Gil G. Rosenthal

A private ultraviolet channel in visual communication.

Although private communication is considered an important diversifying force in evolution, there is little direct behavioural evidence to support this notion. Here, we show that ultraviolet (UV) signalling in northern swordtails (Xiphophorus) affords a channel for communication that is not accessible to their major predator, Astyanax mexicanus, the Mexican tetra. Laboratory and field behavioural experiments with swordtails (X. nigrensis) and predators (A. mexicanus) demonstrate that male UV ornamentation significantly increases their attractiveness to females but not to this predator, which is less sensitive to UV. UV reflectance among swordtail species correlates positively with tetra densities across habitats, and visual contrast estimates suggest that UV signals are highly conspicuous to swordtails in their natural environment. Cross–species comparisons also support the hypothesis that natural selection drives the use of UV communication. We compared two species, one with high (X. nigrensis) and one with low (X. malinche) Mexican tetra densities. Xiphophorus nigrensis males reflect significantly more UV than X. malinche, exhibit significant UV sexual dimorphism, and UV is a salient component of the sexual communication system. In X. malinche, however, males reflect minimally in the UV, there is no UV sexual dimorphism, and UV does not play a part in its communication system.

Alteration of the chemical environment disrupts communication in a freshwater fish.

It is well established that changes to the chemical environment can impair development, physiology and reproductive biology; by contrast, impacts on communication have not been widely reported. This is surprising given that chemical communication is the most widely used sensory modality in nature, and that variation in the chemical composition of the environment is the rule, not the exception. Here, we show that chemically mediated species recognition in a swordtail fish, Xiphophorus birchmanni, can be hindered by anthropogenic disturbance to the signalling environment. Females have a strong preference for conspecific male chemical cues, yet they hybridize in nature with the congener X. malinche. Wild-caught females showed a strong preference for conspecifics when tested in clean water, but failed to show a preference when tested in stream water subject to sewage effluent and agricultural runoff. We hypothesized that this was due to the interaction between chemical communication systems and humic acid (HA), a ubiquitous, natural product elevated to high levels by anthropogenic processes. When exposed to elevated concentrations of HA, female X. birchmanni again lost their preference for conspecific male chemical cues, while visual mating preferences and motivation to mate were retained. Sub-lethal concentrations of seemingly benign substances can thus have a drastic effect on natural populations through their specific impact on communication systems.

Shared preferences by predators and females for male ornaments in swordtails.

Sexually dimorphic traits in many mate recognition systems have evolved in response to preexisting female biases. These biases are often quite general in form and are likely to be shared by predators, thereby imposing a cost on male trait expression. The Mexican tetra Astyanax mexicanus (Pisces: Characidae), a visual predator of swordtail fishes, exhibits the same visual preferences for male body size morphs as do females. Furthermore, tetras in populations where swordtails are absent prefer males with sword ornaments over males with swords removed. The predator preference is thus likely to have arisen prior to contact with fishes bearing the ornament, as has also been suggested for mating preferences for swords.

The vocal sac as a visual cue in anuran communication: an experimental analysis using video playback

Acoustic signals produced by most anurans are accompanied by inflation of a conspicuous vocal sac. We presented female tungara frogs, Physalaemus pustulosus, with acoustic playback of the male advertisement call, synchronized with a video playback of vocal sac inflation. Females significantly preferred a stimulus combination including vocal sac inflation over an identical set of stimuli with the vocal sac inflation removed. Neither a moving rectangle bearing the gross contrast and spatiotemporal properties of the vocal sac inflation sequence, nor the image of a noncalling male significantly enhanced the attractiveness of the acoustic stimulus. Both the form and spatiotemporal properties of the vocal sac thus appear to be salient to females. The results indicate that the vocal sac can serve as a visual cue, which may account for the conspicuous pigmentation found on the vocal sacs of males in many species. Gular inflation in synchrony with a call may function to facilitate female localization of individual males in an aggregation.

PHYLOGENOMICS REVEALS EXTENSIVE RETICULATE EVOLUTION IN XIPHOPHORUS FISHES

Hybridization is increasingly being recognized as a widespread process, even between ecologically and behaviorally divergent animal species. Determining phylogenetic relationships in the presence of hybridization remains a major challenge for evolutionary biologists, but advances in sequencing technology and phylogenetic techniques are beginning to address these challenges. Here we reconstruct evolutionary relationships among swordtails and platyfishes (Xiphophorus: Poeciliidae), a group of species characterized by remarkable morphological diversity and behavioral barriers to interspecific mating. Past attempts to reconstruct phylogenetic relationships within Xiphophorus have produced conflicting results. Because many of the 26 species in the genus are interfertile, these conflicts are likely due to hybridization. Using genomic data, we resolve a high‐confidence species tree of Xiphophorus that accounts for both incomplete lineage sorting and hybridization. Our results allow us to reexamine a long‐standing controversy about the evolution of the sexually selected sword in Xiphophorus, and demonstrate that hybridization has been strikingly widespread in the evolutionary history of this genus.

Female preference for dynamic traits in the green swordtail,Xiphophorus helleri

Analyses of the relationship between female preference and male behaviour have been complicated by correlated variation in factors such as male size and appearance. This study examined the effects of systematically manipulating male behaviour, while holding male morphology constant. Female green swordtails were shown video-recorded sequences of the same male engaging in an active courtship display, performing similar levels of feeding activity, and remaining inactive. Control sequences of moving food particles and of an empty aquarium were also presented. Female responsiveness was significantly different across time intervals (before, during and after the stimulus) for the three stimuli showing a male, but not for the two controls. Analyses of female behaviour patterns during the stimulus presentations revealed that they preferred sequences of courting males to all other stimuli. Females did not respond differently to the feeding and inactive sequences. These results indicate that female interest depends upon a specific set of motor patterns, and suggest that male behaviour and morphology may act synergistically to determine female preference. Video stimuli should prove useful in future experiments seeking to identify the role of specific courtship motor patterns in mate choice.

HOW COMMON IS HOMOPLOID HYBRID SPECIATION

Hybridization has long been considered a process that prevents divergence between species. In contrast to this historical view, an increasing number of empirical studies claim to show evidence for hybrid speciation without a ploidy change. However, the importance of hybridization as a route to speciation is poorly understood, and many claims have been made with insufficient evidence that hybridization played a role in the speciation process. We propose criteria to determine the strength of evidence for homoploid hybrid speciation. Based on an evaluation of the literature using this framework, we conclude that although hybridization appears to be common, evidence for an important role of hybridization in homoploid speciation is more circumscribed.

Assortative preferences for stripes in danios

Decisions about joining social groups are often contingent on one’s own phenotypic characteristics and those of existing group members. Zebrafish and their allies (genus Danio) vary substantially in the expression of conspicuous lateral stripes. We presented wild-type zebrafish (D. rerio), two artificially selected D. rerio strains with reduced stripe expression, as well as the stripeless D. albolineatus and the striped D. nigrofasciatus, with simultaneous choices between synthetic, computer-animated shoals of stripeless and striped Danio. Each species and strain of Danio preferred the phenotype that most resembled its own, with a strong preference for striped stimuli by fish with dark-stripe phenotypes, and a strong preference for no stripes by fish lacking stripes. Stripes are therefore a key shoaling cue in Danio. Individuals appear to acquire a general preference for stripes, or no stripes, as a consequence of experience with a specific stripe phenotype.

Female Disdain for Swords in a Swordtail Fish

Studies of mate choice evolution tend to focus on how female mating preferences are acquired and how they select for greater elaboration of male traits. By contrast, far less is known about how female preferences might be lost or reversed. In swordtail fish Xiphophorus, female preference for the sword ornament is an ancestral trait. Xiphophorus birchmanni, however, is one species that has secondarily lost the sword. Using synthetic animation playback of “virtual” males, we found that female X. birchmanni preferred a swordless conspecific over a sworded heterospecific. Moreover, when offered the choice between a conspecific without a sword and one with a digitally attached sword, females preferred the former. These results suggest female preferences need not always select for elaboration of male traits, and they provide a plausible explanation for the lack of introgression of a sexual trait in a naturally occurring hybrid zone.

The role of sexual selection in maintaining a colour polymorphism in the pygmy swordtail, Xiphophorus pygmaeus

The forces responsible for maintaining genetic polymorphisms within populations are often unclear. Male pygmy swordtails are polymorphic at a single Y-linked locus that regulates body colour. The visually conspicuous, sex-limited polymorphism suggests that sexual selection is important for maintaining genetic variation for these conspicuous phenotypes. We performed a series of behavioural experiments that decoupled body colour from possible correlates, to detect female mating preferences for each morph and differences in male reproductive behaviour. Male body colour, male mating behaviour and male agonistic behaviour all appeared to influence male reproductive success. Females in two of three populations showed mating preferences for blue males, but gold males dominated in agonistic interactions and were more aggressive in pursuing females. Females in those populations preferred blue to gold males under optical filters that masked blue colour, but not under filters that masked gold colour, suggesting that female behaviour reflects an aversion to gold males. Females from one population