Biology assignment

Monogamous pair bonds and mate switching in the Western Australian seahorse Hippocampus subelongatus

C . K V A R N E MO ,*  G . I . M O O R E , * A . G . J O N E S ,ৠW. S . NE L S O Nà & J . C . AV IS Eà *Department of Zoology, the University of Western Australia, Nedlands, WA 6009, Australia

 Department of Zoology, Stockholm University, S-106 91 Stockholm, Sweden àDepartment of Genetics, University of Georgia, Athens, GA 30602, USA §Department of Zoology, Oregon State University, Corvallis, OR 97331, USA

Introduction

Sexual selection on males and females is greatly in¯u-

enced by the relationship between the number of mating

partners and the reproductive success of males and

females (Andersson, 1994; Cunningham & Birkhead,

1998). Higher variance in reproductive success may

indicate a greater potential for sexual selection (Bate-

man, 1948; Arnold & Duvall, 1994). In socially monog-

amous species, the variance in reproductive success for

both sexes is often assumed to be low, and therefore

sexual selection is taken to be weak, but not necessarily

absent (Kirkpatrick et al., 1990). However, the genetic

mating system is more germane than the social mating

system to theory pertaining to the strength of sexual

selection.

Until recently, genetic mating systems of natural popu-

lations were empirically inaccessible, but the introduc-

tion of molecular genetic techniques has provided

important insights into the interpretation of behavioural

data, enabling us to know the actual reproductive

outcomes of observed social interactions (Avise, 1994;

Ketterson et al., 1998; Coltman et al., 1999). Such tech-

niques have become indispensable tools in the study of

parentage in nature, particularly of birds. Among the

avian species previously believed to be monogamous, the

vast majority are genetically polygamous (Birkhead &

Mùller, 1992; Avise, 1994). However, there are some

examples of true monogamy. For example, in Leach's

storm-petrel (Mauck et al., 1995), the common loon

(Piper et al., 1997), and the black vulture (Decker et al.,

1993) monogamy has been demonstrated using DNA

®ngerprinting. In addition molecular techniques have

been used to con®rm genetic monogamy in some non-

avian taxa such as the California mouse (Ribble, 1991),

Kirk's dik-dik (Brotherton et al., 1997), and the Western

Australian seahorse (Jones et al., 1998).

Keywords:

Hippocampus angustus;

H. subelongatus;

interbrood interval;

mating pattern;

monogamy;

polygyny;

seahorse;

Syngnathidae.

Abstract

Apparently monogamous animals often prove, upon genetic inspection, to

mate polygamously. Seahorse males provide care in a brood pouch. An earlier

genetic study of the Western Australian seahorse demonstrated that males

mate with only one female for each particular brood. Here we investigate

whether males remain monogamous in sequential pregnancies during a

breeding season. In a natural population we tagged males and sampled young

from two successive broods of 14 males. Microsatellite analyses of parentage

revealed that eight males re-mated with the same female, and six with a new

female. Thus, in this ®rst study to document long-term genetic monogamy in a

seahorse, we show that switches of mates still occur. Polygynous males moved

greater distances between broods, and tended to have longer interbrood

intervals, than monogamous males, suggesting substantial costs associated

with the breaking of pair bonds which may explain the high degree of social

monogamy in this ®sh genus.

Correspondence: Charlotta Kvarnemo, Department of Zoology, Stockholm

University, S-106 91 Stockholm, Sweden.

Tel.: +46-8-164046; fax: +46-8-167715;

e-mail: [email protected]

882 J . E V O L . B I O L . 1 3 ( 2 0 0 0 ) 8 8 2 ± 8 8 8 Ó 2 0 0 0 B L A C K W E L L S C I E N C E L T D

In seahorses, the male provides all postzygotic paren-

tal care by carrying the eggs on his body. Exclusive

paternal brooding is a shared feature across the ®sh

family Syngnathidae, in which care ranges from the eggs

simply being attached to the skin in the genera Entelurus

and Nerophis, to protection and osmo-regulation in the

highly specialized brood pouches of Hippocampus and

Syngnathus (Dawson, 1985). Despite ubiquitous male

pregnancy in seahorses and pipe®shes, mating patterns

and sex roles differ greatly among species. For example,

the pipe®shes N. ophidion and Syngnathus scovelli are

polyandrous, whereas polygynandry is the dominant

mating system for S. typhle and S. ¯oridae (Berglund

et al., 1988; Vincent et al., 1992; Jones & Avise, 1997a,

b; Jones et al., 1999). In addition, monogamy has been

observed in the pipe®sh genus Corythoichthys (Gronell,

1984) and possibly also in Hippichthys (Watanabe et al.,

1997). In contrast to the mating system diversity

observed in pipe®shes, seahorses invariably have been

reported to exhibit social monogamy. The formation of

pair bonds has been documented by behavioural obser-

vations of H. fuscus (Vincent, 1995), H. whitei (Vincent &

Sadler, 1995), and several other species listed in Vincent

et al. (1992) and Lourie et al. (1999). In H. fuscus

(Vincent, 1995), H. whitei (Vincent & Sadler, 1995)

and H. zosterae (Masonjones & Lewis, 1996), pairs

perform daily greetings throughout the breeding season.

Each morning they twist their tails and nod their heads

to one another, behaviours that recur in their courtship

displays.

Yet, there may be opportunities for extra-pair copula-

tion in seahorses. Although males spend most of their

time in relatively small home ranges, females move over

considerably larger areas. Thus, home ranges of both

males and females frequently overlap with several

individuals of both sexes. This pattern of home range

use, observed in H. whitei (Vincent & Sadler, 1995) and

H. subelongatus, means that an individual is often seen in

the presence of members of the opposite sex other than

its primary social mate (Kvarnemo and Moore, unpub-

lished data).

In contrast to some species of pipe®sh, which are sex

role reversed with respect to the direction of sexual

selection (Berglund et al., 1986, 1988), behavioural stud-

ies of several seahorse species suggest that males are the

predominant competitors for mates (reviewed in Vincent

et al., 1992; also Vincent, 1994a; Masonjones & Lewis,

1996; Moore, unpublished results). Vincent (1994b)

ascribed this to a male biased operational sex ratio

among unmated individuals. However, it may also

suggest that the potential reproductive rate of males

could be higher than that of females. A recent experi-

mental study of H. zosterae supports the latter hypothesis

by showing both a higher potential reproductive rate

of males than of females, and a lower paternal than

maternal energy expenditure (Masonjones, 1997;

Masonjones & Lewis, 2000). Thus, males may be prone

to sequential polygyny, particularly if that behaviour

would reduce the interbrood interval.

The present study focuses on determining whether the

genetic mating system of seahorses conforms to expec-

tations based on observed social monogamy in the

genus. An earlier genetic study of the recently renamed

Western Australian seahorse H. subelongatus (previously

H. angustus; Lourie et al., 1999) demonstrated that males

receive eggs from only one female for each particular

brood (Jones et al., 1998). Our goal was to investigate

whether males of this species are monogamous between

broods, as has been suggested by behavioural observa-

tions of related species (Vincent, 1995; Vincent & Sadler,

1995; Masonjones & Lewis, 1996). The approach was to

sample tagged males repeatedly from a natural popula-

tion and to deduce the maternity of sequential broods

using microsatellite-based markers.

Materials and methods

Our main study site, about 45 km south of Perth, was

monitored by SCUBA in 1998 from the end of January

(well after the H. subelongatus breeding season had

started) to the end of March (when the breeding season

ended). These seahorses occurred near 22 pylons of a

jetty, at a maximum depth of about 5 m, and around 46

stumps (0.1±2.0 m high) of a demolished jetty, in about

10 m of water. Other than the pylons and stumps in the

study area, there were very few sites with habitat

suitable for seahorses within 200 m in any direction.

Two additional males were sampled from a small popu-

lation near a jetty in the saline part of Swan River, at

about 4 m depth. The mean temperatures were 22.9 °C at the main site (range 21±24 °C) and 23.3 °C at the river site (range 23±24 °C).

Pregnant males were collected by SCUBA diving and

brought to the jetty for tagging, body measurements and

sampling of broods. Tagging was accomplished using

miniature tags of waterproof copy paper, with printed

numbers sealed with super glue, attached to a thin nylon

string tied loosely around the males' necks. Mobility and

behaviour were not evidently affected by the tags.

Measurements of head length (coronet to tip of the

snout), and of the head-vent length (coronet to vent),

were taken using callipers, and a ruler was used for the

tail length (vent to tail tip). For brood samples, we used a

capillary tube (whose sharp edges had been reduced

using a Bunsen burner ¯ame) attached to a small bulb.

Pregnant males have a strong muscle with which they

close the pouch. We carefully inserted the tube into the

brood pouch and removed approximately 15 embryos per

brood. From an earlier study, we know that a brood can

contain as many as 720 offspring (mean value

458 young) (Jones et al., 1998). Although a sample of

15 embryos is only a small fraction of all offspring in a

brood, such samples were suf®cient for our purposes,

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because all offspring in a brood share the same mother

(Jones et al., 1998).

We recorded at which pylon or stump each male was

collected. Males were returned to their original location

immediately after sampling. Estimates of interbrood

intervals (i.e. days between parturition of ®rst and

second brood) were based on the developmental state

of the embryos at sampling, in combination with obser-

vations from the ®eld of each male's state of pregnancy.

During each dive, we recorded the position and repro-

ductive status of any tagged males encountered. Invari-

ably, the broods and offspring of studied males developed

normally after sampling. After the sampling of a male's

second brood, a small (<2 mm 2 ) portion of the posterior

edge of his dorsal ®n was clipped for genetic analysis. We

took ®n samples only from males whose successive

broods were sampled. The cut was sterilized by iodine

solution. Fin clips and sampled embryos were stored and

transported in a buffer containing 20% DMSO, 250 mMM

EDTA (pH 7.5), and saturated NaCl.

For the microsatellite assessment, we used three

previously characterized seahorse loci (Han03, Han06,

Han15; Jones et al., 1998). A simple DNA-extraction

protocol was employed (see Jones & Avise, 1997a1 for

details) and polymerase chain reaction (PCR) was per-

formed as described by Jones et al. (1998). Polymerase

chain reaction fragments were separated on sequencing

gels and visualized using a radioactive label and

autoradiography. The fathers and approximately 10±15

embryos per clutch were assayed for all three microsat-

ellite loci (Table 1). The maternal genotype for each

clutch was determined by subtraction. We estimated

allele frequencies using adult genotypes from a previous

study (Jones et al., 1998) combined with the genotypes of

the 14 males assayed in this study.

Results

In total, 34 males were tagged, of which 17 were seen

pregnant at least twice, 15 were seen pregnant once, and

another two males remained unmated. Sequential

broods from 14 males were sampled successfully, whereas

the broods of three other males were sampled prema-

turely and did not provide useful embryos for genetic

analysis. Comparing the 17 males found pregnant twice

with the 17 males seen pregnant only once or not at all,

no difference was detected in any of the morphological

traits measured (t-tests, d.f. � 32: head length, t � 0.65, P � 0.52; head-vent length, t � 0.40, P � 0.69; tail length, t � 0.91, P � 0.37).

In all cases, the genotypes of progeny within a clutch

were consistent with a single mother, whose genotype

was reconstructed unambiguously by subtraction

(Table 1). Thus, our results are consistent with the

previous ®nding that males receive eggs from only one

female during a pregnancy (Jones et al., 1998). Assuming

linkage equilibrium and Hardy±Weinberg proportions,

the expected frequencies of the reconstructed maternal

genotypes ranged from 6.1 ´ 10)8 to 8.7 ´ 10)5, so it is extremely unlikely that eggs from two different females

that shared the same genotype would appear in our

study. Thus, if two clutches had the same maternal

genotype, we considered the eggs in both clutches to

have originated from the same female. This conclusion is

bolstered by the fact that each adult multilocus genotype

determined in this and a previous study (Jones et al.,

1998) has been unique to a single individual.

Of the 14 males from which two sequential broods

were sampled, eight proved by genetic analysis to have

re-mated with the same female (Table 1), thus clearly

indicating a capacity for sequential monogamy. In addi-

tion, two males (males 127 and 143) remained monoga-

mous over three successive broods (Table 1). However,

six males had switched mates between successive broods

(Table 1). These sequentially polygamous males did not

differ from the monogamous males in any morphological

measurement (Fig. 1) (t-tests: head length, t � 0.38, P � 0.71; head-vent length, t � 0.93, P � 0.37; tail length, t � 0.67, P � 0.51).

Males that mated with the same female in two

successive broods stayed within the original home range,

whereas males that mated with new partners had moved

signi®cantly longer distances between broods (Fig. 2)

(Mann±Whitney U-test: U � 3.0, P � 0.006). Further- more, males that mated with a new partner for the

second brood tended to have a longer interbrood interval

than those that re-mated with the same female (Fig. 3)

(t-test: t � 1.91, P � 0.08). One of the monogamous males was not included in this analysis due to an

uncertain estimate of his interbrood interval.

We found one instance in which a divorced female

re-mated with a new male. The ®rst mate of male 139

had the same genotype as the second mate of male 131

(Table 1). Male 139 was ®rst found pregnant on pylon 2

of the submerged jetty, and later on a nearby pylon

during his second pregnancy. Male 131, on the other

hand, was ®rst found pregnant on a far distant pylon. He

was later found pregnant with his second brood on pylon

2 and, later in his pregnancy, moved to another nearby

pylon. Interestingly, both males were probably near

pylon 2 when they received eggs from the shared female.

The estimated time between the two successive matings

of the female was at the most 20 days.

Discussion

This study of the Western Australian seahorse, H. sube-

longatus, is the ®rst to document successive-brood

monogamy in a seahorse using genetic techniques. We

successfully sampled the young from two sequential

broods of 14 males, eight of which proved to have mated

with the same female in both broods, while six had

re-mated with a new female. Our data strongly suggest

884 C . K V A R N E M O E T A L .

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that genetic monogamy is a common mating pattern in

this species of seahorse, and they convincingly support

previous behavioural observations of social monogamy in

other seahorse species.

Nevertheless, almost half of the males switched mates

between pregnancies. These males moved greater dis-

tances between broods, and tended to have longer

interbrood intervals, than the males that remained paired

to a single female. As there is no reason to believe that

the developmental time of the embryos would differ

between these males, the difference in interbrood inter-

val probably is a function of the time from parturition to

re-mating. These results suggest that breaking up a pair

bond may incur a substantial cost for the male in terms of

travel distance and reduced reproductive rate while

searching for a new mate. Vincent & Sadler (1995)

reported similar results for H. whitei. Further, Vincent

(1990) proposed that the formation of a new pair

increases the interspawning interval of both sexes, due

not only to the additional time spent searching for a new

mate but also to the fact that a female must hydrate her

eggs before spawning (Boisseau, 19672 , as cited on p. 16 in

Lourie et al., 1999; Vincent, 1994b). In existing pairs of

H. fuscus, the hydration is synchronized with the brood-

ing cycle of the male, but in newly formed pairs this

process takes about 2 days (Vincent, 1994b, 1995).

In this study, the prediction that seahorse males might

be prone to polygyny due to a higher potential repro-

ductive rate of males was not supported. Under this

model a male should engage in polygyny only if this

behaviour reduces his interbrood interval. The opposite

was found here, as the males that mated polygynously

Table 1 Summary of assayed males. Each

row represents the results for a single clutch

from a single male pregnancy. Shown are

the male ID and clutch number, the number

of embryos genetically assayed per clutch,

the paternal genotype (assayed directly from

a ®n clip) and the maternal genotype

(reconstructed from the progeny array).

Male ID

Number of

embryos

Paternal genotype at locus: Maternal genotype at locus:

(clutch no.) assayed Han15 Han03 Han06 Han15 Han03 Han06

Monogamous males:

117 (1) 13 289/295 248/252 163/233 295/303 206/256 163/187

117 (2) 15 295/303 206/256 163/187

121 (1) 6 257/297 252/254 157/183 297/301 254/258 163/231

121 (2) 10 297/301 254/258 163/231

122 (1) 11 261/309 246/252 163/183 285/303 206/246 163/235

122 (2) 15 285/303 206/246 163/235

127 (1) 15 261/281 256/256 163/235 257/299 248/256 183/183

127 (2) 15 257/299 248/256 183/183

127 (3) 12 257/299 248/256 183/183

128 (1) 11 285/293 206/256 205/235 271/329 252/274 163/237

128 (2) 15 271/329 252/274 163/237

137 (1) 10 287/303 206/256 187/187 303/309 248/250 163/233

137 (2) 10 303/309 248/250 163/233

143 (1) 15 297/313 252/254 163/235 289/295 206/254 163/163

143 (2) 15 289/295 206/254 163/163

143 (3) 15 289/295 206/254 163/163

146 (1) 14 273/343 206/206 163/235 303/309 206/238 163/183

146 (2) 15 303/309 206/238 163/183

Males that switched mates:

130 (1) 15 305/331 250/250 153/237 299/305 206/256 163/237

130 (2) 15 295/309 206/206 183/203

131 (1) 10 285/299 248/254 163/179 295/299 206/248 177/235

131 (2) 10 285/303 248/252 163/163

139 (1) 15 285/307 256/256 163/183 285/303 248/252 163/163

139 (2) 12 303/305 248/264 163/163

147 (1) 14 299/305 206/256 183/183 261/309 252/258 235/237

147 (2) 15 285/297 256/262 183/183

148 (1) 14 271/293 206/256 163/235 277/303 238/252 183/235

148 (2) 15 341/345 206/254 163/237

149 (1) 14 293/343 206/262 163/163 285/303 252/256 163/233

149 (2) 15 261/303 206/254 163/187

Monogamy and split pair-bonds in seahorses 885

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between broods tended to increase their interbrood

interval. Nevertheless, the possibility remains that these

males may still have bene®ted from switching mates in

terms of overall ®tness, if the second female was more

fecund or showed some other preferred trait that might

be naturally or sexually selected.

Of course, mate switching in this seahorse population

may result from either male- or female-mediated strat-

egies. Pregnant males were the major focus of this study,

so we have fewer data regarding female reproductive

behaviours and rates. In the single case for which we

have data on two successive matings by one female, the

interval between broods was at the most 20 days, hinting

that the time cost for a female to switch mates might be

lower than that for a male. In addition, this particular

female appears not to have switched mates to obtain a

more fecund partner, as her second mate (131) was

smaller than her ®rst (139). One goal of future studies of

seahorse mating systems should be to gather additional

information on female behaviour and movement pat-

terns together with genetic mating system data.

In seahorses, individuals of either sex may be pre-

disposed to switch mates as a result of constraints

imposed by male pregnancy. Indeed, substantial costs

may be associated with monogamy, as fecundity is

positively correlated with body size in both males and

females (e.g. Vincent, 1990; Masonjones, 1997). If a

female has a low fecundity relative to a male, he may

only brood with part of his capacity, or vice versa for a

female if only part of her brood would ®t into the male's

pouch. In pipe®shes, brood reduction has been docu-

mented, with males giving birth to fewer young than the

number of eggs received at mating (AhnesjoÈ , 1992), and

this seems to be the case in H. fuscus as well (Vincent,

1994b). Thus, there would be mutual bene®ts of mating

with large mates for both sexes, and hence size assorta-

tive mate choice would be predicted (Johnstone, 1997).

Other studies of seahorses have indeed found a pattern in

that direction, and more strongly so early in the breeding

season (Vincent & Sadler, 1995). Pairs that were formed

later in the season were less well matched, possibly as a

result of a paucity of available mates at that stage

Fig. 1 Body size (mean � SE) of seahorse males that mated

monogamously and polygamously in two successive broods

(HL � head length, HVL � head to vent length, TL � tail length).

Fig. 2 Distances moved (mean � SE) between two successive

broods of seahorse males that mated monogamously and polyga-

mously.

Fig. 3 Interbrood interval (mean � SE) of two successive broods of

seahorse males that mated monogamously and polygamously.

886 C . K V A R N E M O E T A L .

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(Vincent & Sadler, 1995). Hence, if such a pattern is valid

for our population as well, individuals switching mating

partners from the ®rst to the second brood would be more

likely to do this as a `best of a bad job' strategy, rather than

as an attempt to improve their reproductive rate.

A possible explanation for mate switching by either sex

is that it is done by necessity rather than choice, for

example if an individual's initial mate had for some

reason disappeared from the area. We have evidence that

one female remained in the area after her divorce,

whereas other females may have disappeared from the

area due to predation or emigration. We have no data on

risk of predation on females, but extrapolating from the

male data, the risk of predation seems to be fairly low. All

except three tagged males were re-sighted at one time or

another and most of them were seen repeatedly. Exclud-

ing three males that were tagged during the last week of

the study and, thus, were unlikely to be re-sighted, our

observations yielded a maximum predation rate of 9.7%

over the study period. Thus, predation is unlikely to

account for all our cases of split pair bonds.

Emigration of individuals from the area could be an

additional reason for seahorses to switch mates. Our

general impression from this study and other unpublished

observations (Kvarnemo and Moore) is that some new

adults do seem to move into the area during the breeding

season, indicating that some probably moved out as well.

Indeed, three of our tagged males disappeared during the

breeding season, but had returned to the area by the time

we revisited the site about 4 months later. Females, who

tend to have larger home ranges than males, are likely to

move between breeding areas as well.

One could argue that the polygynous matings gen-

etically documented were due to altered behaviours

caused by human handling. We cannot rule out this

possibility completely, but this explanation seems unlikely

because the vast majority of males remained in the study

area. Furthermore, males sampled twice evidently had

re-mated after the ®rst sampling, and, thus, were able to

mate despite the annoyance of tag application and progeny

removal. Another possibility is that the polygynous mat-

ings resulted from disturbances to females caused by our

frequent dives. However, this seems even less likely

because no females were handled during the study.

For monogamy to evolve and be sustained by natural

(or sexual) selection, the bene®ts of a long-term pair

bond must outweigh its costs. A relatively shorter

interbrood interval for a monogamous pair compared

with a polygamous pair, as documented in this study,

could provide such a bene®t to monogamy. Other

seahorse studies have found that a pair's capacity to

produce offspring increases with the time elapsed since

pair formation (Vincent, 1994b), suggesting an additional

possible advantage to long-term monogamy. In one

model, Ranta & Kaitala (1999) investigated the evolution

of polygyny vs. monogamy, and concluded that whereas

polygyny is an evolutionary stable strategy, monogamy is

expected to replace (or coexist with) polygyny only if the

latter entails some ®tness decrement. In two passerine

species of birds studied in this regard, polygynous males

raised considerably fewer nestlings than monogamous

males (Ranta & Kaitala, 1999; references therein). The

current study of seahorses may provide another example

of this phenomenon.

In conclusion, this genetic study of maternity in male

broods clearly shows that the Western Australian sea-

horse often maintains monogamous pair bonds over

successive mating episodes. This ®nding is consistent

with, and extends, behavioural studies on related sea-

horse species that have documented social monogamy

with long-term pair bonding. However, our study also

revealed that a considerable proportion of males had

mated with a new female in the second of the two broods

sampled. Thus, in this species, pair bonds were not

always as long lasting as previously supposed. These

results illustrate how genetic studies can yield unequi-

vocal data on biological parentage and thereby contribute

to a deeper understanding of the causes and consequences

of mating-system evolution.

Acknowledgments

We wish to thank Kate Lessels and Kerstin Johannesson

for valuable comments, the Swedish Research Council of

Natural Sciences, and the Knut & Alice Wallenberg

Foundation for funding CK; the Department of Zoology

at UWA for generously letting us use their diving

equipment, and for funding GIM; AGJ and JCA received

funding from the National Science Foundation, National

Institute of Health Training Grant, the Pew Foundation,

and the University of Georgia. We also wish to thank

Leah Beesley for being an unprecedented research

assistant, and Janne Kotiaho and John Hunt for cheer-

fully helping us out with additional dives.

References

AhnesjoÈ , I. 1992. Consequences of male brood care; weight and

number of newborn in a sex-role reversed pipe®sh. Funct. Ecol.

6: 274±281.

Andersson, M.B. 1994. Sexual Selection. Princeton University

Press, Princeton.

Arnold, S.J. & Duvall, D. 1994. Animal mating systems: a

synthesis based on selection theory. Am. Nat. 143: 317±348.

Avise, J.C. 1994. Three fundamental contributions of molecular

genetics to avian ecology and evolution. Ibis 138: 16±25.

Bateman, A.J. 1948. Intra-sexual selection in Drosophila. Heredity

2: 349±368.

Berglund, A., Rosenqvist, G. & Svensson, I. 1986. Mate choice,

fecundity and sexual dimorphism in two pipe®sh species

(Syngnathidae). Behav. Ecol. Sociobiol. 19: 301±307.

Berglund, A., Rosenqvist, G. & Svensson, I. 1988. Multiple

matings and paternal brood care in the pipe®sh, Syngnathus

typhle. Oikos 51: 184±188.

Monogamy and split pair-bonds in seahorses 887

J . E V O L . B I O L . 1 3 ( 2 0 0 0 ) 8 8 2 ± 8 8 8 Ó 2 0 0 0 B L A C K W E L L S C I E N C E L T D

Birkhead, T.R. & Mùller, A.P. 1992. Sperm Competition in Birds:

Evolutionary Causes and Consequences. Academic Press, London.

Brotherton, P.N.M., Pemberton, J.M., Komers, P.E. & Malarky,

G. 1997. Genetic and behavioural evidence of monogamy in a

mammal, Kirk's dik-dik (Madoqua kirkii). Proc. R. Soc. Lond. B

264: 675±681.

Coltman, D.W., Bancroft, D.R., Robertson, A., Smith, J.A.,

Clutton-Brock, T.H. & Pemberton, J.M. 1999. Male reproduc-

tive success in a promiscuous mammal: behavioural estimates

compared with genetic paternity. Mol. Ecol. 8: 1199±1209.

Cunningham, E.J.A. & Birkhead, T.R. 1998. Sex roles and sexual

selection. Anim. Behav. 56: 1311±1321.

Dawson, C.E. 1985. Indo-Paci®c Pipe®shes, Red Sea to the Americas.

The Gulf Coast Research Laboratory, Ocean Springs, Mississippi.

Decker, M.D., Parker, P.G., Minchella, D.J. & Rabenold, K.N.

1993. Monogamy in black vultures: genetic evidence from

DNA ®ngerprinting. Behav. Ecol. 4: 29±35.

Gronell, A.M. 1984. Courtship spawning and social organization

of the pipe®sh, Corythoichthys intestinalis (Pisces: Syngnathidae)

with notes on two congeneric species. Z. Tierpsychol. 65: 1±24.

Johnstone, R.A. 1997. The tactics of mutual mate choice and

competitive search. Behav. Ecol. Sociobiol. 40: 51±59.

Jones, A.G. & Avise, J.C. 1997a. Microsatellite analysis of

maternity and the mating system in the Gulf pipe®sh

Syngnathus scovelli, a species with male pregnancy and sex-

role reversal. Mol. Ecol. 6: 203±213.

Jones, A.G. & Avise, J.C. 1997b. Polygynandry in the dusky

pipe®sh Syngnathus ¯oridae revealed by microsatellite DNA

markers. Evolution 51: 1611±1622.

Jones, A.G., Kvarnemo, C., Moore, G.I., Simmons, L.W. & Avise,

J.C. 1998. Microsatellite evidence for monogamy and sex-

biased recombination in the Western Australian seahorse

Hippocampus angustus. Mol. Ecol. 7: 1497±1506.

Jones, A.G., Rosenqvist, G., Berglund, A. & Avise, J.C. 1999. The

genetic mating system of a sex-role-reversed pipe®sh (Syn-

gnathus typhle): a molecular inquiry. Behav. Ecol. Sociobiol. 46:

357±365.

Ketterson, E.D., Parker, P.G., Raouf, S.A., Nolan, V., Ziegenfus,

C. & Chandler, C.R. 1998. The relative impact of extra-pair

fertilisations on variation in male and female reproductive

success in dark-eyed juncos (Junco hyemalis). Ornithol. Monogr.

49: 81±102.

Kirkpatrick, M., Price, T. & Arnold, S.J. 1990. The Darwin-Fisher

theory of sexual selection in monogamous birds. Evolution 44:

180±193.

Lourie, S.A., Vincent, A.C.J. & Hall, H.J. 1999. Seahorses: an

identi®cation guide to the world's species and their conservation.

Project Seahorse, London.

Masonjones, H.D. 1997. Sexual Selection in the Dwarf Seahorse,

Hippocampus zosterae (Syngnathidae): an investigation into

the mechanisms determining the degree of male vs. female

intrasexual competition and intersexual choice. PhD Thesis,

Tufts University,3 Medford, MA, USA.

Masonjones, H.D. & Lewis, S.M. 1996. Courtship behavior in the

dwarf seahorse, Hippocampus zosterae. Copeia 1996: 634±640.

Masonjones, H.D. & Lewis, S.M. 2000. Differences in potential

reproductive rates of male and female seahorses related to

courtship roles. Anim. Behav. 59: 11±20.

Mauck, R.A., Waite, T.A. & Parker, P.G. 1995. Monogamy on

Leach's storm-petrel: DNA-®ngerprinting evidence. Auk 112:

473±482.

Piper, W.H., Evers, D.C., Meyer, M.W., Tischler, K.B., Kaplan,

J.D. & Fleischer, R.C. 1997. Genetic monogamy in the

common loon. Behav. Ecol. Sociobiol. 41: 25±31.

Ranta, E. & Kaitala, V. 1999. Punishment of polygyny. Proc. R.

Soc. Lond. B 266: 2337±2341.

Ribble, D.O. 1991. The monogamous mating system of Peromys-

cus californicus as revealed by DNA ®ngerprinting. Behav. Ecol.

Sociobiol. 29: 161±166.

Vincent, A.C.J. 1990. Reproductive Ecology of Seahorses. PhD

Thesis. University of Cambridge, Cambridge.

Vincent, A.C.J. 1994a. Seahorses exhibit conventional sex roles

in mating competition, despite male pregnancy. Behaviour

128: 135±151.

Vincent, A.C.J. 1994b. Operational sex ratios in seahorses.

Behaviour 128: 153±167.

Vincent, A.C.J. 1995. A role for daily greetings in maintaining

seahorse pair bonds. Anim. Behav. 49: 258±260.

Vincent, A., AhnesjoÈ , I., Berglund, A. & Rosenqvist, G. 1992.

Pipe®shes and seahorses: are they all sex role reversed? Trends

Ecol. Evol. 7: 237±241.

Vincent, A.J.C. & Sadler, L.M. 1995. Faithful pair bonds in wild

seahorses, Hippocampus whitei. Anim. Behav. 50: 1557±1569.

Watanabe, S., Watanabe, Y. & Okiyama, M. 1997. Monogamous

mating and conventional sex roles in Hippichthys penicillus

(Syngnathidae) under laboratory conditions. Ichthyol. Res. 44:

306±310.

Received 12 April 2000; revised 16 June 2000; accepted 4 August 2000

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