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The intensity of mating competition and the potential benefits for female of mating with certain males can be influenced by several extrinsic factors, such that behavioral decisions can be highly context-dependent. Short-lived species with a single reproductive season are a unique model to study context-sensitive mating decisions. Through exhaustive sampling in the field and simultaneous choice tests in the laboratory, we evaluated operational sex ratio (OSR) and female mate choice at the beginning and end of the reproductive season in the annual killifish Austrolebias reicherti. We found seasonal change in both OSR and female mate choice. At the start of the reproductive season the OSR did not deviate from parity, and females preferred larger males. Later in the reproductive season, while the proportion of males in the ponds decreased, females became unselective with respect to male size. The particular biological cycle of annual killifish, where both life expectancy and mating opportunities decline sharply over a short timescale, could account for the seasonal change in female choice. Reduction in choosiness could arise from diminished reproductive prospects due to a decline in male availability. Moreover, as the end of the season approaches, any benefits of choosiness are presumably reduced: a female's fitness will be higher if she mates with any male than if she forgoes reproduction and dies. Future work will disentangle the mechanisms underlying seasonal changes in mating preferences, notably direct responses to demographic factors, environmental cues, or intrinsic changes during development.

Mate choice is context dependent, but the importance of current context to interspecific mating and hybridization is largely unexplored. An important influence on mate choice is predation risk. We investigated how variation in an indirect cue of predation risk, distance to shelter, influences mate choice in the swordtail Xiphophorus birchmanni, a species which sometimes hybridizes with X. malinche in the wild. We conducted mate choice experiments to determine whether females attend to the distance to shelter and whether this cue of predation risk can counteract female preference for conspecifics. Females were sensitive to shelter distance independent of male presence. When conspecific and heterospecific X. malinche males were in equally risky habitats (i.e., equally distant from shelter), females associated primarily with conspecifics, suggesting an innate preference for conspecifics. However, when heterospecific males were in less risky habitat (i.e., closer to shelter) than conspecific males, females no longer exhibited a preference, suggesting that females calibrate their mate choices in response to predation risk. Our findings illustrate the potential for hybridization to arise, not necessarily through reproductive \"mistakes\", but as one of many potential outcomes of a context-dependent mate choice strategy.

The differential allocation hypothesis predicts that reproductive investment will be influenced by mate attractiveness, given a cost to reproduction and a tradeoff between current and future reproduction. We tested the differential allocation hypothesis in the swordtail fish Xiphophorus multilineatus, where males have genetically influenced (patroclinous inheritance) alternative mating tactics (ARTs) maintained by a tradeoff between being more attractive to females (mature later as larger courting males) and a higher probability of reaching sexual maturity (mature earlier as smaller sneaker males). Males in X. multilineatus do not provide parental care or other resources to the offspring. Allelic variation and copy number of the Mc4R gene on the Y-chromosome influences the size differences between males, however there is no variation in this gene on the X-chromosome. Therefore, to determine if mothers invested more in offspring of the larger courter males, we examined age to sexual maturity for daughters. We confirmed a tradeoff between number of offspring and female offspring's age to sexual maturity, corroborating that there is a cost to reproduction. In addition, the ART of their fathers significantly influenced the age at which daughters reached sexual maturity, suggesting increased maternal investment to daughters of courter males. The differential allocation we detected was influenced by how long the wild-caught mother had been in the laboratory, as there was a brood order by father genotype (ART) interaction. These results suggest that females can adjust their reproductive investment strategy, and that differential allocation is context specific. We hypothesize that one of two aspects of laboratory conditions produced this shift: increased female condition due to higher quality diet, and/or assessment of future mating opportunities due to isolation from males.

Mimicry can occur in several contexts, including sexual interactions. In some cases, males mimic females to gain access to potential mates. In contrast, there are relatively few examples of species where females mimic males, and we know very little about what drives these patterns. Two hypotheses have been advanced to explain female mimicry of males. The first is that mimicry is used to reduce harassment of females by males. The second is that mimicry is used to display dominance over other females. In this study, we tested these hypotheses in Brachyrhaphis fishes, wherein females of several species have pigmentation on their anal fin of the same coloration and shape, and in the same location, as the genitalia of males. To test if female mimicry of males reduces male harassment, we experimentally manipulated female pigmentation and observed male preference for females with and without male-like pigmentation. To test the effect that female mimicry of males has on female dominance, we observed how females respond to anal fin pigmentation patterns of companion females. We found that neither of these hypotheses was supported by our data. We conclude that similarities in anal fin pigmentation between male and female Brachyrhaphis fishes is not an adaptation to reduce male harassment or to signal dominance between females. Alternative explanations must exist, including the possibility that these similarities are simply non-adaptive.

Chemical communication plays a critical role in sexual selection and speciation in fishes; however, it is generally assumed that most fish pheromones are passively released since most fishes lack specialized scent glands or scent-marking behavior. Swordtails (genus Xiphophorus) are widely used in studies of female mate choice, and female response to male chemical cues is important to sexual selection, reproductive isolation, and hybridization. However, it is unclear whether females are attending to passively produced cues, or to pheromones produced in the context of communication. We used fluorescein dye injections to visualize pulsed urine release in male sheepshead swordtails, Xiphophorus birchmanni. Simultaneous-choice assays of mating preference showed that females attend to species- and sex-specific chemical cues emitted in male urine. Males urinated more frequently in the presence and proximity of an audience (conspecific females). In the wild, males preferentially courted upstream of females, facilitating transmission of pheromone cues. Males in a teleost fish have evolved sophisticated temporal and spatial control of pheromone release, comparable to that found in terrestrial animals. Pheromones are released specifically in a communicative context, and the timing and positioning of release favors efficient signal transmission.

Sexual selection and species recognition play important roles in mate choice; however, sexual selection preferences may overlap with traits found in heterospecifics, producing a conflict between sexual selection and species recognition. We examined female preferences in Xiphophorus pygmaeus for male traits that could provide both types of information to determine how females use multiple cues when preferences for these cues would conflict. We also examined X. pygmaeus behavior in the field to determine if females have the opportunity to choose mates. As no male-male competition was observed in the field, and females occasionally chased males from feeding areas, females apparently have the opportunity to exercise mate choice in their natural habitat. In the laboratory, female X. pygmaeus used body size as a sexual selection cue, preferring large heterospecifics (X. cortezi) to small conspecifics. Females also preferred barless X. cortezi over barred X. cortezi when males were size matched. Because X. pygmaeus males do not have bars, this preference suggests that X. pygmaeus females use vertical bars in species recognition, and that large body size and vertical bars are conflicting cues. However, X. pygmaeus females did not have a preference for males of either species when sexual selection and species recognition cues were presented concurrently. This result was surprising, because preferences for species recognition cues are often assumed to be stronger than sexual selection cues. We suggest that females may be using additional species-specific cues in mate choice to prevent hybridization.

In male swordtails (Xiphophorus nigrensis) there are three size classes that derive from allelic variation at the pituitary locus on the Y chromosome. Progeny analysis and preference tests suggest that females prefer to mate with larger males. In the closely related X. pygmaeus, there is no allelic variation at this locus; this species consists of males similar in size only to smaller X. nigrensis males. In addition to being smaller than most X. nigrensis males, these X. pygmaeus males also lack both the swordtail and a major component of the courtship display common in most X. nigrensis males. Usually, female X. pygmaeus prefer to mate with heterospecific males rather than conspecifics, regardless of body size and the presence of a swordtail. However, the smallest X. nigrensis males lack the same courtship component as do the X. pygmaeus males, and in this comparison female X. pygmaeus show no preference. Although sexual selection, through its action on divergence of courtship displays, has been implicated as a factor leading to speciation, in this case sexual selection could lead to the congealing of gene pools between heterospecifics.

Twenty-nine populations of Xiphophorus fishes representing nine species of northern swordtails, one southern swordtail and a platyfish were assayed electrophoretically for allozyme variation. Phylogenetic relationships were inferred using parsimony and likelihood analysis of gene frequency characters, as well as Fitch-Margoliash, minimum evolution and neighbor-joining analyses of genetic distances. The phylogenetic relationships among species that were well supported in all analyses included (1) monophyly of the northern swordtails, (2) the pygmaeus clade of Xiphophorus nigrensis, X. multilineatus, and X. pygmaeus, and (3) the clade of X. nigrensis and X. multilineatus. Of those species represented by more than one population, all analyses supported monophyly of X. montezumae and weakly supported monophyly of X. nezahualcoyotl and X. birchmanni. Only the distance analyses supported monophyly of X. cortezi, and the support was weak. Finally, all analyses supported a clade including X. nezahualcoyotl from the Río Tamesí drainage and some populations from the Río Pánuco drainage, that is, nonmonophyly of the set of populations from the Río Pánuco drainage. Previously published trees based on morphology, behavior and randomly amplified DNAs were generally congruent with the optimal trees for the allozyme data and were not rejected by those data; in contrast, trees based on DNA sequences were more incongruent with the optimal trees for the allozyme data and were rejected by those data.

Swordtails in the genus Xiphophorus exhibit substantial variation in male body size influenced by genetic variation at the pituitary (P) locus on the Y-chromosome. However, males of Xiphophorus pygmaeus have historically been classified as uniformly small; they were thought to possess one of two P-alleles, both for small size. In 1988, large male X. pygmaeus were discovered at two sites in the Río Huichihuayán, Mexico. Surprisingly, females from these sites have lost the ancestral preference for large males although females from adjacent sites have not. This study investigates the heritability of large body size in male X. pygmaeus to understand its role in the evolution of male mating strategies and female preference in this species. We conducted breeding experiments with X. pygmaeus from two populations to determine whether large size is influenced by the same P-allele system found in other swordtails. Small fathers had a significant effect on the size at maturity of their sons but large fathers did not. All sons were small in size regardless of paternal size. Large fathers were no more successful than small fathers in producing offspring, and we found no significant relationship between age at maturity and size at maturity of the sons. In some cases, there was a significant difference in prematuration growth rate of sons between populations and in postmaturation growth rates within and between populations, yet all sons remained small in size. These results suggest large size is not paternally heritable through a P-allele in this species but could be a result of changes in pre- or postmaturation growth rates. The evolutionary persistence and spread of large male size may not follow current models of sexual selection on P-alleles and the loss of female preference in one population may be explained by the apparent lack of heritability of this male trait.

In the present study, we determine female breeding cycles in three species - Xiphophorus nigrensis, X. multilineatus , and X. pygmaeus - and discuss the relationships between brood size, female size, and time of year. X. nigrensis females were collected from the headwaters of the Rio Choy and X. multilineatus (formerly considered the same species, see Rauchenberger et al., 1990) from the Rio Coy, both in San Luis Potosi, Mexico. X. pygmaeus females were collected from two sites on the Rio Huichihuayan, Mexico, the headwaters near Nacimiento (only sampled on Oct. and Jan.), and at Chimalaco.