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     

Variation in mating systems and sexual size dimorphism between populations of the Australian python Morelia spilota (Serpentes: Pythonidae)

Although adaptationist hypotheses predict a functional relationship between mating systems and sexual size dimorphism, such predictions are difficult to test because of the high degree of phylogenetic conservatism in both of these traits. Taxa that show intraspecific variation in mating systems hence offer valuable opportunities for more direct tests of evolutionary-ecological hypotheses. Based on a collation of published and unpublished records, we document intraspecific geographic variation in mating systems (presence versus absence of male-male combat) within the widely-distributed Australian python Morelia spilota. Radiotelemetric monitoring of 19 free-ranging pythons in a population in north-eastern New South Wales showed that these animals display a mating system of female defence polygyny. Previous studies on a southern population of the same species found that males engaged in long mate-searching movements, showed no overt agonistic behavior, and formed long-term (>2 months) aggregations around reproductive females. In strong contrast, our adult male carpet pythons (i) moved about relatively little (mean displacement <11 m/day) during the mating season, (ii) remained with females only briefly (<5 days), and (iii) engaged in male-male combat in the vicinity of females. This male-male combat included vigorous biting as well as ritualised wrestling matches, resulting in a high incidence of bite scars in adult males. In keeping with predictions from sexual selection theory, males attain larger body sizes than females in this population, whereas females grow larger than males in the previously-studied southern population where males do not engage in physical combat for mating opportunities.     

Factors influencing the degree of sexual size dimorphism within and among calanoid copepod species

Populations of Diaptomus leptopus (Copepoda: Calanoida) and other calanoid copepods exhibit varying degrees of sexual size dimorphism. We examined whether intraspecific or interspecific variation in dimorphism could be explained by allometry, and we examined the relationship between adult size attained and development rate to determine any relationship between the two. We compared the degree of sexual size dimorphism in D. leptopus and in other calanoid copepods inhabiting temporary and permanent habitats. Allometry did not explain variation in sexual size dimorphism within or among populations or among species. Permanence of habitat affected the degree of dimorphism: dimorphism was greater within and among species inhabiting temporary environments. Non-significant differences in development rate were found among populations and significant differences were found between sexes of D. leptopus when reared under identical laboratory conditions: males developed more rapidly than females but there was no general relationship between development rate and adult size. Potential adaptive hypotheses to explain the differences between populations inhabiting temporary and permanent habitats are discussed.     

The adaptive significance of parental role division and sexual size dimorphism in breeding shorebirds

Sexual size dimorphism among 57 species in the shorebird family Scolopacidae is evaluated in relation to parental role division during breeding. Normal size dimorphism, i.e. the female being smaller than the male, occurs in species where the female has the main responsibility for parental care, whereas reverse size dimorphism, the most common pattern among shorebirds, is associated with reversed parental roles. Pronounced dimorphism between sexes occurs, besides in body size, also in bill length, where the sex undertaking the main part of brood attendance has a disproportionately short bill in species adapted for foraging by deep probing. A small body size is of adaptive value to attain high parental efficiency for energetic reasons, because smaller individuals need less energy to maintain themselves. Short bills may be advantageous during brood attendance when feeding mainly takes place in terrestrial habitats together with the chicks. Females released from parental care duties are favoured by a larger body size allowing increased accumulation of energy reserves for egg production. There are obvious parallels between shorebirds and raptors concerning the adaptive significance of reverse sexual size dimorphism and parental role division.     

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.     

Sexual behaviour and evolution of sexual dimorphism in body size in Jaera (Isopoda Asellota)

Individuals of the genus Jaera do not mate at random. In the species from the Mediterranean group, J. italica and. J. nordmanni, large males and medium sized females are at an advantage and their sizes are positively assorted. These effects are attributable to sexual competition between males. In the Ponlo-caspian species J. istri, no advantage of large males exists, but sexual selection could be the cause for a long passive phase prior to copulation and for normalizing selection upon female size at pairing. In the Atlantic species, J. albifrons, no selection can be ascertained.
Differential mating success in males appears as one of the causes of the evolution of sexual dimorphism in body size, which makes males larger, of equal size, or smaller than females according to the species. The reason for this reversal in dimorphism seems to differ in the two sexes. Sexual selection provides an explanation for the evolution of male size, while the interspecific changes in female length are more likely due to ecological factors.     

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.     

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.     

Gamete production and sexual size dimorphism in an insect (Orchesella cincta) with indeterminate growth

Abstract 1. The relationship of growth and body size with reproductive effort in animal species has been studied much less for males than for females. This imbalance applies to Orchesella cincta (L.) (Collembola), an insect with indeterminate growth, in which egg production is related positively to body size and negatively to growth.
2. To allow a comparison of the reproductive effort of male and female O. cincta , development and growth in immature stages of both sexes, and growth and spermatophore production for adult males were studied.
3. Embryonic development time and hatchling size did not differ between prospective males and females, but from hatching on the trajectories diverged, with males growing more slowly and maturing earlier and at a much smaller body size than females.
4. Neither the number of spermatophores deposited in the first adult instar (= inter-moult period) nor the total number of spermatophores deposited during seven instars was related to body size or growth.
5. Differences in growth rate between instars with and without spermatophore deposition indicated that the physiology of spermatophore production inhibits growth, which, however, was compensated for during the next instar.
6. The difference in the relationship of gamete production with body size and growth between males and females explains the divergence of their size at maturity.     

Body size, male combat and the evolution of sexual dimorphism in eublepharid geckos (Squamata: Eublepharidae)

Lizards of the family Eublepharidae exhibit interspecific diversity in body size, sexual size dimorphism (SSD), head size dimorphism (HSD), occurrence of male combat, and presence of male precloacal pores. Hence, they offer an opportunity for testing hypotheses for the evolution and maintenance of sexual dimorphism. Historical analysis of male agonistic behaviour indicates that territoriality is ancestral in eublepharid geckos. Within Eublepharidae, male combat disappeared twice. In keeping with predictions from sexual selection theory, both events were associated with parallel loss of male-biased HSD and ventral scent glands. Eublepharids therefore provide new evidence that male-biased dimorphic heads are weapons used in aggressive encounters and that the ventral glands probably function in territory marking rather than in intersexual communication. Male-biased SSD is a plesiomorphic characteristic and was affected by at least three inversions. Shifts in SSD and male combat were not historically correlated. Therefore, other factors than male rivalry appear responsible for SSD inversions. Eublepharids demonstrate the full scope of Rensch's rule (small species tend to be female-larger, larger species male-larger). Most plausibly, SSD pattern hence seems to reflect body size variation. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 303–314.     

Reversed sexual size dimorphism in microtines: Are females larger than males or are males smaller than females?

Summary We analysed sexual size dimorphism for 21 populations of microtine rodents. Female to male size ratio varied considerably among populations from females significantly larger than males (ratio=1.18) to males larger than females (ratio=0.78). In a multiple regression analysis female to male home range size ratio explained 94% of the total variation in body size dimorphism and was the only one of eight independent variables that was selected in a stepwise regression procedure. When females are the larger sex, males have home range sizes much larger than females. We suggest that the relationship between home range size ratio and body weight size dimorphism reflects different selection pressures on males and females in competition for resources and mates.     

Sexual size monomorphism in the crested porcupine (Hystrix cristata)

Amongst mammals, female-biased sexual size dimorphism (SSD) is rare and it occurs mostly in species where reduced male intrasexual competition is present. Reverse SSD has been reported for Old World porcupines Hystrix spp. We compared weight and six metric body measurements of 40 male and 42 female crested porcupines from Southern Tuscany, Italy. No significant difference was observed between sexes. The monogamous mating system of porcupines, sharing parental care, together with no evidence of territoriality, militate against previous claims of SSD presence, probably due to small sample size and inappropriate statistical analyses.     

Male-biased size dimorphism in ichneumonine wasps (Hymenoptera: Ichneumonidae) – the role of sexual selection for large male size

Abstract.  1. Sexual differences in body size are expected to evolve when selection on female and male sizes favours different optima.
2. Insects have typically female-biased size dimorphism that is usually explained by the strong fecundity advantage of larger size in females. However, numerous exceptions to this general pattern have led to the search for selective pressures favouring larger size in males.
3. In this study, the benefits of large size were investigated in males of four species of ichneumonine wasps, a species-rich group of parasitoids, many representatives of which exhibit male-biased size dimorphism.
4. Mating behaviour of all ichneumonine wasps are characterised by pre-copulatory struggles, in the course of which males attempt to override female reluctance to mate. A series of laboratory trials was conducted to study the determinants of male mating success.
5. A tendency was found for larger males as well as those in better condition to be more successful in achieving copulations. Size dimorphism of the species studied, mostly male-biased in hind tibia length but female-biased in body weight, indicates that sexual selection in males favours longer bodies and appendages rather than larger weight.
6. The qualitative similarity of the mating patterns suggests that sexual selection cannot completely explain the considerable among-species differences in sexual size dimorphism.
7. The present study cautions against using various size indices as equivalents for calculating sexual size dimorphism.
8. It is suggested that female reluctance in ichneumonine wasps functions as a mechanism of female mate assessment.     

Causes of secondary sexual differences in plants — Evidence from extreme leaf dimorphism in Leucadendron (Proteaceae)

In extreme cases leaves in male plants of the dioecious genus Leucadendron (Proteaceae) are up to an order of magnitude smaller than female leaves. This secondary sexual dimorphism (SSD) in leaf size has previously been suggested to be due to intra-male sexual selection, leading to an increase in male allocation to reproduction in dimorphic species. After critically evaluating previous data provided to support this hypothesis, I suggest on both theoretical grounds and on re-analysis that this argument is unlikely and unsupported. Leaf size dimorphism could theoretically evolve directly due to disruptive ecological selection between genders, leading to niche dimorphism either within or between habitats. I test this ecological causation hypothesis by providing data on specific leaf area (sla) and water use efficiency (δ ¹³C) of leaves from males and females of several Leucadendron species. Results confirm the expectation of minimal gender differences. I argue that leaf dimorphism is a consequence of selection on flower size and architecture.     

Variation in oxygen consumption of the western diamondback rattlesnake (Crotalus atrox): implications for sexual size dimorphism

Variation in metabolism affects energy budgets of individuals and may serve as a mechanism that influences variation at whole organism or population levels. For example, sex differences in metabolic expenditure may contribute to bioenergetic sources of sexual size dimorphism. We measured oxygen consumption rates of 48 western diamondback rattlesnakes (Crotalus atrox) from a sexually dimorphic population and tested the effects of body mass, body temperature and time of day, in three groups of snakes: males, non-reproductive females, and vitellogenic females. Metabolic rates of male and non-reproductive female C. atrox were similar to rates reported for other rattlesnakes (mass exponents ranging from 0.645–0.670). Oxygen consumption was affected by body mass, body temperature and time of day, and was approximately 1.4 times greater in vitellogenic females than in non-reproductive females. No differences were found between males and non-reproductive females. Accordingly, differences in metabolic rate apparently do not contribute directly to sexual dimorphism in this population. Nevertheless, estimates of size-dependent maintenance expenditure lead us to hypothesize that adult female body size may represent a compromise between selection for increased litter size (accomplished by increasing body size), and selection for increased reproductive frequency (accomplished by decreasing body size, and, therefore inactive maintenance expenditure); this is a mechanistic scenario suggested previously for some endotherms. Accepted: 20 May 1998     

On the sexual dimorphism in the skull of the tiger (Panthera tigris)

Sexual dimorphism in the skull of the tiger (Panthera tigris) is reviewed and described in detail. The most significant diagnostic differences between the sexes are absolute length of the cranium, breadth of interorbital region and muzzle, zygomatic arch, and occipital region, length of upper carnassial, and the degree of the development of the cranial prominences. The degree of sexual dimorphism is closely related to geographic variation, and its form is rather complex.     

Female-biased sexual size dimorphism in the yellow-pine chipmunk (Tamias amoenus): sex-specific patterns of annual reproductive success and survival

Sexual size dimorphism is ultimately the result of independent, sex-specific selection on body size. In mammals, male-biased sexual size dimorphism is the predominant pattern, and it is usually attributed to the polygynous mating system prevalent in most mammals. This sole explanation is unsatisfying because selection acts on both sexes simultaneously, therefore any explanation of sexual size dimorphism should explain why one sex is relatively large and the other is small. Using mark-recapture techniques and DNA microsatellite loci to assign parentage, we examined sex-specific patterns of annual reproductive success and survival in the yellow-pine chipmunk (Tamias amoenus), a small mammal with female-biased sexual size dimorphism, to test the hypothesis that the dimorphism was related to sex differences in the relationship between body size and fitness. Chipmunks were monitored and body size components measured over three years in the Kananaskis Valley, Alberta, Canada. Male reproductive success was independent of body size perhaps due to trade-offs in body size associated with behavioral components of male mating success: dominance and running speed. Male survival was consistent with stabilizing selection for overall body size and body size components. The relationship between reproductive success and female body size fluctuated. In two of three years the relationship was positive, whereas in one year the relationship was negative. This may have been the result of differences in environmental conditions among years. Large females require more energy to maintain their soma than small females and may be unable to maintain lactation in the face of challenging environmental conditions. Female survival was positively related to body size, with little evidence for stabilizing selection. Sex differences in the relationship between body size and fitness (reproductive success and survival) were the result of different processes, but were ultimately consistent with female-biased sexual size dimorphism evident in this species.