Population structure and sex-change in the coral-inhabiting snailCoralliophila violacea at Hsiao-Liuchiu,Taiwan

Surveys of the coral-inhabiting snailCoralliophila violacea (Lamarck) (=C. neritoidea Kiener) were made on shallow fringing reefs (<8 m deep) around Hsiao-Liuchiu, Taiwan, between July and October 1990. The snails were aggregated into patches on the surface of massive poritid coral colonies. Coral colonies >40 cm in diameter were more likely to bear patches of snails than smaller colonies, and also to have more snails. The coralliophilids ranged from 5 to 30 mm in aperture length. The sex ratio of the population was biased toward males (539:279), with only a few small individuals of indistinguishable sex. Snails between 6 and 10 mm were all males, while most snails with aperture lengths 20 mm were females. Judging from the distinct size ranges of males and females within patches and from the observed degeneration of the penis, the snails may have changed sex from male to female with increasing size. Sex-change may occur across a wide size range (10 to 20 mm). The correlation of smallest female size and largest male size among patches indicates that snail size at sex-change is peculiar to each individual patch. Those females in patches with a single female (but many males) were significantly smaller than females in multiple-female patches. It is likely that in the absence of females males change sex at a smaller size, whereas in the presence of large females males delay sexchange until they have reached a larger size. The plasticity of size at sex-change may be adaptive and a result of natural selection at the individual level.     

Occurrence and reproductive success of feminized males in the polychaete Capitella capitata (Species Type I)

Natural populations of the cosmopolitan polychaete species, Capitella capitata (Species Type I, Grassle and Grassle 1976) contain males, females and hermaphrodites. Hermaphroditic individuals arise through feminization of males when females are rare. The age-specific survivorships and fecundities of females and hermaphrodites were estimated. There were no significant differences between females and hermaphrodites in survivorship, number of offspring per brood, or percentage of aborted eggs per brood. Net reproductive rates were used to estimate fitness, and the relative fitness of a hermaphrodite as a female ranged from 0.09 to 0.31. The fitness differential was due to the difference in the number of broods that females and hermaphrodites produce. The effects of density, sex ratio, age and body size on the timing of the development of hermaphrodites in groups of siblings were also examined. Hermaphrodites appeared when females were rare or when densities were low. Hermaphrodites never developed in cohorts with larger males unless females were rare. These observations suggest that feminization of males occurs when some males are unable to gain access to females because of mate competition. Feminization does not appear to be correlated with a threshold in body size.     

Hermaphrodite sex role preferences: the role of partner body size, mating history and female fitness in the sea slug Chelidonura sandrana

Costs and benefits associated with matings and the effects of mating frequency on fitness commonly differ between the sexes. As a result, outcrossing simultaneous hermaphrodites may prefer to copulate in the more rewarding sex role, generating conflicts over sperm donation and sperm receipt between mates. Because recent sex role preference models remain controversial, we contrast here some of their assumptions and predictions in the sea slug Chelidonura sandrana. For this hermaphrodite with sperm storage and internal fertilisation, risk-averse models assume that fitness pay-offs are constantly higher in the female than in the male function in any single mating. Moreover, excluding mutual partner assessment, these models predict male mating behaviour to be independent of receiver traits. The competing gender ratio hypothesis assumes that relative fitness pay-offs, and thus the preferred mating roles, vary and may reverse between matings and predicts that ejaculation strategies co-vary with receiver quality. We found that field mating rates of C. sandrana substantially exceeded what is required to maintain female fertility and fecundity, indicating large variation in direct female benefits between matings. We further demonstrate that male copulation duration adaptively increased with partner body size (i.e. fecundity) but decreased with recent partner promiscuity. These findings are compatible with the gender ratio hypothesis but contradict risk-averse models.     

Sex adjustments are not functionally costly in simultaneous hermaphrodites

Because of their double sex functions, hermaphrodites are selected to optimize their investment in the two sex functions. Sex allocation (SA) theory predicts that, in promiscuous mating conditions, simultaneous hermaphrodites should adjust their reproductive investment so as to invest an amount of resources into the male relatively larger than that invested into the female function. In contrast, in monogamy, individuals should invest relatively larger amount of resources into the female function at the expenses of the male function. In the study of SA patterns of simultaneous hermaphrodites little attention has been paid to allocate adjustments costs, which may play an important role in determining variations in SA pattern among species. Indeed, the costs paid for such adjustments may constrain sex investment resulting in suboptimal allocation. We evaluated the costs of SA adjustments on individual fitness in each sexual function in the simultaneous, outcrossing hermaphrodite Ophryotrocha diadema. Following a crossover design, we compared the reproductive success in paternal and maternal offspring of focal hermaphrodites, which were put in replicated monogamous and promiscuous regimes. We document that those hermaphrodites that switched mating regimes and altered their sex investment accordingly did not entail large short-term fitness costs in any sexual function compared to those that were in stable mating regimes. Indeed, individuals changed their sex investment quickly and appropriately to current mating conditions. Hermaphrodites, which had to adjust their SA, did not decrease their maternal or paternal reproductive output with respect to those which did not change their SA. Time needed to shift resources from one to the other sex function is 5 days (the time interval between successive egg layings is of 3 days) indicating that selective pressures for SA adjustments may favour great plasticity and quick adjustments of sex investments in simultaneous hermaphrodites.     

Strategic mating effort in a simultaneous hermaphrodite

Sexual selection theory for simultaneously hermaphroditic animals predicts an overall preference for inseminating partners that have a relatively higher female fecundity. Previous work on the link between male mating decisions and female fecundity has primarily focused on the effect of the partners’ body size using existing variation in this trait within a study population. On the assumption that the body size is positively correlated with female fecundity, sperm donors should preferentially inseminate relatively larger individuals to obtain a higher fitness gain through their male sex function. However, empirical evidence for such size-dependent mate choice in simultaneous hermaphrodites is equivocal, possibly because of confounding variables. We studied the mating behavior of the simultaneously hermaphroditic flatworm Macrostomum lignano and tested for a strategic mating effort in response to the feeding status of the partner. We experimentally manipulated the feeding status of potential mating partners in order to generate variation in female fecundity among them and tested whether this affected the copulation number and the number of sperm that the focal worm managed to store in the partner’s sperm storage organ. We found that the manipulation of the feeding status had a strong effect on the body size of the potential mating partners and that focal worms copulated more frequently with, and stored more sperm in well-fed partners compared to unfed partners. Our results suggest that M. lignano adjusts its mating effort in response to the feeding status of the mating partner.     

Population dynamics, sex ratio and size at sex change in a protandric simultaneous hermaphrodite, the spiny shrimp Exhippolysmata oplophoroides

One of the main goals of sex allocation theory is understanding sex ratio evolution. However, theoretical studies predicting sex ratios in species with unusual sexual systems, such as protandric simultaneous (PS) hermaphroditism, are rare. In PS hermaphrodites, juveniles first develop into functional males that mature into simultaneous hermaphrodites later in life. Here, we report on the sex ratio (males/males + hermaphrodites) in the PS hermaphroditic shrimp Exhippolysmata oplophoroides. A 2-year study demonstrated that hermaphrodites dominated the population in two different bays. This skewed sex ratio may be explained by limited encounter rates among conspecifics. In agreement with this idea, the density of shrimps was extremely low (≤1 shrimp km⁻²) at the two study sites. Size at sex phase change and sex ratios remained relatively stable through time at the two bays. The stability of these parameters might be explained by the rather steady population structure of this species during the study period. A review of sex ratios in PS hermaphroditic shrimps (Lysmata and Exhippolysmata) revealed considerable variation; some species have male- and others hermaphrodite-skewed sex ratios. The conditions explaining inter- and intra-specific sex ratio variation in protandric simultaneous hermaphroditic species remain to be addressed.     

Sex expression of an immobile coral-inhabiting snail,<Emphasis Type="Italic"> Quoyula monodonta</Emphasis>

To test the hypotheses explaining the sex expression of the immobile snail Quoyula monodonta, which inhabits the surfaces of the branching coral, Pocillopora eydouxi, the size, sex, gonad development, penis length and the composition of neighboring individuals were investigated between November 1994 and August 1995 in southern Taiwan. Although the snails often aggregated and formed patches, more than 50% were solitary. Females were larger than males both within a snail patch and in the whole population, but the overlap in size range was wide. The males were generally accompanied by females, whereas most females were solitary. Females were rarely (6%) found in the same patch with another female, but 35% of the males had male neighbors. Most juveniles found were also solitary. The composition pattern within a patch cannot be explained by random sampling. Gonad development of an individual was dependent on the presence and the sex of its neighbors within the same patch; the penis length of males also depended on the presence of neighbors. These phenomena suggest that an individual is sensitive to its neighbors. No individuals in the process of sex change were ever found from histological studies of the gonads. Neither the hypothesis that the sex of recruits determines their habitats, nor the hypothesis that there is strict protandric sex-change is supported. The results, however, are all clearly compatible with the hypothesis that the snail has labile sex expression. In the presence of existing females in a patch, recruits are more likely to develop into males, whereas recruits starting a new patch grow to a larger size before developing into females directly. The labile sex expression of Q. monodonta is the only such report in neogastropods.     

Intraspecific density mediates sex-change in the territorial patellacean limpet Lottia gigantea

The effects of intraspecific density and agonistic interactions on sex-change were studied in the territorial limpet Lottia gigantea. In a one-year field experiment (1982–1983) on San Nicolas Island off the southern California coast, USA, male limpets transplanted to large enclosures changed sex more frequently than those transplanted to small enclosures (9 of 13 vs 1 of 10; p=0.013), indicating that intraspecific density can profoundly influence the probability of sex change. Large limpets were more likely to change sex than small ones. Observations of gender-age distributions as well as field behavior suggested that each limpet's territorial status prior to the experiment may have been an important component of this size effect, although other interpretations including an effect of age are possible. Pooling the results with those of two previous studies confirmed that sex-change is enhanced by low density. This enhancement was observed among the largest members of a local population in the first year of each experiment, while among the smaller members the enhancement was delayed until the second or third year. Low density may be a correlate of high mortality, and therefore an adaptive cue for an earlier age of sex change. Dominant territorial status correlates with an individual's size, and therefore egg-producing capacity, relative to its neighbors, and thus may also be a good cue for the initiation of sex-change.     

Effects of frequent mating on sex allocation in the simultaneously hermaphroditic great pond snail (Lymnaea stagnalis)

Simultaneous hermaphrodites have the opportunity to control the allocation of resources to the male and female function depending on the circumstances. Such flexibility also provides the possibility to influence sex allocation in the mating partner. To investigate this idea, we measured egg production (female investment) and sperm production as well as prostate gland size (both are part of male investment) under different mating regimes in the great pond snail Lymnaea stagnalis. We find no evidence for the prediction from sex allocation theory that sperm production increases with mating frequency. However, we do find that animals with more mating opporunities develop smaller prostate glands, in which seminal fluid is produced. Moreover, repeated mating increases egg production, thus shifting allocation towards the female function, and probably decreases growth. So, our data hint at a three-way trade-off between part of the male function (prostate gland), female function, and growth. Interestingly, sex allocation seems to be shifted in the opposite direction from the one predicted by theory. We discuss how this feminization is suggestive of a direct manipulation by the sperm donor, probably to stimulate immediate sperm use.     

Maternal investment, sex-differential prospects, and the sex ratio in wild house mice

In a population of first-generation offspring from wild-caught house mice (Mus musculus domesticus), previous evidence suggested that male fitness is more strongly affected by an increase in body weight than female fitness. This paper shows that in these mice the young are weaned at heavier weights the smaller the litter and the better the maternal body condition. These effects persisted into adulthood and were less pronounced in female young. However, contrary to expectation from conventional sex ratio theory, maternal condition and litter size had no detectable effect on sex ratios. Also, litter size did not affect sex ratios in two populations of laboratory-kept, wild-caught western (M. m. domesticus) and eastern house mice (M. m. musculus). Wild house mice, therefore, appear not to adaptively manipulate the sex ratio of offspring. It is argued that this absence of sex ratio trends might not be maladaptive, but rather that models currently used to predict sex ratio trends in rodents may not be valid. Received: 13 March 1997 / Accepted after revision: 9 August 1997     

Comparative mating success of smaller male-phase and larger male-role euhermaphrodite-phase shrimp, <Emphasis Type="Italic">Lysmata wurdemanni</Emphasis> (Caridea: Hippolytidae)

The protandric simultaneous hermaphrodite shrimp Lysmata wurdemanni (Gibbes 1850) has a pure searching mating system, i.e., males are continually searching for receptive females and copulation is brief. To examine whether size-based advantage in male–male competition occurs and whether the mating ability of male-phase (M) shrimp equals that of euhermaphrodite-phase shrimp serving as males (Em), mating performance, including mating frequency and precopulatory behavior, of M and Em shrimp was compared using two M:Em ratios. Two experiments were carried out from March 2004 to August 2004 at Florida Institute of Technology’s Vero Beach Marine Laboratory using laboratory-cultured shrimp that originated from Port Aransas, TX, USA. In the two experiments, one parturial euhermaphrodite-phase shrimp acting as a female (Ef) was maintained with one M and two Em shrimp (one with and one without an egg mass), and two M and two Em shrimp, respectively. The M shrimp used were always smaller than the Em shrimp. Experiment 1 showed that there was no significant difference in mating ability between Em with and without egg mass. In both experiments, the M shrimp gained mating partners more frequently than the Em shrimp did. In the experiment with two M and two Em shrimp, mating frequencies of the small M and large M shrimp were similar. Precopulatory behaviors of the M shrimp were more active than those of the Em shrimp. Mating between the small M and larger Ef shrimp was sometimes successful even when the size difference was 20.0 mm total length (TL). Mating between a larger M shrimp and smaller Ef shrimp sometimes failed when the size difference was only 13.0 mm TL. Mating frequency of M shrimp over that of Em shrimp with Ef shrimp increased significantly with increasing density and operational sex ratio. The advantage of M over Em shrimp in obtaining mating partners is probably a result of sexual selection and adaptation, and may partially explain the observed delayed sex change in some L. wurdemanni, i.e., some male-phase shrimp grow very large and never become hermaphrodites.     

Female grouping best predicts lekking in blackbuck (Antilope cervicapra)

The study of intraspecific variation can provide insights into the evolution and maintenance of behavior. To evaluate the relative importance of ecological, demographic and social conditions thought to favor lekking, I studied variation in mating behavior among and within populations of the blackbuck, Antilope cervicapra, an Indian antelope. Rather than viewing lekking as a discrete mating strategy, I took a continuous approach and treated lekking as a question of the clustering of mating territories, with leks representing one extreme in a range of territory distributions. I surveyed nine blackbuck populations, which differed in population density and in habitat conditions. For each population, I described the mating system in terms of the clustering of mating territories, and measured various factors suggested to favor lekking. I found that large-scale, among-population variation in territory clustering was most strongly related to female group size. Territory clustering was not related to population density. Female group size, in turn, was best explained by habitat structure. Interestingly, these among-population patterns were repeated at a finer spatial scale within one intensively studied population. These findings suggest that territorial males respond to local patterns in female distribution (represented by group size) when making decisions regarding territory location. Finally, although female distribution may explain territory clustering at the population level and more locally within a population, other selective factors (e.g., female preference, male competition, male harassment) are likely to shape the clustering and size of territories at even finer scales.Electronic Supplementary Material Supplementary material is available in the online version of this article at .Communicated by T. Czeschlik     

Inbreeding and competitive ability in the common shrew (<Emphasis Type="Italic">Sorex araneus</Emphasis>)

Common shrews (Sorex araneus) maintain a foraging territory for most of their immature life. Possessing a high-quality territory is vital for overwinter survival in the harsh boreal climate, and hence, competitive ability in territorial disputes is expected to be an important component of individual fitness. To test possible association between individual inbreeding and fitness, we used neutral arena trials to assess the competitive performance of young common shrews. The experiment involved pairs of individuals originating from small island populations, where breeding must often occur between related individuals, and from large outbred mainland populations. The percentage of neutral arena tests that an individual won was highly significantly explained by internal relatedness, a surrogate measure of individual inbreeding, measured using ten microsatellite markers. Body size, sex, learning, and population type (mainland vs island) made no significant contributions. Even a low level of individual inbreeding may lead to significant adverse consequences in multiple territorial contests, which may represent a significant cause of inbreeding depression in many wild vertebrate populations.     

Sex ratios,mating behavior and sexual size dimorphism of the northern water snake,Nerodia sipedon

Competition among males to mate is generally associated with male-biased size dimorphism. In this study we examine mating behavior in the northern water snake (Nerodia sipedon), a species in which males are much smaller than females despite substantial competition among males to mate. Competition among males was a consequence of a male-biased operational sex ratio due to slightly higher female mortality from a birth sex ratio of 1 : 1, and, in 1 year, more synchronous and longer mating activity by males. Approximately one-third of both males and females appeared not to mate in a given year. Larger males were generally more likely to attempt mating, but size did not explain the variance in the number of aggregations in which individual males participated. Within aggregations, males that were successful at achieving intromission were larger than unsuccessful males in 1 of 2 years. Variation in condition (mass relative to length) and relative tail length were not generally useful predictors of either mating effort or success in males. Because large size was often advantageous to males, sexual size dimorphism appeared not to be a consequence of sexual selection favoring smaller males. Because sexual dimorphism was evident at birth, and both males and females matured sexually at about 4 years, sexual dimorphism was not simply a consequence of one sex growing at the maximum rate for longer. Female fecundity increased with size, and sex differences in size-fecundity relations may underly the pattern of sexual size dimorphism. However, because multiple mating by females is common, sperm competition is likely to be important in determining male reproductive success. Therefore, allocation of energy to sperm rather than growth may also prove to be an important influence on male growth rates and sexual size dimorphism.     

Gender-role alternation in the simultaneously hermaphroditic freshwater snail <Emphasis Type="Italic">Physa acuta</Emphasis>: not with the same partner

In simultaneous hermaphrodites, gender conflicts that arise from two potential mates sharing the same gender preference may be solved through conditional reciprocity (or gamete trading). Conditional reciprocity had initially been considered widespread, but recent studies suggest that its real occurrence may have been overestimated, possibly because most mating observations have been performed on isolated pairs of individuals. Some resulting patterns (e.g., non-random alternation of sexual roles) were indeed compatible with conditional reciprocity but could also have stemmed from the two partners independently executing their own mating strategy and being experimentally enforced to do so with the same partner. Non-random alternation of gender roles was recently documented in the simultaneously hermaphroditic freshwater snail Physa acuta. To distinguish between conditional and unconditional gender alternations, we observed copulations of individually marked snails reared at three contrasted densities. We showed that density affected the overall frequency of copulations during the first 2 days of the experiment with high-density boxes showing more copulations than low density boxes, but it did not affect gender alternation patterns. A change in gender role was observed more often than expected by chance over two successive copulations by the same individual, confirming previous studies. However, gender switches did not preferentially occur with the same partner. We conclude that gender alternation is not due to conditional reciprocity in P. acuta. It may rather stem from each individual having a preference for gender alternation. We finally discuss the mechanisms and the potential extent of this unconditional reciprocity.     

Size at maturity,fecundity and reproductive potential of a protected population of the spiny lobster<Emphasis Type="Italic"> Palinurus elephas</Emphasis> (Fabricius, 1787) from the western Mediterranean

Protected lobster populations are expected to contribute to the replenishment of fished populations through increased egg production. We studied the reproductive biology and egg production potential of a population of the spiny lobster Palinurus elephas protected from fishing since 1990 in the Columbretes Islands Marine Reserve (western Mediterranean). An index of spawning potential was derived to compare egg production potential in the Reserve and in western Mediterranean exploited populations. Females' physiological maturity (ability to reproduce) and functional maturity (ability to mate and bear eggs) occurred at a carapace length (CL) of 76–77 mm. Males' physiological maturity occurred at a slightly larger size, 82.5 mm CL. In the Reserve, P. elephas' individual fecundity increases linearly with body size up to the females' maximum size, although maximum reproductive yield (eggs per body gram) was reached at intermediate sizes. Size-specific fecundity in the protected population was similar to that of lightly fished populations off Ireland and greater than that of western Mediterranean exploited populations. The female size class of 105–110 mm CL contributed most to egg production in the protected population and is well above the minimum landing size (MLS) for western Mediterranean fisheries. Newly mature females (below MLS) generate a very small fraction (1%) of the egg production from the Reserve. Given the pattern of exploitation in western Mediterranean fisheries, egg production potential depends more on the quantity than on the mean size and fecundity of the available females. The role of the greater availability of large males for mating in unfished populations is discussed in terms of the females' individual fecundity and mating success.     

Offspring sex ratio in relation to parental structural size and body condition in the long-lived wandering albatross (<Emphasis Type="Italic">Diomedea exulans</Emphasis>)

Sex ratio theory is one of the most controversial topics in evolutionary ecology. Many deviations from an equal production of males and females are reported in the literature, but few patterns appear to hold across species or populations. There is clearly a need to identify fitness effects of sex ratio variation. We studied this aspect in a population of a long-lived seabird, the wandering albatross (Diomedea exulans), using molecular sex-identification techniques. We report that parental traits affect both (1) fledgling traits in a sex-dependent way and (2) chick sex: Sons are overproduced when likely to be large at fledging and, to a lesser extent, daughters are overproduced when likely to be in good body condition at fledging. Because for the same population, a previous study reported that post-fledging survival was positively affected by size in males and by body condition in females, our results suggest that wandering albatrosses manipulate offspring sex to increase post-fledging survival.     

Male mating opportunities affect sex allocation in a protrandric-simultaneous hermaphroditic shrimp

Sex allocation theory predicts phenotypic adjustments by individuals in their investments into the male and female reproductive function in response to environmental conditions. I tested for phenotypically plastic shifts in sex allocation in a protandric simultaneous hermaphrodite, in which individuals mature and reproduce as males first, and later in life, as simultaneous hermaphrodites. I predicted that initially maturing males should adjust the timing of maturation as hermaphrodites according to male mating opportunities mediated by population size of hermaphrodites. In a first experiment, males maintained with only one hermaphrodite reduced the time they spent as males in comparison to males maintained with no conspecifics, presumably because total reproductive output is maximized by two individuals being simultaneous hermaphrodites when the mating system is a pair. Conversely, males maintained in groups with two or more hermaphrodites increased the time they spent as males in comparison to single males. This delay in maturation was not an effect of resource depletion with increasing shrimp density because the growth rate of males did not differ among most of the experimental treatments. One hypothesis to explain this social mediation of sex allocation is that the smaller males are more successful in mating as males than are the larger hermaphrodites: it will pay reproductively for males to delay maturation as hermaphrodites in large but not in small groups. In agreement with this notion, a second experiment demonstrated that smaller males were four times more successful than were larger hermaphrodites in inseminating shrimps reproducing as females. The informative cue that males may use to perceive different group sizes deserves further attention.     

Local mate competition in the solitary parasitoid wasp <Emphasis Type="Italic">Ooencyrtus kuvanae</Emphasis>

Local mate competition (LMC) occurs when brothers compete with each other for mating opportunities, resulting in selection for a female-biased sex ratio within local groups. If multiple females oviposit in the same patch, their sons compete for mating opportunities with non-brothers. Females, in the presence of other females, should thus produce relatively more sons. Sex ratio theory also predicts a more female-biased sex ratio when ovipositing females are genetically related, and sex-ratio responses to foundress size if it differentially affects fitness gains from sons versus daughters. The mating system of the parasitoid wasp Ooencyrtus kuvanae meets assumptions of LMC. Females insert a single egg into each accessible egg of gypsy moth, Lymantria dispar, host egg masses. Wasps complete development inside host eggs and emerge en masse, as sexually mature adults, resulting in intense competition among brothers. We tested the hypothesis that O. kuvanae exhibits LMC by manipulating the number of wasp foundresses on egg masses with identical numbers of eggs. As predicted by LMC theory, with increasing numbers of wasp foundresses on an egg mass, the proportions of emerging sons increased. In contrast, the presence of a sibling compared to a non-sibling female during oviposition, or the size of a female, did not affect the number or sex ratio of offspring produced. The O. kuvanae system differs from others in that larvae do not compete for local resources and thus do not distort the sex ratio in favor of sons. With no resource competition among O. kuvanae larvae, the sex ratio of emergent son and daughter wasps is due entirely to the sex allocation by ovipositing wasp foundresses on host egg masses.     

Body size,morphological constraints,and mated pair formation in four populations of horseshoe crabs (Limulus polyphemus) along a geographic cline

The relationship of body size to mating success was studied in four populations of horseshoe crabs, Limulus polyphemus L., along the east coast of the U.S.A. in the spring and summer from 1986 to 1989. Crabs of both sexes from Great Bay, New Hampshire, were significantly smaller than crabs from three middle Atlantic coast populations: Sandy Hook Bay and Delaware Bay, New Jersey, and Chincoteague Bay, Virginia. The formation of mated pairs was independent of body size in each population; there were no significant size differences between mated and single individuals, and size assortative mating did not occur. A comparision of male clasper dimensions with the corresponding point of attachment on the female indicates that there are no morphological constraints limiting amplexus between any male and any female within a population. The ratios of male to female prosoma width within amplexed mated pairs averaged from 0.78 to 0.80 in each population, despite the large difference in absolute size between southern and nothern populations. This may suggest a role for natural selection in regulating the relative sizes of each sex.