Female bias for enlarged male body and dorsal fins in Xiphophorus variatus (2023)

Behavioural Processes

Volume 87, Issue 2,

June 2011

, Pages 197-202

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Female preference for male fin elaborations in Poeciliid fishes may be driven by a sensory bias for increased lateral projection area (LPA) that has existed since the lineages diverged from a common ancestor. Previous research supports this hypothesis demonstrating female Poecilia latipinna, Poecilia mexicana, and Poecilia reticulata prefer males of larger body and dorsal fin size, but exhibit no such preferences when controlling for total LPA. In the current study, we further tested this hypothesis by presenting female platys, Xiphophorus variatus, with pairs of dummy males differing in: (1) body size (holding dorsal fin size constant); (2) dorsal fin size (holding body size constant); and (3) dorsal fin: body size ratio (holding total LPA constant). Females spent more time near dummies of greater body and dorsal fin size; however, in the third experiment, neither fin size, body size, nor any particular dorsal fin+body size combination was preferred. These results provide additional support for the LPA and sensory bias hypotheses, demonstrating that female X. variatus not only prefer males with “swords”, but sailfin-like dorsal fins as well when body size is held constant. Shared preference for increased LPA is consistent with common ancestry of the sensory/neural systems in females of all four species.


► Female preference for male fin enhancements in Poeciliid fishes may be driven by a sensory bias for larger male apparent size or lateral projection area. ► Female Xiphophorus variatus in this study preferred males of larger body size. ► Female X. variatus also preferred males with artificially enhanced sailfin-like dorsal fins. ► Females showed no such preferences when male total lateral projection area was held constant. ► These results provide support for the lateral projection area and sensory bias hypotheses.


Females may develop perceptual biases in mate preferences due to a sensory bias in their nervous systems formed by natural selection, sexual selection, or pleiotropic effects (Kirkpatrick and Ryan, 1991, Endler, 1992). Such biases may reflect ecological constraints (Proctor, 1991, Endler, 1992) or basic properties of nervous systems (Ryan and Keddy-Hector, 1992, Rosenthal and Evans, 1998, MacLaren, 2006). Regardless of the bias origin, females are preferentially attracted to males that stimulate their sensory systems more than other males. If a male sends a novel signal which taps into a latent bias, these males are preferred as mates. Thus, a preexisting bias may affect the nature and direction of sexual selection when a new trait arises (Basolo, 1995a).

Support for the preexisting bias hypothesis requires evidence that the female preference evolved within the species’ lineage prior to the evolution of the male trait that exploits the preference (Basolo, 1990a). The role of preexisting biases on female preference for male traits has been well studied in certain species of live-bearing fishes (Poecilidae). For example, males of some species of the genus Xiphophorus possess an elongation of certain ventral caudal fin rays (the sword) that even females of related but unsworded species find attractive (e.g. X. maculatus; Basolo, 1990b, Xiphophorus variatus; Basolo, 1995b, Priapella olmecae; Basolo, 1995a). Phylogenetic information and mate choice tests suggest that female preference for swords arose before the Xiphophorus-Priapella clade diverged, pre-dating the evolution of the sword within Xiphophorus (Basolo, 1996).

Species within the subgenus Poecilia (mollies) provide additional evidence of female bias for male fin elaborations. Male sailfin mollies (Poecilia latipinna) have an enlarged dorsal fin (sailfin) that is presented to females in courtship (Ptacek and Travis, 1996). Female P. latipinna as well as Poecilia mexicana (a species whose males do not naturally possess the sailfin phenotype) prefer to associate and mate with males of larger dorsal fin (MacLaren et al., 2004, MacLaren and Rowland, 2006, Jordan et al., 2006) and body size (Ptacek and Travis, 1997, MacLaren et al., 2004, MacLaren and Rowland, 2006). As with the sword ornament of Xiphophorus, phylogenetic information (Ptacek and Breden, 1998, Breden et al., 1999) suggests that female preference for enlarged dorsal fins arose before the monophyletic sailfin clade diverged from a shortfin ancestor, pre-dating the evolution of the sailfin trait within the subgenus Poecilia.

Additionally, males of numerous guppy populations (Poecilia reticulata) show polymorphism in dorsal and caudal fin length including homoplastic “swords”, elongated tails and enlarged dorsal fins (Houde, 1997, Brooks and Endler, 2001, Karino and Matsunaga, 2002, MacLaren et al., submitted for publication) that along with body size (Endler and Houde, 1995, Karino and Matsunaga, 2002; MacLaren et al., unpublished data) offer a selective advantage in female mate choice in at least some populations (Bischoff et al., 1985, Endler and Houde, 1995, MacLaren et al., submitted for publication).

These elongated caudal and dorsal fins, among other poeciliid fin elaborations may have originated from a shared female preference for larger apparent size in males, a preference that is common in many species (Andersson, 1994) and widespread among the Poecilidae (Ryan, 1998). Because a larger male projects a larger image onto the female's retina at a given viewing distance (O’Brien et al., 1976, O’Brien et al., 1985, Rowland, 1989a, Rowland, 1989b), this could elicit a stronger sexual response (MacLaren, 2006). Fin elaborations may therefore have evolved as a way for males to increase their apparent size or lateral projection area (LPA) and consequent attractiveness to females (Haines and Gould, 1994, Rosenthal and Evans, 1998, Karino and Matsunaga, 2002, MacLaren et al., 2004).

If females respond primarily to greater LPA rather than increased body or fin size per se, then female preference should be proportional to the male's total LPA (body+fin area) regardless of his fin: body size ratio (MacLaren et al., 2004). The LPA hypothesis predicts that females: (1) prefer males of larger body and fin size; (2) show no preference between two males whose total LPAs (fin+body surface areas) are equal (i.e. increases in fin surface area compensate for decreases in body surface area and vice versa); and (3) show no preference for males with the fin elaboration over a finless male of equivalent LPA. Preference experiments with P. latipinna (MacLaren et al., 2004), P. mexicana (MacLaren and Rowland, 2006), and P. reticulata (MacLaren et al., submitted for publication) support all three of the predictions above.

In this study we further test the LPA hypotheses by examining female preferences for male body size, dorsal fin size, and dorsal fin: body size ratio in X. variatus—a series of experiments similar to those previously conducted with P. latipinna, P. mexicana, and P. reticulata. Moreover, we want to know whether this preference stems from a preexisting bias. Female preference for increased male LPA not only in the subgenera Poecilia (“mollies”) and Lebistes (“guppies”) but the more distantly related genus Xiphophorus would support the preexisting bias hypothesis, demonstrating that female X. variatus (a species sexually monomorphic with respect to fin morphology) not only prefer males with artificial “sword” ornaments (Basolo, 1995b) but supernormal size dorsal fins and perhaps other forms of size manipulation that increase male LPA (Haines and Gould, 1994). A shared preference for sailfin-like dorsal fins and/or increased LPA would be consistent with common ancestry of the sensory/neural systems in females of P. latipinna, P. mexicana, P. reticulata, and X. variatus.

Section snippets


Test subjects were X. variatus, Zarco collected from the Arroyo Zarco locality west of Encino, Tamaulipas, Mexico (locale described by Borowsky (1984); stock source: Dr. Steve Kazianis, New York University, 6th of September 1996). The fish were shipped to the laboratory at Merrimack College from the Xiphophorus Genetic Stock Center, Texas State University, San Marcos, TX in the fall of 2008, placed in 378-l and 70-l mixed-sex stock tanks (water temperature 23–25°C; 16h light:8h dark cycle) and


The female behaviors observed during both experiments included unison swimming, circling, and backing toward the male, all of which are activities attributed to mating behavior in the literature for poeciliids (Farr, 1989, Houde, 1997, Basolo, 2002a). There were no significant effects of dummy presentation order on female strength of preference in any of the three experiments (expt I: one-way repeated measures ANOVA, df=2, F=0.327, P=0.723; expt II: one-way repeated measures ANOVA, df=2, F=


As demonstrated in similar studies with P. latipinna, P. mexicana, and P. reticulata, the preferences of female X. variatus satisfy all three predictions of the LPA hypothesis, suggesting that increased dorsal fin size can compensate for decreased body size and vice versa, and that preference is for male LPA rather than for dorsal fin and body size per se. Haines and Gould (1994) explored the basis of female preference for a male fin elaboration – the sword – in X. variatus and came to a


This study was supported in part by a Merrimack College Faculty Development Grant. E. ElAchi, and P. Imbriano offered valuable assistance in data collection and analysis. C. MacLaren and two anonymous reviewers provided helpful comments on the manuscript. We thank D. Tombarelli for her assistance in lab maintenance. We also thank the Xiphophorus Genetic Stock Center, Texas State University, San Marcos, TX. for supplying the fish used in this study. The research methods presented herein were

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      In the present study, a longer dorsal fin was observed in males exposed to the highest level of etonogestrel (320 ng L−1). Worthy of note is that Poecilia latipinna and Poecilia mexicana display a sexual dimorphism of dorsal fin, where females prefer males with longer dorsal fins (MacLaren et al., 2011). In addition, exposure to etonogestrel at both concentrations caused changes in the shape of the gonopodium, namely it reduced the ratio of the 4th to 6th ray.

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