Sexual Dimorphism of the Jilin Clawed Salamander,Onychodactylus zhangyapingi,(Urodela:Hynobiidae:Onychodactylinae) from Jilin Province,China

Sexual dimorphism in size and shape is common in many organisms, and is a key evolutionary feature. In this study, we analyzed morphometric data of the Jilin clawed salamander Onychodactylus zhangyapingi, an endemic Chinese salamander, to examine sexual size and shape dimorphism. The morphometric data included 14 characteristics of 13 females and 11 males and was analyzed using univariate and multivariate methods. Our results showed that sexual dimorphism occurs not only in body size, but also in body shape. Males have a longer snout-vent length than females, a rarely reported pattern of male-biased sexual size dimorphism. Females have a larger space between the axilla and groin than males, while males have longer and larger tails compared to females. The sexual dimorphism in body size and shape can be explained by existing theories, but there is little data for the mating system, behavior, reproduction, or ecology of O. zhangyapingi, so further studies are required.     

Variation in guenon skulls (II): sexual dimorphism

Patterns of size and shape sexual dimorphism in adult guenons were examined using a large sample of skulls from almost all living species. Within species, sexual dimorphism in skull shape follows the direction of size-related shape variation of adults, is proportional to differences in size, and tends to be larger in large-bodied species. Interspecific divergence among shape trajectories, which explain within species sex differences, are small (i.e., trajectories of most species are nearly parallel). Thus, changes in relative proportions of skull regions that account for the distinctive shape of females and males are relatively conserved across species, and their magnitude largely depends on differences in size between sexes. A conservative pattern of size-related sexual dimorphism and a model of interspecific divergence in shape which strongly reflects size differences suggest a major role of size and size-related shape variation in the guenon radiation. It is possible that in the guenons, as in the neotropical primates (with whom they have obvious parallels), size has helped to determine morphological change along lines of least evolutionary resistance, influencing sexual dimorphism. In Miopithecus and Erythrocebus, the smallest and largest guenon genera, it is likely that the interaction of ecology and size contributes significantly to patterns of sexual dimorphism. The results of this study thus emphasise the need to consider allometry and size alongside ecology and behaviour when examining primate sexual dimorphism.     

Development of intraspecific size variation in black coucals,white-browed coucals and ruffs from hatching to fledging

Most studies on sexual size dimorphism address proximate and functional questions related to adults, but sexual size dimorphism usually develops during ontogeny and developmental trajectories of sexual size dimorphism are poorly understood. We studied three bird species with variation in adult sexual size dimorphism: black coucals (females 69% heavier than males), white-browed coucals (females 13% heavier than males) and ruffs (males 70% heavier than females). Using a flexible Bayesian generalized additive model framework (GAMM), we examined when and how sexual size dimorphism developed in body mass, tarsus length and bill length from hatching until fledging. In ruffs, we additionally examined the development of intrasexual size variation among three morphs (Independents, Satellites and Faeders), which creates another level of variation in adult size of males and females. We found that 27–100% of the adult inter- and intrasexual size variation developed until fledging although none of the species completed growth during the observational period. In general, the larger sex/morph grew more quickly and reached its maximal absolute growth rate later than the smaller sex/morph. However, when the daily increase in body mass was modelled as a proportion, growth patterns were synchronized between and within sexes. Growth broadly followed sigmoidal asymptotic models, however only with the flexible GAMM approach, residual distributions were homogeneous over the entire observation periods. These results provide a platform for future studies to relate variation in growth to selective pressures and proximate mechanisms in these three species, and they highlight the advantage of using a flexible model approach for examining growth variation during ontogeny.     

Geography of sexual dimorphism in the tooth size of the red fox Vulpes vulpes (Mammalia, Carnivora)

Geographic variation in sexual dimorphism of tooth size was assessed for the red fox Vulpes vulpes (Linnaeus, 1758) across the whole northern range of the species. Twenty-one measurements of tooth size and skull length were taken from 2849 specimens (1577 males and 1272 females) originating from 12 Nearctic and 25 Palearctic localities. The index of sexual dimorphism was calculated as a quotient of the mean measure of certain characters in males by the respective mean in females ( M _m/ M _f). In the whole range, the males were larger than females and mean dimorphism index of tooth size ranged from 1.01 to 1.06. On average, the tooth measurements in males were 3.6% larger than in females. The highest dimorphism was observed in the canines. Dimorphism of tooth size was higher in the Palearctic than Nearctic. Statistically significant differences between regions were found for lengths of C¹, C₁ and M₁. In the Palearctic, higher values of the dimorphism indices were observed particularly in the southern parts of the Eurasian range of the red fox and in Great Britain. For a few metrical traits, sexual dimorphism indices presented significant relations to some geo-climatic variables. The geographic pattern of size dimorphism in the red fox seems to be shaped by sexual selection, intraspecific and interspecific competition and population density.     

Body size variation in the sexually dimorphic scaphopod Rhabdus rectius (Carpenter, 1864) (Dentaliida: Rhabdidae)

ABSTRACT

Male-biased sexual size dimorphism typically evolves via sexual selection for larger males that are favoured by choosy females or are more successful in mate competition with other males. Among marine invertebrates that broadcast their gametes into the ocean for fertilisation, this form of sexual size dimorphism is rare because such species lack direct interactions among males or between the sexes. However, the broadcast-spawning tusk shell Rhabdus rectius was recently reported to show strong male-biased sexual size dimorphism. That pattern might imply interesting and undiscovered sexual selection in this species. We found instead that the distribution of body size variation (weight, shell length) was similar between males and females of R. rectius, and mean sizes were not different between the sexes. However, we noted a male-biased sex ratio (～1:1.3) in our large sample of individuals. Many live scaphopods (and several dead shells) showed partial or complete boreholes drilled by predatory gastropods. Boreholes were observed on males and females in similar proportions. We collected scaphopods along with multiple individuals of one likely scaphopod predator, the small moon snail Euspira pallida, and in the lab we observed successful attacks by moon snails on tusk shells.     

Variation in sexual size dimorphism among populations: testing the differential‐plasticity hypothesis

Sexual size dimorphism (SSD) is a common phenomenon in animals and varies widely among species and among populations within species. Much of this variation is likely due to variance in selection on females vs. males. However, environmental variables could have different effects on females vs. males, causing variation in dimorphism. In this study, we test the differential‐plasticity hypothesis, stating that sex‐differential plasticity to environmental variables generates among‐population variation in the degree of sexual dimorphism. We examined the effect of temperature (22, 25, 28, and 31 °C) on sexual dimorphism in four populations of the cockroach Eupolyphaga sinensis Walker (Blattaria: Polyphagidae), collected at various latitudes. We found that females were larger than males at all temperatures and the degree of this dimorphism was largest at the highest temperature (31 °C) and smallest at the lowest temperature (22 °C). There is variation in the degree of SSD among populations (sex*population interaction), but differences between the sexes in their plastic responses (sex*temperature interaction) were not observed for body size. Our results indicated that sex‐differential plasticity to temperature was not the cause of differences among populations in the degree of sexual dimorphism in body size.     

Relative growth,ontogeny, and sexual dimorphism in gorilla(Gorilla gorilla gorilla and G. g. beringei): Evolutionary and ecological considerations

Gorillas are the largest and among the most sexually dimorphic of all extant primates. While gorillas have been incorporated in broad-level comparisons among large-bodied hominoids or in studies of the African apes, comparisons between gorilla subspecies have been rare. During the past decade, however, behavioral, morphological, and molecular data from a number of studies have indicated that the western lowland (Gorilla gorilla gorilla) and eastern mountain (Gorilla gorilla beringei) subspecies differ to a greater extent than has been previously believed. In this study I compare patterns of relative growth of the postcranial skeleton to evaluate whether differences between subspecies result from the differential extension of common patterns of relative growth. In addition, patterns of ontogeny and sexual dimorphism are also examined. Linear skeletal dimensions and skeletal weight were obtained for ontogenetic series of male and female G.g. gorilla (n = 315) and G.g. beringei (n = 38). Bivariate and multivariate methods of analysis were used to test for differences in patterns of relative growth, ontogeny, and sexual dimorphism between sexes of each subspecies and in same-sex comparisons between subspecies. Results indicate males and females of both subspecies are ontogenetically scaled for postcranial proportions and that females undergo an earlier skeletal growth spurt compared to males. However, results also indicate that the onset of the female growth spurt occurs at different dental stages in lowland and mountain gorillas and that mountain gorillas may be characterized by higher rates of growth. Finally, data demonstrate lowland and mountain gorilla females do not differ significantly in adult body size, but mountain gorilla males are significantly larger than lowland gorilla males, suggesting mountain gorillas are characterized by a higher degree of sexual dimorphism in body size. Thus, although lowland and mountain gorillas do not appear to have evolved novel adaptations of the postcranium which correlate with differences in locomotor behavior, the present investigation establishes subspecies differences in ontogeny and sexual dimorphism which may be linked with ecological variation. Specifically, these findings are evaluated in the context of risk aversion models which predict higher growth rates and increased levels of sexual dimorphism in extreme folivores. Am. J. Primatol. 43:1–31, 1997. © 1997 Wiley-Liss, Inc.     

Sex‐specific evolution of bite performance in Liolaemus lizards (Iguania: Liolaemidae): the battle of the sexes

Although differential selective pressures on males and females of the same species may result in sex‐specific evolutionary trajectories, comparative studies of adaptive radiations have largely neglected within‐species variation. In this study, we explore the potential effects of natural selection, sexual selection, or a combination of both, on bite performance in males and females of 19 species of Liolaemus lizards. More specifically, we study the evolution of bite performance, and compare evolutionary relationships between the variation in head morphology, bite performance, ecological variation and sexual dimorphism between males and females. Our results suggest that in male Liolaemus, the variation in bite force is at least partly explained by the variation in the degree of sexual dimorphism in head width (i.e. our estimate of the intensity of sexual selection), and neither bite force nor the morphological variables were correlated with diet (i.e. our proxy for natural selection). On the contrary, in females, the variation in bite force and head size can, to a certain extent, be explained by variation in diet. These results suggest that whereas in males, sexual selection seems to be operating on bite performance, in the case of females, natural selection seems to be the most likely and most important selective pressure driving the variation in head size. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 461–475.     

The Role of Sex-specific Plasticity in Shaping Sexual Dimorphism in a Long-lived Vertebrate,the Snapping Turtle <Emphasis Type="Italic">Chelydra serpentina</Emphasis>

Sex-specific plasticity, the differential response that the genome of males and females may have to different environments, is a mechanism that can affect the degree of sexual dimorphism. Two adaptive hypotheses have been proposed to explain how sex-specific plasticity affects the evolution of sexual size dimorphism. The adaptive canalization hypothesis states that the larger sex exhibits lesser plasticity compared to the smaller sex due to strong directional selection for a large body size, which penalizes individuals attaining sub-optimal body sizes. The condition-dependence hypothesis states that the larger sex exhibits greater plasticity than the smaller sex due to strong directional selection for a large body size favoring a greater sensitivity as an opportunistic mechanism for growth enhancement under favorable conditions. While the relationship between sex-specific plasticity and sexual dimorphism has been studied mainly in invertebrates, its role in long-lived vertebrates has received little attention. In this study we tested the predictions derived from these two hypotheses by comparing the plastic responses of body size and shape of males and females of the snapping turtle (Chelydra serpentina) raised under common garden conditions. Body size was plastic, sexually dimorphic, and the plasticity was also sex-specific, with males exhibiting greater body size plasticity relative to females. Because snapping turtle males are larger than females, sexual size dimorphism in this species appears to be driven by an increased plasticity of the larger sex over the smaller sex as predicted by the condition-dependent hypothesis. However, male body size was enhanced under relatively limited resources, in contrast to expectations from this model. Body shape was also plastic and sexually dimorphic, however no sex by environment interaction was found in this case. Instead, plasticity of sexual shape dimorphism seems to evolve in parallel for males and females as both sexes responded similarly to different environments.     

Statural growth in known-age African elephants (Loxodonta africana)

The shoulder heights of 224 females and 170 males, and hindfoot length of 236 female and 217 male known-age African elephants ( Loxodonta africana ) were measured, and growth curves constructed for each measure of size. A linear relationship between foot length and shoulder height was confirmed in simultaneous measures of 97 males and 110 females. Growth curves demonstrated the typical sexual dimorphism in both foot length and shoulder height, with males growing more rapidly than females from birth onwards. The size dimorphism in foot length and shoulder height becomes marked by the age of 10 years, with males on average being 60–70 cm taller than females at 65 years. This size dimorphism is produced through faster growth which continues for longer than does that of females. The variance in growth rates is slightly greater for females than for males. It is proposed that female growth after puberty is affected by a trade-off between growth and reproduction, while males who deviate markedly from typical patterns of growth may be subject either to mortality or energetic constraints limiting their potential variance.     

Female‐biased size dimorphism in a diapausing caddisfly,Mesophylax aspersus: effect of fecundity and natural and sexual selection

1. The effect of mating success, female fecundity and survival probability associated with intra‐sex variation in body size was studied in Mesophylax aspersus, a caddisfly species with female‐biased sexual size dimorphism, which inhabits temporary streams and aestivates in caves. Adults of this species do not feed and females have to mature eggs during aestivation. 2. Thus, females of larger size should have a fitness advantage because they can harbour more energy reserves that could influence fecundity and probability of survival until reproduction. In contrast, males of smaller size might have competitive advantages over others in mating success. 3. These hypotheses were tested by comparing the sex ratio and body size of individuals captured before and after the aestivation period. The associations between body size and female fecundity, and between mating success and body size of males, were explored under laboratory conditions. 4. During the aestivation period, the sex ratio changed from 1 : 1 to male biased (4 : 1), and a directional selection on body size was detected for females but not for males. Moreover, larger clutches were laid by females of larger size. Finally, differences in mating success between small and large males were not detected. These results suggest that natural selection (i.e. the differential mortality of females associated with body size) together with possible fecundity advantages, are important factors responsible of the sexual size dimorphism of M. aspersus. 5. These results highlight the importance of taking into account mechanisms other than those traditionally used to explain sexual dimorphism. Natural selection acting on sources of variation, such as survival, may be as important as fecundity and sexual selection in driving the evolution of sexual size dimorphism.     

Multivariate and geometric morphometrics in the analysis of sexual dimorphism variation in Podarcis lizards

Podarcis bocagei and P. carbonelli are two closely related lacertid species, very similar morphologically and ecologically. We investigated sexual dimorphism patterns presented by both species in allopatry and in sympatry. Sexual size and shape dimorphism patterns were analyzed using both multivariate and geometric morphometric techniques. Multivariate morphometrics revealed a marked sexual dimorphism in both species--males being larger with more robust habitus and females presenting a longer trunk. General patterns of sexual size dimorphism are not modified in sympatry, although there is evidence for some morphological change in male head size. The application of geometric morphometrics offered a more detailed image of head shape and revealed that males present a more developed tympanic area than do females, while females have a more rounded head. Differences in the degree of sexual shape dimorphism were detected in sympatry, but no consistent patterns were observed. From the results of the study, and based on previous knowledge on the populations studied, we conclude that the morphological differences observed are probably not caused by exploitative competition between the species, but rather appear attributable to the modification of the relative influence of sexual and natural selection on both sexes.     

Shape,size, and maturity trajectories of the human ilium

Morphological traits of the ilium have consistently been more successful for juvenile sex determination than have techniques applied to other skeletal elements, however relatively little is known about the ontogeny and maturation of size and shape dimorphism in the ilium. We use a geometric morphometric approach to quantitatively separate the ontogeny of size and shape of the ilium, and analyze interpopulation differences in the onset, rate and patterning of sexual dimorphism. We captured the shape of three traits for a total of 191 ilia from Lisbon (Portugal) and London (UK) samples of known age and sex (0–17 years). Our results indicate that a) there is a clear dissociation between the ontogeny of size and shape in males and females, b) the ontogeny of size and shape are each defined by non‐linear trajectories that differ between the sexes, c) there are interpopulation differences in ontogenetic shape trajectories, which point to population‐specific patterning in the attainment of sexual dimorphism, and d) the rate of shape maturation and size maturation is typically higher for females than males. Male and female shape differences in the ilium are brought about by trajectory divergence. Differences in size and shape maturation between the sexes suggest that maturity may confound our ability to discriminate between the sexes by introducing variation not accounted for in age‐based groupings. The accuracy of sex determination methods using the ilium may be improved by the use of different traits for particular age groups, to capture the ontogenetic development of shape in both sexes. Am J Phys Anthropol 156:19–34, 2015 © 2014 Wiley Periodicals, Inc.     

Sexual dimorphism inAnastrepha suspensa (Loew) and other tephritid fruit flies (Diptera: Tephritidae): Possible roles of developmental rate,fecundity, and dispersal

Larger male Caribbean fruit flies are more likely to be chosen as mates and defeat rivals in territorial contests. Yet males are smaller than females. Adaptive explanations for relatively small male size include (1) acceleration of male development to maximize female encounter rates, (2) selection for greater female size to increase fecundity, and (3) selection for body sizes most suitable for sexually dimorphic degrees of mobility, speed, and distance flight. None of these unambiguously accounts for the degree of sexual dimorphism. Male development is not accelerated relative to that of females. On average, males remain inside fruit longer than females and those males with extended development periods are smaller than more rapidly developing individuals. There is no evidence that female enlargement alone, presumably for greater fecundity, has generated the degree of dimorphism in the Caribbean fruit fly or other fruit flies. The relationship between dimorphism and mean female body size in 27 species of Tephritidae is the opposite of what would be predicted if differences in dimorphism were due to differences in unilateral female enlargement. Larger size in a species or in one sex of a species may be an adaptation for extensive flight. In general, among 32 species of fruit flies, as body size increases, wing shape becomes progressively more suited for distance flight. However, there are important exceptions to this correlation. Both sexual selection and nonadaptive allometries may contribute to the range of dimorphisms within the family.     

Clinal variation in body size and sexual dimorphism in an Indian fruit bat, Cynopterus sphinx (Chiroptera: Pteropodidae)

Geographic variation in body size and sexual dimorphism of the short‐nosed fruit bat (Cynopterus sphinx) was investigated in peninsular India. Bats were sampled at 12 localities along a 1200 km latitudinal transect that paralleled the eastern flanks of the Western Ghats. The geographic pattern of variation in external morphology of C. sphinx conforms to the predictions of Bergmann's Rule, as indicated by a steep, monotonic cline of increasing body size from south to north. This study represents one of the first conclusively documented examples of Bergmann's Rule in a tropical mammal and confirms that latitudinal clines in body size are not exclusively restricted to temperate zone homeotherms. Body size was indexed by a multivariate axis derived from principal components analysis of linear measurements that summarize body and wing dimensions. Additionally, length of forearm was used as a univariate index of structural size to examine geographic variation in a more inclusive sample of bats across the latitudinal transect. Multivariate and univariate size metrics were strongly and positively correlated with body mass, and exhibited highly concordant patterns of clinal variation. Stepwise multiple regression on climatological variables revealed that increasing size of male and female C. sphinx was associated with decreasing minimum temperature, increasing relative humidity, and increasing seasonality. Although patterns of geographic size variation were highly concordant between the sexes, C. sphinx also exhibited a latitudinal cline in the magnitude and direction of sexual size dimorphism. The size differential reversed direction across the latitudinal gradient, as males averaged larger in the north, and females averaged larger in the south. The degree of female‐biased size dimorphism across the transect was negatively correlated with body size of both sexes. Canonical discriminant analysis revealed that male‐ and female‐biased size dimorphism were based on contrasting sets of external characters. Available data on geographic variation in the degree of polygyny in C. sphinx suggests that sexual selection on male size may play a role in determining the geographic pattern of sexual size dimorphism.     

Quantitative analysis of sexual dimorphism and sex ratio in Hyphydrus ovatus (Coleoptera: Dytiscidae)

Sexual dimorphism in size, shape, and mass relative to length, and sex ratios are quantified for populations of Hyphydrus ovatus. a dytiscid beetle Males of H ovatus are not only longer than females, but also significantly larger in elytron length, thorax width, head width, leg length, total width, total depth, and abdominal segment length Two local populations differ slightly but significantly in total depth and abdominal segment length, but sexual dimorphism in size is similar for the two populations Hyphydrus ovatus are also sexually dimorphic in shape, with males having relatively broader heads and thoraxes than females The two populations differed slightly but significantly in relative abdominal segment length, but as with size, sexual dimorphism in shape is similar for the two populations Males are relatively heavier than females, although the slope of the log mass vs log length relationship is the same for the two sexes Sex ratios in field samples vary significantly over the summer, with percent females declining from c 50% to c 15% Sex ratios are significantly below 50% females m two of five monthly samples and in the total pooled set of samples Sexual dimorphism in size, shape, and relative mass, combined with male-biased sex ratios suggest that larger size of male H ovatus is a product of sexual selection     

Sex-specific niche partitioning and sexual size dimorphism in Australian pythons (Morelia spilota imbricata)

Sexual dimorphism is usually interpreted in terms of reproductive adaptations, but the degree of sex divergence also may be affected by sex-based niche partitioning. In gape-limited animals like snakes, the degree of sexual dimorphism in body size (SSD) or relative head size can determine the size spectrum of ingestible prey for each sex. Our studies of one mainland and four insular Western Australian populations of carpet pythons ( Morelia spilota ) reveal remarkable geographical variation in SSD, associated with differences in prey resources available to the snakes. In all five populations, females grew larger than males and had larger heads relative to body length. However, the populations differed in mean body sizes and relative head sizes, as well as in the degree of sexual dimorphism in these traits. Adult males and females also diverged strongly in dietary composition: males consumed small prey (lizards, mice and small birds), while females took larger mammals such as possums and wallabies. Geographic differences in the availability of large mammalian prey were linked to differences in mean adult body sizes of females (the larger sex) and thus contributed to sex-based resource partitioning. For example, in one population adult male snakes ate mice and adult females ate wallabies; in another, birds and lizards were important prey types for both sexes. Thus, the high degree of geographical variation among python populations in sexually dimorphic aspects of body size and shape plausibly results from geographical variation in prey availability.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 113–125.     

The evolution of condition-dependent sexual dimorphism

Theory suggests that the net benefit of allocating resources to a sexual trait depends both on the strength of sexual selection on that trait and on individual condition. This predicts a tight coevolution between sexual dimorphism and condition dependence and suggests that these patterns of within-sex and between-sex variation may share a common genetic and developmental basis. Although condition-dependent expression of sexual traits is widely documented, the extent of covariation between condition dependence and sexual dimorphism remains poorly known. I investigated the effects of condition (larval diet quality) on multivariate sexual dimorphism in the fly Telostylinus angusticollis (Neriidae). Condition determined the direction of sexual size dimorphism and modulated sexual shape dimorphism by affecting allometric slopes and/or intercepts of sexually homologous traits in both sexes. Although the greatest responses to condition manipulation were observed in male sexual traits, both sexual and nonsexual traits exhibited substantial variation in the nature and magnitude of condition effects. Nonetheless, condition dependence and sexual dimorphism were remarkably congruent: variation in the strength of condition effects on male traits explained more than 90% of the variation in the magnitude of sexual dimorphism, whether quantified in terms of trait size or allometric slope. The genetic mechanisms that give rise to multivariate sexual dimorphism in body shape thus function in a strongly condition-dependent manner in this species, suggesting a common genetic basis for body shape variation within and between sexes.     

Brief communication: Morphometric data for adult lion-tailed macaques (Macaca silenus)

Basic morphometric data were collected from 22 adult lion-tailed macaques (M. silenus) of both sexes. M. silenus is a rare primate species from which adequate morphometric data have not heretofore been available for comparative purposes. Data collected include measures of gross body size (weight; crown-rump and rump-heel length), and for males, measures of secondary sexual characteristics (canine tooth and testes size). Degree of sexual dimorphism was marked, with males significantly larger and heavier than females. The three body size measures were correlated for males but not for females. There was substantial variation among individual males in secondary sex characteristics measurements. The data indicate than lion-tailed macaque morphometrics are consonant with the general pattern of positive allometry for body size and sexual dimorphism characteristic of the primate order.     

Body size, age and paternity in common brushtail possums (Trichosurus vulpecula)

Sexual selection should produce sexual size dimorphism in species where larger members of one sex obtain disproportionately more matings. Recent theory suggests that the degree of sexual size dimorphism depends on physical and temporal constraints involving the operational sex ratio, the potential reproductive rate and the trade-off between current reproductive effort and residual reproductive value. As part of a large-scale experiment on dispersal, we investigated the mating system of common brushtail possums inhabiting old-growth Eucalyptus forest in Australia. Paternity was assigned to 20 of 28 pouch-young (maternity known) genotyped at six microsatellite loci. Male mating success was strongly related to body size and age; male body weight and age being highly correlated. Despite disproportionate mating success favouring larger males, sexual size dimorphism was only apparent among older animals. Trapping and telemetry indicated that the operational sex ratio was effectively 1 : 1 and the potential reproductive rate of males was at most four times that of females. Being larger appeared to entail significant survival costs because males 'died-off' at the age at which sexual size dimorphism became apparent (8-9 years). Male and female home ranges were the same size and males appeared to be as sedentary as females. Moreover, longevity appears to be only slightly less important to male reproductive success than it is to females. It is suggested that a sedentary lifestyle and longevity are the key elements constraining selection for greater sexual size dimorphism in this 'model' medium-sized Australian marsupial herbivore.