Male mating speed promote hybridization in the Rana lessonae–Rana esculenta waterfrog system

Hybridization is a widespread phenomenon in many vertebrate groups. Prezygotic isolating mechanisms, probably caused by selection against hybrids with reduced fitness, reduce the likelihood of such events. Although hybrid-reduced fitness relatively to parental species is common, hybridization can also be beneficial, and hybrids sometimes outperform the pure species type. In this study, we examined two potential processes, Hubbs’s principle and male–male competition, which could enhance hybridization in the waterfrog complex and thus explain the proportion of heterospecific pairs collected in a natural pond. Firstly, by collecting 791 frogs in the field to study pair and chorus composition, we showed that in a mixed Rana lessonae–Rana esculenta population, the scarcity of hybrid R. esculenta males did not account for the proportion of heterospecific pairs: indeed, when examining pairing composition in six different choruses, we found that hybrid males were always under-represented and that R. esculenta females were found paired with R. lessonae males. Secondly, we investigated experimentally whether or nor male–male competition mechanism could explain pair formation in waterfrogs. Our mating speed experiment highlights mechanisms that could explain heterospecific pairs in a context of promiscuous mating where scramble competition was intense. To measure the rapidity with which a male grasps a female, we placed males in a grid cage with a female, and the dynamics of pair formation was monitored. R. esculenta males showed a lower pairing success than R. lessonae males as a smaller proportion of them amplexed females, and more time was needed for them to get amplexed. Thus, a less adaptative mechanism than female mate choice may also explain the mating pattern observed in waterfrog species.     

Female choice on male quantitative traits in lizards — why is it so rare?

Female choice on the basis of male traits has been described in an array of taxa but has rarely been demonstrated in reptiles. In the sand lizard (Lacerta agilis), and possibly in other non-territorial reptiles, a male's contribution to a female's fitness is restricted to his genes. In order to choose males of high genetic quality, females have to trade the fitness gain against the costs of active choice. In a Swedish population of sand lizards, long-lived males sired offspring with higher embryonic survival compared to offspring sired by short-lived males. In spite of this female sand lizards did not mate selectively with older and/or larger males. There appeared to be mo reliable cues to male longevity; age-specific male body size was highly variable. Furthermore, estimates of male nuptial coloration did not covary with ectoparasite load and, hence, females cannot use male coloration as a cue to heritable resistance to pathogenic parasite effects. When cues to male genetic quality are poor, or inaccurate, and males make no parental investment, we predict that female choice will be rare. Sand lizard females mating with many partners lay clutches with higher hatching success. Thus, females may obtain good genes for their young by multiple mating, thereby avoiding costs associated with mate choice.     

Selective males and ardent females in pipefishes

Summary In the pipefishes Syngnathus typhle and Nerophis ophidion, males have been shown to limit female reproductive rate, and females to compete for access to males. Hence, these species fit the criteria for sex-role reversal. Males brood the eggs and provide the offspring with nutrients, oxygen and an osmoregulated environment. Moreover, in S. typhle both sexes prefer a larger mate when given a choice. Sexual selection theory predicts that males should be more choosy than females, and that was experimentally demonstrated in this study. We predicted that S. typhle males should be less eager to copulate than S. typhle females with an unattractive (i.e. small) mate. We measured eagerness as the time from the start of the experiment until copulation occurred. Males with unattractive partners took significantly longer to copulate than females with unattractive partners. Moreover, females invariably initiated the courtship dance, and resumed it quicker after copulation than did the males, again suggesting reproductive hesitation in males. Neither male nor female size per se was correlated with time until copulation. In N. ophidion, where we have previously shown that males prefer larger to smaller females, we found that females did not select males with regard to size. Our results are consistent both with earlier findings (males limit female reproduction and females compete for males) and with operational sex ratios in nature: in seven annual field samples in June, the numbers of S. typhle females with ripe eggs always significantly exceeded numbers of receptive males. Hence, the potential cost of being choosy in terms of lost matings is much higher in females than in males. In conclusion, S. typhle females were somewhat choosy, but less so than males, whereas N. ophidion females were not choosy at all. Correspondence to: A. Berglund     

Sequential female choice and the previous male effect in sticklebacks

Summary Female choice, identified as a major force in sexual selection theory, has recently been demonstrated in a number of species. These tests concentrated on simultaneous choice situations although females have to compare males sequentially in most territorial species, which is the more demanding task. Here it is shown that female three-spined sticklebacks, Gasterosteus aculeatus L., rate sequentially presented males according to their brightness. With increasing costs of sampling the females become less choosy. Furthermore, a male's attractiveness has a significant effect on the female's rating of the next male; a given male is rated higher when preceded by a duller male than by a brighter one and vice versa. Female sticklebacks use a stochastic decision rule in sequential mate choice that is attuned to the attractiveness of the present and previously encountered male. This demonstration of a previous male effect not only indicates an efficient mechanism for finding the best of a number of males but also extends the applicability of sexual selection theory.Offprint requests to: T.C.M. Bakker     

Re-copulation and post-copulatory mate guarding increase immediate female reproductive output in the dragonfly Nannophya pygmaea Rambur

After copulation, male Nannophya pygmaea dragonflies mate guard by hovering over ovipositing females and repelling conspecific males. Copulation is not always a prerequisite for oviposition in the females of this species because females can store the sperm received during previous visits/copulations. An oviposition episode consists of several bouts of oviposition separated by periods of perching. We conducted two types of male-removal experiments to examine the effects of mating and post-copulatory mate guarding on the oviposition behaviour of females. In the first experiment, we removed all males from the habitat to eliminate the effect of re-copulation, mate-guarding and harassment by males. In the second experiment, we removed males immediately after copulation to eliminate the effects of guarding and other post-copulatory male-female interactions. We compared these experimental data with data obtained under natural conditions. The dipping rate in an oviposition bout was not influenced by copulation or guarding. However, guarded females made more dips per episode than did solitary females. The proportion of time actually spent ovipositing (total bout duration/oviposition episode duration) of guarded females was higher than that of solitary females. Solitary females often oviposited in more than one territorial site, while guarded females usually oviposited within a single territorial site during an oviposition episode. Because males tend to hold territories at sites where egg survival is high, guarded females (and the male guardian) benefit from guarding in terms of egg hatchability. The possible benefits for solitary females are discussed.     

Variation in fertilisation abilities between hemiclonal hybrid and sexual parental males of sympatric water frogs (<Emphasis Type="Italic">Rana lessonae</Emphasis>,<Emphasis Type="Italic"> R. esculenta</Emphasis>,<Emphasis Type="Italic"> R. ridibunda</Emphasis>)

In many species, males and females mate with multiple partners, which gives rise to sperm competition and multiple paternity. The experiments on water frogs presented here demonstrate that such sperm competition can affect the structure and dynamics of mixed-species communities. The hybrid frog Rana esculenta (LR) mates with one of its parental species, usually R. lessonae (LL), although in some areas R. ridibunda (RR), to regain the premeiotically eliminated parental genome ("hybridogenesis"). Mixed LL/LR-populations are stable although hybrid numbers should continuously increase at the expense of parental animals, because of differences in female fecundity and other factors. This would finally lead to the extinction of the sexual host, followed by that of the sexual parasite, unless the reproductive superiority of R. esculenta is reduced by other factors, such as lower hybrid male fertility. Eggs from LL- and LR-females were fertilised in vitro by single- and multi-male sperm suspensions of LL-, LR- and RR-males. In all experiments, the proportion of offspring sired by R. esculenta sperm was significantly lower than that sired by R. lessonae or R. ridibunda sperm. Gonad mass, sperm morphology, sperm swimming velocity, and sperm survival did not explain these differences in fertilisation success; nor did gamete recognition and compatibility. Sperm density was the only trait that paralleled fertilisation success, but it offers no explanation either, because densities were equalised for the in-vitro fertilisations. In natural LL/LR populations, the significantly smaller amount, poorer competitive ability and lower long-term survival of R. esculenta compared to R. lessonae sperm will reduce the initial reproductive superiority of hybrids and contribute to the stabilisation of mixed water-frog populations. Differences in fertilisation ability are also likely to be relevant for the structure and dynamics of several other systems with encounters between eggs and sperm from different genotypes, ecotypes, ploidy levels and/or species.     

Fighting success and attractiveness as predictors of male mating success in the black field cricket, Teleogryllus commodus: the effectiveness of no-choice tests

Females are generally assumed to prefer larger, more dominant males. However, a growing number of studies that control for male-male competition have shown no correlation between dominance and attractiveness. Aggressive males can interfere with female mate preference either by physically coercing females into mating or by driving submissive males away and restricting mate choice. The most common method of assessing female mate choice is by using simultaneous two-choice tests. These control for male-male interactions, but usually interfere with physical and chemical cues involved in mate selection or alter male behaviour. They are therefore unsuitable for many study species, especially insects. Another method is the no-choice test that measures a females latency to mating when placed with a single male as an indication of male attractiveness. No-choice tests control for male-male aggression while allowing full contact between pairs (they allow actual mating to be directly observed rather than to occur based on a correlated behaviour). So far, however, no study has confirmed that males that entice females to mate sooner actually enjoy increased longer-term mating success. As such, the accuracy of no-choice tests as a method of examining mate choice remains untested. Here, we used no-choice tests on the black field cricket, Teleogryllus commodus, to show that (1) females did not prefer males that won fights (dominant males), and (2) latency to mating predicts actual mating success. We have clearly demonstrated the usefulness of no-choice tests and, considering the advantages of this method, they should be more often considered for a wider variety of taxa.Communicated by D. Gwynne     

Roles of interplant movement,acoustic communication,and phototaxis in mate-location behavior of the leafhopper Graminella nigrifrons

Summary Male Graminella nigrifrons leafhoppers (Cicadellidae: Homoptera) employ a call-fly strategy to find virgin females on oat host plants. Males observed in isolation during daylight hours exhibit a high rate of interplant movement, calling from the lower canopy on each plant visited. Virgin and mated females exhibit little interplant movement. They differ from one another in that virgin females perch on the upper half of plants, whereas mated females perch on the lower half of plants. The positioning of females in the plant canopy is influenced by light. Unlike mated females, virgin females respond to male calls by emitting their own acoustic signals. When virgin females are present on plants visited by males, interplant movement of males ceases, and a localized-upward search of the female bearing plant ensues. Male search is influenced by light. Regardless of whether virgin females were confined to the upper or lower portion of plants, direction of male search was towards a light source used to illuminate above or below the plant canopy. These findings suggest that interplant movement by males and sedentary behavior by females prior to mate recognition and their use of acoustic and phototactic sensory modalities after mate recognition represent previously unrecognized adaptations to problems associated with the use of vibrational signals on plants. Offprint requests to: R.E. Hunt at the current address     

Temporal variation and distribution of trace metals in freshwater and fish from Calabar River, S.E. Nigeria

An investigation on the abundance and distribution of trace metals (Fe, Cu, Zn, Mn, Cr, Cd and Pb) in water, and nine species of fish samples from Calabar river was carried out in 1992. The concentrations of iron (6000–7240gl^–1), zinc (4910–7230gl^–1), and cadmium (3–7gl^–1) showed moderate pollution while those of copper (420–630gl^–1), manganese (23–48gl^–1), chromium (<10–20gl^–1) and lead (<1–10gl^–1) in water were well below WHO permissible levels. Significant seasonal changes (0.001p0.25) were obtained for iron, copper, zinc, manganese and cadmium in water. Furthermore, iron, zinc and cadmium showed statistically significant spatial changes (0.005p0.10). Of the nine fish species studied, no statistically significant relationship between body weight and the concentrations of the metals was observed. The concentrations of the metals per mean total body weight apparently decreases in the order Fe>Zn>Cu>Mn>Pb>Cd=Cr and were within the limits that were safe for consumption.     

Survival of Nucella lapillus in a tributyltin-polluted area in west Brittany: a further example of a male genital defect (Dumpton syndrome) favouring survival

A male genital defect was reported in a Nucella lapillus (L.) population at Dumpton Gap (England). This defect was termed Dumpton syndrome (DS) and appears to be a genetic feature. Its main characteristic is the absence of penis (aphally) in males and in females. In 1992, such a phenomenon was discovered in populations in the vicinity of Brest (Brittany, France). DS-affected females exhibit fewer tributyltin-induced imposex characteristics than expected in normal individuals. The percentage of female sterilization is thus lower, favouring population survival. In consequence, the DS is considered to be a pollution-resistance feature. Comparison with the Dumpton population revealed similarities and differences in the DS characteristics. It is thus hypothesized that the DS observed at the two locations is due to two different biological mechanisms. Indeed, aphallic males with a split prostate were observed but no underdevelopment of their vas deferens and testis was noted in the present study. In this gonochoristic gastropod species, the most DS-affected males in Brest possess an ovotestis and it is thus hypothesized that feminity is remnant in N. lapillus. Incidence of abnormality is ten times higher in females than in males. This suggested that a sex-difference operates in the Brest phenomenon.     

Parasite distribution and virulence: implications for parasite-mediated sexual selection

The Hamilton-Zuk hypothesis states that females choosing males with more developed secondary sexual traits, i.e. brighter males, achieve greater fitness if variability in brightness reflects heritable variation in resistance to parasites. However, several factors will affect the likelihood that parasites play a role in sexual selection in given species. Here, using simple models, we show that because of parasite aggregation on a few hosts, only few breeding males would suffer from reductions in brightness due to parasites. Only in cases where parasites are abundant and show low levels of aggregation among their hosts would there be sufficient variability in brightness among breeding males for female choice of bright, resistant males to evolve. In addition, sufficient parasite-induced variability in brightness among breeding males will only occur in host-parasite systems where pathology is linearly related to the number of parasites per host. The presence of males that are uninfected and bright but genetically susceptible to parasites will also influence the fitness advantages obtained by females choosing bright males. If genetic immunity against parasites is rare in the host population, females can probably only benefit from choosing bright males if parasites are common and little aggregated among males. These results greatly limit the generality of the Hamilton-Zuk hypothesis, and suggest that only a small fraction of host-parasite associations could promote the evolution of host mate choice for resistance based on brightness. Correspondence to: R. Poulin     

Photosynthetic performance of Laminaria solidungula measured in situ in the Alaskan High Arctic

Photosynthetic performance in the kelp Laminaria solidungula J. Agardh was examined from photosynthesis irradiance (P-I) parameters calculated from in situ ¹⁴C uptake experiments, using whole plants in the Stefansson Sound Boulder Patch, Alaskan Beaufort Sea, in August 1986. Rates of carbon fixation were determined from meristematic, basal blade, and second blade tissue in young and adult sporophytes. Differences in saturating irradiance (I _k, measured as photosynthetically active radiation, PAR), photosynthetic capacity (P _max), and relative quantum efficiency () were observed both between young and adult plants and between different tissue types. I _k was lowest in meristematic tissue (20 to 30 E m^–2 s^–1) for both young and adult plants, but consistently 8 to 10 E m^–2 s^–1 higher in young plants compared to adults in all three tissues. Average I _k for non-meristematic tissue in adult plants was 38 E m^–2 s^–1. Under saturating irradiances, young and adult plants exhibited similar rates of carbon fixation on an area basis, but under light limitation, fixation rates were highest in adult plants for all tissues. P _max was generally highest in the basal blade and lowest in meristematic tissue. Photosynthetic efficiency () ranged between 0.016 and 0.027 mol C cm^–2 h^–1/E m^–2 s^–1, and was highest in meristematic tissue. The relatively lower I _k and higher exhibited by L. solidungula in comparison to other kelp species are distinct adaptations to the near absence of light during the eight-month ice-covered period and in summer when water turbidity is high. Continuous measurement of in situ quantum irradiance made in summer showed that maximum PAR can be less than 12 E m^–2 s^–1 for several days when high wind velocities increase water turbulence and decrease water transparency.The Univeristy of Texas Marine Science Institute Contribution No. 695     

Lead and cadmium in urban allotment and garden soils and vegetables in the United Kingdom

In order to assess the intake of lead and cadmium by consumers of home grown vegetables in urban areas, replicated experimental plots of uniform size, comprising summer and winter crops, were established in 94 gardens and allotments in nine towns and cities in England.The geometric mean lead and cadmium concentrations for the soils (n = 94) were 217 g g^–1 (ranging from 27 to 1,676 g g^–1) and 0.53 g g^–1 (<0.2–5.9 g g^–1), respectively. Compared with agricultural soils, the garden and allotment soils contained elevated levels of lead but not cadmium.Lead concentrations in the vegetables ranged from <0.25 g g^–1 to 16.7 g g^–1 dry weight and cadmium concentrations ranged from <0.025 g g^–1 to 10.4 g g^–1 dry weight. Lead concentrations were higher than reported background levels, although <1% exceeded the statutory limit for saleable food in the UK (1 g g^–1 fresh weight). Cadmium concentrations were generally similar to background levels.     

Mating system of the commensal marine isopod Jaera hopeana (Crustacea)

The commensal marine isopod Jaera hopeana Costa, 1853 was reared in the laboratory through 30 generations in the absence of its host, Sphaeroma serratum, to study its development and reproductive behaviour. It was found that adult males take virtually any opportunity to associate with a young conspecific in manca-I stage (first free-living stage) and carry it around in a characteristic position. This male-manca(I) amplexus ends during the manca's first postmarsupial molt (manca-I to manca-II) at an average age of about 9 d after hatching from the marsupium. Entry into amplexus does not alter a manca's molt timing, the duration of the amplexus thus depending exclusively on the manca's age when it joined by a male. Given a choice, males do not prefer manca-I close to molting over those just hatched. With the first postmarsupial molt, long before sexual maturity, female J. hopeana become receptive to mating. Before a female manca is released from amplexus, it is inseminated by its male partner. Sperm transferred to young immature females are stored within special sperm stores until they are needed for fertilization. The male-manca(I) amplexus seems to be a precopula, wherein males wait for their partners to molt and become sexually receptive. Nevertheless, it is inevitable that every second amplexus on average ends unsuccessfully: although manca-I have already been determined irreversibly (and probably genetically) as males and females in equal numbers, adult males are unable to predict which manca-I are potential mates and which are not. Males accept any manca-I as a precopula partner, and even when given a choice they do not prefer female over male manca-I. From the manca-II stage onward, females are continuously receptive to mating for the rest of their life, and mating can occur immediately upon contact of the partners without any significant investment in energy and time (en passant copulation).     

Bat serenades—complex courtship songs of the sac-winged bat (<Emphasis Type="Italic">Saccopteryx bilineata</Emphasis>)

Vocalisations of many songbirds, anurans, and insects are shaped by sexual selection. Males acoustically compete for territories, and females choose their mates by means of male courtship songs. In courtship, richness and complexity of elements are often favoured characters. Only a few examples of complex songs are known in mammals. Males of the harem-polygynous sac-winged bat (Saccopteryx bilineata, Emballonuridae) have an uncommonly complex vocal repertoire, and different song types of males are used in the context of territorial defence and in courting females. We classified the daytime vocalisations of 16 male S. bilineata from a colony in Costa Rica, both on the basis of their acoustical properties and the social context in which they occurred. Seven vocalisation types were differentiated: echolocation pulses, barks, chatter, whistles, screeches, territorial songs and courtship songs. Territorial songs were short, rather stereotyped and not obviously directed towards a certain conspecific. They appear to be of importance in male competition for harem territories, in which females roost during the day. Courtship songs were exclusively observed when males displayed towards a female; they were long and complex, and consisted of highly variable elements (calls). We classified the calls in courtship songs of six males into call types, based on acoustical properties, mainly spectral purity and duration. Four call types are described in detail: trills, noise-bursts, short tonal calls, and quasi constant frequency calls. Twelve parameter values were extracted from the most common call type, the trill. Discriminant function analysis of trills showed that different males had different repertoires. This could allow females to use trill parameters for recognition of individual males and thus for mate choice.Communicated by G. Wilkinson     

Optimal divorce and re-mating strategies for monogamous female birds: a simulation model

Although the extensive variation in divorce rates among monogamous bird species has stimulated several theoretical accounts, the mechanisms underlying divorce strategies remain poorly understood. Here, we use an individual-based simulation model to investigate the adaptiveness of mechanisms of mate choice in the context of remating. Our model compares the fitness of females that choose a mate during each breeding season using one of two different decision rules; best-of-n females sample n potential partners and then select the male with the highest quality, whereas better option females choose a mate whose quality is maximal among the non-mated individuals they sampled the season before. It is assumed in the model that best-of-n females have no a priori information about the quality of potential partners and systematically decide to divorce at the beginning of each breeding season before searching for a new mate. Conversely, better option females use the information they gained the season before, and may retain their previous partner if they have no opportunity to mate with an individual of better quality. Results from simulations indicate that the best-of-n decision rule should be favoured when there is a large variation in male quality and low costs of mate sampling. On the other hand, the probability that the better option rule may invade the population is predicted to increase with male survival rate. However, changes in male mortality had no marked influence on the expected proportion of divorcing pairs, contrary to previous theoretical expectations.Communicated by H. Kokko     

Environmental Accumulation of Airborne Fluorides in Romania

The nature and extent of pollution from an aluminium smelter and a fertiliser factory in Romania were studied. These are large industrial complexes, and both types of industry are known to release fluorides into the atmosphere. In grass samples collected from around the aluminium smelter, the maximum fluoride levels were found to be 4023mgkg^–1 and 162mgkg^–1 in unwashed and washed grass samples respectively, and 89mgkg^–1 in soils. For the fertiliser factory, the maximum levels in washed grasses were found to be 207mgkg^–1, and 11mgkg^–1 in the soils. In both locations, these maximum values were obtained in samples collected from within 200m of the factory limits, and compare with regional background levels of less than 10mgkg^–1 for grasses and 2mgkg^–1 for soils. The high fluoride levels of fluoride in the grasses are sufficient to give cause for concern for the effects that these could have on the local population and on grazing animals.     

Parental care of nestlings by male red-winged blackbirds

Summary Nestling feeding by males is less common among birds with polygynous mating systems than in monogamous species, because of the pronounced trade-off between parental behavior and the attraction of additional mates. In this study, however, we found that male red-winged blackbirds (Agelaius phoeniceus) commonly assisted females in feeding nestlings in several Ontario marshes. Male parental care was additional to that provided by females, and it significantly enhanced the fledging success of nests (Table 2). Male redwings did not help to feed all nests on their territories: primary and secondary nests were much more likely to receive male parental care than tertiary and later nests. Contrary to expectation, male parental care was not restricted to the nests of primary females: a greater proportion of secondary than primary nests were assisted (Tables 4a and b). The presence of new females settling on the territory at the same time that a resident female had nestlings, resulted in males deferring parental care until a later brood. This suggests that males trade off the recruitment of females against parental care to an existing brood. Although the number of nestlings fledging from a male's territory was strongly influenced by the number of females in the harem, males could additionally increase their reproductive success by feeding nestlings in one or more nests on their territories (Fig. 2). The reproductive success of females was significantly enhanced by male assistance in feeding nestlings (Table 3). However, those females not receiving male assistance on territories of feeding males did not suffer a significantly reduced reproductive success in comparison to females on territories of non-feeding males (Table 2). Males varied considerably in the quality of brood care given. We therefore suggest that the quality of male parental care may be a factor considered by females in choosing a breeding situation.     

Male characteristics,parental quality and the study of mate choice in the red-winged blackbird (Agelaius phoeniceus)

Summary A demonstration of adaptive mate choice by females in resource-defence mating systems requires clear predictions as to how females should rank breeding situations (defined by the quality of both the resident male and the territory he defends) so as to maximize their fitness. Since male quality is only weakly correlated with territory quality in red-winged blackbirds (Agelaius phoeniceus), ranking breeding situations in this species will require a consideration of both those parameters independently of each other as long as both vary among males, are predictable before mating, and affect female fitness. Data from a two year study of an eastern Ontario population of this species suggested that two components of male parental quality, nest defence effort and provisioning of nestlings with food, both varied among males and were somewhat predictable. Two measures of nest defence effort were correlated with an index of epaulet size (a reliable predictor of captive dominance rank in this species) (Table 5), and provisioning appeared to be predictable on the basis of both courtship behavior and breeding experience. Our data also suggest that these two components of male parental quality do not covary. Since male provisioning tends to be restricted to the nestlings of the primary and secondary mates in this species, breeding situations must be ranked not only with respect to their manifold quality but also with respect to the mating status of individual females.     

“Alternative” mating behavior in the swordtails Xiphophorus nigrensis and Xiphophorus pygmaeus (Pisces: Poeciliidae)

Summary The swordtail Xiphophorus nigrensis exhibits three relatively discrete male body-size classes that derive from allelic variation at the Y-linked pituitary (P) locus. Previous studies have shown that larger males have greater relative reproductive success, and that females prefer large males. We describe the mating behavior utilized by males of each size class during individual encounters with females. Small males rely on chase behavior, similar to the alternative mating behavior classified as sneaker in small males of other species. Large males court and intermediate-sized males court or chase, depending on their body size. There is a strong correlation between P alleles for small size (s) and large size (L) with chasing and courting, respectively. The relationship between mating behavior of males of the genotype I is ambiguous. In the closely related species X. pygmaeus, males are of size similar to smaller X. nigrensis males. Paradoxically, these males do not show the courtship display that typifies larger male X. nigrensis and many other species of swordtails, but instead often employ chase behavior identical to the alternative mating behavior in small male X. nigrensis. We suggest that historical and genetic constraints, in addition to current selection forces, might be important factors in explaining the existence of alternative mating behavior in X. pygmaeus.