Sexual size dimorphism, growth and maturity of the Japanese fluvial sculpin,Cottus pollux (large egg type), in the Inabe River, Mie prefecture, central Japan

The population structure of the Japanese fluvial sculpin,Cottus pollux (large egg type), in the upper reaches of the Inabe River, Mie Prefecture, central Japan, was investigated by a mark-and-recapture method from July 1989 to January 1991. Breeding of the species occurred from mid February to early May, peaking from mid February to late March. The mean size of mature males observed in March 1990 was significantly larger than that of females, showing apparent sexual size dimorphism. Data analysis of the growth of 1658 marked individuals revealed that the species matured at 2 years of age in both sexes. Whereas 1 year old males reached ca. 50–70 mm SL, females were less than 50 mm SL at the same age, size dimorphism already being apparent. Immature males exhibited higher growth rates than females during their first and second years, some of the former outstripping mature males of the preceding year class in total length. After attaining sexual maturity, both males and females grew mainly from July to December, with no significant differences in mean growth rate between them. Sexual size dimorphism of the species seems to be attributable to different growth rates between the sexes during their immature stage.     

Spawning site of the fluvial eight-barbel loach, Lefua sp., in the natural environment

The spawning site of the fluvial eight-barbel loach, Lefua sp. (sensu Hosoya, 1993; Japanese name: nagare-hotoke-dojo), an endangered species, was investigated by searching for fertilized eggs in June and July 1995–1997 in an upper reach of the Kako River, Hyogo Prefecture, Japan. We obtained one egg each by a random sampling with agitating the river bottom (80 trials) and by collecting drift samples (89 trials). This finding indicates that the eggs were not scattered on the bottom or into the water column after being spawned. Two individual males were observed to patrol around particular interstices occurring under the buried cobbles and under the boulders that formed the riverbank. Males pecked other individuals that approached around the interstices. We obtained 9 and 15 eggs, respectively, from two such interstices in only two collection trials. The interstices were suggested to function as the spawning sites of this endangered fish in nature.     

Spawning habits of the fluvial eight-barbel loach Lefua sp. observed in captivity

The spawning behavior and reproductive characteristics of the fluvial eight-barbel loach (nagare-hotoke-dojo) Lefua sp. (sensu Hosoya, 1993) were examined in 11 male–female pairs of 31.5–61.5 mm in standard length (SL) (males) and 34.1–70.9 mm SL (females) in experimental tanks, each with a layer of charcoal sand. Spawning of Lefua sp. began at a water temperature between 13.4° and 15.9°C. For each pair, on the days when spawning occurred, the male was observed to patrol the bottom briskly, whereas the female swam up and down along the tank glass. Just before spawning, the females attempted to burrow into the sand or under a stone using beats of the caudal fin. When stimulated by being pecked at or being bitten by the male, the female redoubled her effort to find cover and the male followed her, after which they finally succeeded in finding cover. Muscular spasms, resembling trembling, were observed and they remained adjacent to each other. A moment later they both swam up out of the sand, completing the spawning. Each spawning event with a sequence of these behaviors took place within a 24-h cycle, with mating behavior not being observed to continue into a second day in this study. A total of 69 batches of eggs laid were confirmed in the experiments. Lefua sp. showed basically batch-spawning to a maximum of 13 batches. With the increasing number of batches, the number of eggs spawned in each ensuing batch decreased and the interval between spawning events lengthened. The spawned eggs tended to increase in diameter as the size of the female increased.     

Sexual bimaturation and sexual size dimorphism in animals with asymptotic growth after maturity

Many animal taxa exhibit a positive correlation between sexual size dimorphism and sex differences in age at maturity, such that members of the larger sex mature at older ages than members of the smaller sex. Previous workers have suggested that sexual bimaturation is a product of sex differences in growth trajectories, but to date no one has tested this hypothesis. The current study uses growth-based models to study relationships between sexual size dimorphism and sexual bimaturation in species with asymptotic growth after maturity. These models show that sex differences in asymptotic size would produce sexual bimaturation even if both sexes approach their respective asymptotic sizes at the same age, mature at the same proportion of asymptotic size and have otherwise equivalent growth and maturation patterns. Furthermore, our analyses show that there are three ways to reduce sexual bimaturation in sexually size-dimorphic species: (1) higher characteristic growth rates for members of the larger sex, (2) larger size at birth, hatching or metamorphosis for members of the larger sex or (3) smaller ratio of size at maturity to asymptotic size (relative size at maturity) for members of the larger sex. Of these three options, sex differences in relative size at maturity are most common in size-dimorphic species and, in both male-larger and female-larger species, members of the larger sex frequently mature at a smaller proportion of their asymptotic size than do members of the smaller sex. Information about the growth and maturation patterns of a taxon can be used to determine relationships between sexual size dimorphism and sexual bimaturation for the members of that taxon. This process is illustrated for Anolis lizards, a genus in which both sexes exhibit the same strong correlation (r 0.97) between size at maturity and asymptotic size, and in which the relative size at maturity is inversely related to asymptotic size for both sexes. As a result, sexually size-dimorphic species of anoles exhibit the expected pattern of a smaller relative size at maturity for members of the larger sex. However, for species in this genus, sex differences in the relative size at maturity are not strong enough to produce the same age at maturity for both sexes in sexually size-dimorphic species. Members of the larger sex (usually males) are still expected to mature at older ages than members of the smaller sex in Anolis lizards.     

Sexual size dimorphism in species with asymptotic growth after maturity

If animals mature at small sizes and then grow to larger asymptotic sizes, many factors can affect male and female size distributions. Standard growth equations can be used to study the processes affecting sexual size dimorphism in species with asymptotic growth after maturity. This paper first outlines the effects of sex differences in growth and maturation patterns on the direction and degree of sexual dimorphism. The next section considers the effects of variation in age structure or growth rates on adult body sizes and sexual size dimorphism. Field data from a crustacean, fish, lizard and mammal show how information on a species' growth and maturation patterns can be used to predict the relationships between male size, female size and sexual size dimorphism expected if a series of samples from the same population simply differed with respect to their ages or growth rates. The last section considers ecological or behavioural factors with different effects on the growth, maturation, survival or movement patterns of the two sexes. This study supports earlier suggestions that information on growth and maturation patterns may be useful, if not essential, for comparative studies of sexual size dimorphism in taxa with asymptotic growth after maturity.     

Sexual dimorphism in proconsul

One of the more important sources of variability in primate species is sexual dimorphism. Most Primates heavier than five kilos bodyweight are sexually dimorphic, both in body size and in shape of certain hard tissues. Despite these facts, most of the fossil Primates from East African Miocene deposits were originally perceived as being monomorphic, a perception which has propogated through the literature. Re-examination ofProconsul from various sites in Western Kenya results in the view that it was as dimorphic in its splanchonocranium and in bodyweight as chimpanzees and gorillas. The clearest evidence comes from Rusing Island, where adequate samples are known of two morphs, traditionally identified as two species, but more likely to represent two sexes of a single species,P. nyanzae. Co-occurrence of the two morphs is 100% at the various Rusinga sites. Less complete samples have been collected from the Tinderet sites os Koru and Songhor, yet what is available shows that similar patterns of dimorphism characterise the speciesP. africanus andP. major, and that the co-occurrence of the two morphs in each species is 100%. The identification of fossils taking into consideration the role of sexual dimorphism clarifies many of the old debates in which individual specimens frequently shifted between different species, mainly on the basis of metric rather than morphologic evidence. Consequently, the distribution of the species ofProconsul is rather different after accounting for dimorphism, than it was before.     

蝙蝠体型性别二态性研究现状与展望

动物体型性别二态性(Sexual size dimorphism,SSD)是存在于动物界的普遍现象,作用于某一性别体型的选择压力与作用于另一性别体型的选择压力大小或方向的不同被认为是SSD 产生的原因。伦施法则认为,在雄性体型比雌性体型大的动物类群中,SSD 随体型增大而增大,相反地,在雌性体型比雄性体型大的生物类群中随体型增大而减小。本文从动物体型性别二态性产生的原因及规律方面概述了其研究现状,以及蝙蝠性别二态性研究的进展,并提出关于蝙蝠体型性别二态性尚未解决的科学问题及未来的研究展望。     

Environmental influences on the sexual dimorphism in body size of western bobcats

Sexual size dimorphism might be influenced by environmental constraints on sexual selection or by intraspecific competition between males and females. We studied bobcats (Lynx rufus) in collections of museum specimens from western North America to examine these hypotheses. Structural body size was estimated from several measurements of the skull, ln-transformed and indexed through principal components analysis. Sexual dimorphism in body size was estimated from the difference in size index of males and females, and compared to geographic and climatic variables associated with biotic provinces (ecoregions). Of several climatic variables that were associated with bobcat body size, only seasonality of climate was associated with sexual dimorphism. Sexual size dimorphism, longitude, elevation, and seasonality were intercorrelated. As longitude decreased (moving inland from west-coastal ecoregions), sexual dimorphism decreased with the increased elevation and seasonality of continental climates of the Rocky Mountains. We suggest that increased seasonality and the need for fasting endurance by females may place constraints on the degree of sexual dimorphism in bobcats. Sexual dimorphism of body size and sexual size dimorphism of trophic structures (teeth) exhibited a strong positive association over geography, thus indirectly supporting the hypothesis that intrasexual competition for prey could account for the geographic variation in sexual size dimorphism. Thus, both environmental constraints on sexual selection of body size and intersexual competition were supported as possible explanations of the degree of sexual size dimorphism that occurs in populations of bobcats.     

Sexual size dimorphism in the ontogeny of the solitary predatory wasp Symmorphus allobrogus (Hymenoptera: Vespidae)

Sex-specific patterns of individual growth, resulting in sexual size dimorphism (SSD), are a little studied aspect of the ontogeny related to the evolutionary history and affected by the ecology of a species. We used empirical data on the development of the predatory wasp Symmorphus allobrogus (Hymenoptera, Vespidae) to test the hypotheses that sexual differences of growth resulting in the female-biased SSD embrace the difference in (1) the egg size and the starting size of larva, (2) the larval development duration, and (3) the larval growth rate. We found that eggs developing into males and females have significant differences in size. There was no significant difference between the sexes in the duration of larval development. The relative growth rate and the food assimilation efficiency of female larvae were significantly higher than compared to those of male larvae. Thus, the SSD of S. allobrogus is mediated mainly by sexual differences in egg size and larval growth rate.     

Patterns of growth and sexual size dimorphism in two species of box turtles with environmental sex determination

An adaptive explanation for environmental sex determination is that it promotes sexual size dimorphism when larger size benefits one sex more than the other. That is, if growth rates are determined by environment during development, then it is beneficial to match developmental environment to the sex that benefits more from larger size. However, larger size may also be a consequence of larger size at hatching or growing for a longer time, i.e., delayed age at first reproduction. Therefore, the adaptive significance of sexual size dimorphism and environmental sex determination can only be interpreted within the context of both growth and maturation. In addition, in those animals that continue to grow after maturation, sexual size dimorphism at age of first reproduction could differ from sexual size dimorphism at later ages as growth competes for energy with reproduction and maintenance. I compared growth using annuli on carapace scales in two species of box turtles (Terrapene carolina and T. ornata) that have similar patterns of environmental sex determination but, reportedly, have different patterns of sexual size dimorphism. In the populations I studied, sexual size dimorphism was in the same direction in both species; adult females were, on average, larger than adult males. This was due in part to males maturing earlier and therefore at smaller sizes than females. In spite of similar patterns of environmental sex determination, patterns of growth differed between the species. In T. carolina, males grew faster than females as juveniles but females had the larger asymptotic size. In T. ornata, males and females grew at similar rates and had similar asymptotic sizes. Sexual size dimorphism was greatest at maturation because, although males matured younger and smaller, they grew more as adults. There was, therefore, no consistent pattern of faster growth for females that may be ascribed to developmental temperature. Received: 20 March 1996 / Accepted: 10 March 1998     

Sexual dimorphism of the canine roots in theMacaca fuscata

In situ radiographic analysis of the maxillary canines ofMacaca fuscata was conducted on 88 specimens in 44 individuals (23 dry skulls and 21 live animals) in order to examine the number of roots. The left canines were then extracted from ten female skulls for measurement, further radiographic examination, and visual morphological observation. The results showed a clear sexual dimorphism in root morphology: all male canines were clearly distinguished as single-rooted from the radiograph, whereas more than 40% of the female canines were double-rooted. Variation was also found among the single-rooted female canines, in that some of these teeth appeared to have a bifurcated canal. This sexual dimorphism in the number of maxillary canine roots and the individual variation found among the females in root and canal morphology are previously unreported for this species. No observations were attempted on mandibular canines, however, because of the incomplete nature of the sample.     

Sexual dimorphism in the growth of the cranium

The major sexual dimorphisms in body size appear at puberty but, by then, 95% of the growth of the cranium is completed. As sexual dimorphism in the cranium is as great as for other parts of the body, this suggests that it must appear at an earlier age, and that cranium/body size ratios for the two sexes will vary during growth. Results from a longitudinal study of Montreal children are used to investigate this phenomenon. The effect is expressed quantitatively by proportional growth and growth velocity curves, based on the final size of boys, which show that the dimorphism indeed makes an early appearance. The data are also analyzed on an age scale relative to the ages of peak growth velocity in stature, derived from the individual growth curves. This shows that although there is a minor pubertal spurt in growth for the external cranial dimensions of boys, it contributes relatively little to the final dimorphism in cranial size. To summarize this aspect of growth, an index of cephalization is calculated: head length × head width/stature. Cross-sectional standards for the change of the mean index with age show a linear decline for boys and girls until puberty, with a constant difference between them. After puberty, the index becomes equal in the two sexes. Individual development curves for the index are however not linear.     

Sexual size dimorphism and selection in the wild in the waterstrider Aquarius remigis: Body size,components of body size and male mating success

Sexual size dimorphism is assumed to be adaptive and is expected to evolve in response to a difference in the net selection pressures on the sexes. Although a demonstration of sexual selection is neither necessary nor sufficient to explain the evolution of sexual size dimorphism, sexual selection is generally assumed to be a major evolutionary force. If contemporary sexual selection is important in the evolution and maintenance of sexual size dimorphism then we expect to see concordance between patterns of sexual selection and patterns of sexual dimorphism. We examined sexual selection in the wild, acting on male body size, and components of body size, in the waterstrider Aquarius remigis, as part of a long term study examining net selection pressures on the two sexes in this species. Selection was estimated on both a daily and annual basis. Since our measure of fitness (mating success) was behavioral, we estimated reliabilities to determine if males perform consistently. Reliabilities were measured as ? statistics and range from fair to perfect agreement with substantial agreement overall. We found significant univariate sexual selection favoring larger total length in the first year of our study but not in the second. Multivariate analysis of components of body size revealed that sexual selection for larger males was not acting directly on total length but on genital length. Sexual selection for larger male body size was opposed by direct selection favoring smaller midfemoral lengths. While males of this species are smaller than females, they have longer genital segments and wider forefemora. Patterns of contemporary sexual selection and sexual size dimorphism agree only for genital length. For total length, and all other components of body size examined, contemporary sexual selection was either nonsignificant or opposed the pattern of size dimporhism. Thus, while the net pressures of contemporary selection for the species may still act to maintain sexual size dimorphism, sexual selection alone does not.     

Sexual dimorphism in bone growth as a function of body size in moderately malnourished Guatemalan preschool age children

The sexual dimorphism in second metacarpal bone growth was investigated in 710 malnourished Guatemalan children one to seven years old to determine if the sex differences seen are only the result of differences in stature and weight. The study sample was mixed-longitudinal and consisted of 1,586 annual examinations. Boys have greater mean stature, weight, periosteal diameter, medullary diameter and cortical area than girls the same age, while girls have greater age specific mean cortical thickness and percent cortical area than boys. When the effects of stature, weight and age are removed boys still have significantly larger periosteal and medullary diameters and less cortical thickness and percent cortical area than girls. These differences between boys and girls therefore cannot be explained by sex differences in body size. However, no sex differences in cortical area remain after accounting for differences in stature, weight and age.     

Sexual dimorphism in the emergence of deciduous teeth: its relationship with growth components in height

Sexual dimorphism in the emergence of the deciduous dentition of French-Canadian children may be explained by differences in recumbent length. Relative to the chronological age scale, boys are longer and their teeth emerge earlier than girls. Recumbent lengths attained at the exact age of emergence, as estimated by fifth-order polynomials fitted to each subject's serial data, are comparable between the sexes. Multi- and univariate analyses of variance show no significant sex differences in the lengths attained at the age of emergence of the deciduous teeth. These findings suggest that clinical standards for emergence of deciduous teeth scaled relative to length rather than chronological age are more accurate and efficient.     

Sexual size and shape dimorphism and allometric scaling patterns in head traits in the New Zealand common gecko Woodworthia maculatus

Sexual dimorphism in shape and size is widespread across animal taxa and arises when natural or sexual selection operates differently on the sexes. Male and female common geckos (Woodworthia maculatus; formerly Hoplodactylus maculatus) in New Zealand do not appear to experience different viability selection pressure, nor do males appear to be under intense pre-copulatory sexual selection. It was therefore predicted that this species would be sexually monomorphic with regard to body size and the size and shape of the head. In line with the prediction, there was no sexual difference in head width, depth, or length or in lateral head shape. However, contrary to prediction, males had a larger body and lateral head size than females. This study suggests that males, at least on Maud Island, NZ, might be under stronger pre-copulatory sexual selection than previously recognized and thus have evolved larger heads (i.e. lateral head size) for use in male combat for females. Allometric scaling patterns do not differ between the sexes and suggest that head width and depth are under directional selection whereas lateral head size is under stabilizing selection. Diet ecology – an agent of natural selection common to both sexes – is likely largely responsible for the observed patterns of head size and shape and the lack of sexual dimorphism in them.     

Sexual dimorphism in cuticular hydrocarbons of the Australian field cricket Teleogryllus oceanicus (Orthoptera: Gryllidae)

Sexual dimorphism is presumed to reflect adaptive divergence in response to selection favouring different optimal character states in the two sexes. Here, we analyse patterns of sexual dimorphism in the cuticular hydrocarbons of the Australian field cricket Teleogryllus oceanicus using gas chromatography. Ten of the 25 peaks found in our chromatographs, differed in their relative abundance between the sexes. The presence of sexual dimorphism in T. oceanicus is discussed in reference to a review of sexual dimorphism in cuticular hydrocarbons of other insects. We found that this trait has been examined in 103 species across seven different orders. Seventy-six of these species (73%) displayed sex specificity of cuticular hydrocarbons, the presence/absence of which does not appear to be directly linked to phylogeny. The occurrence of sexual dimorphism in cuticular hydrocarbons of some but not other species, and the extent of variation within genera, suggest that this divergence has been driven primarily by sexual selection.     

Sexual dimorphism and the influence of artificial elevated temperatures on body size in the imago of Nemoura trispinosa (Plecoptera: Nemouridae)

Our study site was composed of a spring that was divided lengthwise from its source into two equally wide channels, with water temperature in the experimental channel being increased by 2.0–3.5°C relative to that of the control channel. We examined sexual dimorphism in overall body size and eight morphological traits in the stonefly Nemoura trispinosa Claassen, and whether body size varies according to site of emergence (upstream versus downstream) and/or the thermal regime in which they undergo nymphal development. Univariate tests showed that females were significantly larger than males in all eight traits, and multivariate analyses (Principal Components Analysis) demonstrated that females were larger in overall body size. Both univariate and multivariate tests showed that adults emerging from the control and downstream portions of the stream were larger than those from heated and upstream reaches, respectively. We discuss possible hypotheses contributing to the female-biased sexual dimorphism observed in this species.     

Sexual dimorphism in <Emphasis Type="Italic">Sebastes owstoni</Emphasis> (Scorpaenidae) from the Sea of Japan

Morphological and genetic differences between red and yellow morphotypes of Sebastes owstoni were investigated, utilizing 277 males [84.0–194.3 mm in standard length (SL)] and 542 females (92.3–251.5 mm SL) from the Sea of Japan. All males smaller than 120 mm SL were characterized by red body color. The frequency of specimens with yellow body color thereafter increased gradually with SL, all specimens larger than 170 mm SL being yellow. The specimens with yellow body color were observed throughout the year. All females smaller than 170 mm SL were characterized by red body color, the frequency of specimens with yellow body color tending to slightly increase with SL. However, most females had red body color, except for 16 specimens (177.7–241.5 mm SL) that were yellow, growth-related color change from red to yellow being uncommon. Morphological analysis of 49 males (107.6–193.3 mm SL) and 68 females (108.7–241.5 mm SL) showed the head length, orbit diameter, lower jaw length, and predorsal length to be relatively greater, but the distance between the pelvic and anal fins less, in males. A discriminant analysis using Mahalanobis distances resulted in 100% correct assignment of specimens to sex, regardless of SL and body color. In addition, no genetic differences were apparent between red and yellow individuals in mitochondrial DNA sequence analyses from the threonine tRNA to the first half of the control region (498 bp). Accordingly, the differences in body color, maximum size, and the five morphometric characters listed above were considered to represent sexual dimorphism. That evidenced by body color was considered to appear after that shown by morphometric characters, some exceptions in the former occurring in females. This is the first report of permanent sexual dimorphism in body color in Sebastes.