Behavioral Ecology Vol. 13 No. 1: 59-64
© 2002 International Society for Behavioral Ecology
Color patterns and species recognition in four closely related species of Lake Malawi cichlid
Ecophysiological Studies Research Group, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
Address correspondence to V. Couldridge. E-mail: vanessa{at}gecko.biol.wits.ac.za .
Received 12 September 2000; revised 16 March 2001; accepted 16 March 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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We examined interspecific female mating preferences in four closely related species of cichlid belonging to the Pseudotropheus zebra species complex of Lake Malawi. These species differ in coloration but are similar in other respects, suggesting that male color patterns may be important to female mate choice in species recognition. To test this hypothesis, we presented females from each species with a choice of four males, one of her own species and three others that were each of a different species. We also gave each female a choice between the three heterospecific males only. In all four species, females showed a significant preference for conspecific males in the four-way choice and chose the male with the most similar color pattern to the conspecific male in the three-way choice. These results are discussed with reference to the theory of sexual selection on color patterns as a means of sympatric speciation in cichlids.
Key words: cichlids, Lake Malawi, mate choice, Pseudotropheus zebra, sexual selection, speciation.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Cichlids are a family of fish that have evolved very rapidly over time into an enormous number and diversity of species (Johnson et al., 1996
;
Meyer et al., 1990;
Moran et al., 1994;
Owen et al., 1990;
Schliewen et al., 1994). For
example, the 500 or more cichlid species endemic to Lake Malawi
(Martens, 1997) are estimated
to have arisen within the past 700,000 years
(Meyer et al., 1990). There
has been much debate over the speciation mechanisms that have been responsible
for this remarkable radiation. Intralacustrine allopatric speciation aided by
the high evolutionary plasticity of the feeding apparatus
(Liem, 1973) has traditionally
been cited as an explanation for the high rate of speciation. Several factors
are likely to have been important in contributing to the process of allopatric
speciation. These include certain life-history traits of cichlids, such as
habitat fidelity and low rates of dispersion and the repeated creation and
reflooding of small, isolated, and peripheral bodies of water due to
fluctuating lake levels (Fryer and Iles,
1972; Ribbink et al.,
1983; Sturmbauer,
1998). However, there are many species that have arisen in the
absence of any obvious habitat barriers
(Schliewen et al., 1994).
Furthermore, many recent speciation events have not been accompanied by
significant changes in morphology or ecology and thus may be attributed to
nonadaptive processes (Albertson et al.,
1999; Genner et al.,
1999).
Dominey (1984) was the first
to propose sexual selection based on color patterns as a means of speciation
in cichlids. He based this proposal on the observation of the huge color
diversity among cichlids, with closely related species differing most
noticeably in male breeding colors. Many cichlid species are also predisposed
to sexual selection because they have a polygynous mating system with
exclusive female parental care in the form of mouthbrooding. Although color
often varies geographically within a species, color markings within a
population usually show little variation among sexually active males
(Ribbink et al., 1983), and
colors also intensify during courtship. This implies that color may play a
role in the species recognition process and may act as a reproductive barrier
between species. Although these ideas were initially met with skepticism,
there is now mounting evidence in the form of both empirical studies
(Knight et al., 1998;
Seehausen and van Alphen,
1999; Seehausen et al.,
1998,
1999) and theoretical models
(Dieckmann and Doebeli, 1999;
Kondrashov and Kondrashov,
1999; Turner and Burrows,
1995; van Doorn et al.,
1998) that disruptive sexual selection on male color patterns is a
common method of speciation among cichlids.
If sexual selection on color patterns does facilitate speciation among
certain types of cichlids by acting as the first step in divergence, then it
would be expected that among closely related, and therefore recently diverged,
species, color pattern is an important factor in interspecific mate choice. To
test this prediction, we conducted an interspecific female mate choice
experiment on four closely related haplochromine cichlid species from Lake
Malawi. Our aim was to determine if females are able to distinguish males of
their own species from those of closely related species on the basis of color
pattern and whether females respond to similarly patterned males in the
absence of conspecific males. Several sibling species complexes of cichlids
exist in Lake Malawi, and these represent natural groups of species that are
monophyletic in origin, are closely related to each other, and have speciated
recently (Ribbink et al.,
1983). We used cichlids belonging to one such complex, the
Pseudotropheus zebra species complex, in our study. There is a great
deal of color diversity among the members of this complex, but they are
otherwise similar, both in their morphology and in their behavior
(Ribbink et al., 1983;
Stauffer et al., 1997). We
gave females of each species a choice between males of all four species to
establish preference for conspecifics and a choice between three
heterospecific males to determine preferences when conspecifics are
unavailable.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Study animals
The four species of cichlid that we used in this experiment were P. callainos, P. lombardoi, P. zebra "gold" and P. zebra "red dorsal." The latter two species are referred to by their common names, as P. zebra gold is an undescribed species, P. zebra red dorsal has recently been split into three species (Stauffer et al., 1997
), and
the origin of our laboratory population is uncertain. Males of P.
callainos are pale blue with no barring. Males of P. lombardoi
are yellow with five vertical black bars. Males of P. zebra gold are
pale orange with faint brown bars. Males of P. zebra red dorsal are
pale blue with darker blue bars and an orange dorsal fin. We used fish that either had been obtained from local pet shops and born in captivity from wild-caught parents or had been reared in the laboratory (second-generation captive bred). They were maintained in the laboratory under a 12:12 h light:dark cycle and kept in single species groups in 3-foot aquariums fitted with air-driven box filters. A layer of gravel covered the bottom of the tanks, and rocks and terracotta pots acted as refuges. The water temperature was kept at approximately 27°C. The fish were fed once or twice a day on flake food and were fed live food (bloodworms and brine shrimp) every 2 weeks.
Measuring female preference
The experimental setup consisted of a square tank (600 x 600 x
300 mm) with four sheets of glass (315 x 300 mm) partitioning off the
corners. A layer of gravel covered the bottom of the aquarium, and a
terracotta pot was placed on its side in each of the four corner enclosures to
act as a territory. A 70-mm area was marked out in the gravel and along the
sides of the tank around each of the four enclosures as a female choice
zone.
We tested female choice by placing a female in the center of the tank and placing four males, one conspecific and three different heterospecific, behind the four partitions. The fish were allowed to acclimatize for 10 min before the trial began. Each trial lasted 20 min, and during this period we recorded the amount of time spent by the female in the marked area around each of the four males' enclosures. The trial was then repeated with the same fish, but with the conspecific male removed so that the female only had a choice between the three heterospecific males. The amount of time that the female spent near the empty enclosure was recorded, but this time was either zero or very short and was not included in the analysis. The order in which a female was presented first with four males or first with three males was alternated between females to eliminate the effects of previous experience. However, there were no significant differences in female choice based on the order of male presentation (one-way ANOVA with order as a covariate; p >.050 in all cases). The fourway choice served to establish that females have the greatest preference for males of their own species, and the three-way choice was used to determine which male a female will choose if her own species is unavailable. We tested 15 females of each species (60 females in total), and the species were tested in random order. We used 15 males of each species to test female preferences, such that no two females within a species were presented with the same male, but the same set of males was used to test the preferences of each species. The positions of the males in the choice tank were randomly assigned and were changed after each female was tested (i.e., after every two trials).
Statistical analysis
Because the data did not deviate from normality in any case except one
(Lilliefors test; two-tailed p >.050 in 27 cases, p =.002
in one case), we used ANOVAs (one way; species as the independent variable)
with Tukey multiple comparison tests to test for differences in the amount of
time (calculated as a percentage of total time) that females spent near each
male in the four-way choice and in the three-way choice. A 5% level of
significance was used in all tests. The results of each species were analyzed
separately.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In all four species, there were significant differences in the amount of time that females spent with males, both in the four-way choice and in the three-way choice (Table 1).
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Females of P. callainos spent significantly more time near conspecific males than each of the other three males in the four-way choice, and significantly more time near P. zebra red dorsal, the male with the closest color pattern to the conspecific male, than either of the other two males in the three-way choice (Figure 1 and Table 2). In addition, there was a further significant difference between the preferences for P. zebra red dorsal and P. zebra gold in the four-way choice.
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In P. lombardoi, females spent a significantly greater amount of time with conspecific males than any of the other three males in the four-way choice and spent significantly more time with P. zebra gold, which is the male most similar in color to P. lombardoi, than with the two other heterospecific males in the three-way choice (Figure 2 and Table 3).
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Females of P. zebra gold spent a significantly greater percentage of time near their own males than any of the other three males in the four-way choice and spent significantly more time near P. lombardoi, the male with the most similar color pattern to the conspecific male, than near the two other males in the three-way choice (Figure 3 and Table 4). However, in P. zebra red dorsal, the pattern is not as clear. Females still spent significantly more time near conspecific males than each of the three heterospecific males in the four-way choice, but in the three-way choice the amount of time females spent with P. callainos, the most similarly colored male to the conspecific male, was only significantly higher than the amount of time spent with males of P. zebra gold and not with males of P. lombardoi (Figure 4 and Table 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Females of all four species had a significant preference for males of their own species in the four-way choice and chose the male with the most similar color pattern to this male in the three-way choice. These results strongly suggest that color patterns play a role in species recognition among the members of this species complex. However, the pattern of female preference is not as clear in P. zebra red dorsal as it is in the other three species, and the reason for this may be that, although males have a blue body color, they have an orange dorsal fin, and this fin color may form part of their species recognition process.
It is not surprising that females would prefer the heterospecific male with
a similar color pattern to the conspecific male in the three-way choice, as
this male would appear greatly contrasted to the other two males. Of the four
species used in this experiment, two had males with a basic body color of blue
(P. callainos and P. zebra red dorsal), and two had males
with a basic body color of yellow or orange (P. lombardoi and P.
zebra gold). Although the Mbuna of Lake Malawi are very colorful fish,
they have evolved within a very limited color spectrum, with blue and yellow
being the predominant colors. Possible reasons for this include genetic or
physiological constraints, female bias in color vision, or the fact that clear
water transmits blue most effectively, and blue has the greatest contrast with
yellow (Deutsch, 1997).
Our finding that females will so readily transfer their attention to a
heterospecific male, albeit a similarly colored one, in the absence of a
conspecific male, raises questions about the validity of describing these fish
as fully distinct species. However, because the choice tests did not include
actual mating, it is not known whether or not hybridization would have
occurred. Hybridization is rare in the field under natural conditions, but may
occur when mates of the same species are rare or unavailable. For example, a
lone P. zebra male in a population of P. greshakei produced
hybrid offspring with a female of this species
(Konings, 1990). Furthermore,
hetero-specific matings under natural conditions always occur between two
species with similar color patterns
(Seehausen, 1996;
Stauffer et al., 1996).
Hybridization has occurred between translocated populations of P.
zebra and Cynotilapia afra in Lake Malawi, and these two species
strongly resemble each other in terms of color patterns but are otherwise
dissimilar, indicating the importance of color patterns to species recognition
(Stauffer et al., 1996).
We measured female preference as the amount of time spent in close proximity to a male, not as female response to male displays or actual mating, which may give a more reliable indication of preference. However, this experimental design also has the advantage of eliminating any possible species-specific behavior cues, such as differences in courtship, which may have influenced female choice, and ensuring that female choice was based on male appearance alone. Furthermore, the amount of time spent with a male has been shown to be a reliable method of measuring female preference in these cichlids (Couldridge and Alexander, manuscript submitted).
A recent study by Genner et al.
(1999) found that three
sympatrically occurring members of the Pseudotropheus zebra species
complex, P. zebra, P. zebra gold, and P. callainos, are
similar ecologically: no differences in feeding habits or micro-habitat could
be found. This implies that mate recognition is probably the only factor
maintaining divergence among these three species, and possibly also among
other members of the complex that occur sympatrically. Apart from ecological
similarities, members of the P. zebra species complex also have
morphological and behavioral similarities. The complex is typified by P.
zebra, and all members share certain anatomical features that distinguish
them from other cichlids belonging to the genus Pseudotropheus as
well as from other Mbuna (Stauffer et al.,
1997). No differences in reproductive behavior among these species
have ever been reported, and, if they do occur, they are likely to be
extremely subtle. Courtship behavior has been found to be highly conserved
among the cichlids of Lake Malawi and is unlikely to have influenced
speciation (McElroy and Kornfield,
1990). Variation in color patterns is the only obvious difference
between the species and is therefore likely to form the major component of the
mate recognition system.
The results of our study are in agreement with work conducted on Lake
Victoria cichlids (Seehausen,
1997; Seehausen and van
Alphen, 1998; Seehausen et
al., 1999), where color pattern was shown to be important in
initiating speciation and in species recognition among closely related
species. Matechoice experiments on color morphs representing incipient species
of Neochromis omnicaeruleus revealed that color polymorphisms with
restricted gene flow occurred before morphological and ecological
differentiation (Seehausen et al.,
1999). Another study by Seehausen and van Alphen
(1998) examining female mating
preferences under different lighting conditions found that females of two
closely related cichlid species from the Haplochromis nyererei
complex of Lake Victoria could identify conspecific males only on the basis of
color. Furthermore, many of the Lake Victorian cichlid species are becoming
extinct as a result of human activity on the lake that increases water
turbidity (Seehausen et al.,
1997) because females use color to recognize conspecific males. In
turbid water they can no longer distinguish males of their own species from
those of closely related species, resulting in hybridization and consequent
loss of species diversity (Seehausen et
al., 1997).
Our study is different from previous studies examining interspecific female
mate choice in cichlids in that we measured female choice both in the presence
and in the absence of conspecific males, whereas in previous studies
conspecific males have always been present
(Knight et al., 1998). It was
only in the absence of conspecific males that preference for heterospecific
males became apparent, as females always preferred their own males when given
the choice. Further study would be needed to determine whether female
preference for males similar in color to conspecific males when these are
unavailable is due to mistaken identity or due to a preference for females to
mate with the most similar heterospecific male rather than not to mate at all.
Our results indicate that color patterns are important in species recognition
among closely related species and imply that sexual selection on color
patterns may have played a role in speciation.
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ACKNOWLEDGEMENTS |
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Ethical clearance for this study was obtained from the University of the Witwatersrand Animal Ethics Screening Committee (clearance certificate number 99/30/1). Funding for the project came from a Prestigious Masters Scholarship from the National Research Foundation and from the University Research Committee of the University of the Witwatersrand.
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