Size and scaling of sexually-selected traits in the lizard, Uta palmeri

Differences between the sexes in overall body size and in the size of other morphological traits, relative to overall body size, are common in many animals. In this study, patterns of growth and scaling of sexually dimorphic tratis are assessedin a lilzard and then used to sugest general developmental mechanisms responsible for sexual size dimorphism (SSD). Adult make Uta palmeri lizards are larger than adult females inoverall body size (snout-vent length, SVL), body mass, jaw length head width, and head depth. Two general growth processes produce this adult SSD. First, juvenile males have greater annual SVL growth rates than do juvenile females, contributing to adult SSD because males will be larger than females in any trait positively correlated with SVL. Secondly, males and females differ in age-related changes in growth of the three head size traits, relative to growth in SVL. Comparing slopes from reduced major axis regressions of each trait on SVL reveals that the sexes do not differ in the scaling of these traits as juveniles, but as adults males have greater slopes than adult females, indicating ontogenetic differences in scaling of these traits in males. Two other topics in SSD are addressed with these data. First, comparing these data on scaling to those of an earlier analysis that used ordinary least squares regression reveals that conclusions about underlying mechanisms in an analysis of scaling can be altered by the choice of a regression model. Secondly, these data indicate that postmaturational differences in scaling contribute to adult sexual size differences, contrary to an earlier study. Shine (1990) found that for many ectotherms, which continue to grow after sexual maturation, post-maturational events contribute little to sexual differences in overall body size. Results for U. palmeri suggest that these findings may only hold for measures of overall body size (e.g. SVL) and may not generalize to traits that exhibit sex difference in scaling.     

Male chimpanzee development focusing on adolescence: Integration of behavioral with physiological changes

Intensive longitudinal behavior observations of male chimpanzees in stable mixed-sex social groups in a semi-natural environment were analyzed by state of development as defined by dental, growth and hormonal signals. Initial increase of sexual behaviors preceded adolescent hormonal changes. Peak prevalence of sexual behaviors coincided with the adolescent period. This period is also characterized by peak activity level and feeding. The major increase in aggression and stabilization of submissive behavior at adult level occurred concomitant with achievement of adult hormone levels and body size, and these were accompanied by increase of grooming affiliative behaviors.     

树鼩(Tupaia belangeri chinensis)生长发育的初步观察

本文对39只树鼩的个体生长发育作了描述。新生幼仔生长发育迅速,在出生后头一个月内,体重可增长3—6倍,其体重的增长与饮奶量有关。出生后4个月性成熟,7个月龄时可繁殖产仔。性器官的重量有明显的季节性变化,1—6月间睾丸、子宫、卵巢重量最大,性行为活跃,其余月份重量下降,性功能出现渐进性衰退。辜丸重和体重的比值小于0.01,表明树鼩是单雄交配繁殖。乳齿的萌发为下颚先于上颚,门齿最先萌发,其次是犬齿,前臼齿,最后是臼齿。与Lyon(1913)和Shigehara(1980)提出的恒齿萌发次序不同。     

Ontogeny of sexual size dimorphism in monitor lizards: males grow for a longer period, but not at a faster rate

Monitor lizards belong to the largest and the most sexually dimorphic lizards in terms of size, making this group an ideal model for studies analyzing ontogenetic causes of sexual dimorphism. Understanding of these ontogenetic factors is essential to the current discussion concerning patterns of sexual dimorphism in animals. We examined the ontogenetic trajectories of body weight and snout-vent length to analyze the emergence of sexual size dimorphism. Experimental animals were 22 males and 13 females of mangrove-dwelling monitors (Varanus indicus) hatched at the Prague Zoo. They were regularly weighed and measured up to the age of 33-40 months, and subsequently sexed by ultrasonographic imaging. The logistic growth equation was used to describe and analyze the observed growth patterns. Our results confirm considerable sexual size dimorphism in the mangrove monitor. The mean asymptotic body weight of males was nearly three times higher than that of females. As the body size of male and female hatchlings is almost equal, and the growth rate parameter (K) of the logistic growth equation as well as the absolute growth rate up to the age of 12 months do not differ between the sexes, size differences between fully grown males and females should be attributed to timing of the postnatal growth. Males continue to grow several months after they reach the age when the growth of females is already reduced. Therefore, the sexual size dimorphism emerges and sharply increases at this period.     

Relationship of body size and body mass to blood pressure: sex-specific and developmental influences.

The relationship of body mass and body fat distribution to blood pressure has been recognized for many years. This relationship has formed the basis for much additional research, including the impact of growth and developmental factors on blood pressure levels. Blood pressure in children is related to somatic growth and is tied to increases in height, skeletal maturation, and sexual maturation. Sexual and ethnic differences in blood pressure levels are already apparent during childhood and may also be related to the process of growth and sexual maturity. Body size exerts a profound influence on a variety of physiological functions, including blood pressure and the onset of sexual maturity. In general, studies have reported a strong linear relationship between height and blood pressure and between body mass and blood pressure such that tracking correlations from childhood to adulthood for both blood pressure and body mass index are significant for most sex and ethnic groups. Studies evaluating the effects of hormone replacement therapy on post-menopausal women have thus far generated results suggesting that the age-related rise of blood pressure is not due directly to hormonal changes associated with menopause. The interrelated effects of growth, maturation, body weight, and body fat are influenced by both genetic and environmental factors. Environmental influences may modify relationships established much earlier, perhaps as early as prenatally, during infancy, or during early childhood. Directions for future research and implications resulting from the complex relationship between body weight and blood pressure are discussed.     

Foraging effort, food intake, fat deposition and puberty in female mice

A novel caging system was used to study the interrelationships between foraging effort, food intake, growth and sexual maturation of peripubertal female mice. Females housed in these cages were forced to work (forage) at various intensities in order to obtain food pellets. It is argued that this is a biologically more meaningful approach to understanding the energetics of sexual development than the traditional approach of simple underfeeding. Female mice exhibited a cascade of developmental adjustments and deficits when challenged to forage harder for less food. The functions most sensitive to increased foraging effort were sexual development and growth in body length; growth in body weight was intermediate and fat deposition was least sensitive of all. The relative insensitivity of fat deposition to higher foraging costs suggests a strategy for survival during the postweaning dispersal movements of the wild ancestors of the laboratory mouse. Finally, regression analyses suggested that heavier females who had less than average body fat and higher than average food intake achieved their pubertal ovulation most rapidly.     

Hormonal regulation of hair follicles exhibits a biological paradox

Hair's importance for insulation and camouflage or human communication means that hairs need to change with season, age or sexual development. Regular, regenerating hair follicle growth cycles produce new hairs which may differ in colour and/or size, e.g., beard development. Hormones of the pineal-hypothalamus-pituitary axis coordinate seasonal changes, while androgens regulate most sexual aspects with paradoxically different effects depending on body site; compare beard growth and balding! Hormones affect follicular mesenchymal-epithelial interactions altering growing time, dermal papilla size and dermal papilla cell, keratinocyte and melanocyte activity. Greater understanding of these mechanisms should improve treatments for poorly controlled hair disorders, alopecia and hirsutism.     

Growth and sexual dimorphism in a population of hybrid macaques

Growth and sexual dimorphism have long been the focus of investigation for researchers interested in the life history and socioecology of nonhuman primates. Previous research has shown that sex differences in the duration of growth, or bimaturism, are primarily responsible for the sexual dimorphism observed in anthropoid primates with multimale–multifemale social structure, such as macaques. The present study investigates sex differences in patterns of craniofacial and somatometric growth relative to head and body size and relative to dental development in a population of hybrid macaques (Cercopithecidae: Macaca ) from Sulawesi, Indonesia. How these patterns may contribute to sexual dimorphism in this hybrid population is also examined. The results of the study suggest that there is no substantial effect on the levels of sexual dimorphism associated with hybridization in these macaques. Although sex differences in patterns of size-related, or allometric, growth patterns play a significant role in the development of sexual dimorphism for some cranial dimensions in these hybrids, bimaturism seems to be the primary component in the ontogeny of sexual dimorphism in this hybrid population. The observed levels of hybrid dimorphism and the predominant ontogenetic pattern of bimaturism characterized by prolonged male growth are consistent with previously published reports on dimorphism and growth in other cercopithecine primates.     

The influence of growth patterns on sexual size monomorphism in lemurs

The lack of sexual size dimorphism among lemurs is puzzling given the high degree of polygyny in this clade. It has been proposed that the unique ecological conditions of Madagascar favour rapid completion of growth, limiting the opportunities for bimaturism and sexual size dimorphism in lemurs. Using recently compiled large data sets on many species across the lemur clade, I examined the prevalence of sexual size monomorphism of body mass among lemurs and tested the hypothesis that limited growth durations constrain sexual size dimorphism. I used segmented regression analyses to accurately model growth in each species. The majority of species analysed exhibited a period of rapid growth followed by a distinct period of slow growth prior to attainment of adult body mass. Whereas the first period of growth was constrained by the need to attain the majority of adult body mass prior to the onset of the infant's first dry season, the subsequent period of slow growth was unconstrained and sufficiently long to promote sexual bimaturism. Sex differences in the duration and rate of growth during this second growth phase appeared to account for the sexual size dimorphism exhibited by three lemur species. Therefore, constraints on growth processes do not limit sexual size dimorphism in lemurs, and other explanations for the prevalence of sexual size monomorphism in this clade should be examined. The importance of considering ontogeny in future investigations of sexual size monomorphism in lemurs is highlighted.     

Sexual development and the effects of active immunization against GnRH in Chinese Tanyang ram lambs

Tanyang is one of the most important sheep breeds in the northwest of China. However, there is no detailed data on sexual development and timing of puberty/sexual maturity of this breed. This experiment was designed to study the relationships between growth rate and sexual development by measuring body weight, testis diameter, and plasma testosterone concentrations, and observing sexual behavior through puberty from 3 to 8 months old of entire Tanyang ram lambs. At same time, to assess the potential efficacy and welfare benefits of non-surgical castration, the effect of active immunization against gonadotrophin-releasing hormone (GnRH) on sexual development and growth rate on ram lambs was evaluated, and compared with surgically castrated and entire lambs. The results showed that body weight increased at a constant rate from 3 to 8 months old of age. Testis diameter increased rapidly between 3 and 5 months, followed by a period of slow growth. Plasma testosterone concentrations increased sharply from 3 to 6 months of age (P<0.05), after which there were no significant changes from 6 to 8 months of age. Sexual behavior started from 5 months of age. After ram lambs were immunized against GnRH at 3 months of age, there was no increase in testis diameter in the first 4 months after immunization. Plasma testosterone concentrations remained at similar levels to surgically castrated lambs until 6 months of age, and the onset of sexual behavior was delayed, compared with entire ram lambs. GnRH immunization and surgical castration had no significant effect on the body weight by the end of this experiment. These results suggest that 4-5 months of age is a critical period for pubertal changes and testosterone plays an important role in the initiation of puberty and sexual maturity of Tanyang ram lambs in China. In addition, a single shot GnRH immunization at 3 months, acting as an alternative to surgical castration, is a practical method for Chinese Tanyang ram lambs, and provides animal welfare benefits.     

Effect of ovarectomy of females and oestrogen administration to males during the neonatal critical period on salt intake in adulthood in rats.

The effect of interference in the neonatal critical period on water and salt solution (3% NaCl) intake by adult rats given a free choice of these fluids was studied. Consumption was expressed per animal, per 100 g body weight and as the NaCl concentration in the total daily fluid intake volume [NaCl]I. Newborn female rats were ovarectomized or sham-ovarectomized. Ovarectomy markedly reduces consumption and [NaCl]I in adulthood and brought these values close to the values in normal males. Newborn male rats were injected with 1 mg oestradiol propionate dissolved in oil or just with oil. Oestradiol propionate severely inhibited growth, but produced no changes in fluid intake per animal. The [NaCl]I values were likewise unaltered, but owing to the lower body weight comsumption values per 100 g b.w. were higher in rats given oestradiol propionate than in those given oil. The relationship of the given growth changes to the regulation of salt intake and to hypothalamic function and its sexual correlates is discussed.     

Laboratory observations of moulting,growth and maturation in Antarctic krill (Euphausia superba Dana)

Summary Observations of intermoult period, growth and maturation were made on krill which were transported from Antarctic waters and maintained in the laboratory in Australia over a three year period. The mean intermoult period (IP) for each of 10 specimens, with initial body lengths of 24.7=46.8 mm, kept at -0.5° C varied from 22.0 to 29.8 days (overall mean = 26.6 days). These measurements of IP are significantly longer than those obtained in some previous studies. Differences in experimental temperatures, light, body sizes and growth patterns of the specimens between studies are unlikely to be causes of these dissimilar results. The pattern of changes in body length (BL) varies from one individual to the next. The greatest increase in BL over a series of 4–5 moults ranged from 0.024 to 0.070 mm/day, which is equivalent to 0.0020 to 0.0086/day in body weight, assuming exponential growth. This maximum growth rate is about half the rate predicted from the growth scheme of Mauchline (1980) for wild krill. Comparison of growth data for other euphausiids suggests that Mauchline's scheme produces anomalous growth rate. The slower growth rate observed in the present study would extend the estimated life span of krill from 3–4 years, as calculated by Mauchline (1980), to 4–7 years. If krill undergo body shrinkage during the Antarctic winter the estimated life span might be even longer. Examination of the external sexual characters of moults showed both progression and regression of maturity stage in association with changes in BL.     

Dietary influences on growth and sexual maturation in premenarchial rhesus monkeys

The effect of a high-fat diet on growth, sexual maturation, and developmental changes in serum levels of estradiol (E2), growth hormone (GH), somatomedin-C (Sm-C), and insulin were examined in outdoor-housed premenarchial rhesus monkeys. From 16 to 32 months of age, females were fed either a high-fat diet (HFD, N = 5) with 31% calories from fat or a control diet (commercial laboratory chow, N = 10) with 12% of the calories from fat. Maintenance on a HFD did not accelerate physical growth, as all animals exhibited similar increments in body weight, crown-rump length, and weight/height ratios. In contrast, the HFD group exhibited an earlier onset of perineal swelling and menarche despite lower body weights during that time. Moreover, 80% of the HFD animals exhibited an early first ovulation (31-32 months of age) compared with 40% of control animals. These reproductive changes were associated with significant differences in the endocrine profiles of HFD animals. Fasting serum levels of insulin were significantly elevated within 2 months of diet treatment and remained elevated throughout the study period. Levels of Sm-C were elevated relative to those of controls after 3 months of diet treatment, but not thereafter. Serum GH increased after 6 months on the HFD and, overall, concentrations were higher in HFD animals. A significant rise in E2 was observed after only 45 days on the HFD treatment. These concentrations did not differ from 18 to 21 months of age but again were elevated at 27 months in the HFD females. Since these endocrine and physical changes associated with reproductive physiology occurred in the absence of enhanced growth, these data suggest that a high-fat diet may influence the rate of sexual maturation through changes in certain metabolic factors which may act on the developing neuroendocrine system.     

Relative growth and morphological sexual maturity of the caridean shrimp Nematopalaemon schmitti (Decapoda: Caridea: Palaemonidae) in an upwelling region in the Western Atlantic*

In crustaceans, successful reproductive processes, such as the transition from juvenile to adult, exhibit important morphological changes that can be detected by analyzing relative growth. This study describes the relative growth of body structures in Nematopalaemon schmitti and its secondary sexual characteristics, and also estimates the morphological sexual maturity of this species in a region influenced by upwelling. The carapace length (CL), second pleuron length (PlL), cheliped carpus length (CaL), cheliped propodus length (PrL) and the length of appendix masculina (AML) of the shrimp were measured. The relationships that best demonstrated the changes in allometric coefficient between demographic categories were AML vs. CL for males, and PlL vs. CL for females. The estimated CL for morphological sexual maturity in males was 8.51 mm and 9.30 in females. Our results showed the appendix masculine and the second pleuron were secondary sexual characteristics that play roles in reaching the morphological sexual maturity necessary for reproductive success and to assure the life cycle of this species.     

SEXUAL SIZE DIMORPHISM AS A CORRELATED RESPONSE TO SELECTION ON BODY SIZE: AN EMPIRICAL TEST OF THE QUANTITATIVE GENETIC MODEL

We artificially selected for body size in Drosophila melanogaster to test Lande's quantitative genetic model for the evolution of sexual size dimorphism. Thorax width was used as an estimator of body size. Selection was maintained for 21 generations in both directions on males only, females only, or both sexes simultaneously. The correlated response of sexual size dimorphism in each selection regime was compared to the response predicted by four variants of the model, each of which differed only in assumptions about input parameters. Body size responded well to selection, but the correlated response of sexual size dimorphism was weaker than that predicted by any of the variants. Dimorphism decreased in most selection lines, contrary to the model predictions. We suggest that selection on body size acts primarily on growth trajectories. Changes in dimorphism are caused by the fact that male and female growth trajectories are not parallel and termination of growth at different points along the curves results in dimorphism levels that are difficult to predict without detailed knowledge of growth parameters. This may also explain many of the inconsistent results in dimorphism changes seen in earlier selection experiments.     

BODY SIZE AND SEXUAL SIZE DIMORPHISM IN MARINE IGUANAS FLUCTUATE AS A RESULT OF OPPOSING NATURAL AND SEXUAL SELECTION: AN ISLAND COMPARISON

Body size is often assumed to represent the outcome of conflicting selection pressures of natural and sexual selection. Marine iguana (Amblyrhynchus cristatus) populations in the Galápagos exhibit 10-fold differences in body mass between island populations. There is also strong sexual size dimorphism, with males being about twice as heavy as females. To understand the evolutionary processes shaping body size in marine iguanas, we analyzed the selection differentials on body size in two island populations (max. male mass 900 g in Genovesa, 3500 g in Santa Fé). Factors that usually confound any evolutionary analysis of body sizes—predation, interspecific food competition, reproductive role division—are ruled out for marine iguanas. We show that, above hatchlings, mortality rates increased with body size in both sexes to the same extent. This effect was independent of individual age. The largest animals (males) of each island were the first to die once environmental conditions deteriorated (e.g., during El Niños). This sex-biased mortality was the result of sexual size dimorphism, but at the same time caused sexual size dimorphism to fluctuate. Mortality differed between seasons (selection differentials as low as –1.4) and acted on different absolute body sizes between islands. Both males and females did not cease growth when an optimal body size for survival was reached, as demonstrated by the fact that individual adult body size phenotypically increased in each population under favorable environmental conditions beyond naturally selected limits. But why did marine iguanas grow “too large” for survival? Due to lek mating, sexual selection constantly favored large body size in males (selection differentials up to +0.77). Females only need to reach a body size sufficient to produce surviving offspring. Thereafter, large body size of females was less favored by fertility selection than large size in males. Resulting from these different selection pressures on male and female size, sexual size dimorphism was mechanistically caused by the fact that females matured at an earlier age and size than males, whereafter they constantly allocated resources into eggs, which slowed growth. The observed allometric increase in sexual size dimorphism is explained by the fact that the difference between these selective processes becomes larger as energy abundance in the environment increases. Because body size is generally highly heritable, these selective processes are expected to lead to genetic differences in body size between islands. We propose a common-garden experiment to determine the influence of genetic factors and phenotypic reaction norms of final body size.     

Ontogeny and diachronic changes in sexual dimorphism in the craniofacial skeleton of rhesus macaques from Cayo Santiago, Puerto Rico

Insight into the ontogeny of sexual dimorphism is important to our understanding of life history, ecology, and evolution in primates. This study applied a three-dimensional method, Euclidean Distance Matrix Analysis, to investigate sexual dimorphism and its diachronic changes in rhesus macaque (Macaca mulatta) skulls. Twenty-one landmarks in four functional areas of the craniofacial skeleton were digitized from macaques of known age and sex from the Cayo Santiago collections. Then, a series of mean form matrices, form difference matrices, and growth matrices were computed to demonstrate growth curves, rates and duration of growth, and sexual dimorphism within the neurocranium, basicranium, palate, and face. The inclusion of fully adult animals revealed a full profile of sexual dimorphism. Additionally, we demonstrate for the first time diachronic change in adult sexual dimorphism caused by extended growth in adult females. A quicker growth rate in males from ages 2 to 8 was offset by a longer duration of growth in adult females that resulted in diminished dimorphism between the ages of 8 and 15. Four functional areas showed different sex-specific growth patterns, and the rate and duration of growth in the anterior facial skeleton contributed most to the changing profiles of sexual dimorphism. The late maturation in size of the female facial skeleton corresponds to later and less complete fusion of facial sutures. The prolongation of growth in females is hypothesized to be an evolutionary response to high levels of intrasexual competition, as is found in other primate species such as common chimpanzees with similar colony structure and reproductive behavior. Further investigation is required to determine (1) if this phenomenon observed in craniofacial skeletons is linked to sexual dimorphism in body size, and (2) whether this diachronic change in sexual dimorphism is species specific. The changing profile of sexual dimorphism in adult rhesus macaques suggests caution in studying sexual dimorphism in fossil primate and human forms.     

Genomic regulation of sexual behavior

Estrogen receptors are distributed in discrete areas of the hypothalamus, preoptic area and amygdala of the rat brain, and in some of these areas estrogens induce progestin receptor sites. Estradiol (E), followed by progesterone (P), induce feminine sexual behavior in female, but not in male, rats. This induction takes time (on the order of hours, not minutes, so that the hormone may be cleared from the body) and is dependent on RNA and protein synthesis. Within the hypothalamic ventromedial nuclei (VMN), E and P induce changes in RNA and protein synthesis and also induce morphological changes indicative of cellular growth, genomic activation, and either new synapse formation or morphological rearrangement of existing synapses. Neurochemically, a number of neurotransmitter systems are implicated in the control of feminine sexual behavior, including acetylcholine, serotonin, GABA, and the neuropeptides, oxytocin and CCK. One of the means by which E and P may exert their influence on sexual behavior, aside from the morphological alterations, is by regulating levels of receptors for certain of these neurotransmitters. The critical differences which underlie the inability of male rats to display high levels of feminine sexual behavior after E plus P priming may depend on sex differences in the ability of E to induce particular neurochemical products as well as P receptors and upon differences in neural circuitry in the VMN.     

THE EVOLUTION OF FEMALE-BIASED SEXUAL SIZE DIMORPHISM: A POPULATION-LEVEL COMPARATIVE STUDY IN HORNED LIZARDS (PHRYNOSOMA)

Female-biased sexual size dimorphism is uncommon among vertebrates and traditionally has been attributed to asymmetric selective pressures favoring large fecund females (the fecundity-advantage hypothesis) and/or small mobile males (the small-male advantage hypothesis). I use a phylogenetically based comparative method to address these hypotheses for the evolution and maintenance of sexual size dimorphism among populations of three closely related lizard species (Phrynosoma douglasi, P. ditmarsi, and P. hernandezi). With independent contrasts I estimate evolutionary correlations among female body size, male body size, and sexual size dimorphism (SSD) to determine whether males have become small, females have become large, or both sexes have diverged concurrently in body size during the evolutionary Xhistory of this group. Population differences in degree of SSD are inversely correlated with average male body size, but are not correlated with average female body size. Thus, variation in SSD among populations has occurred predominantly through changes in male size, suggesting that selective pressures on small males may affect degree of SSD in this group. I explore three possible evolutionary mechanisms by which the mean male body size in a population could evolve: changes in size at maturity, changes in the variance of male body sizes, and changes in skewness of male body size distributions. Comparative analyses indicate that population differentiation in male body size is achieved by changes in male size at maturity, without changes in the variance or skewness of male and female size distributions. This study demonstrates the potential of comparative methods at lower taxonomic levels (among populations and closely related species) for studying microevolutionary processes that underlie population differentiation.     

Incorporating an ontogenetic perspective into evolutionary theory of sexual size dimorphism

Sexual size dimorphism (SSD) describes divergent body sizes of adult males and females. While SSD has traditionally been explained by sexual and fecundity selection, recent advances in physiology and developmental biology emphasize that SSD would occur proximately because of sexual differences in ontogenetic growth trajectories (i.e., growth rate and duration). Notably, these ontogenetic traits are subject to energetic or time constraints and thus traded off with fitness components (e.g., survival and reproduction). To elucidate the importance of such ontogenetic trade‐offs in the evolution of SSD, we developed a new theoretical framework by extending quantitative genetic models for the evolution of sexual dimorphism in which we reinterpret the trait as body size and reformulate sex‐specific fitness in size‐dependent manners. More specifically, we assume that higher growth rate or longer growth duration leads to larger body size and higher reproductive success but incurs the cost of lower survivorship or shorter reproduction period. We illustrate how two sexes would optimize ontogenetic growth trajectories in sex‐specific ways and exhibit divergent body sizes. The present framework provides new insights into the evolutionary theory of SSD and predictions for empirical testing.