Sex Differences In Size Research Articles

Age and Body Size in Populations of Two Syntopic Spadefoot Toads (GenusPelobates) at the Limit of Their Ranges

Abstract Age and size at sexual maturity are major life-history traits that affect growth rate and reproductive output. The differences in these life-history traits can be better assessed in syntopic populations of related species, where environmental variability in time and space is removed. The Spadefoot Toads (Pelobates spp.) are highly specialized burrowing and nocturnal species with a narrow ecological niche. We tested if age-related parameters are responsible for sexual size dimorphism and size differences in two syntopic populations from the Danube Delta Biosphere Reserve (Romania). The two populations differed significantly in size and body mass, and showed distinct sexual dimorphism, with females significantly larger in P. fuscus, and males slightly larger in P. syriacus. We also found differences in age structure: P. fuscus was on average 5.0 yr old, whereas P. syriacus was on average 7.4 yr old. Both species reached sexual maturity at a similar age. Different growth rates before sexual maturity...

Photoperiod, temperature, and food availability as drivers of the annual reproductive cycle of the sea urchin Echinometra sp. from the Gulf of Aqaba (Red Sea)

In spite of the efforts invested in the search for the environmental factors that regulate discrete breeding periods in marine invertebrates, they remain poorly understood. Here, we present the first account of the annual reproductive cycle of the pantropical sea urchin Echinometra sp. from the Gulf of Aqaba/Eilat (Red Sea) and explore some of the main environmental variables that drive echinoid reproduction. Monthly measurements of gonado-somatic indexes and histological observations of 20 specimens revealed a single seasonal reproductive cycle, with gametogenesis in males and females being highly synchronized. Gametogenesis commenced in June and peak spawning occurred between September and October. Gonado-somatic indexes were significantly correlated with seawater temperatures but not with photoperiod. The latter cycle lagged behind the gonado-somatic cycle by two months, suggesting that the onset of gametogenesis corresponds to shortening day length, while spawning may be driven by warming seawater temperatures. Gonads remained quiescent throughout the winter and spring (January through May) when temperatures were at their lowest. Chlorophyll-a concentrations increased significantly in the months following spawning (October through January). These high concentrations are indicative of high phytoplankton abundance and may reflect the increase in food availability for the developing larvae. Of the external test dimensions, length presented the highest correlation to body weight, indicating length as the best predictor for body size in Echinometra. Neither sexual dimorphism nor size differences between males and females were detected, and the sex ratios were approximately 1:1 in three distant Echinometra populations. Environmentally regulated reproduction, as occurs in sea urchins, might face severe outcomes due to anthropogenic disturbances to the marine environment. Consequently, there is a need to deepen our understanding of the mechanisms that drive and regulate this process in broadcast-spawning species.

Natural History of a Madagascan GeckoBlaesodactylus ambonihazoin a Dry Deciduous Forest

Abstract: Several aspects of natural history of Blaesodactylus ambonihazo, a gecko distributed in a dry forest of northwestern Madagascar, were investigated in the rainy and dry seasons between 2000 and 2006. There were no significant sexual size differences in snout-vent length, and no sexual dimorphism was detected either in head width or body mass. Females ceased oogenesis during the rainy season and were recrudescent at the beginning of the dry season. They probably lay eggs in the middle of the dry season. Hatchlings were observed at the beginning of the rainy season and probably reach adult size in the subsequent dry season. At night geckos perched on tree trunks and buildings and exhibited typical sit-and-wait foraging. During the day they retreated to shelters, mainly crevices between buttress roots. The gecko was thermally passive to environmental temperatures, showing a wide range of cloacal temperatures (15–30 C), but they selected relatively higher substrate temperatures at low air temperature...

Patterns of sexual dimorphism in the Persian Leopard (Panthera pardus saxicolor) and implications for sex differentiation

Extant felids show a high degree of inter-sexual dimorphism, meaning significant size differences between males and females. Such a differentiation may have various ecological, behavioural and evolutionary implications, at both species and subspecies levels. We have investigated the sexual size differences in one of the most dimorphic felids, i.e. the Leopard (Panthera pardus), based on 63 craniometric and 55 morphometric samples from Iran which belong to the subspecies Persian Leopard (P. p. saxicolor). In order to explore patterns of sexual dimorphism, multivariate statistical analysis on 24 skull variables as well as univariate approaches for two body measurements were applied. We found significant inter-sexual differences in skull size whereas it was not meaningful after removing the effect of size to address skull shape. Moreover, inter-sexual differentiation was also remarkable when comparing morphometric body measurements in adults, showing that the males possess a larger head mass and longer body, but sub-adults did not show any remarkable differentiation between sexes. A combination of craniometric and morphological features is proposed for sex differentiation in Leopards.

Mechanisms causing variation in sexual size dimorphism in three sympatric, congeneric lizards.

Sexual differences in adult body size (sexual size dimorphism, or SSD) ultimately can be favored by selection because larger males are more likely to be successful competitors for females, because larger females bear larger clutches, or because intersexual size differences reduce resource competition. Natural selection during juvenile development can influence sexual dimorphism of adults, and selection on adults and juveniles may differ. Studies that address the relative contributions of adult body shape dimorphism and sexually dimorphic patterns of growth and maturity are particularly useful in understanding the evolution of size dimorphism, yet they are rare. We investigated three sympatric, congeneric lizard species with different degrees and directions of adult sexual dimorphism and compared their growth patterns, survival probabilities, and intersexual trophic niche differences. Different mechanisms, even within these closely related, sympatric species, acted on juvenile lizards to produce species differences in adult SSD. Both degree and direction of dimorphism resulted from differences between the sexes in either the duration of growth or the rate of growth, but not from differences in rates of survival or selection on juvenile growth rate. Species- and sex-specific trade-offs in the allocation of energy to growth and reproduction, as well as differential timing of maturation, thus caused the growth patterns of the sexes to diverge, producing SSD. The differences that we observed in the direction of SSD among these species is consistent with their different social systems, suggesting that differential selection on adult body size has been responsible for the observed species-specific differences in juvenile growth rates and maturational timing.

The ontogeny of sexual dimorphism in the skull of the springbok antelope (Antidorcas marsupialis) (918.7)

Antelope (Artiodactyla: Bovidae) provide many classic examples of sexual dimorphism, but the ontogenetic pathway to adult sexual dimorphism has not been systematically studied. Moreover, it is unknown whether the development of obvious sexual differences in horn size correlate with the development of sexual differences in other horn traits, skull size, or skull shape. To better understand springbok skull development in general, and the development of sexual dimorphism in particular, I compared skulls from more than 50 springbok (Antidorcas marsupialis), which had been harvested, sexed and age‐estimated from a wild population in northern Namibia from 1992‐1994, and ranged from 7 months to >5 years old. Seven traits were examined: horn length, circumference, and number of horn rings, and skull size and shape in ventral and lateral views. Skulls were photographed in ventral and lateral views, landmarks were digitized, and 2D geometric morphometric analysis was performed using standard software and procedures. For each trait, male and female values were compared in yearlings and in adults to test the hypothesis that springbok sexual dimorphism increases after one year of age. By one year of age, males were significantly larger than females in horn length, horn circumference, and the sexes differed significantly in lateral skull shape. After one year of age, sexual differences increase in horn circumference and lateral skull size, and decrease and then reverse in the number of horn rings. Sexual differences were observed in nearly all trait studied. However, sexual differences arose at various times for the different traits. Whether these differences increased, were maintained, or reversed with age also depended on the trait. The developmental timing of sexual differences in size and shape indicates that sex‐specific changes in skull shape, and not allometry through male growth, are the primary mechanism for achieving sexual differences in skull shape.Grant Funding Source: Supported by an NSF‐GRF pre‐doctoral fellowship.

Neither season nor sex affects the cost of terrestrial locomotion in a circumpolar diving duck: the common eider (Somateria mollissima)

Locomotion accounts for a significant proportion of the energy budget in birds, and selection is likely to act on its economy, particularly where energy conservation is essential for survival. Birds are capable of different forms of locomotion, such as walking/running, swimming, diving and flying, and adaptations for these affect the energetic cost [cost of locomotion (CoL)] and kinematics of terrestrial locomotion. Furthermore, seasonal changes in climate and photoperiod elicit physiological and behavioural adaptations for survival and reproduction, which also influence energy budget. However, little is understood about how this might affect the CoL. Birds are also known to exhibit sex differences in size, behaviour and physiology; however, sex differences in terrestrial locomotion have only been studied in two cursorially adapted galliform species in which males achieved higher maximum speeds, and in one case had a lower mass-specific CoL than females. Here, using respirometry and high-speed video recordings, we sought to determine whether season and sex would affect the CoL and kinematics of a principally aquatic diving bird: the circumpolar common eider (Somateria mollissima). We demonstrate that eiders are only capable of a walking gait and exhibit no seasonal or sex differences in mass-specific CoL or maximum speed. Despite sharing identical limb morphometrics, the birds exhibited subtle sex differences in kinematic parameters linked to the greater body mass of the males. We suggest that their principally aquatic lifestyle accounts for the observed patterns in their locomotor performance. Furthermore, sex differences in the CoL may only be found in birds in which terrestrial locomotion directly influences male reproductive success.

Sexual differences in colour and size in the Great Spotted CuckooClamator glandarius

The appearance of plumage in brood parasites represents an evolutionary conflict between sexual selection that favours colourful plumages, and parasite–host coevolution that favours crypsis. In this study we quantified the degree of sexual dimorphism from a sample of 179 Great Spotted Cuckoos and determined which features facilitate accurate sex discrimination. In addition, we collected spectrophotometric measures of two colour patches (the crown and the throat) and ran visual models to test for physical and bird-perceivable sexual differences in coloration. We found that males are bigger and brighter than females in both colour patches. Using visual modelling techniques we demonstrate for the first time that adult Great Spotted Cuckoos are sexually dichromatic in an avian visual framework.

Sexual Differences in Post-Hatching Saunders's Gulls: Size, Locomotor Activity, and Foraging Skill

Various selection pressures induce the degree and direction of sexual size dimorphism in animals. Selection favors either larger males for contests over mates or resources, or smaller males are favored for maneuverability; whereas larger females are favored for higher fecundity, or smaller females for earlier maturation for reproduction. In the genus of Larus (seagulls), adult males are generally known to be larger in size than adult females. However, the ontogeny of sexual size dimorphism is not well understood, compared to that in adults. The present study investigates the ontogeny of sexual size dimorphism in Saunders's gulls (Larus saundersi) in captivity. We artificially incubated fresh eggs collected in Incheon, South Korea, and measured body size, locomotor activity, and foraging skill in post-hatching chicks in captivity. Our results indicated that the sexual differences in size and locomotor activity occurred with the post-hatching development. Also, larger males exhibited greater foraging skills for food acquisition than smaller females at 200 days of age. Future studies should assess how the adaptive significance of the sexual size dimorphism in juveniles is linked with sexual divergence in survival rates, intrasexual contests, or parental effort in sexes.

Ecology of the Rough-tailed Gecko,Cyrtopodion scabrum(Squamata: Gekkonidae) in the Suez Canal Zone, Egypt

Abstract Field observations and lab studies were carried out on the Rough-tailed Gecko (Cyrtopodion scabrum) in the Suez Canal Zone to assess sexual size dimorphism, activity, habitat selection, growth rate, diet, and reproduction. Cyrtopodion scabrum showed no significant sexual difference in body size and head shape characters. The lizard's maximum activity was in autumn and the minimum in winter, with peaks of daily activity during the first 3 h following sunset; air temperature was a major factor controlling daily and seasonal activity of C. scabrum. Lizards were solitary and had small home ranges. They occupied all available substrates and assumed different positions at heights ranging from 5 to 500 cm above the ground. The greatest rate of growth occurred in spring, and growth ceased altogether during winter. Lizards with the smallest snout–vent length had the highest growth rate. A total of 14 arthropod orders were recorded in the diet of C. scabrum; dipterans and hymenopterans dominated in number ...

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 Dimorphism in the Shell Shape of the Golden Apple Snail, Pomacea canaliculata (Lamarck) Using Geometric Morphometric Analysis

It has been accounted that the extent of sexual dimorphism in Pomacea canaliculata, a serious rice pest in Asia, has caused considerable taxonomic confusion. This study was conducted to explore possible sexual shell dimorphism in P. canaliculata by the aid of geometric morphometric measurements, since traditional measurements have problems in terms of accuracy. Relative Warp Analysis and Discriminant Function Analysis have been used in the study to determine patterns of sexual shell dimorphism in the golden apple snail. Results demonstrated sexual differences in size, spire height, and shell opening. These morphological differences could be attributed to ecological responses of the gastropod. The results clearly indicate that geometric morphometric methods are useful in detecting subtle differences between sexes.

Sex‐biased resource allocation in ovo in a sexually size‐dimorphic species

Sex‐biased resource allocation in eggs is increasingly recognized as one strategy oviparous mothers can employ to invest differentially in one sex, depending on nutritional requirements. Previous studies have used egg size as an index of nutrient allocation, but few have examined egg contents directly. We used molecular sexing of early‐stage ring‐billed gull Larus delawarensis embryos, a species with sexual size dimorphism, to test whether sex‐specific nutrient allocation occurs in ovo. Despite no sex difference in size, eggs with male embryos contained more albumen, while eggs with female embryos contained more yolk, lipid and non‐lipid (protein and carbohydrate). It is unclear why such sex‐biased resource allocation in ovo is utilized by ring‐billed gulls. However, our data indicate that a cursive examination of egg mass or size may not necessarily reflect nutrient allocation strategies mothers use in ovo, and that sex‐biased investment in ovo may be more widespread than currently appreciated.

Shiny cowbirdMolothrus bonariensisegg size and chick growth vary between two hosts that differ markedly in body size

In birds, egg size affects chick growth and survival and it is an important component of reproductive success. The shiny cowbirdMolothrus bonariensisis an extreme generalist brood parasite that uses hosts with a wide range of body masses. Survival of cowbird chicks decreases with host body mass, as competition for food with nestmates is more intense in large than in small hosts. We studied variation in shiny cowbird egg size and chick growth in two hosts that differ markedly in body size: the chalk‐browed mockingbirdMimus saturninus(70–75 g), and the house wrenTroglodytes aedon(12–13 g). We analyzed: 1) if females parasitizing mockingbirds lay larger eggs than those parasitizing wrens, and 2) the association between egg size and chick growth. We experimentally controlled for time of parasitism and number of host chicks and evaluated growth rate of male and female parasite chicks. Shiny cowbirds parasitizing mockingbird nests laid larger eggs than those parasitizing wren nests. Chick body mass after hatching was positively associated with egg size until chicks were five days of age, but there was no association between egg size and growth rate, or asymptotic mass. There were no sexual differences in egg size or body mass at the time of hatching, but growth rate was higher in males than in females leading to sexual dimorphism in asymptotic mass. Differences in egg size between hosts and the effect of egg size on body mass after hatching support the hypothesis that different females are specialized in the use of hosts that differ in body mass.

Non‐sex‐biased Dominance in a Sexually Monomorphic Electric Fish: Fight Structure and Submissive Electric Signalling

AbstractMales and females commonly compete for limited resources. When interaction costs are similar for both sexes and there are no sexual differences in resource value estimation, a non‐sex‐biased dominance is expected. Moreover, only non‐sex‐biased assessment of contenders fighting ability (Resource Holding Potential, RHP) should influence contest decisions. To test these predictions, we evaluated non‐breeding agonistic intra‐ and intersexual dyadic interactions in the weakly electric fish, Gymnotus omarorum. During the non‐breeding season, resource value is not expected to depend on individuals’ reproductive status and should thus be equal for males and females. In addition, as G. omarorum presents no sexual differences in body size, interaction costs can be considered symmetric between sexes. We confirmed that body size differences, but not individuals’ gender, is the best predictor of dominance. We correlated RHP asymmetries with contest duration and evidenced that body size but not sex influences assessment in intrasexual and intersexual encounters. All dyads tested engaged in agonistic interactions (N = 33) in which a clear dominant emerged. The analysis of conflict phases evidenced the submissive role of electric displays. Electric organ discharge (EOD) interruptions appear early in the contest as an electric hiding attempt, whereas chirps are post‐resolution signals of subordinate status. Interestingly, the decision of interrupting the EOD was also influenced by RHP asymmetries, whereas chirping activity was influenced by the intensity of the attacks received. Our results confirm that body size is the best RHP proxy in non‐breeding intra‐ and intersexual contests of this monomorphic species and demonstrated a sequential pattern of submissive signalling by means of two different electric displays.

Are there real sexual morphometric differences in yellow-bellied toads (Bombina spp.; Bombinatoridae)?

In this study, we explored the level and pattern of sexual size dimorphism and sexual shape dimorphism in two closely related Bombina species that have low levels of sexual dimorphism in body size and shape. We applied an experimental protocol to explore sexual variations in morphological traits, including a preliminary evaluation of the measurement error. Mean measurement error (MME) and measurement error index (MEI) were estimated on each of the eleven morphometric variables to exclude any possible subjective factor in measuring and to perform, for the first time, an objective functional and statistical evaluation of sexual size differences in the two species. Even if statistically significant, each difference that lies below the level of uncertainty of the measure could not be reliable. Therefore, statistically significant differences in head shape have been rejected, due to an average difference between males and females smaller than the possible MME. We detected significantly longer distal segments of the hind limbs in males, which could account for their use in mating behaviour (e.g. scramble competition, water-wave communication). However, major and more reliable evidences of sexual dimorphism have been found on forelimb measures (MEI > 1), in particular humerus length and amplexus, which are significantly larger in males than in females. These results indicate a mating related sexual dimorphism, when larger and stronger forelimbs can give an advantage during coupling as well as during male-male fighting. The mean measurement error values and formulas provided in this work could be applied to future morphometric studies on Bombina species.

Sexual dimorphism in the skull geometry of newt species of Ichthyosaura, Triturus and Lissotriton (Salamandridae, Caudata, Amphibia)

In this study, we applied geometric morphometrics to explore variations in the level and pattern of sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) of the ventral cranium in three different Modern Eurasian newt taxa (Ichthyosaura alpestris, Triturus species group and Lissotriton vulgaris). The ventral cranium is the part of the skull that is more directly related to foraging and feeding. Our results indicate that the level and pattern of sexual dimorphism in the ventral cranium differ among Modern Eurasian newt taxa. Regarding sexual dimorphism in skull size, Ichthyosaura alpestris and Triturus species show female-biased patterns (females are larger than males), whereas Lissotriton vulgaris appears to be non-dimorphic in skull size. In I. alpestris and Triturus species, SShD is mostly absent, whereas in L. vulgaris, SShD is more pronounced. A high level of variation between populations in both SSD and SShD indicates that local conditions may have a profound effect on the magnitude and direction of sexual dimorphism. The significant sexual differences in ventral cranium size and shape indicate possible subtle intersexual differences in ecological demands due to diet specialisation, in spite of similar general ecological settings.

Parentally biased favouritism in relation to offspring sex in zebra finches

Family conflicts over parental care result in offspring attempting to exert control using solicitation behaviours, whilst the parents are potentially able to retaliate through provisioning rules. However, the evolutionary interests of one parent may not necessarily support the evolutionary interests of the other parent, and such conflicts of interest may be expressed in how the two parents allocate the same form of parental care to individual offspring. Theory suggests that such parentally biased favouritism is a universally predicted outcome of evolutionary conflicts of interest, and empirical evidence suggests that parentally biased favouritism occurs in relation to offspring size and solicitation behaviours. However, unequivocal empirical evidence of parentally biased favouritism in relation to offspring sex is absent, due to being strongly confounded by sex differences in size and solicitation behaviours. Here, we present strong evidence for parentally biased favouritism in relation to offspring sex in zebra finches (Taeniopygia guttata), independent of the effects of chick size and begging intensity. Mothers preferentially provisioned sons over daughters, whilst fathers showed no bias, meaning that sons received more food than daughters. Parentally biased favouritism in relation to offspring sex facilitates parental control over evolutionary conflicts of interest and is probably more widespread than previously realised.

Analysis of the reproductive strategy of Jenynsia multidentata (Cyprinodontiformes, Anablepidae) with focus on sexual differences in growth, size, and abundance

Jenynsia multidentata is one of the most abundant freshwater fishes in the subtropical region of South America and when abundant it appears to have a major impact on lake ecosystems function. We studied the life-history traits and population structure of J. multidentata in a shallow lake in Uruguay, and collected literature data along a subtropical to tropical freshwater gradient in South America. Our general focus was on reproductive strategy, particularly on sexual differences in growth, size and abundance. In Lake Blanca, we found strong reverse size dimorphism (RSD) and a feminized sex ratio. Both are attributed to sexual differences in growth patterns and longevity. RSD and a feminized sex ratio seem to characterise J. multidentata regardless of latitude, and together with other life traits such as small size, rapid growth, low age of maturity, the capacity of the females to keep sperm alive to fertilize several successive litters, high natural mortality and fertility, resistance to extreme environmental conditions and omnivory, they are indicative of high productivity to biomass ratio. The very high population growth rate helps to explain why fish removal, aiming to promote an increase in the zooplankton grazing pressure on phytoplankton, may not be a useful tool in eutrophic-turbid subtropical South American lakes. We also found that the duration of the breeding season deviated markedly with latitude, increasing towards the tropics, which may have major consequences for population growth rates and differential impact of this species on lake ecosystems in the different climate zones.

Geographical and sexual differences in body size of common genets,Genetta genetta(Viverridae, Carnivora), in south-western Europe (Iberian Peninsula)

Animals' body size varies intra-specifically and geographically among populations, and many species (including small carnivores) show sexual dimorphism and larger individuals (lower superficial area/volume ratio) inhabiting cooler climates complying with Bergman's rule. In the present study we analyse data of common genets wild-caught in three different regions of the Iberian Peninsula, searching for variations in size and weight between males and females, testing for sexual dimorphism, as well as for micro-scale geographical variations among populations in biometrics and sexual dimorphism. We use field measurements such as length (body and tail) and weight, to characterise the three populations in the Iberian Peninsula. Our results show that Iberian genets present significant differences between sexes, although sexual dimorphism is lower than in other small carnivores, and that they comply with Rensch's rule, males size showing greater variation. Iberian genets also follow the Bergman's rule, being bigger and heavier in colder and northern regions. Although we have detected morphometric differences among studied populations, sexual dimorphism indexes varied little. We discuss our results in the light of the different hypotheses given to explain the sexual dimorphism in carnivores, trying to identify the mechanisms that might play a role in the dimorphism of genets.