BACKGROUND
sexual selection can be used to explain the evolution of various male secondary sexual characters which are used as courtship signals  <cit> .
a number of experimental studies provide evidence that females prefer males with conspicuous sexual traits  <cit> .
females may benefit from choosing males with strong signals directly by reducing their own predation risk  <cit>  or by providing better parental care to their offspring  <cit> .
females also may acquire indirect benefit by transmitting high genotypic quality of males to their offspring  <cit> .
the preference for the existence of the signal and the preference for signal intensity are often regarded to have the same origin  <cit> .
however, the preferences for a signal per se and for intensity of the signal may have different origins when the two components serve different functions.
no studies to date have tried to separate the origins of preferences for these two components of courtship signals.
fiddler crabs of the genus uca are semi-terrestrial animals that live on intertidal mud or sand flats  <cit> .
courting males of approximately  <dig> species sometimes build various kinds of mud or sand structures at their burrows such as hoods, pillars, and lips  <cit> .
it has been demonstrated that the mud pillars in u. beebei  <cit>  and the sand hoods in u. musica  <cit>  function as sexual signals in attracting females.
the sensory trap hypothesis has been suggested to explain the female preference for particular courtship structures  <cit> .
it was recently found that the preference for hoods in u. terpsichores  <cit>  and for pillars in u. beebei  <cit>  could increase with the perceived predation risk, because the structures provided a direct survival benefit to females by allowing them to escape predation.
however, female u. terpsichores did not show a directional preference for exaggerated artificial hoods which were conspicuously larger than average-sized natural hoods  <cit> .
nevertheless, females may prefer larger structures within natural variation and this possibility has not yet been explored.
here we tested if both the preference for a courtship structure and the preference for the size of the structure in the fiddler crab uca lactea were shaped by the need of females to escape predation.
males of u. lactea build semidomes using mud at their burrows and wave their large claws to attract females for mating.
previous studies suggest that semidome building in u. lactea is related to courtship signaling  <cit>  and the semidome has a function to attract females  <cit> .
given the evidence of previous studies, females may prefer larger semidomes over smaller ones if the courtship structure could provide a greater survival benefit to them.
our first objective in this study is to determine if female u. lactea prefer males with larger sized courtship structures.
if male attractiveness increases with the semidome size, we can predict that males with larger semidomes would mate more successfully than those with smaller semidomes.
the second objective of this study is to examine if the preference for structures and preference for structure size have the same origin to function in predation avoidance  <cit> .
to test this, we conducted an arena experiment in the non-courtship context for females to choose burrows with different-sized semidomes in the presence and absence of predation cue.
a previous pilot study found that female u. lactea without predator threat preferentially moved to burrows with semidomes during reproductive periods, but did not show any orientation bias during non-reproductive periods  <cit> .
thus, we conducted an experiment with and without a predator cue during both reproductive and non-reproductive periods to determine if the reproductive cycle influences female responses to male courtship structures.
RESULTS
female preference in the context of mate choice
we observed  <dig> interactions between  <dig> females and semidome-building males with full-sized semidomes, half-sized semidomes and without semidomes .
on average each female visited  <dig>  ±  <dig>  males and passed  <dig>  ±  <dig>  males before choosing one of them as a potential mate.
generalized linear mixed model  analyses revealed a significant difference in attractiveness among semidomes of different size .
the visiting frequency of mate-searching females showed a linearly increasing trend with semidome size  in the chi-square test.
females preferred males with semidomes of any size to males without semidomes .
also, males with full-sized semidomes were significantly more attractive than males with half-sized semidomes .
the effect of predation risk on female choice for different sized-semidomes
an ordinal logistic regression test revealed that predation risk is a significant factor which influences the female choice on semidome size .
however, there was no marked effect of reproductive cycle  on female choice  in the ordinal regression test.
the effect of interaction between predation pressure and reproductive cycle was not significant either .
under high predation risk, female choices did not differ significantly between reproductive periods and non-reproductive periods .
under low predation risk, however, there was significant difference between two periods .
more females moved to burrows with full-sized semidomes than to burrows with half-sized semidomes or without semdiomes in the presence of a mock predator .
in the absence of a mock predator, however, females showed different orientation biases varying with the reproductive cycle : females during reproductive periods preferred to move to burrows with semidomes rather than to burrows without semidomes , but did not discriminate between the sizes of semidomes ; during non-reproductive periods, females did not show any orientation bias to different semidome treatments .
discussion
our study provides the experimental evidence that sexual attractiveness of male fiddler crabs increases with size of the courtship structures within natural variation.
compared to males with half-sized semidomes and without semidomes, those with full-sized semidomes were more attractive to mate-searching females.
males with half-sized semidomes were more attractive to females than those without semidomes.
the directional preference for the size of structures suggests that there is a positive correlation between male attractiveness and semidome size within the normal range of variation.
why do females prefer males with larger structures?
given that females in the non-courtship context showed a directional orientation bias for larger semidomes under predation threat regardless of the reproductive cycle, larger semidomes may be more salient visual cues to provide protection to females.
the consistency between female choice in the mate-searching context and female orientation response in the predation context  <cit>  indicates that the preference for the larger signal may have originated from the female bias to escape predation.
in other words, mate choice is linked to male-built structures that can serve as a landmark to keep females safe against predators.
characteristics of the fiddler crab's visual system may also explain why females prefer larger semidomes.
the horizontal resolving power of fiddler crabs' eyes is poor compared to their vertical resolving power  <cit> .
accordingly, fiddler crabs are better able to resolve objects in the vertical than in the horizontal plain.
semidomes which are tall enough to be imaged within the zone of the acute vertical resolution of their eyes may be most conspicuous to fiddler crab females  <cit> .
in the absence of predator cues, however, females responded differently to different-sized semidomes depending on the reproductive cycle.
during the non-reproductive periods, females did not show any difference in the preference for burrows with different treatments.
in contrast, during the reproductive periods, they showed different responses.
although females did not more preferentially move to full-sized structures than half-sized structures, they did prefer moving to burrows with structures over moving to burrows without structures.
this result presents a striking contrast to the case of u. musica  where females have an orientation preference to structures, even without predator cue  <cit> .
therefore, the predator-induced sensory trap hypothesis  <cit>  cannot fully explain the female preference on the structure building in u. lactea, as the effect of large structures decreased in the absence of a predator.
rather, structure building itself may have been selected by female choice independent of predation pressure.
this female preference for structures may have started because fiddler crab courtship structures may signal male nutritional condition.
in previous studies in which males were supplemented with food, it was shown that structure building depends on male condition  <cit> .
the semidome building behavior may signal phenotypic condition of a male crab, which could be affected by food availability.
females may choose better males using semidomes as an indicator of genetic quality.
later, the predation pressure might enhance the selection on larger structures, which may help reduce predation risk for female crabs.
CONCLUSIONS
in conclusion, we have decoupled the origins of the preference on the sexual signal and signal intensity by manipulating predation risk.
the preference for semidomes in u. lactea may have been selected by female choice not related with predation pressure.
however, the preference for increased semidome size could have been strengthened to reduce the predation risk for females.
our results suggest that sexual signal per se and signal intensity is not necessarily selected by the same selection pressure.

 sexual selection can be used to explain the evolution of various male secondary sexual characters which are used as courtship signals. a number of experimental studies provide evidence that females prefer males with conspicuous sexual traits. females may benefit from choosing males with strong signals directly by reducing their own predation risk or by providing better parental care to their offspring. females also may acquire indirect benefit by transmitting high genotypic quality of males to their offspring. the preference for the existence of the signal and the preference for signal intensity are often regarded to have the same origin. however, the preferences for a signal per se and for intensity of the signal may have different origins when the two components serve different functions. no studies to date have tried to separate the origins of preferences for these two components of courtship signals. fiddler crabs of the genus uca are semi-terrestrial animals that live on intertidal mud or sand flats. courting males of approximately species sometimes build various kinds of mud or sand structures at their burrows such as hoods, pillars, and lips. it has been demonstrated that the mud pillars in u. beebei and the sand hoods in u. musica function as sexual signals in attracting females. the sensory trap hypothesis has been suggested to explain the female preference for particular courtship structures. it was recently found that the preference for hoods in u. terpsichores and for pillars in u. beebei could increase with the perceived predation risk, because the structures provided a direct survival benefit to females by allowing them to escape predation. however, female u. terpsichores did not show a directional preference for exaggerated artificial hoods which were conspicuously larger than average-sized natural hoods. nevertheless, females may prefer larger structures within natural variation and this possibility has not yet been explored. here we tested if both the preference for a courtship structure and the preference for the size of the structure in the fiddler crab uca lactea were shaped by the need of females to escape predation. males of u. lactea build semidomes using mud at their burrows and wave their large claws to attract females for mating. previous studies suggest that semidome building in u. lactea is related to courtship signaling and the semidome has a function to attract females. given the evidence of previous studies, females may prefer larger semidomes over smaller ones if the courtship structure could provide a greater survival benefit to them. our first objective in this study is to determine if female u. lactea prefer males with larger sized courtship structures. if male attractiveness increases with the semidome size, we can predict that males with larger semidomes would mate more successfully than those with smaller semidomes. the second objective of this study is to examine if the preference for structures and preference for structure size have the same origin to function in predation avoidance. to test this, we conducted an arena experiment in the non-courtship context for females to choose burrows with different-sized semidomes in the presence and absence of predation cue. a previous pilot study found that female u. lactea without predator threat preferentially moved to burrows with semidomes during reproductive periods, but did not show any orientation bias during non-reproductive periods. thus, we conducted an experiment with and without a predator cue during both reproductive and non-reproductive periods to determine if the reproductive cycle influences female responses to male courtship structures.  female preference in the context of mate choice we observed interactions between females and semidome-building males with full-sized semidomes, half-sized semidomes and without semidomes. on average each female visited ± males and passed ± males before choosing one of them as a potential mate. generalized linear mixed model analyses revealed a significant difference in attractiveness among semidomes of different size. the visiting frequency of mate-searching females showed a linearly increasing trend with semidome size in the chi-square test. females preferred males with semidomes of any size to males without semidomes. also, males with full-sized semidomes were significantly more attractive than males with half-sized semidomes. the effect of predation risk on female choice for different sized-semidomes an ordinal logistic regression test revealed that predation risk is a significant factor which influences the female choice on semidome size. however, there was no marked effect of reproductive cycle on female choice in the ordinal regression test. the effect of interaction between predation pressure and reproductive cycle was not significant either. under high predation risk, female choices did not differ significantly between reproductive periods and non-reproductive periods. under low predation risk, however, there was significant difference between two periods. more females moved to burrows with full-sized semidomes than to burrows with half-sized semidomes or without semdiomes in the presence of a mock predator. in the absence of a mock predator, however, females showed different orientation biases varying with the reproductive cycle: females during reproductive periods preferred to move to burrows with semidomes rather than to burrows without semidomes, but did not discriminate between the sizes of semidomes; during non-reproductive periods, females did not show any orientation bias to different semidome treatments. discussion our study provides the experimental evidence that sexual attractiveness of male fiddler crabs increases with size of the courtship structures within natural variation. compared to males with half-sized semidomes and without semidomes, those with full-sized semidomes were more attractive to mate-searching females. males with half-sized semidomes were more attractive to females than those without semidomes. the directional preference for the size of structures suggests that there is a positive correlation between male attractiveness and semidome size within the normal range of variation. why do females prefer males with larger structures? given that females in the non-courtship context showed a directional orientation bias for larger semidomes under predation threat regardless of the reproductive cycle, larger semidomes may be more salient visual cues to provide protection to females. the consistency between female choice in the mate-searching context and female orientation response in the predation context indicates that the preference for the larger signal may have originated from the female bias to escape predation. in other words, mate choice is linked to male-built structures that can serve as a landmark to keep females safe against predators. characteristics of the fiddler crab's visual system may also explain why females prefer larger semidomes. the horizontal resolving power of fiddler crabs' eyes is poor compared to their vertical resolving power. accordingly, fiddler crabs are better able to resolve objects in the vertical than in the horizontal plain. semidomes which are tall enough to be imaged within the zone of the acute vertical resolution of their eyes may be most conspicuous to fiddler crab females. in the absence of predator cues, however, females responded differently to different-sized semidomes depending on the reproductive cycle. during the non-reproductive periods, females did not show any difference in the preference for burrows with different treatments. in contrast, during the reproductive periods, they showed different responses. although females did not more preferentially move to full-sized structures than half-sized structures, they did prefer moving to burrows with structures over moving to burrows without structures. this result presents a striking contrast to the case of u. musica where females have an orientation preference to structures, even without predator cue. therefore, the predator-induced sensory trap hypothesis cannot fully explain the female preference on the structure building in u. lactea, as the effect of large structures decreased in the absence of a predator. rather, structure building itself may have been selected by female choice independent of predation pressure. this female preference for structures may have started because fiddler crab courtship structures may signal male nutritional condition. in previous studies in which males were supplemented with food, it was shown that structure building depends on male condition. the semidome building behavior may signal phenotypic condition of a male crab, which could be affected by food availability. females may choose better males using semidomes as an indicator of genetic quality. later, the predation pressure might enhance the selection on larger structures, which may help reduce predation risk for female crabs.  in conclusion, we have decoupled the origins of the preference on the sexual signal and signal intensity by manipulating predation risk. the preference for semidomes in u. lactea may have been selected by female choice not related with predation pressure. however, the preference for increased semidome size could have been strengthened to reduce the predation risk for females. our results suggest that sexual signal per se and signal intensity is not necessarily selected by the same selection pressure. 
first, we observed responses of mate-searching female u. lactea to courting males with full-sized, half-sized and no semidomes which were experimentally manipulated. results indicate that selection for the size of courtship structures in u. lactea may have an origin in the function to reduce predation risk, but that the preference for males with structures may have evolved by female choice, independent of predation pressure. thereafter, we tested the effect of predation risk on the female bias in the non-courtship context. if both components have the identical origin, females should show the same response to different-sized courtship structures regardless of predation risk. females had a directional preference for males with bigger semidomes within normal variation. when the predator cue was absent, however, females preferred burrows with semidomes without discriminating structure size during reproductive periods but did not show any bias during non-reproductive periods. it was suggested that the female fiddler crabs prefer males with courtship structures because of direct benefit to escape predation. here we tested if female preference for both components of the courtship structure in uca lactea is from the sensory bias to escape predation. a female preference for intense sexual visual signals is widespread in animals.
 a female preference for intense sexual visual signals is widespread in animals. although the preferences for a signal per se and for the intensity of the signal were often regarded to have the identical origin, no study has demonstrated if this is true. it was suggested that the female fiddler crabs prefer males with courtship structures because of direct benefit to escape predation. here we tested if female preference for both components of the courtship structure in uca lactea is from the sensory bias to escape predation. if both components have the identical origin, females should show the same response to different-sized courtship structures regardless of predation risk.  first, we observed responses of mate-searching female u. lactea to courting males with full-sized, half-sized and no semidomes which were experimentally manipulated. females had a directional preference for males with bigger semidomes within normal variation. thereafter, we tested the effect of predation risk on the female bias in the non-courtship context. when threatened by an avian mock predator, females preferentially approached burrows with full-sized semidomes regardless of reproductive cycles. when the predator cue was absent, however, females preferred burrows with semidomes without discriminating structure size during reproductive periods but did not show any bias during non-reproductive periods.  results indicate that selection for the size of courtship structures in u. lactea may have an origin in the function to reduce predation risk, but that the preference for males with structures may have evolved by female choice, independent of predation pressure. 