Molly R. Morris

A Genetic Polymorphism in the Swordtail Xiphophorus nigrensis: Testing the Prediction of Equal Fitnesses

When a genetically determined polymorphism is selectively maintained in a population, the different morphs should have equal fitnesses at equilibrium. We empirically examined this prediction for the size polymorphism of the swordtail Xiphophorus nigrensis, in which a single locus on the Y chromosome controls male size. Small males mature earlier and chase females, whereas large males mature later and court females. We analyze our data with a model that uses the differential mating success and the ages at sexual maturity of the two morphs to calculate the per capita death rate necessary for them to have equal fitness. We demonstrate how female fecundity data can be used to determine whether the estimated death rate is biologically realistic. Our data support the hypothesis that morph fitnesses are equal, and the model is fairly robust to changes in population growth rate and differential death rates of morphs. However, the confidence intervals for our estimates are large, which suggests that the null hypothesis only be accepted with caution. We show that in many circumstances very large sample sizes will be needed to distinguish between alternative hypotheses concerning the relative fitnesses of the two morphs. We emphasize that despite the popularity of alternative mating behaviors, specifically, and mixed evolutionarily stable strategies, in general, there is almost no empirical evidence that alternative behavioral morphs have equal fitnesses. Also, the conclusion that morph fitnesses are equal does not address the hypothesis that frequency-dependent mating success is the mechanism maintaining the equilibrium of fitnesses. This requires additional evidence directly demonstrating the fitness effect of changes in morph frequency.

Male-Male Competition and Access to Females in the Swordtail Xiphophorus nigrensis

1982 Ed. Statistical Analysis Systems Institute Inc., Cary, North Carolina. SCHOENER, T. W., AND A. SCHOENER. 1978. Estimating and interpreting body-size growth in some Anolis lizards. Copeia 1978:390-405. SEXTON, O. J. 1984. Life history notes on terrestrial vertebrates inhabiting some glades of eastern Missouri with emphasis on amphibians and reptiles, p. 21-65. In: Proceedings of Cedar Glade Symposium. W. Davis (ed.). School of the Ozarks, Point Lookout, Missouri, 23-24 April 1982. Missouri Acad. Sci. Occ. Pap. No. 7. SOKAL, R. R., AND F. J. ROHLF. 1981. Biometry. 2d ed. W. H. Freeman and Co., San Francisco, California.

A polymorphism in female preference for a polymorphic male trait in the swordtail fish Xiphophorus cortezi

Xiphophorus cortezi males are polymorphic for the pigment pattern vertical bars. In this study, we determined whether X. cortezi females are polymorphic in their preference for this trait by examining both within- and between-individual variation in female preference. There was significantly more variation in female preference within than between individuals using both video animations and live males as stimuli; repeatability measures were 0.86 and 0.5, respectively. Some females had a strong preference for males with bars, some for males without bars, and some had either a weak preference or no preference at all. We also found a significant difference in the strength of preference for bars between females with and without bars, suggesting the potential for a genetic correlation between preference and trait. Finally, we examined female preference for bar symmetry in the same females tested for preference for bars. We detected a significant preference for bar symmetry in the population of females as a whole, and a positive relationship between the strength of this preference and a preference for the presence/absence of bars among the subset of females that preferred ‘no bars’. We discuss these results in light the possibility that these two preferences are interrelated. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Assessment and individual recognition of opponents in the pygmy swordtails Xiphophorus nigrensis and X. multilineatus

We examined factors that determine the outcome of agonistic encounters between male pygmy swordtail fish. Xiphophorus nigrensis and X. multilineatus males formed dominance relationships based on body size in staged laboratory encounters. There was a significant negative correlation between size asymmetry and fight intensity, suggesting that males assessed size in the encounters. However, a significant proportion of the variation in fight intensity in contests that escalated to bites could not be explained by size asymmetry. Aggressive motivation may also influence the outcome of contests and could be assessed in agonistic encounters. Theory suggests that signals of aggressive intention will be evolutionarily stable if individuals can recognize opponents and encounter one another repeatedly. In addition, individual recognition is one way that dominance hierarchies can be maintained. Here we demonstrate that males from both species can recognize individuals. In addition, at least some X. nigrensis males were site-faithful in the field, suggesting males encounter the same opponents repeatedly.

Sexual selection and species recognition in the pygmy swordtail, Xiphophorus pygmaeus: Conflicting preferences

Abstract. 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.

Female choice of large males in the treefrog Hyla chrysoscelis: the importance of identifying the scale of choice

SummaryIn this study, I detected the presence of female choice for larger males in the treefrog Hyla chrysoscelis. Mated males were larger than unmated males at the scales females could have assessed males: local groups (males found within 2 meters of a mated pair) and the nightly chorus. In addition, females were observed to initiate amplexus with the larger males in local groups. Mated males were larger than unmated males for two of the four seasons studied. Seasonal mating success was also analyzed with a multiple regression model that included size and chorus attendance (nights spent at the breeding site). The partial selection coefficients, which represent the relative magnitude of directional selection, were significant for size in only one season and for chorus attendance in all four seasons. Therefore, there are two possible advantages to being a large male: increased likelihood of obtaining a mate due to female preference, and increased likelihood of mating even if no female preference because of a greater number of opportunities to obtain a mating if matings occurred at random.

Mate choice for more melanin as a mechanism to maintain a functional oncogene

The mechanisms by which cancer evolves and persists in natural systems have been difficult to ascertain. In the Xiphophorus melanoma model, a functional oncogene (Xiphophorus melanoma receptor kinase Xmrk) has been maintained for several million years despite being deleterious and in an extremely unstable genomic region. Melanomas in Xiphophorus spp. fishes (platyfishes and swordtails) have been investigated since the 1920s, and, yet, positive selection that could explain the maintenance of Xmrk has not been found. Here, we show that Xiphophorus cortezi females from two populations prefer males with the spotted caudal (Sc) melanin pattern, which is associated with the presence of the Xmrk oncogene and serves as the site of melanoma formation within this species. Moreover, X. cortezi females prefer males with an enhanced Sc to males with a reduced Sc pattern. RT-PCR analysis confirms tissue-specific Xmrk expression within the Sc pattern in X. cortezi. Because of the association of Xmrk with the Sc pigment pattern and the fact that melanoma formation augments this visual signal, sexual selection appears to be maintaining this oncogene because of a mating preference for Sc, as well as the exaggeration of this male trait. At the individual level, decreases in viability and fecundity because of Xmrk and subsequent melanoma formation may be mitigated via increases in mate acquisition. At the population level, maintenance of this oncogene appears to be under frequency dependent selection, as we detected female preference for males without Sc in a third population that had higher frequencies of Sc in females.

A negative correlation between trait and mate preference inXiphophorus pygmaeus

Variation in female mating preferences was compared with variation in male traits across populations of Xiphophorus pygmaeus. Females of this species prefer the large courting males of X. nigrensis to their own small, non-courting males. It was predicted that if large, courtingX. pygmaeus males were to occur, they would be preferred. The discovery, distribution and subsequent spread, of large X. pygmaeus males provided an unusual opportunity to analyse how sexual selection might act on a trait not currently fixed in a species. The present study showed variation between populations in female preference for large male body size that was negatively correlated with the presence of large males. This variation in preference persisted over a 5-year period. Female preference for larger body size and courtship are evolutionarily decoupled from the male traits of large size and courtship in X. pygmaeus. This pattern is not consistent with indirect selection models (e.g. runaway selection), which predict that female mating preferences and male traits evolve in concert due to a genetic correlation between preference and trait. ? 1996 The Association for the Study of Animal Behaviour

A mechanism for female choice of large males in the treefrog Hyla chrysoscelis

SummaryIn this study, we combine both field and laboratory experiments to address the effects of female preference for certain call characteristics on a large-male mating advantage in the treefrog H. chrysoscelis. In laboratory-choice experiments, females always chose the call with the lower fundamental frequency when call rate and call intensity were held constant and the difference in frequency between the two calls was 15%. The lower frequency call was preferred by 8 out of 12 females when the difference in fundamental frequency was 7.4%. These results are consistent with field comparisons of the size of unmated males calling within 2 m of a mated male: Male body size was negatively correlated with fundamental frequency and the greater the size difference, the more likely that the larger male mated. In field choice experiments, females preferred males with higher call rates. Since size differences between males used in this experiment averaged only 2.3 mm, we would not expect the fundamental frequency of a males call to be the best predictor of mating success. Laboratory results demonstrated that call rate could override female preference for the ‘low frequency’ call over the ‘high frequency’ call, while intensity could at least dilute this preference. However, individual males in the field varied both call rate and the call intensity as perceived by the female. We suggest that the interaction between call rate, male size and mating success should be studied further through the use of field-choice experiments.

Female swordtail fish prefer symmetrical sexual signal

It has been proposed that females use the symmetry of secondary sexual traits to differentiate between potential mates. The vertical bars on male swordtail fish function as a signal that attracts females and deters rival males in one swordtail species. In addition, male courtship behaviour of most Xiphophorus species incorporates serial lateral presentations, which provide females with a clear opportunity to assess males for bilateral symmetry. We tested the hypothesis that X. cortezi females prefer males with a symmetrical bar number by determining whether females switched their preference between two males when we switched which male had a symmetrical number of bars. The ability to manipulate the bars without influencing other male traits allowed us to control for male characters correlated with bar symmetry that females might prefer. The degree of asymmetry in bar number we used was within the degree of asymmetry found for this trait in nature. Females switched their preference between a pair of males when we switched which male was symmetrical for bar number. We discuss the possibility that females prefer a symmetrical bar number as well as an alternative hypothesis. Copyright 1998 The Association for the Study of Animal Behaviour.