Scaling peak VO2 to body mass in young male and female distance runners.

This study examined age- and sex-associated variation in peak oxygen consumption (VO2) of young male and female distance runners from an allometric scaling perspective. Subjects were from two separate studies of 9- to 19-yr-old distance runners from the mid-Michigan area, one conducted between 1982 and 1986 (Young Runners Study I, YRS I) and the other in 1999-2000 (Young Runners Study II, YRS II). Data from 27 boys and 27 girls from YRS I and 48 boys and 22 girls from the YRS II were included, and a total of 139 and 108 measurements of body size and peak VO2 in boys and girls, respectively, were available. Subjects were divided into whole year age groups. A 2 x 9 (sex x age group) ANOVA was used to examine differences in peak VO2. Intraindividual ontogenetic allometric scaling was determined in 20 boys and 17 girls measured annually for 3-5 yr. Allometric scaling factors were calculated using linear regression of log-transformed data. Results indicated that 1) absolute peak VO2 increases with age in boys and girls, 2) relative peak VO2 (ml x kg(-1) x min(-1)) remains relatively stable in boys and in girls, 3) relative peak VO2 (ml x kg(-0.75) x min(-1)) increases throughout the age range in boys and increases in girls until age 15 yr, and 4) peak VO2 adjusted for body mass (ml/min) increases with age in boys and girls. The overall mean cross-sectional scaling factor was 1.01 +/- 0.03 (SE) in boys and 0.85 +/- 0.05 (SE) in girls. Significant age x sex interactions and significant scaling factors between sexes identify the progressive divergence of peak VO2 between adolescent male and female distance runners. Mean ontogenetic allometric scaling factors were 0.81 [0.71-0.92, 95% confidence interval (CI)] and 0.61 (0.50-0.72, 95% CI) in boys and girls, respectively (P = 0.002). There was considerable variation in individual scaling factors (0.51-1.31 and 0.28-0.90 in boys and girls, respectively). The results suggest that the interpretation of growth-related changes in peak VO2 of young distance runners is dependent upon the manner of expressing peak VO2 relative to body size and/or the statistical technique employed.     

Sexual segregation in childhood: a review of evidence for two hypotheses

In this paper, I review the evidence associated with two hypotheses, both ultimately derived from sexual selection theory, put forth to explain social sexual segregation in human juveniles. More proximately, I posit that segregation is motivated by sex differences in body size, physical activity and competitiveness. The first hypothesis, the energetic/behavioural hypothesis, suggests that difference in energetics is a way in which males differentiate themselves from females and develop muscle and brain systems implicated in reproductive roles. The second hypothesis, the social roles hypothesis, suggests that segregation is related to learning male and female reproductive roles. The reviewed evidence supports the view that there are differences in energetics in male and female groups. With development, the vigorous behaviours shown by males take the form of specific social roles, related to dominance and competitiveness for males and maternal roles for females.     

Sex differences in lactate and glycerol levels during maximal aerobic and anaerobic running

Lactate, glycerol, adrenaline, and noradrenaline in venous blood following 400 m and 3000 m runs were measured in 6 untrained male students, 5 female handball players, 6 female sprinters and 6 female long-distance runners. Physical performance in the two events by the untrained males was the same as for the female handball players, but was less than that by the female sprinters and female long-distance runners. Peak blood lactate levels obtained after 400 m sprinting, and glycerol concentration following the 3000 m run were not significantly different between the untrained males and the female handball players. On the other hand, both peak blood lactate concentrations after 400 m sprinting for female sprinters and peak blood glycerol levels following a 3000 m run for female long-distance runners were significantly higher than those in the untrained male subjects. In both runs there was no significant difference in adrenaline and noradrenaline between the untrained male group and the female handball players. These results suggest that blood lactate in a 400 m run, and glycerol in a 3000 m run might be a reflection of physical performance level but not of sex difference.     

Nature Needs Nurture: The Interaction of Hormonal and Social Influences on the Development of Behavioral Sex Differences in Rhesus Monkeys

Thirty years of research on early social and hormonal environments and their relationship to the expression of behavioral sex differences in rhesus monkeys are reviewed. These studies demonstrate that whether aggressive and submissive behaviors are sexually dimorphic depends primarily on the social and not the hormonal environment. Early rearing environments without mothers or allowing brief periods of peer interaction produced higher levels of male aggression and female submission. Presenting behavior was expressed more by females than males in environments with high male aggressivity and female submissiveness. No sex differences in presenting occurred in low aggressivity environments, unless monkeys were reared isosexually, when males presented more than females. Rough and tumble play and foot-clasp mounting were consistently exhibited more by males than females across all rearing environments studied, but rearing environment affected the degree of the sex difference. When reared isosexually males displayed less, and females more, foot-clasp mounting than when heterosexually reared. No social environment increased the low frequency of female rough and tumble play. Suppressing neonatal androgen in males did not effect any sexually dimorphic behavior. Prenatal androgen administration to genetic females masculinized many aspects of their juvenile behavior, consistently increasing rough and tumble play and foot-clasp mounting across different social environments. Thus the sexually dimorphic behaviors which showed the smallest variability across social contexts were the most profoundly affected by the prenatal hormonal environment. These studies demonstrate that the expression of consistent juvenile behavioral sex differences results from hormonally induced predispositions to engage in specific patterns of juvenile behavior whose expression is shaped by the specific social environment experienced by the developing monkey.     

Sex differences in adult chimpanzee positional behavior: The influence of body size on locomotion and posture

Focal animal instantaneous sampling of adult male and female chimpanzee positional behavior was conducted during a 7-month study in the Tai Forest, Ivory Coast, in order to determine whether there are sex differences in the locomotion, posture, substrate use, and height preference of sexually dimorphic adult chimpanzees, and if so, whether these differences support predictions based on body size differences. Results indicate that as predicted, adult male and female chimpanzees differ in their arboreal locomotor behavior, with the larger males using less quadrupedalism and more climbing, scrambling, and aided bipedalism than females during feeding locomotion. There is a sex difference in height preference as well, with female chimpanzees consistently using more arboreal behavior than males, primarily during resting. Although it has been previously demonstrated that separate primate species of differing body size differ in locomotor and postural activities (Fleagle and Mittermeier, 1980; Crompton, 1984), this study clearly demonstrates that body size differences within a species can also be correlated with differences in locomotor behavior. These findings may influence how we interpret sex differences in body size of extinct species. © 1993 Wiley-Liss, Inc.     

Sex-specific spawning behavior and its consequences in an external fertilizer

Identifying the target of sexual selection in externally fertilizing taxa has been problematic because species in these taxa often lack sexual dimorphism. However, these species often show sex differences in spawning behavior; males spawn before females. I investigated the consequences of spawning order and time intervals between male and female spawning in two field experiments. The first involved releasing one female sea urchin's eggs and one or two males' sperm in discrete puffs from syringes; the second involved inducing males to spawn at different intervals in situ within a population of spawning females. In both, fertilization success was measured as the fraction of eggs fertilized and the paternity share of each male. The results indicate that spawning after females imposes a cost on males but only during sperm competition. Further, the optimal interval between the initiations of male and female spawning depends on degree of sperm competition, distance between males and females, and water velocity. The results show that sex differences in spawning timing of marine invertebrates can be explained on the basis of the differential costs and benefits of spawning out of synchrony with the other sex and that the result of sexual selection on external fertilizers may be behavioral rather than morphological differentiation of the sexes.     

Spatial cognition in rhesus monkeys: male superiority declines with age.

Twelve young (4-7 years of age) and 14 old (20-27 years of age) male and female rhesus monkeys were tested on seven cognitive tasks. Males and females performed similarly on tasks of object memory and executive function, but young males outperformed young females on a spatial memory task (Delayed Recognition Span Test) that requires the identification of a new stimulus among an increasing array of serially presented stimuli. This superior level of spatial ability in young males declined sharply with age, so that old males did not perform significantly better than old females. These findings in the nonhuman primate suggest that biological rather than sociocultural factors underlie the sex differences in cognition and their diminution with age.     

Evolution of male longevity bias in nematodes

Many animal species exhibit sex differences in aging. In the nematode Caenorhabditis elegans, under conditions that minimize mortality, males are the longer-lived sex. In a survey of 12 independent C. elegans isolates, we find that this is a species-typical character. To test the hypothesis that the C. elegans male longevity bias evolved as a consequence of androdioecy (having males and hermaphrodites), we compared sex-specific survival in four androdioecious and four dioecious (males and females) nematode species. Contrary to expectation, in all but C. briggsae (androdioecious), males were the longer-lived sex, and this difference was greatest among dioecious species. Moreover, male lifespan was reduced in androdioecious species relative to dioecious species. The evolutionary theory of aging predicts the evolution of a shorter lifespan in the sex with the greater rate of extrinsic mortality. We demonstrate that in each of eight species early adult mortality is elevated in females/hermaphrodites in the absence of food as the consequence of internal hatching of larvae (matricide). This age-independent mortality risk can favour the evolution of rapid aging in females and hermaphrodites relative to males.     

Male mate choice relies on major histocompatibility complex class I in a sex‐role‐reversed pipefish

Mate choice for compatible genes is often based on genes of the major histocompatibility complex (MHC). Although MHC‐based mate choice is commonly observed in female choice, male mate choice remains elusive. In particular, if males have intense paternal care and are thus the choosing sex, male choice for females with dissimilar MHC can be expected. Here, we investigated whether male mate choice relies on MHC class I genes in the sex‐role reversed pipefish Syngnathus typhle. In a mate choice experiment, we determined the relative importance of visual and olfactory cues by manipulating visibility and olfaction. We found that pipefish males chose females that maximize sequence‐based amino acid distance between MHC class I genotypes in the offspring when olfactory cues were present. Under visual cues, large females were chosen, but in the absence of visual cues, the choice pattern was reversed. The use of sex‐role reversed species thus revealed that sexual selection can lead to the evolution of male mate choice for MHC class I genes.     

Female-limited X-chromosome evolution effects on male pre- and post-copulatory success

Intralocus sexual conflict arises when the expression of shared alleles at a single locus generates opposite fitness effects in each sex (i.e. sexually antagonistic alleles), preventing each sex from reaching its sex-specific optimum. Despite its importance to reproductive success, the relative contribution of intralocus sexual conflict to male pre- and post-copulatory success is not well-understood. Here, we used a female-limited X-chromosome (FLX) evolution experiment in Drosophila melanogaster to limit the inheritance of the X-chromosome to the matriline, eliminating possible counter-selection in males and allowing the X-chromosome to accumulate female-benefit alleles. After more than 100 generations of FLX evolution, we studied the effect of the evolved X-chromosome on male attractiveness and sperm competitiveness. We found a non-significant increase in attractiveness and decrease in sperm offence ability in males expressing the evolved X-chromosomes, but a significant increase in their ability to avoid displacement by other males'' sperm. This is consistent with a trade-off between these traits, perhaps mediated by differences in body size, causing a small net reduction in overall male fitness in the FLX lines. These results indicate that the X-chromosome in D. melanogaster is subject to selection via intralocus sexual conflict in males.     

Experimental evolution under varying sex ratio and nutrient availability modulates male mating success in Drosophila melanogaster

Biased population sex ratios can alter optimal male mating strategies, and allocation to reproductive traits depends on nutrient availability. However, there is little information on how nutrition interacts with sex ratio to influence the evolution of pre-copulatory and post-copulatory traits separately. To address this omission, we test how male mating success and reproductive investment evolve under varying sex ratios and adult diet in Drosophila melanogaster, using experimental evolution. We found that sex ratio and nutrient availability interacted to determine male pre-copulatory performance. Males from female-biased populations were slow to mate when they evolved under protein restriction. By contrast, we found direct and non-interacting effects of sex ratio and nutrient availability on post-copulatory success. Males that evolved under protein restriction were relatively poor at suppressing female remating. Males that evolved under equal sex ratios fathered more offspring and were better at supressing female remating, relative to males from male-biased or female-biased populations. These results support the idea that sex ratios and nutrition interact to determine the evolution of pre-copulatory mating traits, but independently influence the evolution of post-copulatory traits.     

Sex dimorphism in the thyroid gland. IV. Cytologic aspects of sex dimorphism in the rat thyroid gland

This study was designed to explain whether the sex-dependent differences in the structure of the thyroid gland of adult male and female rats depend on quantitative or qualitative changes in the thyroid follicular cells. Absolute thyroid gland weight was similar in male and female rats, but its relative weight was markedly higher in females however. Volume fractions of epithelium and stroma were higher and that of colloid lower in male than in female rats and the epithelium/colloid ratio was higher in the males. Also absolute the volumes (in mm3) of epithelium and stroma were higher in the males; the thyroid gland of females contained more colloid. The average volume of a thyroid follicular cell, estimated by stereology, was higher in males than in females, although the thyroid gland contained similar numbers of follicular cells in both sexes. Also, thyroid glands from both male and female rats contained a similar DNA quantity. Results of the present study show that the sex dimorphism in the rat thyroid depends upon a difference in the mean volume of thyroid follicular cells, with males having larger cells than females. However, in both sexes the thyroid gland contains a similar quantity of these cells.     

Reproductive energetics of the role reversing bushcricket,Kawanaphila nartee (Orthoptera: Tettigoniidae: Zaprochilinae)

Theory predicts that sexual differences in reproductive strategies arise because of differences in the magnitude of investment made by males and females in reproduction. In some bushcrickets, the typical sex role of competitive male and choosy female is reversed when populations are subject to nutrient stress. Here I present an energetic analysis of reproduction for the role reversing bushcricket, Kawanaphila nartee, that supports the contention that this sex role reversal is a consequence of reversal in the pattern of relative reproductive investment. When fed ad libitum, males spent 16% of their daily energy reserves on the spermatophore compared with 26% spent on calling to attract a mate. Females spent 29% of their daily energy reserves in producing and laying eggs. However, when allowed only limited access to food, female expenditure in eggs was reduced to 23% of daily reserves while male expenditure remained unchanged. After accounting for the incorporation of male nutrients into eggs, female energy expenditure in reproduction exceeded male expenditure when animals were fed ad libitum, but male expenditure exceeded female expenditure when diet was limited. This role reversal in relative energy expenditure that is associated with courtship role reversal supports classical and contemporary theories on the control of sexual selection.     

Increased hip adduction during running is associated with patellofemoral pain and differs between males and females: A case-control study

Patellofemoral pain is common amongst recreational runners and associated with altered running kinematics. However, it is currently unclear how sex may influence kinematic differences previously reported in runners with patellofemoral pain. This case-control study aimed to evaluate lower limb kinematics in males and females with and without patellofemoral pain during running. Lower limb 3D kinematics were assessed in 20 runners with patellofemoral pain (11 females, 9 males) and 20 asymptomatic runners (11 females, 9 males) during a 3 km treadmill run. Variables of interest included peak hip adduction, internal rotation and flexion angles; and peak knee flexion angle, given their previously reported association with patellofemoral pain. Age, height, mass, weekly run distance and step rate were not significantly different between groups. Mixed-sex runners with patellofemoral pain were found to run with a significantly greater peak hip adduction angle (mean difference = 4.9°, d = 0.91, 95% CI 1.4–8.2, p = 0.01) when compared to matched controls, but analyses for all other kinematic variables were non-significant. Females with patellofemoral pain ran with a significantly greater peak hip adduction angle compared to female controls (mean difference = 6.6°, p = 0.02, F = 3.41, 95% CI 0.4–12.8). Analyses for all other kinematic variables between groups (males and females with/without PFP) were non-significant. Differences in peak hip adduction between those with and without patellofemoral pain during running appear to be driven by females. This potentially highlights different kinematic treatment targets between males and females. Future research is encouraged to report lower limb kinematic variables in runners with patellofemoral pain separately for males and females.     

Sex-Specific Posttranslational Regulation of the Gamete Fusogen GCS1 in the Isogamous Volvocine Alga Gonium pectorale

Male and female, generally defined based on differences in gamete size and motility, likely have multiple independent origins, appearing to have evolved from isogamous organisms in various eukaryotic lineages. Recent studies of the gamete fusogen GCS1/HAP2 indicate that this protein is deeply conserved across eukaryotes, and its exclusive and/or functional expression generally resides in males or in male homologues. However, little is known regarding the conserved or primitive molecular traits of males and females within eukaryotes. Here, using morphologically indistinguishable isogametes of the colonial volvocine Gonium pectorale, we demonstrated that GCS1 is differently regulated between the sexes. G. pectorale GCS1 molecules in one sex (homologous to male) are transported from the gamete cytoplasm to the protruded fusion site, whereas those of the other sex (females) are quickly degraded within the cytoplasm upon gamete activation. This molecular trait difference might be conserved across various eukaryotic lineages and may represent male and female prototypes originating from a common eukaryotic ancestor.     

The effects of sex on chemosensory communication in a terrestrial salamander (Plethodon shermani)

Although much evidence reveals sexually dimorphic processing of chemosensory cues by the brain, potential sex differences at more peripheral levels of chemoreception are understudied. In plethodontid salamanders, the volume of the vomeronasal organ (VNO) is almost twice as large in males as compared to females, both in absolute and relative size. To determine whether the structural sexual dimorphism in VNO volume is associated with sex differences in other peripheral aspects of chemosensation, we measured sex differences in chemo-investigation and in responsiveness of the VNO to chemosensory cues. Males and females differed in traits influencing stimulus access to VNO chemosensory neurons. Males chemo-investigated (“nose tapped”) neutral substrates and substrates moistened with female body rinses more than did females. Compared to females, males had larger narial structures (cirri) associated with the transfer of substrate-borne chemical cues to the lumen of the VNO. These sex differences in chemo-investigation and narial morphology likely represent important mechanisms for regulating sex differences in chemical communication. In contrast, males and females did not differ in responsiveness of VNO chemosensory neurons to male mental gland extract or female skin secretions. This important result indicates that although males have a substantially larger VNO compared to females, the male VNO was not more responsive to every chemosensory cue that is detected by the VNO. Future studies will determine whether the male VNO is specialized to detect a subset of chemosensory cues, such as female body rinses or female scent marks.     

Group mating among Norway rats I. Sex differences in the pattern and neuroendocrine consequences of copulation

The copulatory pattern of groups of rats (Rattus norvegicus) was studied in the laboratory in a seminatural environment. In a given mating session, every oestrous female copulated with each male; likewise, every male copulated with each oestrous female. While individual males and females experienced similar amounts of copulation, there were dramatic sex differences in sequence and temporal pattern. Males mated in a multiple intromission pattern and had more ejaculatory series when several females were in oestrus. In contrast, females received intromissions and ejaculations in a random order, not in the sequence of a male ejaculatory series. Males copulated at shorter intervals than females did, a temporal sex difference that was determined by the pattern of female solicitations and male approaches. These sex differences are used to discuss the different units of analysis that are appropriate for male and female sexual behaviour in this species. Furthermore, the sex differences in the temporal pattern of copulation which emerged during group mating parallel the known sex differences in the temporal parameters of the neuroendocrine reflexes which mediate successful reproduction in the domestic strain.     

Adaptation to experimental alterations of the operational sex ratio in populations of Drosophila melanogaster

Theory predicts that males adapt to sperm competition by increasing their investment in testis mass to transfer larger ejaculates. Experimental and comparative data support this prediction. Nevertheless, the relative importance of sperm competition in testis size evolution remains elusive, because experiments vary only sperm competition whereas comparative approaches confound it with other variables, in particular male mating rate. We addressed the relative importance of sperm competition and male mating rate by taking an experimental evolution approach. We subjected populations of Drosophila melanogaster to sex ratios of 1:1, 4:1, and 10:1 (female:male). Female bias decreased sperm competition but increased male mating rate and sperm depletion. After 28 generations of evolution, males from the 10:1 treatment had larger testes than males from other treatments. Thus, testis size evolved in response to mating rate and sperm depletion, not sperm competition. Furthermore, our experiment demonstrated that drift associated with sex ratio distortion limits adaptation; testis size only evolved in populations in which the effect of sex ratio bias on the effective population size had been compensated by increasing the numerical size. We discuss these results with respect to reproductive evolution, genetic drift in natural and experimental populations, and consequences of natural sex ratio distortion.     

Sex Differences in Captive Olive Baboon Behavior During the First Fourteen Days of Life

I discuss newborn baboon behavioral and proximity sex differences in a population of captive olive baboons (Papio anubis) living in a social group of >500 individuals. The data are based upon 20-min focal observations of 42 mother-newborn pairs (n = 27, n = 15) for infant-days 1–7 and 36 pairs (n = 23, n = 13) for infant-days 8–14 collected late-May through late-November 2001. I examined the first two weeks of infant life via behavioral, proximity, and approach-leave/contact analyses in order to determine whether behavioral sex differences exist during the first few days of life. I examine and analyze these 2 weeks independently due to different sample sizes. I used data from the total available sample population of 57 infants (n = 36, n = 21) to discuss birth, survivorship, and infant weight. Statistically significant age and/or age-sex interactions exist for all of the behavior and proximity measures during either infant-week 1 or 2. Moreover, there is a statistically significant difference in the birth sex ratio in the sample population but no significant difference in infant mortality by sex. There are also relative and significant differences in mothers' treatment of their newborn males and females. There are also some general tendencies for female newborns on average to suckle less and to explore more per focal observation than male infants do as they age. Conversely, male newborns average slightly more time per focal observation 1 m from the mother than do female infants. However, the observed differences may be influenced by maternal behavior in that mothers have higher rates of contact with their female than their male infants.     

On the evolutionary pathway of parental care in mouth-brooding cichlid fishes

Evolutionary theory predicts that differences in parental care patterns among species arose from interspecific differences in the costs and benefits of care for each sex. In Galilee St Peter''s fish, Sarotherodon galilaeus (Cichlidae), male care, female care and biparental care all occur in the same population. We exploit this unusual variability to isolate conditions favouring biparental versus uniparental mouth-brooding by males or females. We first review a game-theoretic model of parental care evolution, predictions of which we test experimentally in this paper. Manipulations of the operational sex ratio show that males and females desert their offspring more frequently when the costs of care are high (in terms of lost mating opportunities). Breeding trials with males of different sizes show that small fathers desert more frequently than large fathers. We attribute this to the associated difference in the fitness benefit of biparental care relative to female-only care. Our experimental results confirm that in St Peter''s fish the probability of caring is determined facultatively according to current conditions at each spawn. The experiments and model together suggest that interspecific variation in remating opportunities and clutch size may be responsible for differences in care patterns within the sub-family Tilapiini. Our results support the hypothesis that biparental mouth-brooding was the ancestral state of both male and female uniparental mouth-brooding in cichlid fishes.