Significant Sexual Size Dimorphism Research Articles

Ecological notes on two sympatric Psammophis species from East Africa

We investigated the ecology of two sympatric whipsnakes from East Africa (Psammophis punctulatus trivirgatus and P. biseriatus) by examining preserved museum specimens (n = 40). Both species lacked any significant sexual size dimorphism and displayed seasonal reproductive cycles. These findings are consistent with previous investigations of allopatric congeners using similar methodologies. While sample sizes for this study are small, our data lend support to the notion that many ecological traits of this speciose genus are constant across the heterogeneous environments of Africa and Asia they inhabit.

Intra- and interspecific skull variation in two sister species of the subterranean rodent genusCtenomys(Rodentia, Ctenomyidae): coupling geometric morphometrics and chromosomal polymorphism

The great and rapid process of chromosomal diversification of the genus Ctenomys has highlighted these rodents as an excellent model for studying evolution. Despite their karyotypic diversity, tuco-tucos have a conservative skull form related to their subterranean habit. In this study, 221 karyotyped specimens were analyzed with geometric morphometric methods for dorsal, ventral, and lateral views of the skull in two sister species occurring in southern Brazil and Uruguay: Ctenomys torquatus (2n = 40, 44, and 46) and Ctenomys pearsoni. The geometric descriptors showed significant sexual size dimorphism and significant shape differences between sexes, species, and chromosomal groups. The differences between C. pearsoni chromosomal groups (2n = 66 and 70) were of the same order of magnitude as the interspecific differences. We detected a large unexpected morphometric difference between Brazilian and Uruguayan C. torquatus populations with 2n = 44, showing that the morphometric differentiation is not necessarily associated with karyotypic evolution. In conclusion, geometric morphometrics revealed more skull shape variation than expected, both at the intra- and interspecific levels. We propose that a combination of drift and selection on the skull may be at the origin of this diversity. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155, 220-237.

Geographic trend in sexual size dimorphism and body size of Osteopilus septentrionalis (Cuban treefrog): implications for invasion of the southeastern United States

We collected Osteopilus septentrionalis from Central Florida for analysis of sexual size dimorphism and compared our data to published information from populations across the native and introduced range of the species. We found significant sexual size dimorphism (females larger), with the degree of dimorphism decreasing with increasing degrees north latitude, a trend largely driven by a pronounced decrease in female mean snout to vent length. Potential explanations for this trend include reduced growth rates, increased time to maturity, and reduced life expectancy, all of which may be tied to climatic variation. A trend of decreasing female mean size toward the northernmost extent of the current introduced range of this species may be indicative of diminished impacts and invasive success. Geographical variation in morphology and its associated implications should be evaluated when considering the potential impacts of invasive species.

Life-history traits in green toad (Bufo viridis) populations: indicators of habitat quality

Five life-history traits (age and size at maturity, longevity, potential reproductive life span, age-dependent growth rate) were investigated in four Bufo viridis Laurenti, 1768 (= Pseudepidalea viridis (Laurenti, 1768)) populations that inhabited localities at similar altitude (60–100 m above sea level) and latitude (50°N–51°N, Germany), but that differed in habitat quality (i.e., human land use within a radius of 1 km around the breeding site). The age of 374 males and of 127 females collected during the breeding period was estimated using skeletochronology on phalange bones. We tested the hypothesis that sex and habitat quality account for detectable amounts of local variation in life-history traits. Significant sexual size dimorphism was present in all populations. Gender-specific variation in size was mainly accounted for by age, but also to a minor extent by habitat quality. In males, age at maturity varied between 1 and 3 years and was the only life-history trait that was significantly related to the intensity of human land use. In contrast, land-use indices covaried significantly with female longevity (6–15 years) and potential reproductive life span (5–12 years). Our pilot study suggests that, in B. viridis, life-history traits derived from the local age structure may be useful as indicators of habitat quality.

Reproductive Behaviour of Japanese Tubesnout, Aulichthys japonicus (Gasterosteiformes), in the Natural Habitat Compared with Relatives

We observed significant sexual dimorphism in size (females > males) and in colour in Japanese tubesnout, Aulichthys japonicus, in Aburatsubo Bay, central Japan, and Otsuchi Bay, northern Japan. This species is known to spawn its eggs into ascidians. We observed reproductive behaviour of tubesnout in the natural environment in both bays. All the males established territories at harbours or rocky shores and showed conspicuous nuptial colouration with metallic green or blue on their snout and bright yellow on the body. Females had a dark brown body with remarkable white spots. The male territories varied somewhat in location and extent on a daily basis. The individual male size and territory size were not correlated. The females usually stayed in the seaweed beds where no males were present. Males and females were distributed separately, and the gravid females visited the male territories in groups to spawn. The territorial males courted the gravid females by rushing and returning, and tried to lead the females to the particular place of their territories by tail beating. We discuss nuptial colouration, courtship behaviour and territoriality in comparison to the related species; Aulorhynchus flavidus, Hypoptychus dybowskii, and gasterosteid fishes.

Ecological aspects of the casque-headed frog Aparasphenodon brunoi (Anura, Hylidae) in a Restinga habitat in southeastern Brazil

We describe some aspects of the ecology of Aparasphenodon brunoi, a species associated with bromeliads. We comment on the relationships of this species with bromeliad size, microhabitat use, diet and sexual dimorphism. This study was conducted on a Restinga habitat near Presidente Kennedy, state of Espírito Santo, southeastern coast of Brazil. When the animals were found inside the bromeliads, we measure bromeliad and head size of frogs. We analyzed stomach contents and determined the sex and reproductive condition. We found 17 individuals (58.6%) in bromeliad leafs, six (20.7%) in Cactaceae, three (10.3%) in liana and three (10.3%) on trunks. The correlation between head measurements and bromeliad size were high, indicating that animals apparently use bromeliads based on their size, which could be related to the minimization of water loss. The most common prey items were beetles, ants, and insect larvae, suggesting that the species is relatively generalist in prey consumption. Aparasphenodon brunoi showed significant sexual size and shape dimorphism with females having larger bodies than males (size) and females having tibia, eye diameter and SVL larger than males (shape), but larger sample size and more detailed ecological and life history data are needed to elucidate the factors that have led to sexual size dimorphism.

Growth, reproduction and recruitment of the endangered brackish water snail Iravadia (Fairbankia) sakaguchii (Gastropoda : Iravadiidae)

The population structure and reproductive activity of the brackish water snail Iravadia (Fairbankia) sakaguchii, which lives under partially buried stones on intertidal mudflats, were investigated monthly in the Waka River Estuary, Wakayama, central Japan. Observations of the reproductive organs indicated the main reproductive season as being from June to July. Maturation size was estimated to be approximately 4.0 mm in shell length (SL) in females and 3.9 mm SL in males, there being no significant sexual dimorphism in adult body size. The occurrence of snails smaller than 2 mm SL indicated that recruitment occurred between August and October, all the recruits co-occurring with adults under stones. The recruits grew to a large size (>3 mm SL) and reached maturity in August of the second year (1 year old), longevity being estimated as at least 37–38 months. Throughout the study period of 28 months, the population mostly comprised individuals between 3 and 5 mm SL, with recruits being fewer than older snails. The population density decreased each year, this being attributable to a low level of recruitment.

Feeding relationships between sympatric Afrotropical tree frogs (genus Hyperolius): The effects of predator body size and season

Abstract The feeding relationships of four sympatric, ecologically nearly unknown, tree frogs of southern Nigeria (Hyperolius sylvaticus, H. guttulatus, H. fusciventris burtoni, and H. concolor) were studied during both the dry and the rainy seasons. Stomach contents were obtained by stomach flushing. The largest species were H. guttulatus and H. concolor, whereas the other two species were of smaller average size. A statistically significant sexual size dimorphism was observed in H. guttulatus, H. concolor, and H. sylvaticus (in all species the females were larger), but not in H. fusciventris. In all four species, the frequency of animals with empty stomachs was significantly lower during the rainy season than during the dry season. The four frog species were mainly arthropodeating: Araneidae and Blattoidea were preyed upon mainly by the two largest species, Acarina and Lepidoptera were taken mainly by the two smaller species, and Diptera and Formicoidea were eaten frequently by all species. The diet composition was similar between seasons in three of the four species, and H. guttulatus was the only species showing considerable diet variation between seasons. We concluded that 1) the intensity of interspecific competition should be particularly high between the two larger species on the one hand, and between the two smaller species on the other hand; and 2) the intensity of the interspecific competition should be relatively constant throughout the year.

Ecology of the Gecko Gymnodactylus geckoides amarali in a Neotropical Savanna

We describe the ecology of the gecko Gymnodactylus geckoides amarali in the Cerrado of central Brazil and using published data, compare it with the conspecific Gymnodactylus geckoides geckoides from the Caatinga of northeastern Brazil. Gymnodactylus geckoides amarali is rupicolous, living primarily in rock crevices in “campos rupestres” (rocky fields), which are highly patchy in Cerrado. Field body temperatures were low and associated with environmental temperatures, suggesting thermoconformity. Termites were the most important food item, and consumption was greatest during the dry season. We suggest that access to termites is facilitated by the shallow soils used by G. g. amarali and by its well-developed chemosensory apparatus. Reproduction was concentrated in the dry season, a pattern known for very few Cerrado squamates, possibly because of the higher availability of termites during that period. Clutch size was significantly correlated with female size, a pattern unknown in any other gecko, and there was no association between egg size and female size. Gymnodactylus geckoides amarali produced larger clutches of smaller eggs relative to G. g. geckoides, presumably because of the high climatic unpredictability of the Caatinga. We detected a significant sexual dimorphism in both body size and shape: females were larger and had smaller heads than males. We suggest that large female size is likely a plesiomorphic character of gekkonids and that large head size in males results from sexual selection theory.

Do snakes exhibit shifts in feeding ecology associated with the presence or absence of potential competitors? A case study from tropical Africa

Two species of rain-forest snakes, the colubrids Natriciteres fuliginoides and Natriciteres variegata, are potential competitors, because they attain, on average, the same body size (20–30 cm long), have similar habits and ecological requirements (they are both diurnal and nocturnal, often semi-aquatic), and have similar dietary spectra ( including both small vertebrates and invertebrates). I studied prey type and predator size – prey size relationships of these snakes in four swamp-forest areas of southeastern Nigeria (West Africa): only N. fuliginoides was found in two of these areas, whereas both species were common in the other two areas. The feeding ecology of N. fuliginoides shifted greatly in the presence of its potential competitor, N. variegata, in terms of prey-type preferences and predatory behaviour as well as in terms of predator size – prey size relationships. In particular, the effects of the potential competitor on the feeding ecology of N. fuliginoides were (i) a shift toward eating many invertebrates from a diet based on many small vertebrates, (ii) a shift toward eating many terrestrial organisms from a diet based on many aquatic organisms, and (iii) a shift in mean prey size for females toward relatively larger prey from a diet based on smaller prey. The resulting effect was a clear partitioning of food resources between the two predator species in both the areas where they are sympatric, whereas N. fuliginoides had a wider dietary spectrum (covering that of the two species together) in the areas where its competitor is not found. The role of intraspecific competition seems less strong, as no apparent resource partitioning in prey type or prey size was observed between males and females of either species or in any study area. However, mean prey size was significantly greater for females than for males of both species and in all study areas, but depended on the significant reverse sexual size dimorphism observed in both species.

Morphometric study of the Pissodes strobi complex (Coleoptera: Curculionidae)

AbstractMorphometric data were collected on 15 characters in adults of the species of the Pissodes strobi complex (P. nemorensis (Germar), P. schwarzi Hopkins, P. strobi (Peck), and P. terminalis Hopping). Data were examined using stepwise discriminant analysis to determine if all characters contribute significantly to the diagnosis of taxa, and their relative amounts of resolving power were illuminated using canonical variates analysis. Discriminant functions were generated to diagnose species. Separate discriminant functions were generated for each sex for populations from eastern and western North America. Significant sexual dimorphism and variability in body size decrease the utility of morphological characters for diagnosis, but did not prevent the formulation of useful discriminant functions, particularly when selected ratios that control for body size were added.

Population variation in second metacarpal sexual size dimorphism.

This paper contrasts levels of sexual size dimorphism in second metacarpal osteometric and geometric morphology in two bioculturally distinctive populations: 19th century Euro-Canadian settlers, and proto/historic central Canadian Inuit. Significant within-group sexual size dimorphism is found for all variables, though few show significant interpopulation differences. However, in every case the Euro-Canadian sample is more dimorphic than the Inuit sample. Notably, differences reside in geometric measures (total area, Imax) sensitive to variation in functional strain, and thus are interpretable in light of proximate causal models, i.e., activity profiles distinct from generalized mode of subsistence. Other proximate factors, such as nutritional stress acting to diminish Inuit sexual size dimorphism, may also play a role. However, models often cited to explain dimorphism, such as marriage practice (e.g., polygyny) or division of labor situated in mode of subsistence, do not. The higher sexual size dimorphism in the 19th century settler sample belies the notion that technological progress inevitably leads to reduced dimorphism.

Geographic variation in sexual size dimorphism in the rock agama, Agama atra (Sauria: Agamidae)

Significant sexual dimorphism in overall size occurs in the rock agama, Agama atra (Sauria; Agamidae), with males growing larger than females. Geographic variation in the degree of sexual size dimorphism also exists, males growing significantly larger than females in Namaqualand and Namibia compared to populations in other areas. Sexual differences in scaling of head, limb and tail dimensions were mainly the result of differential asymptotic sizes reached by the sexes. Head size was also influenced by a faster increase in head dimensions with increasing snout to vent length in males compared to females, probably as a result of sexual selection. In females, scaling of limb and tail dimensions was decreased compared to males, possibly a result of differential energy allocation to reproduction.

The growth pattern of the African tortoise Geochelone pardalis and other chelonians

Growth rings were measured in dead African leopard tortoises, Geochelone pardalis, collected in the seasonal tropics of Zimbabwe over an 11-year period. A series of Ford-Walford plots using growth measured from annuli showed that growth fitted a logistic by mass curve best, logistic by length and Gompertz curves less well, and a Bertalanffy curve least well. The Bertalanffy curve, often fitted to growth of chelonians, is characterised by particularly high growth rates of juveniles compared with larger individuals. It is suggested that this growth pattern is likely to be found in species showing a marked decrease in diet quality with size. This hypothesis is supported by a review of growth patterns in chelonians: Bertalanffy curves are associated with an omnivorous (and thus potentially variable) diet and other growth patterns with an obligate carnivorous or herbivorous diet. Geochelone pardalis in Zimbabwe showed significant sexual size dimorphism, the mean asymptotic mass of females being 1.7 times that of males, unlike populations with larger body sizes to the north and south. Annual survival estimated from age-frequency distributions was significantly higher in males (0.80) than in females (0.72), the difference being sufficient to account for the male-biased sex ratio of live animals.

Is there any interspecific competition between dwarf crocodiles (Osteolaemus tetraspis) and Nile monitors (Varanus niloticus ornatus) in the swamps of central Africa? A study from south-eastern Nigeria

Aspects of coexistence between the dwarf crocodile Osteolaemus tetraspis and the Nile monitor lizard Varanus niloticus were studied in swamp rain forests of south-eastern Nigeria, central Africa. Crocodiles were significantly smaller than monitor lizards. There was no significant sexual size dimorphism in the examined sample of Varanus, whereas the female size exceeded the male size in Osteolaemus. The food niche breadth was narrower in young than in adults of both species, and V. niloticus showed a narrower niche breadth than O. tetraspis. Crabs constituted the main prey type category for both species, whereas vertebrates were more rarely preyed upon. The diet composition of males and females was similar in V. niloticus, but it was different in O. tetraspis. The general diet composition of the two species was similar (78.2% of overlap), and even the habitats where these two species were found were similar. These interspecific similarities suggest that O. tetraspis and V. niloticus could be potential competitors in the freshwater ecosystems of the Nigerian rainforest. However, it is suggested that the main prey type for these species (crabs) is not limited in the environment, which may minimize interspecific competition. Interference competition can also occur between these species, as suggested by one case of direct predation of the one species (V. niloticus) towards the other species (O. tetraspis). A case of cannibalism was observed in V. niloticus. This could indicate that intraspecific competition can be important for regulating the dynamics of monitor populations in the rainforests of south-eastern Nigeria.

Is there any interspecific competition between dwarf crocodiles (Osteolaemus tetraspis) and Nile monitors (Varanus niloticus ornatus) in the swamps of central Africa? A study from south‐eastern Nigeria

AbstractAspects of coexistence between the dwarf crocodile Osteolaemus tetraspis and the Nile monitor lizard Varanus niloticus were studied in swamp rain forests of south‐eastern Nigeria, central Africa. Crocodiles were significantly smaller than monitor lizards. There was no significant sexual size dimorphism in the examined sample of Varanus, whereas the female size exceeded the male size in Osteolaemus. The food niche breadth was narrower in young than in adults of both species, and V. niloticus showed a narrower niche breadth than O. tetraspis. Crabs constituted the main prey type category for both species, whereas vertebrates were more rarely preyed upon. The diet composition of males and females was similar in V. niloticus, but it was different in O. tetraspis. The general diet composition of the two species was similar (78.2% of overlap), and even the habitats where these two species were found were similar. These interspecific similarities suggest that O. tetraspis and V. niloticus could be potential competitors in the freshwater ecosystems of the Nigerian rainforest. However, it is suggested that the main prey type for these species (crabs) is not limited in the environment, which may minimize interspecific competition. Interference competition can also occur between these species, as suggested by one case of direct predation of the one species (V. niloticus) towards the other species (O. tetraspis). A case of cannibalism was observed in V. niloticus. This could indicate that intraspecific competition can be important for regulating the dynamics of monitor populations in the rainforests of south‐eastern Nigeria.

Ecology of the Chinese Stripe-Necked Turtle, Ocadia sinensis (Testudines: Emydidae), in the Keelung River, Northern Taiwan

An intensive ecological study of a large population of the Chinese stripe-necked turtle, Ocadia sinensis, was conducted in northern Taiwan from February 1995 to May 1996. Of 661 turtles captured and measured, there were 136 unsexedjuveniles, 289 males, and 233 females; the sex ratio was significantly male-biased. Significant sexual size dimorphism was evident. Females grew larger than males; mean body size of females over four years was consistently larger than that of coeval males. Xray photography revealed that females were gravid from March to May. Clutch size averaged 12.6 and varied from 7 to 17. Stomach contents collected from June to October 1995 revealed sexual differences in diet: males tended to consume animal

Effects of Environment on Phenotypic Variation and Sexual Dimorphism in Dipodomys simulans (Rodentia: Heteromyidae)

The magnitude of sexual dimorphism in size among populations of some small mammals living in different habitats may be a reflection of the degree of habitat partitioning between sexes or nutritional quality of available food. We tested these hypotheses by quantifying between-sex differences in size in populations of the Dulzura kangaroo rat ( Dipodomys simulons ) from several macrohabitat types at regional and local geographic scales. On a regional scale, our results indicated a significant shift in morphology of males and females along a north-south latitudinal gradient. Large kangaroo rats inhabited warm and arid southernmost latitudes, whereas small animals inhabited relatively cool and moist regions to the north. This morphologic gradient represented an increase in size with less seasonal variation in precipitation and temperature. This pattern was clearly counter to Bergmann's rule, but consistent with the hypothesis that temporal variation in availability of food may be an important factor leading to geographic variation in size. External and cranial measures of size dimorphism were not correlated significantly with any major geographic factor, and only cranial measures were associated significantly with variation in climate. At a local level, kangaroo rats living in different vegetation communities within the same latitudinal zone also exhibited significant sexual dimorphism in size; however, macrohabitat heterogeneity appeared to affect males and females equally. Both sexes were larger in Sierran Montane Conifer Forest and Californian Chaparral macrohabitats. Smaller animals occupied Californian Grassland and Coastal-scrub, and Vizcaino vegetation communities. This pattern of variation was observed in all three-way comparisons and suggests the possibility of habitat-mediated phenotypic responses. Finally, only one of eight comparisons involving kangaroo rats living in adjacent plant communities showed a significant two-way interaction between dimorphism and macrohabitat type. Our results, therefore, did not provide strong evidence to substantiate the hypotheses that the magnitude of dimorphism among populations of D. simulans living in different macrohabitats is a reflection of the degree of partitioning between sexes or nutritional quality of available food.

Morphometric Inferences on Sibling Species and Sexual Dimorphism in Neochlamisus Bebbianae Leaf Beetles: Multivariate Applications of the Thin-Plate Spline

Nominally polyphagous species of herbivorous insects sometimes are comprised of multiple morphologically similar biological species with more specialized appetites. When meristic morphological traits cannot be found to distinguish such suspected sibling species, molecular data are increasingly sought as a source of evidence. A role for morphology in distinguishing such taxa might be reclaimed, however, by recent advances in geometric morphometric methods, such as the statistical analysis of partial-warp scores from the thin-plate spline. We employed this method to detect and characterize subtle shape differences among populations and between sexes of the nominal leaf beetle species Neochlamisus bebbianae. Using the thin-plate spline, the shapes of specimens from seven beetle populations collected from five host plants in five eastern North American localities were calculated. These shapes were analyzed by MANOVA, revealing significant variation in both uniform and nonuniform components of shape among test populations. Significant sexual dimorphism in size, shape, and allometric relationships were also documented across these populations. More interestingly, our study provided evidence of sibling species where traditional taxonomic approaches have failed. Individual MANOVAs revealed significant shape variation between sympatric populations from different host plants in each of three localities. Because these sympatric shape differences were significant when adjusted for size, they cannot be attributed to allometric consequences of size variation among test populations. Because certain beetle populations differed significantly in size and shape when reared in a common environment, these morphometric traits may have a genetic basis. Together, these results are consistent with an earlier suggestion that N. bebbianae represents a complex of host-specific races or sibling species, a hypothesis that has received additional support from recent studies on host use traits, sex ratios, and mitochondrial DNA. In sum, these analyses demonstrate the power and utility of the thinplate spline as a morphological means of discriminating among closely related and anatomically homogeneous taxa.

Variation in cranial form and sexual dimorphism among five European populations of the otter Lutra lutra

Craniometric variation between otters Lutra lutra (L.) from Ireland, Scotland, Shetland, Norway, and Czechoslovakia was examined using multivariate statistical analyses. Significant sexual dimorphism in cranial size and shape occurred in all samples, although the degree of dimorphism varied between samples, with the Irish sample exhibiting the most. Morphological differences were also observed across samples. Three groups were observed; Irish, Scottish, (Mainland and Shetland), and Mainland European, though within the two composite groups the samples were still differentiable. There was a good agreement between the patterns of variation between male and female samples, but morphological differentiation was not significantly correlated with geographic separation. These results are discussed in the light of our current knowledge of the ecology of the species in Europe. It is argued that, while there is morphological evidence for subspecific status of the Irish otter (as L. l. roensis), further investigations of morphological and genetic variation across Eurasia are clearly required.