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Behavioral Ecology Vol. 13 No. 6: 832-837
© 2002 International Society for Behavioral Ecology
Congruence between the sexes in preexisting receiver responses
School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588-0118, USA
Address correspondence to A.L. Basolo. E-mail: basolo{at}cricket.unl.edu.
Received 9 October 2001; revised 12 April 2002; accepted 13 April 2002.
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ABSTRACT |
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Preexisting receiver biases have been shown to affect how females detect and respond to new conspecific traits in a mate choice context. Although preexisting biases have often been discussed in the context of female mate choice, these biases need not be sex limited. In the genus Xiphophorus, swordtail males possess a sexually selected trait, the sword. Here I consider evidence that the state of a bias favoring sworded conspecifics may be generally shared by the sexes in taxa in which the sword has not arisen. In three unsworded species of poeciliid fishes, both males and females prefer members of the other sex with swords. In a fourth species, males and females share the absence of a response to a sword. This congruity between the sexes suggests that response biases may not be sex limited and that the sexes could historically share common mechanisms producing shared mating responses. Alternatively, selection may tend to result in parallel changes in biases in the sexes. This work expands our understanding of receiver biases by using a phylogenetic approach to examine whether biases are historically shared by the sexes and suggests that there can be general congruence between the sexes in such biases.
Key words: poeciliid fishes, preexisting biases, receiver biases, shared traits.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Receiver biases established in one context could affect fitness in other contexts (Basolo, 1990b
,
2000;
Endler and McLellan, 1988;
Kirkpatrick, 1987;
Ryan, 1990). Such correlated
fitness effects may be disassociated in evolutionary time from the original
context in which a trait evolved. In terms of mate choice, for example, if
females have biases that cause them to be more sensitive to certain classes of
environmental stimuli, and if new traits arise in males that these biases
differentially recognize, females may be more likely to mate with males
expressing these new traits. The same applies for biases in males for female
traits. In a diverse array of animals, including fish, frogs, birds, crabs,
spiders, and water mites, historical information suggests that preexisting
receiver biases have played a role in the evolution of female preferences,
which in turn appear to have affected the evolution of male signals (reviewed
in Endler and Basolo,
1998).
Although the study of receiver biases has concentrated on females, biases
need not be sex limited. Lande
(1981) suggested a mechanism
for the evolution of shared traits between the sexes; a trait selected in one
sex can be expressed in the other sex, simply through a correlated response to
selection in the other sex. This mechanism has been proposed as an explanation
for shared mating tendencies of males and females within a species
(Halliday and Arnold, 1987).
Thus, traits shared by the sexes can result from selection on one sex within a
given species. Traits may also be shared by the sexes either due to a
non—sex-limited mutation being carried to a high frequency as a result
of genetic drift, or due to selection acting similarly on the sexes. When a
shared ancestral receiver bias is established by mutation with genetic drift,
the state of the bias is not expected to differ for the sexes, unless there is
a differential cost in carrying the mutation. When sensory or cognitive
processing features were favored in the past due to selection acting in the
same manner on both sexes, we might expect the sexes to presently share a
response to a specific stimulus. Alternatively, the sexes may respond
similarly in a mating context, not due to the common descent of the response,
but as a result of convergent evolution.
Shared biases that arose in an ancestor may result in the sexes exhibiting
the same response to a trait in conspecifics. Alternatively, the responses of
the sexes may differ because a shared bias has been co-opted by males and
females in different selective contexts (e.g., intrasexual selection vs.
intersexual selection; Basolo,
2000). Some studies have suggested that within a species, the
sexes exhibit different responses to the same stimulus
(Morton, 1990;
Narins and Capranica, 1976;
Rowland, 1989;
Smith, 1972) as a result of
differential selection. Other studies have indicated that shared biases result
in similar responses by male and females
(Basolo and Delaney, 2001;
Jones and Hunter, 1993;
Ryan and Rand, 1998). Yet we
need not expect congruence between the sexes in the state of biases
(Basolo, 2000). Receiver biases
could change in one or both of the sexes and in one or more taxa following
their appearance. And, if a trait arises in one sex that is favored by an
ancestral bias, sexual selection may subsequently act differently in males and
females, resulting in differences between the sexes in their responses to the
trait (Basolo and Delaney,
2001).
Swordtails and unsworded platyfish are poeciliid fishes that compose the
genus Xiphophorus (Rosen,
1979; Rosen and Bailey,
1963). Male swordtails possess a sword, a colored extension of the
lower caudal fin, except for three species in which sword components appear to
have been secondarily lost (Basolo,
1996; Rauchenberger et al.,
1990; Rosen, 1979;
Rosen and Bailey, 1963). Based
on the available phylogenetic evidence
(Borowsky et al., 1995;
Haas, 1993;
Meyer et al., 1994;
Rosen, 1979), it appears that
this sword arose after the divergence of the sister genera Priapella
and Xiphophorus. Evidence also suggests that a female mating bias for
males with swords was present before the divergence of the two clades; females
of four species in the Xiphophorus/Priapella clade favor conspecific
males with swords, including the green swordtail, X. helleri
(Basolo, 1990a;
Rosenthal and Evans, 1998;
Trainor and Basolo, 2000), the
variable platyfish, X. variatus (Basolo,
1990c,
1995a;
Haines and Gould, 1994), the
southern platy-fish, X. maculatus
(Basolo, 1990b), and the
guayacón olmeca, Priapella olmecae
(Basolo, 1995b). Males of two
of these species have been tested for a response to sworded females as well;
Pr. olmecae males prefer sworded females, but X. helleri
males discriminate against sworded females
(Basolo and Delaney, 2001).
Thus, the sister genera Priapella and Xiphophorus appear to
share a bias in females favoring sworded males, but not a bias in males
favoring sworded females. Although the results for females suggest that a bias
was present before the appearance of a sword in swordtails, the ancestral
condition for males is unclear.
Although a number of studies have investigated whether conspecific males and females share similar responses to specific signals, a phylogenetic approach has not previously been used to investigate whether a response to a trait is historically shared between the sexes. Here, to test whether the sexes generally share receiver responses for traits not present in their evolutionary history, a receiver bias favoring a sword is further investigated by testing the sword response in male X. maculatus, in male and female sailfin mollies, Poecilia latipinna, and in male and female Mexican mollies, Poecilia mexicana. The results of this study are combined with the results of previous studies to assess whether there is general congruence or noncongruence between the sexes in the state of preexisting receiver responses. It appears that in species in which the sword has not arisen, there is congruence between the sexes in their responses to sworded conspecifics of the other sex.
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MATERIALS AND METHODS |
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The test fish were either field collected or first- or second-generation offspring of field-collected fish. The P. mexicana originated from Reptile Pond, Belize. This species was collected under a permit awarded by the Fisheries Ministry of Belize. The X. maculatus originated from the Rio Palma, Vera Cruz, Mexico. This species was collected under a permit awarded by the Secretaria de Relaciones Exteriores and the Secretaria de Medio Ambiente Recursos Naturales y Pesca, Mexico. The P. latipinna originated from a canal in Harvey, Lousiana, under a collection permit awarded by the Department of Wildlife and Fisheries, State of Louisiana.
Five choice experiments were conducted: (1) male preference for sworded
female X. maculatus, (2) male preference for sworded female P.
mexicana, (3) female preference for sworded male P. mexicana,
(4) male preference for sworded female P. latipinna, and (5) female
preference for sworded male P. latipinna. For each of these five
experiments, a set of conspecific individuals were paired for sex, body size,
and morphology. One member of each pair received a surgically attached plastic
sword consisting of an upper stripe, a lower stripe, internal coloration, and
elongation, typical of male green swordtails. The other member of each pair
received a clear attachment of equal length that simulated the visual absence
of a sword, but controlled for the effect of the attachment on behavior.
(Details concerning this procedure have previously been described in Basolo,
1990b,
1995b.) For each of the five
mate choice experiments, a set of conspecific test fish was given a choice
between paired individuals of the other sex. For each experiment, test fish
and test pairs were used only once.
Mating preferences were tested using a general method for poeciliid fish
species (Basolo, 1995b;
Bischoff et al., 1985;
Morris et al., 1995;
Ryan and Wagner, 1987;
Schlupp et al., 1991). This
general method was first used by Bischoff et al.
(1985) to test the response of
female guppies, Poecilia reticulata, to males differing in
coloration; they found that their measure of female preference, amount of time
a female spent in association with males, accurately predicted male
reproductive success based on number of offspring. This general design has
also been used to show that male P. latipinna prefer conspecific
females to heterospecific females (Schlupp
et al., 1991); the results from this experiment are in agreement
with the results obtained by using male mating attempts in P.
latipinna and P. mexicana
(Ryan et al., 1996) as the
measure of male preference. Male mating preferences in P. reticulata
have also been measured using a similar design
(Benz and Leger, 1993) and
verify results from the field and laboratory that indicate that males prefer
larger females (Endler and Houde,
1995; Houde, 1997;
Reynolds and Gross, 1992). For
the five experiments reported here, mate choice trials were performed for each
species by presenting a matched pair of conspecifics to other sex test fish in
a divided tank that did not allow chemical or physical cues between the test
subjects. The time that a test fish spent showing mating interest to each of
the paired stimulus fish was recorded. Mating interest was defined as being
within three body lengths of a stimulus fish while attending to the stimulus
fish and interacting by exhibiting mating behavior. The behaviors observed for
the five tests conducted here (e.g., circling, backing toward a conspecific,
unison swim, presenting a flexed flank) are attributed to mating behavior in
the literature for poeciliids (Basolo,
1995b; Clark et al.,
1954; Farr, 1989;
Franck, 1964;
Hemens, 1966;
Houde, 1997;
Schlosberg et al., 1949); the
aggressive behaviors that have been described do not include these behaviors.
Mating interest was preceded by the test subject ceasing other activities
(e.g., swimming around the tank foraging on the bottom, swimming vertically up
and down the front of the aquarium, positioned near the companion fish,
hanging motionless at the surface), looking over at one of the paired
individuals, then swimming directly toward that individual. To control for the
potential effect of association behavior on the measurement of male and female
choice used here, the test fish could also interact with the companion fish of
the same sex in a tank adjacent to the center section (as described in Basolo,
1995b,
2002). For each of the five
different choice experiments, I summed and compared the amount of mating
interest time for two 10-min test periods for each test subject
(Basolo, 1995b) using a
Wilcoxon matched-pairs signed-rank test.
I calculated the statistical power of the nonsignificant analyses using
NCSS Pass 2000 software. These power calculations are approximations that use
the asymptotic relative efficiency of the Wilcoxon matched-pairs signed-rank
tests relative to the paired t test (per
Lehmann, 1975). The average
effect size was calculated separately for each sex using those comparisons in
which there was a significant difference. For the comparisons that were not
significantly different (both sexes of one species), the power to detect the
average effect size observed for that sex in the four other species was
calculated (e.g., Bee,
2002).
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RESULTS |
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Male X. maculatus spent significantly more time with sworded females than with unsworded females (Wilcoxon matched-pairs signed-rank test: n = 15, p = .03, Figure 1a). Thus, in this unsworded species, males prefer conspecific females with swords to those with clear attachments.
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Female sailfin mollies, P. latipinna, spent significantly more time with sworded males than unsworded males (Wilcoxon matched-pairs signed-rank test: n = 15, p = .04; Figure 1b). Likewise, male sailfin mollies spent significantly more time with sworded conspecific females than with unsworded females (Wilcoxon matched-pairs signed-rank test: n = 17, p = .04; Figure 1b). Thus, in this unsworded species, females and males preferred conspecifics of the other sex with swords to those with clear attachments.
Female Mexican mollies, P. mexicana, did not discriminate between sworded males and unsworded males based on mating interest (Wilcoxon matched-pairs signed-rank test: n = 14, p = .98; Figure 1c). Likewise, male Mexican mollies did not discriminate between sworded females and unsworded females (Wilcoxon matched-pairs signed-rank test: n = 14, p = .36; Figure 1c). Thus, in this unsworded species, neither females nor males appear to prefer conspecifics of the other sex with swords to those with clear attachments.
The power to detect the average effect size observed in the other four species was calculated for P. mexicana. The mean effect size for females for the other four species was 1.44, and the power to detect this expected effect size in female P. mexicana was > 0.99. The mean effect size for males for the other four species was 0.67, and the power to detect this expected effect size in male P. mexicana was 0.61.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Male and female responses to sworded conspecifics of the other sex have now been examined in four related poeciliid species that lack a sword, and in one species with sworded males. In the four historically unsworded species, there appears to be congruence between the sexes in response to a sword (Figure 2). In one of the unsworded species, P. mexicana, neither sex exhibited a differential response to sworded conspecifics, suggesting that a bias favoring a sword is absent in both sexes. The power to detect the expected effect size was modest for male P. mexicana and high for females. In the other three unsworded species, both females and males prefer sworded conspecifics of the other sex. The results from the experiments presented here, combined with previous findings, suggest that there is general congruence between the sexes in the state of a bias when the trait is not present (Figure 2). Although there does not appear to be an intersexual difference in the state of the bias within the two Poecilia species, there was an interspecies difference in the state of the bias; male and female P. latipinna responded positively to sworded individuals of the other sex, while male and female P. mexicana did not respond differentially to sworded individuals of the other sex. This result suggests that biases can change over time in both sexes, possibly with simultaneous change in males and females. However, as indicated by previous findings for the green swordtail, X. helleri, when the trait is currently present in males, congruence may not exist between the sexes.
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Congruence between the sexes in a response bias is expected to exist as a
result of one of four evolutionary scenarios. First, a bias could arise
simultaneously in the sexes and evolve simultaneously in both via drift.
Second, a bias could simultaneously evolve in both sexes via selection because
selection should often act similarly on the sensory and cognitive systems of
both sexes. Third, the sexes may have similar, but nonhomologous biases. This
would be the case if different sources of selection independently favor the
same type of response in both sexes or if genetic drift results in the
establishment of similar mutations in the sexes. Fourth, a bias could evolve
in one sex via selection and evolve in the other sex as a correlated effect
(Basolo, 1996;
Halliday and Arnold, 1987;
Lande and Arnold, 1985); this assumes that there is little cost to the trait
being carried along in the sex in which the bias is not selected.
Natural selection can favor sensory and cognitive systems to process data
in certain ways, and this results in biases toward certain kinds of signals
and signal properties (Endler and Basolo,
1998). If the bias favoring a sword is homologous for the sexes,
there are two equally parsimonious hypotheses for the evolutionary history of
the sword response in the five species of poeciliid fishes discussed here.
First, assuming that P. mexicana males really do not have a bias
toward swords, the bias could have arisen in a common ancestor of all five,
with a single evolutionary event resulting in the lack of a response in P.
mexicana and a second change resulting in the negative response exhibited
by X. helleri males (Figure
3a). Alternatively, there may have been two independent origins of
a sword bias, one in P. latipinna and one in an ancestor of the
Priapella/Xiphophorus clade, with a subsequent change in the bias in
X. helleri males (Figure
3b). These alternative hypotheses would both require three
independent evolutionary events. It is also possible that the responses for
the five species are not homologous or that the responses are not homologous
for the sexes. However, if the response to a sword is not homologous for the
sexes, a minimum of five evolutionary events is required to explain the
distribution of the bias. If parsimony
(Hennig, 1966) is assumed,
then a shared, homologous bias is one explanation for the congruence between
the sexes in the response to the sword.
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Given the female preference for swords, why might male X. maculatus, P.
latipinna, and Pr. olmecae lack swords? Regardless of the
context in which a bias originates, it may be specific, or it may be general
for the types of traits it can subsequently influence. Depending on the types
of variation that arise after a bias has been established, different lineages
could have different traits that have evolved as a result of the same general
ancestral bias (Basolo, 2000).
If this is the case, then the same ancestral bias could have contributed to
the evolution of the sword in male swordtails, the elongated, colorful dorsal
fins in male P. latipinna, the orange gonopodium, caudal fin and
dorsal fin expressed by male Pr. olmecae, and the red, yellow,
orange, and black fins expressed by male X. maculatus. This
possibility is intriguing, but further investigation in both sexes across the
clade using surgical manipulations or altered video presentations
(Rosenthal and Evans, 1998;
Trainor and Basolo, 2001) are needed to explore the bases of the bias favoring
a sword.
It seems clear why female green swordtails do not presently have swords;
males discriminate against sworded females. But why do female X.
maculatus, P. latipinna, and Pr. olmecae lack swords when
sworded females are preferred by males? This question may be addressed by
considering potential costs to females of producing and maintaining a sword or
swordlike structures and colors. Although female Pr. olmecae express
light yellow fins and some female X. maculatus express red, yellow,
orange, and black fins, expression of these colors is usually weaker than
expression in males. Investment in a sword can include extending the length of
lower caudal fin rays as well as allocating pterin, melanin, and carotenoid
pigments to these rays. Allocation to sword components may be costly to
younger females at a time when they are under pressure to mature and
reproduce. In addition, if the sword affects swimming ability, changes in the
caudal fin may be particularly costly to gravid females in terms of capturing
prey or avoiding predators (gestation of young results in a change in shape,
producing a less streamlined body). In such cases, a sword combined with the
temporary change in female body shape might impede rapid maneuvers used in
capture and escape. Finally, although males prefer sworded females, the
strength of the preference may differ for the sexes. Tests investigating the
preference functions (Wagner,
1998) in both sexes could establish whether males and females
differ in the strength of this sword preference; a weaker male preference
would provide another explanation for the lack of swords in females.
Although this work includes only five species, the results are provocative.
For either evolutionary scenario (Figure
3), the combined evidence suggests that the female ancestral state
for these taxa is a receiver bias that results in a sword preference; the same
ancestral bias could explain the preference found in males as well. In one
species that exhibits no differential response to a sword, the sexes share
this response. Only in a species that presently possesses a sword is there a
negative response to the sword in a mate choice context, and only in this
species were the sexes found to differ in their response to a sword. If the
ancestral state for males is a favorable response to a sword, it appears that
somewhere along the receiver signal processing/response system, there has been
a change in the bias in male X. helleri; males do detect the sword,
but they respond negatively to sworded conspecific females. The sword
currently appears to play a role in female mate choice in swordtails, and the
sword could function in alternative contexts in males, such as sex
recognition, maturation status, or male—male competition
(Basolo, 2000). Such other
evolved functions could preclude the co-option of the bias in an intersexual
context in swordtail males, suggesting that once a bias has been co-opted by a
sex in one context, there may be constraints on its co-option in a different
context. The work presented here adds to our growing understanding of receiver
biases, but it remains to be determined whether congruence between the sexes
in the state of receiver biases extends to more distantly related species.
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ACKNOWLEDGEMENTS |
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I thank W. E. Wagner, Jr. for insightful input during the course of these experiments, J. A. Endler, D. Leger, S. Louda, and W. E. Wagner, Jr. for comments on various manuscript versions, A. Aspbury and K. Delaney for assistance with the fish, D. Westneat and two anonymous reviewers for constructive criticism of earlier drafts, and the Fisheries Ministry of Belize, the Secretaria de Relaciones Exteriores, the Secretaria de Medio Ambiente Recursos Naturales y Pesca, Mexico, and the Louisiana Department of Wildlife and Fisheries for permission to collect fish. This work was supported by National Science Foundation grant IBN9629318.
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