BACKGROUND
males often rely on elaborate sexual displays to attract females.
such displays can reveal important information about the quality or motivation of the signaller  <cit> .
courtship displays are therefore commonly used by females as cues when selecting a mate and, in so doing, affect the regime of sexual selection  <cit> .
considerable research attention has been given to understanding among-male variation in signal expression due to its potential in influencing male mating opportunities and reproductive success.
less well studied - but just as important - is the variation in signal intensity that can occur within individuals  <cit>  due to behavioural plasticity .
such variation can arise, for example, in response to life-history trade-offs between present and future signalling effort, as in drosophila  <cit>  and three-spined sticklebacks, gasterosteus aculeatus  <cit> , or between different components of male reproductive investment , as in collared flycatchers, ficedula albicollis  <cit> .
males might also be expected to adjust their courtship effort strategically to maximise their reproductive payoffs, especially if mating costs are high, if females vary greatly in reproductive value, and if there is a good chance of attracting high quality mates in the future  <cit> .
in general, both among- and within-individual variation in courtship intensity can affect the evolutionary potential of sexual selection by contributing to variance in reproductive success  <cit> .
one useful approach for investigating sexual displays at the population level is to measure the display intensity of several individuals more than once  <cit> .
such data can then be used to estimate the fraction of variation in display behaviour that is due to differences among individuals, that is, the 'repeatability' of the behaviour  <cit> .
repeatability has been widely used to understand evolutionary processes , with high repeatability values indicating high consistency within, and large differences among, individuals  <cit> .
low values, by contrast, would indicate the opposite pattern.
quite remarkably, while many studies of sexual selection have assessed repeatability of male courtship effort  <cit> , very few have dissected different sources of variation in male courtship and compared them directly, despite the potential value of such comparisons .
the desert goby, chlamydogobius eremius, is a freshwater fish endemic to the lake eyre drainage basin in central australia.
this small , sexually-dimorphic species  is locally abundant throughout its range where it inhabits both permanent and temporary bodies of water, from spring-fed pools to ephemeral desert streams.
male desert gobies establish nests under rock crevices and rely on conspicuous displays to attract passing females for mating.
the courtship displays involve the raising of the colourful dorsal and anal fins coupled with occasional jerky body movements .
previously, males were shown to adjust their courtship effort strategically by courting larger females more intensely  <cit> .
the aim of the current study was to simultaneously assess potential sources of individual variation in male courtship effort.
specifically, if male courtship is to be a potentially useful sexual signal, we predict that, for a given point in time, variation in signalling effort observed among males should be greater relative to the variation attributed to female identity, an important source of within-male variation  <cit> .
if, in turn, differences among males in courtship were small compared to each male's tendency to adjust his courtship effort, or relative to short term within-male variability due to other reasons, male courtship effort would be less reliable as a signal to females.
we would also expect courtship effort to be repeatable .
accordingly, we designed an experiment that could compare the different sources of variation in male courtship  directly, as well as assess the repeatability of courtship effort .
methods
fish collection and housing
experimental trials took place from january to may  <dig>  desert gobies were collected as juveniles from waterholes and springs west of lake eyre in south australia using dip and seine nets.
during a two-day surface transport to the laboratory, fish were kept in insulated 50-litre plastic tubs , filled with water to a depth of  <dig> cm and provided with constant motorised aeration.
back in the laboratory, fish were housed in  <dig> -  <dig> l aquaria and were separated by sex after maturation.
aquaria were maintained at a temperature of  <dig> - 26°c, a salinity of  <dig> - 7‰ and on a  <dig> h light:dark cycle.
all fish were fed  <dig> -  <dig> times a day ad libitum on a diet of commercially prepared pellets and frozen brine shrimp .
after completion of the study, fish were retained as stock for unrelated research.
measurement of courtship effort and fish size
male desert gobies were introduced into individual compartments  containing a halved clay flowerpot as a nest .
the entrance of the nest was positioned to face a small , adjoining female compartment separated by a clear perspex divider .
the bottom of both compartments was covered with a  <dig> cm layer of sand.
after the male  was introduced into his compartment, he was given three days to acclimate before we introduced a stimulus female  into the adjacent compartment.
to standardise female reproductive state, only ripe females were used, as determined by their distended bellies.
data on fish behaviour was collected following previously published methods  <cit> .
tanks were brightly lit from above, with the observer seated in the dark away from the tank to prevent disturbance to the fish.
three minutes after the introduction of the stimulus female, we measured the amount of courtship directed by the male towards the female.
this was achieved by conducting spot samples every  <dig> seconds over a  <dig> minute period.
during each spot sample, a male was recorded as courting the female if he was within  <dig> cm of her compartment, with his body oriented unambiguously towards her whilst engaged in courtship behaviour   <cit> .
at the end of the trial, we tallied the total number of times the male was courting the female as a measure of his signalling effort.
male courtship effort was measured over two subsequent days so that two different males were each presented with the same pair of stimulus females.
this combination of four fish is hereafter referred to as an experimental unit .
during the first day, both males were tested once with each of the two stimulus females within their experimental unit.
hence, during the first day, each fish performed in two trials.
there was at least  <dig> minutes between each trial and the order of trials was randomised.
the same procedure  was repeated on the following day.
our experimental design provides more detailed information than the coefficient of intra-class variation  alone and, importantly, allows testing of multiple hypotheses within a single experimental set up.
specifically, in order to estimate the variance components of male courtship effort , we analysed the courtship data with linear mixed effects models, using the 'lme4' package in the r  <dig> .
<dig> software  <cit> .
a model was fitted using day  as a fixed factor and female id and male id as random factors .
the statistical significance of random and fixed effects was assessed by comparing a full model with reduced models not containing the factor of interest, using log-likelihood tests  <cit> .
when comparing models differing in their fixed factors, maximum likelihood was used to fit the models.
in all other cases, restricted maximum likelihood was used.
total lengths were measured by photographing each fish after the experiment in a container with  <dig> cm of water and a piece of grid paper as a scale.
we used an olympus c- <dig> digital camera for photographs and image tool  <dig>   software for the image analysis.
the size difference between the two females within each experimental unit ranged from zero to moderate .
similarly, large size differences between each of the two males were avoided .
we tested whether differences in body size, as measured over the whole data-set, predicted patterns in male courtship effort by regressions of courtship effort as a function of body size, both for females and males.
one male did not engage in any courtship behaviour and remained inactive over the whole period of the experiment, and was therefore excluded from analyses.
repeatability of courtship effort
in order to get an estimate for repeatability of male courtship effort , we analysed the total amount of courtship each male performed during days  <dig> and  <dig>  following the procedure of becker  <cit> .
specifically, a one-way anova was used to partition the total variance of courtship intensity into the variance among males  and the variance within males .
the ratio of the variance among males  to the total phenotypic variance  then gives an estimate of repeatability, also called 'the coefficient of intra-class correlation' sensu lessells & boag  <cit> .
ethical note
this study complies with all the relevant federal and state laws of australia, adheres to the asab/abs guidelines for the use of animals in research, and was conducted under ethics permit 'bsci/2007/12' from the biological sciences animal ethics committee of monash university.
RESULTS
the linear mixed effects model revealed a significant effect of 'day' on male courtship .
there was also a significant contribution of male id  and of female id  to the fit of the model.
'male id' explained  <dig> % of the total variance in male courtship, while 'female id'  explained  <dig> % and 'day' explained  <dig> %.
the remaining  <dig> % was left unexplained by the model .
the Δaic is the difference in aic values compared to the estimated best model .
there was no significant relationship between the size of males and their total courtship effort as expressed over the two days .
similarly, there was no significant relationship between the size of females and the amount of courtship that males directed toward them .
male courtship effort, averaged towards the two females, was highly and significantly repeatable .
discussion
our results show that desert goby males exhibit considerable among-individual variation in courtship effort.
moreover, over a short time frame , these individual differences were also highly consistent.
this is important because such consistency provides a potential for female choice  to work on traits that affect courtship rate  <cit> .
in this regard, both the level of heritable variation and the degree of condition dependence are potentially relevant  <cit> .
substantial individual variation in male courtship effort seems to be widespread among taxa  although this variation is usually not as pronounced as what the current study has shown for desert gobies .
the extensive variation among desert goby males is unlikely to be due to differences in body condition  as all males were fed ad libitum.
moreover, we did not find any effect of male body size on courtship.
within-male differences in the expression of sexual signals can also be important.
for example, previous studies have found that environmental factors  <cit>  and variation in female reproductive state  <cit>  can be important sources of within-male variation in male signalling intensity.
indeed, in desert gobies, males are known to adjust their courtship effort strategically towards females of different reproductive value .
in the current study, however, controlled laboratory conditions minimised environmental variation, while the experimental design controlled for order effects.
we also deliberately avoided large size differences between the two females seen by each male, as well as variation in their reproductive state .
consequently, female size differences did not predict male courtship effort.
we nevertheless found a significant effect of female identity on the level of male courtship effort, indicating that the males nonetheless perceived some difference between the females.
these between-female differences could, for example, be related to colouration or behaviour , and provide an interesting avenue for future research.
indeed, female identity, as a source of within-individual variation in male courtship effort, may be especially important because plasticity in the expression of sexual signals can influence both signal reliability  <cit>  and the power of sexual selection  <cit> .
even though desert goby males are capable of strategically adjusting their courtship , the current results indicate that courtship effort of a male is consistent when he experiences a similar situation on two adjacent days.
indeed, the differences among males were more pronounced than short term variation within males .
the consistency in male courtship effort was also demonstrated by a high estimate for its repeatability .
thus, even though a male desert goby can strategically adjust its courtship effort, his displays were highly consistent when re-encountering the same females.
if among-male differences in courtship are more important than variation within males, females have the potential to reliably assess males based on courtship intensity.
indeed, in many species, female mating decisions seem to be biased towards males that exhibit high rates of courtship  <cit> , and among-male differences in courtship effort often translate into differences in male mating success.
however, in species where female traits affect male courtship intensity, we might expect courtship to be less informative of male attributes  than in systems where courtship displays are less flexible.
furthermore, even when courtship effort plays a role in mate choice, the relationship between courtship intensity and female preference is not always clear-cut  <cit> .
females, for instance, could be using a suite of different cues in mate choice  <cit>  or they may be inconsistent in their choice of potential mating partners  <cit> .
for example, in the sand goby, a species with a mating system very similar to that of desert gobies, females exhibit individually varying mate preferences  <cit>  that are dependent on social context  <cit>  and also fluctuate over time  <cit> .
CONCLUSIONS
we found that among-male differences in courtship effort in desert gobies were more pronounced than differences due to female identity or other within-male sources.
thus, males were consistent in their investment in courtship displays when confronted with the same two females on adjacent days.
the consistent among-male differences also resulted in a high repeatability estimate for courtship effort.
females, therefore, have the potential to reliably assess males based on intensity of their courtship.
our study underscores the importance of paying attention to each of the different sources of variation in the expression of male sexual displays, and we offer a robust experimental design for simultaneously estimating these parameters.
future investigations should consider the importance of male courtship for female mate choice relative to other sexually-selected traits that may also be important in guiding female mating decisions.
authors' contributions
tkl designed the study, conducted the experiment, participated in data analyses and wrote the bulk of the manuscript.
pas conducted the image measurements, performed most of the data analyses and participated in writing of the manuscript.
bbmw guided the project, provided ideas throughout the study and contributed to writing of the manuscript.
all authors read and approved the final manuscript.
supplementary material
additional file 1
desert goby courtship behaviour.
video footage on courtship displays by a male desert goby .
click here for file
acknowledgements
we thank s. hamilton-brown for logistical support, and anonymous referees for helpful comments.
this study was supported by funding from the australian research council, nature conservation south australia, the academy of finland, and the department of biology at the university of turku.

 males often rely on elaborate sexual displays to attract females. such displays can reveal important information about the quality or motivation of the signaller. courtship displays are therefore commonly used by females as cues when selecting a mate and, in so doing, affect the regime of sexual selection. considerable research attention has been given to understanding among-male variation in signal expression due to its potential in influencing male mating opportunities and reproductive success. less well studied - but just as important - is the variation in signal intensity that can occur within individuals due to behavioural plasticity. such variation can arise, for example, in response to life-history trade-offs between present and future signalling effort, as in drosophila and three-spined sticklebacks, gasterosteus aculeatus, or between different components of male reproductive investment, as in collared flycatchers, ficedula albicollis. males might also be expected to adjust their courtship effort strategically to maximise their reproductive payoffs, especially if mating costs are high, if females vary greatly in reproductive value, and if there is a good chance of attracting high quality mates in the future. in general, both among- and within-individual variation in courtship intensity can affect the evolutionary potential of sexual selection by contributing to variance in reproductive success. one useful approach for investigating sexual displays at the population level is to measure the display intensity of several individuals more than once. such data can then be used to estimate the fraction of variation in display behaviour that is due to differences among individuals, that is, the'repeatability' of the behaviour. repeatability has been widely used to understand evolutionary processes, with high repeatability values indicating high consistency within, and large differences among, individuals. low values, by contrast, would indicate the opposite pattern. quite remarkably, while many studies of sexual selection have assessed repeatability of male courtship effort, very few have dissected different sources of variation in male courtship and compared them directly, despite the potential value of such comparisons. the desert goby, chlamydogobius eremius, is a freshwater fish endemic to the lake eyre drainage basin in central australia. this small, sexually-dimorphic species is locally abundant throughout its range where it inhabits both permanent and temporary bodies of water, from spring-fed pools to ephemeral desert streams. male desert gobies establish nests under rock crevices and rely on conspicuous displays to attract passing females for mating. the courtship displays involve the raising of the colourful dorsal and anal fins coupled with occasional jerky body movements. previously, males were shown to adjust their courtship effort strategically by courting larger females more intensely. the aim of the current study was to simultaneously assess potential sources of individual variation in male courtship effort. specifically, if male courtship is to be a potentially useful sexual signal, we predict that, for a given point in time, variation in signalling effort observed among males should be greater relative to the variation attributed to female identity, an important source of within-male variation. if, in turn, differences among males in courtship were small compared to each male's tendency to adjust his courtship effort, or relative to short term within-male variability due to other reasons, male courtship effort would be less reliable as a signal to females. we would also expect courtship effort to be repeatable. accordingly, we designed an experiment that could compare the different sources of variation in male courtship directly, as well as assess the repeatability of courtship effort. methods fish collection and housing experimental trials took place from january to may desert gobies were collected as juveniles from waterholes and springs west of lake eyre in south australia using dip and seine nets. during a two-day surface transport to the laboratory, fish were kept in insulated 50-litre plastic tubs, filled with water to a depth of cm and provided with constant motorised aeration. back in the laboratory, fish were housed in - l aquaria and were separated by sex after maturation. aquaria were maintained at a temperature of - 26°c, a salinity of - 7‰ and on a h light:dark cycle. all fish were fed - times a day ad libitum on a diet of commercially prepared pellets and frozen brine shrimp. after completion of the study, fish were retained as stock for unrelated research. measurement of courtship effort and fish size male desert gobies were introduced into individual compartments containing a halved clay flowerpot as a nest. the entrance of the nest was positioned to face a small, adjoining female compartment separated by a clear perspex divider. the bottom of both compartments was covered with a cm layer of sand. after the male was introduced into his compartment, he was given three days to acclimate before we introduced a stimulus female into the adjacent compartment. to standardise female reproductive state, only ripe females were used, as determined by their distended bellies. data on fish behaviour was collected following previously published methods. tanks were brightly lit from above, with the observer seated in the dark away from the tank to prevent disturbance to the fish. three minutes after the introduction of the stimulus female, we measured the amount of courtship directed by the male towards the female. this was achieved by conducting spot samples every seconds over a minute period. during each spot sample, a male was recorded as courting the female if he was within cm of her compartment, with his body oriented unambiguously towards her whilst engaged in courtship behaviour. at the end of the trial, we tallied the total number of times the male was courting the female as a measure of his signalling effort. male courtship effort was measured over two subsequent days so that two different males were each presented with the same pair of stimulus females. this combination of four fish is hereafter referred to as an experimental unit. during the first day, both males were tested once with each of the two stimulus females within their experimental unit. hence, during the first day, each fish performed in two trials. there was at least minutes between each trial and the order of trials was randomised. the same procedure was repeated on the following day. our experimental design provides more detailed information than the coefficient of intra-class variation alone and, importantly, allows testing of multiple hypotheses within a single experimental set up. specifically, in order to estimate the variance components of male courtship effort, we analysed the courtship data with linear mixed effects models, using the'lme4' package in the r. software. a model was fitted using day as a fixed factor and female id and male id as random factors. the statistical significance of random and fixed effects was assessed by comparing a full model with reduced models not containing the factor of interest, using log-likelihood tests. when comparing models differing in their fixed factors, maximum likelihood was used to fit the models. in all other cases, restricted maximum likelihood was used. total lengths were measured by photographing each fish after the experiment in a container with cm of water and a piece of grid paper as a scale. we used an olympus c- digital camera for photographs and image tool software for the image analysis. the size difference between the two females within each experimental unit ranged from zero to moderate. similarly, large size differences between each of the two males were avoided. we tested whether differences in body size, as measured over the whole data-set, predicted patterns in male courtship effort by regressions of courtship effort as a function of body size, both for females and males. one male did not engage in any courtship behaviour and remained inactive over the whole period of the experiment, and was therefore excluded from analyses. repeatability of courtship effort in order to get an estimate for repeatability of male courtship effort, we analysed the total amount of courtship each male performed during days and following the procedure of becker. specifically, a one-way anova was used to partition the total variance of courtship intensity into the variance among males and the variance within males. the ratio of the variance among males to the total phenotypic variance then gives an estimate of repeatability, also called'the coefficient of intra-class correlation' sensu lessells & boag. ethical note this study complies with all the relevant federal and state laws of australia, adheres to the asab/abs guidelines for the use of animals in research, and was conducted under ethics permit'bsci/2007/12' from the biological sciences animal ethics committee of monash university.  the linear mixed effects model revealed a significant effect of'day' on male courtship. there was also a significant contribution of male id and of female id to the fit of the model.'male id' explained % of the total variance in male courtship, while'female id' explained % and'day' explained %. the remaining % was left unexplained by the model. the Δaic is the difference in aic values compared to the estimated best model. there was no significant relationship between the size of males and their total courtship effort as expressed over the two days. similarly, there was no significant relationship between the size of females and the amount of courtship that males directed toward them. male courtship effort, averaged towards the two females, was highly and significantly repeatable. discussion our results show that desert goby males exhibit considerable among-individual variation in courtship effort. moreover, over a short time frame, these individual differences were also highly consistent. this is important because such consistency provides a potential for female choice to work on traits that affect courtship rate. in this regard, both the level of heritable variation and the degree of condition dependence are potentially relevant. substantial individual variation in male courtship effort seems to be widespread among taxa although this variation is usually not as pronounced as what the current study has shown for desert gobies. the extensive variation among desert goby males is unlikely to be due to differences in body condition as all males were fed ad libitum. moreover, we did not find any effect of male body size on courtship. within-male differences in the expression of sexual signals can also be important. for example, previous studies have found that environmental factors and variation in female reproductive state can be important sources of within-male variation in male signalling intensity. indeed, in desert gobies, males are known to adjust their courtship effort strategically towards females of different reproductive value. in the current study, however, controlled laboratory conditions minimised environmental variation, while the experimental design controlled for order effects. we also deliberately avoided large size differences between the two females seen by each male, as well as variation in their reproductive state. consequently, female size differences did not predict male courtship effort. we nevertheless found a significant effect of female identity on the level of male courtship effort, indicating that the males nonetheless perceived some difference between the females. these between-female differences could, for example, be related to colouration or behaviour, and provide an interesting avenue for future research. indeed, female identity, as a source of within-individual variation in male courtship effort, may be especially important because plasticity in the expression of sexual signals can influence both signal reliability and the power of sexual selection. even though desert goby males are capable of strategically adjusting their courtship, the current results indicate that courtship effort of a male is consistent when he experiences a similar situation on two adjacent days. indeed, the differences among males were more pronounced than short term variation within males. the consistency in male courtship effort was also demonstrated by a high estimate for its repeatability. thus, even though a male desert goby can strategically adjust its courtship effort, his displays were highly consistent when re-encountering the same females. if among-male differences in courtship are more important than variation within males, females have the potential to reliably assess males based on courtship intensity. indeed, in many species, female mating decisions seem to be biased towards males that exhibit high rates of courtship, and among-male differences in courtship effort often translate into differences in male mating success. however, in species where female traits affect male courtship intensity, we might expect courtship to be less informative of male attributes than in systems where courtship displays are less flexible. furthermore, even when courtship effort plays a role in mate choice, the relationship between courtship intensity and female preference is not always clear-cut. females, for instance, could be using a suite of different cues in mate choice or they may be inconsistent in their choice of potential mating partners. for example, in the sand goby, a species with a mating system very similar to that of desert gobies, females exhibit individually varying mate preferences that are dependent on social context and also fluctuate over time.  we found that among-male differences in courtship effort in desert gobies were more pronounced than differences due to female identity or other within-male sources. thus, males were consistent in their investment in courtship displays when confronted with the same two females on adjacent days. the consistent among-male differences also resulted in a high repeatability estimate for courtship effort. females, therefore, have the potential to reliably assess males based on intensity of their courtship. our study underscores the importance of paying attention to each of the different sources of variation in the expression of male sexual displays, and we offer a robust experimental design for simultaneously estimating these parameters. future investigations should consider the importance of male courtship for female mate choice relative to other sexually-selected traits that may also be important in guiding female mating decisions. authors' contributions tkl designed the study, conducted the experiment, participated in data analyses and wrote the bulk of the manuscript. pas conducted the image measurements, performed most of the data analyses and participated in writing of the manuscript. bbmw guided the project, provided ideas throughout the study and contributed to writing of the manuscript. all authors read and approved the final manuscript. supplementary material additional file 1 desert goby courtship behaviour. video footage on courtship displays by a male desert goby. click here for file acknowledgements we thank s. hamilton-brown for logistical support, and anonymous referees for helpful comments. this study was supported by funding from the australian research council, nature conservation south australia, the academy of finland, and the department of biology at the university of turku.
accordingly, we set out to dissect sources of variation in male courtship effort in a fish, the desert goby. male sexual displays play an important role in sexual selection by affecting reproductive success. however, for such displays to be useful for female mate choice, courtship should vary more among than within individual males. in this regard, a potentially important source of within male variation is adjustment of male courtship effort in response to female traits. in addition, male courtship effort towards a pair of females was highly repeatable over a short time frame. despite the plasticity in male courtship effort, courtship displays had the potential to reliably convey information about the male to mate-searching females. although males adjusted their courtship depending on the identity of the female, among-male differences were considerably greater. our experiment therefore underscores the importance of addressing the different sources contributing to variation in the expression of sexually-selected traits.
 male sexual displays play an important role in sexual selection by affecting reproductive success. however, for such displays to be useful for female mate choice, courtship should vary more among than within individual males. in this regard, a potentially important source of within male variation is adjustment of male courtship effort in response to female traits. accordingly, we set out to dissect sources of variation in male courtship effort in a fish, the desert goby. we did so by designing an experiment that allowed simultaneous estimation of within and between male variation in courtship, while also assessing the importance of the males and females as sources of courtship variation.  although males adjusted their courtship depending on the identity of the female, among-male differences were considerably greater. in addition, male courtship effort towards a pair of females was highly repeatable over a short time frame.  despite the plasticity in male courtship effort, courtship displays had the potential to reliably convey information about the male to mate-searching females. our experiment therefore underscores the importance of addressing the different sources contributing to variation in the expression of sexually-selected traits. 