that ventral colour is a discrete trait and that morphs differ in body size, prevalence of infection by parasites and infection intensity. We also find that morphs differ along multivariate phenotypic axes and experience different multivariate selection pressures. Our results suggest that multivariate selection pressures may favour alternative optimal morph‐specific phenotypes in P. muralis.     

Molecular determination of paternity in a natural population of the multiply mating polygynous lizard Eulamprus heatwolei

We studied the mating system of the southern water skink, Eulamprus heatwolei, during spring and summer (encompassing the breeding season) in a population in southeastern Australia. We examined potential attributes that might influence the mating system and male reproductive success including home range size, physical proximity of adults and body size, and then genotyped all mothers, offspring and potential sires. Home range overlap of both sexes was extensive, with adult females sharing the greatest amount of space with each other and adult males the least amount of space with each other. However, not all adults hold home ranges. We classified approximately one quarter of adult males as home range holders and the rest as 'floaters'. Adult females occupy home ranges more than males, with approximately three-quarters classified as home range holders. Home range ownership is not correlated with body size for either sex, however, male body size is positively correlated with the number of adult female home ranges that his home range overlaps and adult male home ranges are larger than those of females. We used microsatellite genotyping to assign paternities to 55 offspring from 17 litters and then compared this data with our home range and behavioural observations. This species displays extreme levels of multiple paternity given the small mean clutch size of three. Multiple paternity was confirmed in 11 (64.7%) of 17 clutches but three other clutches (for a total of 82.4%) also may display multiple paternity. A total of 30 offspring from 12 litters were assigned to 10 of the 32 genotyped adult males from our study site. Of these 10 adult males, half were home range holders. Five complete clutches and a total of 25 out of the 55 offspring could not be positively assigned to any male surveyed as part of the study and were attributed to floater males or resident males adjacent to our study site that had not been genotyped. While sample sizes are small, neither male home range ownership nor body size is significantly correlated with the number of paternities a male obtained. Our study suggests a polygynous mating system for this species.     

Viability,behavior, and color expression in the offspring of matings between common wall lizard Podarcis muralis color morphs

Color polymorphisms are widely studied to identify the mechanisms responsible for the origin and maintenance of phenotypic variability in nature. Two of the mechanisms of balancing selection currently thought to explain the long-term persistence of polymorphisms are the evolution of alternative phenotypic optima through correlational selection on suites of traits including color and heterosis. Both of these mechanisms can generate differences in offspring viability and fitness arising from different morph combinations. Here, we examined the effect of parental morph combination on fertilization success, embryonic viability, newborn quality, antipredator, and foraging behavior, as well as inter-annual survival by conducting controlled matings in a polymorphic lacertid Podarcis muralis, where color morphs are frequently assumed to reflect alternative phenotypic optima (e.g., alternative reproductive strategies). Juveniles were kept in outdoor tubs for a year in order to study inter-annual growth, survival, and morph inheritance. In agreement with a previous genome-wide association analysis, morph frequencies in the year-old juveniles matched the frequencies expected if orange and yellow expressions depended on recessive homozygosity at 2 separate loci. Our findings also agree with previous literature reporting higher reproductive output of heavy females and the higher overall viability of heavy newborn lizards, but we found no evidence for the existence of alternative breeding investment strategies in female morphs, or morph-combination effects on offspring viability and behavior. We conclude that inter-morph breeding remains entirely viable and genetic incompatibilities are of little significance for the maintenance of discrete color morphs in P. muralis from the Pyrenees.     

Mechanistic and experimental analysis of condition and reproduction in a polymorphic lizard

Abstract The importance of genetic and environmental variation in condition in shaping evolutionary trade‐offs have recently been subject to much theoretical discussion, but is very difficult to investigate empirically in most field‐based systems. We present the results from mechanistic experimental manipulations of reproductive investment and condition in two female colour morphs (‘orange’ and ‘yellow’) of side‐blotched lizards (Uta stansburiana). We investigated the interactions between throat colour morphs, condition, local social environment and female survival using path‐analysis. Using follice‐ablation experiments, we show that large clutch size has a negative effect on field survival among yellow females, and that this effect is partly mediated by immunosuppressive effects of large clutches. In orange females these effects were less pronounced, and there was a negative survival effect of strong antibody responses. Hence, we experimentally confirmed our previous findings of correlational selection between female morphotype and immunocompetence, an important condition trait. Manipulation of corticosterone revealed multiple (‘pleiotropic’) direct and indirect effects of this hormone on both condition and reproductive traits. We argue that interaction effects (e.g. between local environments and genotypes) could explain a substantial fraction of variation in condition and reproduction in natural populations. Increased attention to such interaction effects and their fitness consequences will provide novel insights in field studies of selection and reproductive allocation.     

Mating system variation and morph fluctuations in a polymorphic lizard

In polymorphic male painted dragon lizards (Ctenophorus pictus), red males win staged contests for females over yellow males, and yellow males have greater success in staged sperm competition trials than red males. This predicts different reproductive strategies in the wild with red males being more coercive or better mate guarders than yellow males. Yellow males would be expected to sire more offspring per copulation and have a greater proportion of offspring from clutches with mixed paternity. However, here we show using microsatellites that the frequency of mixed paternity in the wild is low (< 20% on average across years), that all morphs on average have the same number of offspring sired per year, and that mating system variation (polyandry vs. monandry) is strongly correlated with perch density on male territories. Furthermore, a logistic regression on male successful vs. unsuccessful mate acquisition showed that red males were under negative selection when they dominated the population, which suggests ongoing frequency dependent selection on male colouration.     

Female aggression predicts mode of paternity acquisition in a social lizard

Individual differences in behaviour are ubiquitous in nature. Despite the likely role of selection in maintaining these differences, there are few demonstrations of their fitness consequences in wild populations and, consequently, the mechanisms that link behavioural variation to variation in fitness are poorly understood. Specifically, the consequences of consistent individual differences in behaviour for the evolution of social and mating strategies have rarely been considered. We examined the functional links between variation in female aggression and her social and mating strategies in a wild population of the social lizard Egernia whitii. We show that female Egernia exhibit temporally consistent aggressive phenotypes, which are unrelated to body size, territory size or social density. A female''s aggressive phenotype, however, has strong links to her mode of paternity acquisition (within- versus extra-pair paternity), with more aggressive females having more offspring sired by extra-pair males than less aggressive females. We discuss the potential mechanisms by which female aggression could underpin mating strategies, such as the pursuit/acceptance of extra-pair copulations. We propose that a deeper understanding of the evolution and maintenance of social and mating systems may result from an explicit focus on individual-level female behavioural phenotypes and their relationship with key reproductive strategies.     

Morph‐dependent fitness and directional change of morph frequencies over time in a Dutch population of Common buzzards Buteo buteo

How genetic polymorphisms are maintained in a population is a key question in evolutionary ecology. Previous work on a plumage colour polymorphism in the common buzzard Buteo buteo suggested heterozygote advantage as the mechanism maintaining the co‐existence of three morphs (light, intermediate and dark). We took advantage of 20 years of life‐history data collected in a Dutch population to replicate earlier studies on the relationship between colour morph and fitness in this species. We examined differences between morphs in adult apparent survival, breeding success, annual number of fledglings produced and cumulative reproductive success. We found that cumulative reproductive success differed among morphs, with the intermediate morph having highest fitness. We also found assortative mating for colour morph, whereby assortative pairs were more likely to produce offspring and had longer‐lasting pair bonds than disassortative pairs. Over the 20‐year study period, the proportion of individuals with an intermediate morph increased. This apparent evolutionary change did not just arise from selection on individual phenotypes, but also from fitness benefits of assortative mating. The increased frequency of intermediates might also be due to immigration or drift. We hypothesize that genetic variation is maintained through spatial variation in selection pressures. Further studies should investigate morph‐dependent dispersal behaviour and habitat choice.     

Extreme male leg polymorphic asymmetry in a new empidine dance fly (Diptera: Empididae)

A new dance fly (Empididae: Empidinae) with hugely modified male fore tarsus, either on the right, left, both or neither sides, is described from Japan. Such massive polymorphic asymmetry occurring with so high an incidence in a population is previously unreported. In view of the courtship behaviour of other Empidinae, we hypothesize that the oversized tarsus is a secondary sexual character employed by males for attracting females. Alternative hypotheses are also discussed. We suggest that this extraordinary new species is a potential model for the study of mating biology in Empidinae and the evolution of mating systems in general.     

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.     

Increased energy investment in testes following territory acquisition in a maternal mouthbrooding cichlid

Underwater observations conducted in Lake Tanganyika showed that males of a maternal mouthbrooding cichlid, Petrochromis fasciolatus, defended a mating territory for at least several months, but left at approximately noon every day. After the experimental removal of males from their territories, new owners occupied the vacated territories within several days. New owners exhibited higher body-condition factors and fat indices than the original owners. These results suggest that the new owners had not previously occupied a territory, considering the physical exhaustion of owners owing to the energy expenditure on territory maintenance and mating behaviors and a limited time for feeding. Original owners had heavier testes than new owners, despite similar body sizes. This disparity in testis weight suggests that energy investment in testes increases following territory acquisition. Among males that newly occupied a territory during the observation period, larger individuals occupied territories sooner, suggesting that large body size facilitates competition for territory. It is concluded that energy investment in testes is delayed for somatic growth until territory acquisition.