Monogamy in the maternally mouthbrooding Lake
Tanganyika cichlid fish Tropheus moorii
Bernd Egger1, Beate Obermu ¨ller1, Harris Phiri2, Christian Sturmbauer1
and Kristina M. Sefc1,*
1Department of Zoology, University of Graz, Universita ¨tsplatz 2, 8010 Graz, Austria
2Fisheries Research Division, Department of Research and Specialist Services, Ministry of Agriculture,
Food and Fisheries, PO Box 55, Mpulungu, Zambia
Supported by evidence for assortative mating and polygynandry, sexual selection through mate choice was
suggested as the main force driving the evolution of colour diversity of haplochromine cichlids in Lakes
Malawi and Victoria. The phylogenetically closely related tribe Tropheini of Lake Tanganyika includes the
genus Tropheus, which comprises over 100 colour variants currently classified into six morphologically
similar, polyphyletic species. To assess the potential for sexual selection in this sexually monochromatic
maternal mouthbrooder, we used microsatellite-based paternity inference to investigate the mating system
of Tropheus moorii. In contrast to haplochromines in Lake Malawi, multiple paternity is rare or even absent
in broods of T. moorii. Eighteen of the 19 analysed families were consistent with genetic monogamy, while
either a mutation or more than one sire explained the genotype of one offspring in another brood.
We discuss the differences in breeding behaviour between T. moorii and the Lake Malawi haplochromines,
and evaluate additional factors or alternatives to sexual selection as promoters of colour diversification.
A preliminary survey of other Tropheini species suggested that multiple paternity is infrequent in the
Keywords: mating system; paternity; sexual selection; colour diversification; social competition
The cichlid fishes in the East African Great Lakes embrace
a broad diversity of mating systems, breeding behaviours
and forms of parental care (Fryer & Iles 1972; Barlow
1991; Kuwamura 1997). So far, the study of cichlid
mating behaviour has relied mainly on field and laboratory
observations, and molecular methods for parentage
analysis have been applied to a few selected species only
(Kellogg et al. 1995, 1998; Parker & Kornfield 1996;
Dierkes et al. 1999; Taylor et al. 2003; Maan et al. 2004;
Knight & Turner 2004; Katoh et al. 2005). Numerous
examples from various animal groups, notably birds,
mammals and fish, illustrate the shortcomings of observa-
tional characterization of mating systems due to the
inaccessibility of the mating location for the observer
and discrepancies between social and genetic mating
systems (the true parentage as opposed to the apparent
social interaction), e.g. through sneaking and cuckoldry
(e.g. Hughes 1998; Avise et al. 2002; Griffiths et al. 2002).
However, it is the genetic rather than the social mating
system that has ramifications for several evolutionarily
important parameters. Modes and rates of speciation, a
topic of particular interest in the rapidly radiating
lacustrine Cichlidae, are influenced by mating behaviour
through effective population size (Sugg & Chesser 1994),
reproductive bias in one or both sexes (Jones et al. 2001;
Avise et al. 2002), with consequences for sexual selection
(West-Eberhard 1983; Dominey 1984; Panhuis et al.
2001) and rates of gene flow and assortative mating
(Kondrashov & Shpak 1998; Van Oppen et al. 1998).
Indeed, one explanation for the enormous species richness
and variety in secondary sexual characters of haplochro-
mine cichlids in Lake Malawi and Lake Victoria is that
sexual selection, associated with polygynous mating
systems, drove diversification of spatially segregated
(e.g. Danley & Kocher 2001; Knight & Turner 2004;
Pauers et al. 2004) or even of sympatric populations
(e.g. Seehausen et al. 1998; Seehausen & van Alphen
1999; Maan et al. 2004; but see Arnegard & Kondrashov
2004; Coyne & Orr 2004). In contrast, the morphological
and behavioural diversity of the lineages of Lake
Tanganyika cichlids was largely attributed to natural
selection on allopatric populations occupying different
ecological niches (Sturmbauer 1998).
Many of the Lake Tanganyika species display geo-
graphical variation in coloration (Konings 1998), most
strikingly realized in the over 100 colour variants within
the genus Tropheus (Schupke 2003), a member of the
endemic tribe Tropheini (belonging to the ‘modern
haplochromines’ sensu Salzburger et al. 2005), and
currently classified into six morphologically similar, albeit
polyphyletic, species (Poll 1986; Sturmbauer et al. 2005).
Sexual selection has been included in explanations of the
rapid and sometimes convergent evolution of distinct
colour patterns in the mostly allopatrically distributed
variants (Yanagisawa & Nishida 1991; Sturmbauer &
Meyer 1992; Salzburger et al. 2006), but apart from
extreme parental investment by mouthbrooding females
Proc. R. Soc. B (2006) 273, 1797–1802
Published online 30 March 2006
The electronic supplementary material is available at http://dx.doi.
org/10.1098/rspb.2006.3504 or via http://www.journals.royalsoc.ac.
*Author for correspondence (firstname.lastname@example.org).
Received 15 December 2005
Accepted 29 January 2006
q 2006 The Royal Society
(Schu ¨rch & Taborsky 2005), most Tropheus species lack
some characteristics of sexually selected species, notably
sexual dimorphism and social polygamy. Female Tropheus
establish a pair bond with a chosen mate and draw
resources from his territory for a period of up to three
weeks before spawning. Feeding rate and several indices of
physical condition, including the gonadosomatic index,
are higher in paired than in solitary females, indicating
that females cannot mature their ovaries in their own,
smaller and possibly inferior, territories and depend on the
nutritional resources of a male’s large territory, where they
forage actively under their mate’s protection (Yanagisawa&
courtship and spawning behaviour in Tropheus are similar
to those displayed by other haplochromines (McElroy &
Kornfield 1990), and include lead swimming by the male,
quivering by both sexes, release and snapping up of eggs
by the female, and nuzzling of the male’s anal fin by the
female (Nelissen 1976). Upon spawning, the female
abandons the male territory and settles in an unoccupied
site to mouthbrood. Females usually remain at the
breeding site after release of the fry, and expand their
territories as their feeding activity increases (Yanagisawa &
Nishida 1991). Unlike in many polygynous haplochro-
mines, Tropheus females are fully included in territorial
and social interactions of the community, which also
involves communication via sex-independent (colour)
signals (Wickler 1969; Sturmbauer & Dallinger 1995).
The present study employs microsatellite markers for
paternity analysis of Tropheus moorii broods, and deter-
mines whether the social pair bonds concur with genetic
monogamy within breeding efforts, or whether extra-pair
fertilization could increase the potential for sexual
selection by enhancing the variance in male reproductive
success (Jones et al. 2001). In several other Tropheini
species, sneaking males have been observed to mingle with
spawning pairs (Kuwamura 1987; Ochi 1993a,b), and we
include a survey of paternity in several species of the tribe.
2. MATERIAL AND METHODS
(a) Sample collection and laboratory methods
Maternal families of T. moorii were collected from five
locations along the southern shore of Lake Tanganyika,
Zambia: Muzumwa (nZ5; 08841.970S, 31812.030E), Tonga
(nZ10; 08843.80S, 3188.40E), Mbita Island (nZ2;
08845.060S, 3186.240E), Chituta (nZ1; 08843.710S,
3189.350E) and Kalambo (nZ1; 08836.510S, 31811.650E).
Embryos or fry and a fin clip of the mother were preserved in
99% ethanol, and total length of offspring was measured prior
to DNA extraction. Tropheus moorii population samples from
Muzumwa (nZ33) and Wonzye (nZ23) were used to
evaluate marker polymorphism. Mouthbrooding mothers of
seven additional species were also analysed: Simochromis
pleurospilus, Simochromis diagramma, Simochromis babaulti,
Petrochromis fasciolatus (nZ2), Petrochromis orthognatus,
Gnathochromis pfefferi and Ctenochromis horei.
DNA was extracted by proteinase K digestion, sodium
chloride extraction and ethanol precipitation (Bruford et al.
1998) from fin clips and embryos after removal of the yolk
sac. The families were genotyped at four microsatellite loci.
TmoM11 (Zardoya et al. 1996) and UNH130 (Lee & Kocher
1996), as well as UME003 (Parker & Kornfield 1996) and
Pzeb1 (Van Oppen et al. 1997), were simultaneously
amplified in multiplex PCR reactions containing 1 U DNA
polymerase (BioTherm), 1! reaction buffer (BioTherm)
with 1.5 mM MgCl2in the reaction, 0.5 mM of each primer
and 62.5 mM of each dNTP, under the following temperature
regime: 2 min initial denaturation at 94 8 C; 45 cycles of 30 s
at 92 8 C, 1 min at 50 8C, 1 min at 72 8C; 90 min final
extension at 72 8 C. PCR products and TAMRA 500 internal
size standard (Applied Biosystems; ABI) were loaded on an
ABI 377 automatic sequencer, and gels were analysed in
GENESCAN v. 3.1.2. (ABI).
(b) Data analysis
Paternal alleles were inferred from the genotypes of the
offspring and their mothers. In T. moorii (but not in the other
members of the tribe Tropheini), locus Pzeb1 was proble-
matic to score by allele size due to multiple stutter bands,
addition of extra nucleotides (Magnuson et al. 1996) and
allele size differences of one basepair; therefore, identity with
the maternal alleles and the number of paternal alleles was
determined for each T. moorii progenyarraywithout sizing the
peaks. An assessment of the efficacy of the markers to detect
multiple paternity requires an estimate of allele frequencies in
the populations from which the clutches were sampled. We
calculated the exclusion probabilities (GERUD1.0; Jones
2001) and the likelihood to detect multiple paternity
(GERUDSIM1.0; Jones 2001) for the clutches sampled from
Muzumwa and Tonga, based on allele frequency data
obtained from population samples from Muzumwa and
Wonzye. The Wonzye population was used as reference for
the Tonga families, as the two sampling locations are in close
proximity, and no differentiation was detected between the
Tonga sample consisting of maternal and inferred paternal
genotypes and the Wonzye population (FSTZ0.008;
pO0.05), whereas significant genetic differentiation exists
(FSTZ0.06; p!0.0001). Population differentiation was
estimated in ARLEQUIN (Schneider et al. 2000). Genotype
frequencies in all locus-population combinations complied
with Hardy–Weinberg expectations (GENEPOP v. 3.4;
Raymond & Rousset 1995).
GERUDSIM creates simulated progeny arrays for a given
number of progenyand fathers, based on the population allele
frequencies, and then reconstructs the number of fathers
contributing to the array from the simulated genotypes. The
proportion of iterations, in which the reconstructed number
of fathers is less than the actual number of fathers used to
create the progeny array, indicates the probability to under-
estimate the number offathers with the employed marker set.
For each clutch size observed in the families from Muzumwa
and Tonga, we estimated the likelihood to falsely infer single
paternity when all but one offspring in the brood weresired by
the same male from 10 000 iterated simulations. With only
one offspring sired by an additional male, the simulations
consider the most difficult scenario for the detection of
multiple paternity, and provide a conservative evaluation of
the marker efficacy, potentially underestimating the confi-
dence in the analysis.
(a) Paternity analysis in Tropheus moorii broods
Clutch sizes of mouthbrooding T. moorii ranged from 5 to
13 offspring (mean 8.7, s.d. 2.52). Within each clutch,
total length (LT) varied by less than 0.1 cm, and broods
1798B. Egger and others
Genetic monogamy in Tropheus moorii
Proc. R. Soc. B (2006)
ranged from 0.9 to 2.3 cm (mean 1.7 cm, s.d. 0.35 cm).
No correlation was found between LTand clutch size
(Pearson’s correlation coefficient rZK0.09), suggesting
that mothers do not progressively lose offspring during the
The three markers tested in the reference populations
(TmoM11, UNH130 and UME003) were highly variable
(table 1) and achieved high exclusion probabilities
(cumulative PE of 0.993 and 0.969 in Wonzye and
Muzumwa, respectively). A fourth marker, Pzeb1, was
not evaluated at population level, but scored in the
maternal families (see §2). Therefore, allele frequency-
based estimates of polymorphism were not calculated for
Pzeb1, but high-observed heterozygosity (69.4% among
the maternal and inferred paternal genotypes) indicates
that the marker is informative for the present paternity
study. In all butone of the surveyedfamilies, the genotypes
at these four loci were consistent with motherhood of the
brooding female and a single, but different, sire for each
clutch (see electronic supplementary material). In family
T04-#7, genotypes at three loci were consistent with
single paternity of the clutch, whereas at UME003, one
offspring carried a third, non-maternal allele. Since at the
other three loci the non-maternal alleles of this juvenile
matched the paternal alleles inferred from the remaining
brood, it is possible that the extra allele originated from a
mutation rather than multiple paternity.
The probability of not detecting a second father when it
sired only one offspring of the clutch ranged from 0.019 to
0.138 in the families from Tonga and Muzumwa (see
electronic supplementary material). Accordingly, the
probability to falsely infer single paternity for each of
these clutches, when more than one male were involved in
each case, is below 1.47!10K21based on the three-locus
genotypes. Note that the fourth marker applied to the
families (Pzeb1) further increased the power of the
analysis. We conclude that single paternity is prevalent in
broods of T. moorii.
(b) Polyandry in the tribe Tropheini
A survey of the occurrence of polyandry in the tribe
Tropheini was conducted in maternal families of seven
additional species. The minimum number of fathers
inferred from the four-locus genotypes of mothers and
offspring was one in all broods except for one clutch of
P . fasciolatus, in which up to four paternal alleles per locus
contributed to the offspring genotypes (see electronic
supplementary material). A second P. fasciolatus family,
however, complied with single paternity of the brood.
High levels of polymorphism among the observed
maternal and inferred paternal genotypes (mean of three
alleles per family and locus; percentage of heterozygous
genotypes:TmoM11, 41.2%; UNH130,88.2%;
UME003, 76.5%; Pzeb1, 64.7%) suggest that the markers
are informative for paternity studies in these species.
(a) Monogamy, mating behaviour and parental
With one possible exception, the T. moorii broods surveyed
in this study were sired bya single male each, implying that
multiple paternity within broods occurs infrequently or
not at all in this species. Among the other Tropheini
surveyed in this study, only one brood of P . fasciolatus was
sired by at least two males.Sneakers have been observed in
related rock-dwelling Tropheini species such as Pseudosi-
mochromis curvifrons and S. diagramma (Kuwamura 1987),
C. horei (Ochi 1993a) and G. pfefferi (Ochi 1993b), but we
know of no field observations of spawning in P. fasciolatus,
and it remains unclear whether deliberate polyandry or
sneaking underlie multiple paternity in this brood.
In contrast to the prevalence of single paternity in the
studied Tropheini, multiple paternity is the rule rather
than the exception in mouthbrooding cichlids of Lake
Malawi (Kellogg et al. 1995, 1998; Parker & Kornfield
1996). Despite their close phylogenetic relatedness and
otherwise similar life history, mating behaviour differs
between Tropheus and the Lake Malawi haplochromines.
Females of sand-dwelling cichlids in Lake Malawi have
been observed to mate with multiple males for a single
brood (McKaye 1991), and molecular analyses revealed
multiple paternity in clutches of rock-dwelling species
(mbuna), where spawning with different males had not
been suspected from observational data (Parker &
Kornfield 1996). Predation pressure and sneaky males
have been discussed as causes of polyandry in Lake
Malawi cichlids, especially with regard to sand-dwelling
species, where disturbances by predators and sneakers
reduce the length of individual mating encounters and
encourage switches into other males’ territories (Kellogg
et al. 1995). By spawning in concealed breeding sites, the
rock-dwelling mbuna of Lake Malawi are less vulnerable to
intruders, and multiple paternity in some of these species
has been ascribed to females actively seeking multiple
mating partners as a bet-hedging strategy against
imperfect mate choice (Kellogg et al. 1995; Parker &
Kornfield 1996; Genner & Turner 2005). Tropheus always
remain close to the substrate, browse epilithic algae,
spawn openly on rock surfaces and dart into small caves
and crevices for cover instantly when threatened. In
contrast to the mbuna cichlids, the bonding that precedes
spawning in Tropheus makes it more likely that a pair
resumes spawning after a disturbance than that the female
joins another male to complete her clutch. Furthermore,
females are ‘fickle’ before pairing with one male, paying
Table 1. Polymorphism and paternity exclusion probability of three microsatellite loci in the reference populations from Wonzye
number of alleles gene diversity exclusion probability
Wonzye MuzumwaWonzyeMuzumwa WonzyeMuzumwa
Genetic monogamy in Tropheus mooriiB. Egger and others1799
Proc. R. Soc. B (2006)
repeated visits to several males’ territories to assess the
quality of the territory and/or male, and whereas no
correlation between male body size and success of pairing
was detected, removal experiments showed that previously
solitary males acquired mates soon after they expanded
their territories to incorporate parts of the now vacant
neighbouring area (Yanagisawa & Nishida 1991). While
the attachment to a particular male and his territory poses
a constraint on polyandry, it is also conceivable that the
prolonged courtship period and the attention to the value
of a male’s territory offer sufficient cues for a reliable
assessment of male quality and thus render bet-hedging
against suboptimal mates unnecessary.
Monogamy in fishes is generally correlated with bi-
parental care of eggs and fry (Barlow 1986; Mock &
Fujioka 1990; DeWoody et al. 2000; but see Reavis &
Barlow 1998). Among cichlids, social monogamy is found
in bi-parentally guarding substrate brooders, as well as in
mouth brooding species in which males take over the
wrigglers from females (Kuwamura 1986; Barlow 2000),
but molecular proof of genetic monogamy has so far been
delivered only for the Tanganyikan bi-parental mouth-
brooder Eretmodus cyanostictus (Taylor et al. 2003).
Maternal mouthbrooding entails highly skewed female
investment in reproduction and is associated with
polygamous mating systems, lekking and pronounced
sexual dichromatism in other cichlid species (Kuwamura
1986). Tropheus males, however, invest in their mate’s
fertility by allowing females to harvest the nutritional
resources essential for ovary development from their
territory (Yanagisawa & Nishida 1991), and thus share
in the costs of reproduction albeit without directly
providing parental care.
(b) Sexual poly- and monochromatism and the
role of colour signals in courtship, mating and
Male nuptial coloration of many sexually dichromatic
haplochromines includes conspicuous yellow to orange
spots surrounded by an outer ring on the anal fin. Wickler
(1962) suggested that these markings stimulate the
females to nuzzle the male’s anal fin during spawning.
When the female tries to grasp these ‘egg-dummies’, she
inhales the sperm ejected by the male and ensures
fertilization of the eggs within her mouth (Mrowka
1987). However, the function of egg spots is not yet fully
understood (Barlow 2000), and there is evidence for
alternative roles in attracting females and enhancing
female fertility (Hert 1989) in some species. Elaborate
egg mimics on the anal fin are considered a synapomorphy
of the ‘modern haplochromines’ and their sister clade
(Astatoreochromis allaudi),but have been lostsecondarily in
some deep-water lineages (Salzburger et al. 2005). Small
spots lacking an outer ring on the anal fin are present in
most variants of Tropheus (a ‘modern haplochromine’),
and probably represent the remnants of the—now lost—
true eggs spots present in the ancestors of Tropheus
(Barlow 2000). Similarly, most haplochromines are
sexually dimorphic, and dichromatism may have secon-
darily been lost in Tropheus (except for Tropheus brichardi
and Tropheus annectens). Aside from its significance during
courtship and mating, coloration in Tropheus has import-
ant communicative functions during social interactions,
and both genders exhibit concurrent repertoires of
context-dependent colour patterns signalling motivation
and social status (Wickler 1969; Nelissen 1976; Sturm-
bauer & Dallinger 1995). The social pressure involved in
obtaining and maintaining territories (West-Eberhard
1983; Dominey 1984) seems to promote conspicuous
dominant coloration in both males and females, and
sexual dichromatism may not be affordable to females who
must participate in the competition for territories for
sustenance outside their courtship period (Kawanabe
1986; Yanagisawa & Nishida 1991).
(c) Excessive geographical colour variation in the
absence of strong sexual selection
Although the degree of colour variation within Tropheus
rivals that observed in the mbuna haplochromines of Lake
Malawi, it was probably attained through a different
mechanism. In Lakes Malawi and Victoria, closely related
species differing solely in colour often coexist in sympatry
(Genner et al. 1999; Seehausen & van Alphen 1999) and
reproduce assortatively due to divergent female prefer-
ences (Van Oppen et al. 1998). It has been suggested that
divergent or disruptive selection on male coloration could
explain the diversity and geographical distribution of
colour patterns within genera (e.g. Allender et al. 2003;
but see Arnegard & Kondrashov 2004). In contrast, most
of the Tropheus variants are distributed allopatrically, and
where different morphs occur in the same location they
belong to distinct mitochondrial lineages, suggesting
secondary contact (Sturmbauer et al. 2005). Closely
related, but allopatrically distributed, variants of T. moorii
mate colour-assortatively both in the field following
human-mediated admixis (Salzburger et al. 2006) and
under experimental laboratory conditions (B. Egger 2005,
unpublished data), but given the low potential for sexual
selection in the species, it is unlikely that female
preferences could trigger colour diversification in the
absence of geographical isolation and give rise to
sympatric ‘sister morphs’. In the light of the above
considerations, it is even questionable whether intersexual
selection for male traits could have contributed signifi-
cantly to the allopatric evolution of the manifold Tropheus
By demonstrating genetic monogamy in T. moorii, the
present study confirms behavioural observations of
temporal pair bonding and social monogamy in T. moorii.
The absence of extra-pair fertilizations that would
introduce male reproductive variance into a socially
monogamous system, the prevalence of territory size
and/or quality over male morphology in determining
female mate choice, the males’ contribution to reproduc-
tion over and above the insemination of eggs, and the
similar social roles of males and females possibly
underlying sexual monochromatism in the taxon dis-
tinguish T. moorii from many other ‘modern haplochro-
mines’, and make it unlikely that sexual selection played a
similarly important role in the diversification of Tropheus as
is hypothesized for the haplochromine radiation of Lakes
Malawi and Victoria.
We thank the team at the Mpulungu Station of the Ministryof
Agriculture, Food and Fisheries, Republic of Zambia, for
their cooperation during fieldwork, and fishermen in
1800 B. Egger and others
Genetic monogamy in Tropheus moorii
Proc. R. Soc. B (2006)
Mpulungu and at Kalambo Lodge for help with catching
mouthbrooding females. We are grateful to the reviewers for
comments and suggestions on a previous version of this
manuscript. Thanks to Nina Duftner, Ismene Fertschai and
Stephan Koblmu ¨ller for help and companionship in the field.
The work was supported by the Austrian Science Foundation
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