Male Secondary Sexual Characters Research Articles

Freshwater CichlidCrenicara punctulataIs a Protogynous Sequential Hermaphrodite

Abstract Several papers have suggested that the Freshwater Cichlid Crenicara punctulata is capable of protogynous sex change. These studies provide behavioral data and descriptive details of external morphology but lack information about gonadal histology, which is essential for definitive evidence of sequential hermaphroditism. Therefore, this study was designed to determine whether C. punctulata is a sequential hermaphrodite. The present study included a behavioral experiment, an isolation/transformation experiment, as well as a detailed analysis of gonadal structure. The behavioral experiment established social hierarchies in four groups of juvenile female C. punctulata. The hypothesis that the dominant female in each group would be the only individual to develop male secondary sexual characteristics in that group was verified. The isolation experiment tested the hypothesis that female C. punctulata would change sex without the presence of conspecifics. After social isolation, females that were previou...

Sympatric speciation by sexual selection.

There is increasing evidence for the process of sympatric speciation, in which reproductive isolation of species occurs without physical isolation. Theoretical models have focused on disruptive natural selection as the crucial pressure for splitting a species. Here we report the theoretical finding that sympatric speciation may be caused by sexual selection even without disruptive natural selection. Specifically, we show that variation in a male secondary sexual character with two conspicuous extremes and the corresponding variance in female mating preference around no preference may jointly evolve into bimodal distributions with increasing modal divergence of the male and female traits, pulling a population apart into two prezygotically isolated populations. This mode of speciation, driven by two runaway processes in different directions, is promoted by an increase in the efficiency of females in discriminating among males or a decrease in the cost of male conspicuousness, indicating that sympatric speciation may occur more readily if barrier-free or predator-free conditions arise. Although even a slight cost of female preference would cancel the runaway process of sexual selection, it would not cancel the divergent runaway processes of sympatric speciation.

Effects of waterborne exposure of 17 β-estradiol on secondary sex characteristics and gonads of fathead minnows ( Pimephales promelas)

Environmental contaminants with estrogenic activity have recently received attention because of their potential effects on the reproductive efficiency of humans and wildlife. This study was conducted with the endogenous estrogen, 17 β-estradiol (E2), to establish the histologic response of the fathead minnow ( Pimephales promelas) as a model organism. Sexually mature fathead minnows were exposed for 14 days to waterborne concentrations of 1000, 100, 10, 2, 1, 0.5, 0.25, 0.125, 0.1 or 0.0625 nM E2. Exposure to E2 caused a reduction in size of the prominent male secondary sex characteristics, the fatpads and nuptial breeding tubercles. Histological lesions observed in the testes included proliferation of Sertoli cells and degenerative changes. Electron microscopy of seminiferous tubules and their Sertoli cells revealed large phagolysosomes filled with degenerating spermatozoa and other cellular debris. Females had ovaries in which most of the follicles were in the primary stage of development. There were also more atretic follicles and fewer secondary and Graafian follicles than in unexposed females. These findings demonstrate components of sexually mature fish which may be altered by compounds that mimic E2. To determine if lesions observed in males were permanent, 50 sexually mature males and females were exposed to a single concentration of 10 nM E2 for 10 days. Samples were collected from males on the final day of E2 exposure and over a period of 16 weeks after the exposure was stopped. No E2-induced lesions were observed beyond 16 weeks post E2 exposure. Results of these studies suggest that histological lesions could occur at ecologically-relevant exposures to ‘estrogenic’ compounds. However, certain lesions caused by exposure of adult fathead minnows are not permanent.

Naturally Occurring Mutations of the Luteinizing-Hormone Receptor: Lessons Learned about Reproductive Physiology and G Protein–Coupled Receptors

The luteinizing hormone (LH) receptor (LHR) is a heptahelical receptor present primarily in the ovaries of females and the testes of males. This same receptor can bind with high affinity either pituitary LH or the nearly identical placental hormone human chorionic gonadotropin (hCG). In both males and females, the levels of LH remain quite low during childhood years, until puberty, at which time the hypothalamic-pituitary-gonadal axis matures. After puberty, the functions of LH are critical to normal reproductive function. In postpubertal males, LH stimulates testosterone synthesis in the Leydig cells of the testes, which, in turn, is necessary for both formation of male secondary sexual characteristics and spermatogenesis. In nonpregnant postpubertal females, LH plays several roles. During the follicular phase of the ovarian cycle, LH stimulates theca cells to synthesize androgens, which are then aromatized into estradiol in granulosa cells under the influence of follicle-stimulating hormone (FSH). The midcycle surge of LH induces follicular maturation and ovulation. Subsequently, during the luteal phase, LH induces the formation of the corpus luteum and stimulates progesterone synthesis. In the pregnant female, placental hCG binds to the LHR on ovarian luteal cells and causes the corpus luteum, which otherwise undergoes atresia, to be maintained and to continue steroid synthesis, which is necessary for the continuation of pregnancy. During pregnancy, if the fetus is male, placental hCG also stimulates fetal testicular Leydig cells to produce testosterone, which, in turn, mediates the differentiation of the external genitalia and induces the descent of the testes (see Roberts et al. 1999 [in this issue]). Clearly, the LHR plays a critical role in reproductive physiology in both males and females. The importance of the LHR signal-transduction pathway in normal reproductive functioning has been further underscored by the discovery, in recent years, of naturally occurring mutations of the human LHR gene, hLHR.

Effects of exposure to municipal wastewater in situ on the reproductive physiology of the fathead minnow (Pimephales promelas)

AbstractEffects of representative central Michigan wastewater treatment plant (WWTP) effluents on the reproductive physiology of fathead minnows (Pimephales promelas) were assessed in situ by measuring plasma concentrations of vitellogenin (VTG), 17β‐estradiol (E2), and testosterone (T), and evaluating gonad histology and male secondary sex characteristics. Caged adult male and female fathead minnows were exposed for 3 weeks to WWTP effluent. One riverine site and one lacustrine site were included as references for comparison to WWTP sites. Plasma concentrations of VTG, E2, and T in female fish, quantified by enzyme‐linked immunosorbent assay (ELISA), were greater at the lacustrine site than the riverine site or WWTP sites. None of the male fish had mean plasma VTG concentrations similar to that observed in females. Most VTG concentrations in male fish were less than the VTG ELISA detection limit. Female plasma concentrations of E2 were similar among riverine and WWTP sites. Plasma concentrations of E2 in male fish were greater than the riverine site at all WWTP sites except the Williamston site, but were not significantly different from the lacustrine site. Concentrations of T in female and male fish were similar among riverine and exposure sites. No trends in hormone concentrations, male secondary sex characteristics, or gonad histology could be attributed to putative endocrine disrupter exposure in WWTP effluent. The results indicate that the risk for estrogen agonist exposure below these central Michigan WWTPs is small.

POPULATION DIVERGENCE IN SEXUAL ORNAMENTS: THE WHITE FOREHEAD PATCH OF NORWEGIAN PIED FLYCATCHERS IS SMALL AND UNSEXY.

Models of sexual selection suggest that populations may easily diverge in male secondary sexual characters. Studies of a Spanish population of the pied flycatcher, Ficedula hypoleuca, and a Swedish population of the closely related collared flycatcher, F. albicollis, have indicated that the white forehead patch of males is a sexually selected trait. We studied the white forehead patch of male pied flycatchers (n = 487) in a Norwegian population over seven years. Males with large forehead patches were in general more brightly colored, but patch height was not correlated to body mass, body size, or parasite loads. Conditions during the nestling period did not seem to influence patch height as an adult. Patch height increased slightly from the first to the second year as adults, but then remained relatively constant at higher ages. Patch height was not related to survival. Year-to-year changes showed that males who increased in patch height also increased in body mass, suggesting that expression of the forehead patch may be partly condition dependent. However, changes in body mass explained only a small proportion of the variance in patch height between males. Thus, patch height would not be a good indicator of male quality. Furthermore, patch size was also not related to male ability to feed nestlings, indicating that females would not obtain direct benefits by choosing males with large patches. However, patch height could be a Fisher trait, but this requires heritability and there was no significant father-son resemblance in patch height. Comparisons of the males visited by each female during the mate sampling period indicated that chosen males did not have larger forehead patches than rejected males. Experimental manipulation of patch height did not affect male mating success. These results indicate that females do not use patch size as a mate choice cue. Finally, patch height did not predict the outcome of male contests for nestboxes, suggesting that the forehead patch is not an intrasexually selected cue of status. Norwegian pied flycatchers have smaller forehead patches than both Spanish pied flycatchers and Swedish collared flycatchers. We suggest that this pattern may be explained by the lack of sexual selection on the forehead patch in the Norwegian population as compared to the other populations, where the patch is apparently sexually selected. We discuss possible reasons for these population divergences, such as female choice on an alternative secondary sexual character (general plumage color) and speciation among Ficedula flycatchers.

A Test for Female Attraction to Male Orange Coloration in Poecilia picta

Examining the distribution of female preference and attractive male secondary sexual characters in closely related species can help evaluate the various models for the evolution of female preference. We examined whether orange coloration in males was attractive to females of Poecilia picta, a species in the same subgenus as the guppy, P. reticulata. Females did not respond differently to males with or without a conspicuous orange stripe on the dorsum of the tail, a characteristic ornamentation of the male of this species. This lack of preference in a taxon that possesses a conspicuous, secondary sexual character directly contradicts the predictions of the sensory exploitation hypothesis for the evolution of female preference, but may support a model of antagonistic sexual selection proposed by Rice (1996).

Long-term exposure to 4-nonylphenol affects sexual differentiation and growth of the amphipod Corophium volutator (#Pallas, 1766)

Concern has been growing recently over the aquatic toxicity and reproductive effects of the non-ionic surfactant 4-nonylphenol (NP). NP appears to interact with development in some crustaceans. Using exposure response experiments based on nominal NP concentrations, the 96-h LC 50 for adult Corophium volutator, was 1670 μg l −1 (95% Cl 1370–2170 μg l −1). Juveniles were subsequently exposed to sublethal concentrations of NP for 120 days and effects on growth, sexual differentiation and reproduction were assessed. At the lowest NP exposure of 10 μg l −1 the density of surviving amphipods was reduced and growth was retarded. Fertility of female C. volutator increased in NP exposed populations. Sex ratio was not affected by NP exposure, however, the second antennae of exposed male animals were significantly longer than those of control animals ( P<0.001). It is hypothesised that NP may act via the androgenic gland which controls the development of male secondary sex characteristics in crustaceans. Males with large antennae are potentially at a selective disadvantage in the wild being more vulnerable to predation.

Is avian humoral immunocompetence suppressed by testosterone?

A key issue in sexual selection theory is how a correlation between male secondary sexual characters and male genetic quality can be maintained. The immunocompetence-handicap hypothesis proposes that testosterone-dependent male characters remain honest signals because of the immunosuppressive effect of elevated steroid hormone levels. The hypothesis requires that physiological levels of testosterone depress immune system function. We quantified testosterone titers and humoral immunocompetence of captive male and female red-winged blackbirds (Agelaius phoeniceus) at three points in the annual cycle (autumn, prebreeding, and breeding). We also conducted an implant experiment on the males to assess the effects of prolonged, above-normal testosterone titers on humoral immune responses. Humoral immunocompetence was measured as secondary antibody production to a non-pathogenic protein antigen, keyhole limpet hemocyanin, using an enzyme-linked immunosorbent assay we developed for A. phoeniceus. Secondary antibody responses of individuals were highly repeatable between sampling periods. Neither physiological nor above-normal levels of plasma testosterone suppressed secondary antibody production. In paired tests of the same individuals between prebreeding and breeding, and between breeding and implant, plasma testosterone increased significantly but secondary antibody responses were unaffected. We are confident in these results because with 80% power, an 11–14% difference in antibody titers would have been detected. There was no relationship between plasma testosterone levels and humoral immunocompetence in free-ranging males tested at the peak of breeding. These results cast doubt on a key assumption of the immunocompetence-handicap hypothesis.

Parasitism, host immune function, and sexual selection.

Parasite-mediated sexual selection may arise as a consequence of 1) females avoiding mates with directly transmitted parasites, 2) females choosing less-parasitized males that provide parental care of superior quality, or 3) females choosing males with few parasites in order to obtain genes for parasite resistance in their offspring. Studies of specific host-parasite systems and comparative analyses have revealed both supportive and conflicting evidence for these hypotheses. A meta-analysis of the available evidence revealed a negative relationship between parasite load and the expression of male secondary sexual characters. Experimental studies yielded more strongly negative relationships than observations did, and the relationships were more strongly negative for ectoparasites than for endoparasites. There was no significant difference in the magnitude of the negative effect for species with and without male parental care, or between behavioral and morphological secondary sexual characters. There was a significant difference between studies based on host immune function and those based on parasite loads, with stronger effects for measures of immune function, suggesting that the many negative results from previous analyses of parasite-mediated sexual selection may be explained because relatively benign parasites were studied. The multivariate analyses demonstrating strong effect sizes of immune function in relation to the expression of secondary sexual characters, and for species with male parental care as compared to those without, suggest that parasite resistance may be a general determinant of parasite-mediated sexual selection.

Asymmetry in a visual signaling character and sexual selection in a wolf spider

Fluctuating asymmetry (FA) can indicate developmental instability in bilaterally symmetric organisms, and studies have shown that the degree of asymmetry in male secondary sexual characters influences female mate choice in a number of taxa. In male Schizocosa ocreata wolf spiders, conspicuous tufts of bristles on the forelegs are a critical component of visual courtship signals, which appear to play a role in female mate choice. Previous studies have shown that females exhibit reduced receptivity to males with regenerative asymmetry, a consequence of leg loss and regeneration that causes males to be grossly asymmetric with respect to this important signaling character. We provide data on the occurrence of FA in the tufts of S. ocreata, and examine further the influence of asymmetry on female mate choice. The distribution of tuft area asymmetry values from a sample of field-collected males was normal, with a mean value of zero, indicating true FA. For a subset of males measured directly after field collection and prior to feeding, tuft asymmetry was significantly negatively correlated with measures of body size (body length) and condition (abdomen volume/cephalothorax width). Receptivity responses of females to visual signals from live males of similar size varied with the degree of asymmetry in male tufts. Since FA covaries with male body size and condition, which may also influence behavioral vigor, we used video image manipulation to alter the degree of asymmetry in tufts of a courting male while holding size and condition constant. Asymmetry treatments represented values within the range of natural FA variation as well as more extreme values characteristic of regenerative asymmetry. With the confounding effects of male size, condition, and behavior held constant, female spiders exhibited reduced receptivity responses to all experimental asymmetric video images relative to a control video stimulus. There were no differences in the frequency of female receptivity among the various asymmetry treatments, suggesting that discrimination against asymmetry in conspecific male signal characters is not simply a rejection of extreme phenotypes. Results suggest that asymmetry in a key male secondary character used in visual signaling, independent of any concomitant behavioral or size factor, is an important criterion in mate choice.

Sexual selection resulting from extrapair paternity in collared flycatchers

Extrapair paternity has been suggested to represent a potentially important source of sexual selection on male secondary sexual characters, particularly in birds with predominantly socially monogamous mating systems. However, relatively few studies have demonstrated sexual selection within single species by this mechanism, and there have been few attempts to assess the importance of extrapair paternity in relation to other mechanisms of sexual selection. We report estimates of sexual selection gradients on male secondary sexual plumage characters resulting from extrapair paternity in the collared flycatcherFicedula albicollis, and compare the importance of this form of sexual selection with that resulting from variation in mate fecundity. Microsatellite genotyping revealed that 15% of nestlings, distributed nonrandomly among 33% of broods (N=79), were the result of extrapair copulations. Multivariate selection analyses revealed significant positive directional sexual selection on two uncorrelated secondary sexual characters in males (forehead and wing patch size) when fledgling number was used as the measure of fitness. When number of offspring recruiting to the breeding population was used as the measure of male fitness, selection on these traits appeared to be directional and stabilizing, respectively. Pairwise comparisons of cuckolded and cuckolding males revealed that males that sired young through extrapair copulations had wider forehead patches, and were paired to females that bred earlier, than the males that they cuckolded. Path analysis was used to partition selection on these traits into pathways via mate fecundity and sperm competition, and suggested that the sperm competition pathway accounted for between 64 and 90% of the total sexual selection via the two paths. The selection revealed in these analyses is relatively weak in comparison with many other measures of selection in natural populations. We offer some explanations for the relatively weak selection detected.

Parasites, Morphology, and Blood Characters in Male Red Jungle Fowl during Development

Parasites have significant effects on the fitness of their hosts. We used an intestinal nematode, Ascaridia galli, as an experimental treatment of young male Red Jungle Fowl (Gallus gallus) to investigate the effect of this parasite on their growth and development through sexual maturity. After treatment, 26% of those fed parasites and 31% of controls became infected. Infected males had lower body mass, lower hematocrit, a higher percentage of lymphocytes among their white blood cells, and smaller combs than uninfected males. Infected and uninfected males did not differ in plasma levels of testosterone (T). The differences between parasitized and unparasitized males persisted from 5 months of age through sexual maturity. The classic example of hormonal action is the effect of testosterone on the comb of male chickens, but the difference in comb length between parasitized and unparasitized male Red Jungle Fowl along with no difference in T suggests that the effect of parasites on male secondary sexual characters does not necessarily involve a change in circulating levels of testosterone.

Differential Regulation of Androgen and Glucocorticoid Receptors by Retinoblastoma Protein

The androgen receptor (AR) plays a major role in the development and maintenance of male primary and secondary sexual characteristics. The growth promoting effects of androgens are clearly seen in prostate cancer where treatment by androgen ablation usually leads to tumor regression, followed sometime later, by growth of tumor cells that are resistant to endocrine therapy. We have found that the level of pRB in cells controls AR activity. Overexpression of pRB leads to increased transcriptional activity of the AR. This is similar to the previously reported potentiation of glucocorticoid receptor activity by pRB. In contrast, loss of pRB activity inhibits AR but not glucocorticoid receptor activity. This inhibition correlates with the unique ability of the AR to form a protein-protein complex with pRB in vitro. The site of interaction with pRB lies within the N-terminal domain of the AR and co-localizes with the region of the AR that specifies a requirement for pRB. Thus, the AR has a novel requirement for pRB raising the possibility that the growth promoting activity of AR is due to its direct interaction with pRB. Furthermore, loss of pRB activity during progression of prostate cancer may directly result in a decreased response to androgens.

Reproductive seasonality and maturation throughout the complete life-cycle in the mouflon ram (Ovis musimon).

To document the process of physical and reproductive maturation in a long-lived mammal, the long-term changes in endocrine (follicle stimulating hormone-FSH, prolactin and testosterone) and morphological parameters (body weight, testis diameter, neck girth, sex skin colour, horn growth, wool growth, moult and pelage characteristics) were measured throughout the full life-span in a group of captive mouflon rams, Ovis musimon (n=4, 12-year study). Live body weight increased to a maximum at 7 years of age and declined in old age, while horn size (length and volume) increased throughout the full life-span. Every year there was a marked and consistent cycle in the blood plasma concentrations of FSH and testosterone, and growth of the testes, with peak reproductive function in September and October associated with the rut. The annual maximum in plasma FSH and testosterone concentrations, and the size of the testes increased progressively from 1-6 years of age (or later into the life-cycle for testosterone). There was also a marked annual cycle in plasma prolactin concentrations (peak in May and June), and growth of the horns and pelage. The annual maximum in prolactin concentrations increased throughout the life-cycle, while the rate of horn growth (annual increments) decreased. The overall conclusion is that neuroendocrine mechanisms regulate both the annual and the age-related changes in the reproductive axis, and reproductive maturation proceeds gradually through the life-cycle. Because the acquisition of adult characteristics (large body size, large horns, conspicuous male secondary sexual characteristics) greatly affects male competitiveness, sexual maturity is closely linked to physical maturity, and this occurs long after the initial onset of fertility at puberty.

Acquired resistance affects male sexual display and female choice in guppies

Is resistance to parasites related to the expression of male secondary sex characters? Handicap models predict a positive relationship, proposing that males displaying extravagant sex characters may be honestly signalling their resistance to females. However, no current evidence addresses whether individual changes in immunity (acquired resistance) are reflected in sexual traits. In this experiment I use guppies to compare male orange colour, sigmoid display and female preferences for individual males, before and after a primary challenge infection of males. Challenge infections were terminated chemically and fish were given ten days9 recovery time before proceeding with the second measurements. The degree of acquired resistance was quantified a posteriori , by exposing males to a secondary infection. Sigmoid display rates and female preference for males differed for males of different resistance groups after challenge infection only. This difference was due to resistant males displaying more than non–resistant ones. No differences were detected in male orange colour, but this may be because colour needs a longer time than ten days to be recovered and adjusted. The results show that the level of acquired resistance affects sexual display and attractiveness in guppies. They suggest that once an effective immunity is built up by a male, he can afford to incur higher costs for sexual characteristics, whereas a male that lacks the ability to build up effective resistance cannot. These costs probably consist of higher energy expenditure and/or higher circulating levels of testosterone, which may be needed to increase display. Priming and effective establishment of an individual9s resistance to parasitic infection could eventually result in a higher availability of resources for sexual functions.

Male secondary sexual characteristics, sexual selection, and molecular divergence in fanged ranid frogs of Southeast Asia

A group of Southeast Asian frogs of the family Ranidae are characterized by a suite of unusual secondary sexual characteristics. Males are larger than females, possess bony odontoid processes (fangs) and have enlarged heads. While all members of this clade are sexually dimorphic in this suite of characters, the degree of development of the secondary sexual characteristics in males varies widely across species. One of the most common species,Rana blythiiBoulenger, has an extremely broad geographical range. This frog lacks an advertisement call in Borneo but calling has been reported in populations from peninsular Malaysia and Vietnam. This study examines the relationship between the expression of male secondary sexual characteristics, molecular divergence and number of speciation events in widely separated populations ofR. blythii, and betweenR. blythiiand other species of closely related fanged frogs belonging to thegrunniensspecies group. Results from morphometric and sequence data indicate that (1) there is no correlation between morphological and molecular distance in these frogs and (2) there is no correlation between morphological distance and the number of speciation events separating taxa. Systematic analysis demonstrates thatR. blythii, as currently recognized, is not a single species nor is it monophyletic.

Male secondary sexual characteristics, sexual selection, and molecular divergence in fanged ranid frogs of Southeast Asia

Abstract A group of Southeast Asian frogs of the family Ranidae are characterized by a suite of unusual secondary sexual characteristics. Males are larger than females, possess bony odontoid processes (fangs) and have enlarged heads. While all members of this clade are sexually dimorphic in this suite of characters, the degree of development of the secondary sexual characteristics in males varies widely across species. One of the most common species, Rana blythii Boulenger, has an extremely broad geographical range. This frog lacks an advertisement call in Borneo but calling has been reported in populations from peninsular Malaysia and Vietnam. This study examines the relationship between the expression of male secondary sexual characteristics, molecular divergence and number of speciation events in widely separated populations of R. blythii , and between R. blythii and other species of closely related fanged frogs belonging to the grunniens species group. Results from morphometric and sequence data indicate that (1) there is no correlation between morphological and molecular distance in these frogs and (2) there is no correlation between morphological distance and the number of speciation events separating taxa. Systematic analysis demonstrates that R. blythii , as currently recognized, is not a single species nor is it monophyletic.

Male courtship song frequency as an indicator of male genetic quality in an insect species, Drosophila montana.

Most theoretical models on evolution of male secondary sexual characters and female preferences for these characters suggest that the male characters evolve in response to female preferences that may themselves evolve in response to direct or indirect benefits of choice. In Drosophila montana (a species of the D. virilis group), females use male song in their mate choice, preferring males that produce songs with short sound pulses and a high carrier frequency. We demonstrate here that the females get indirect benefits from their choice: in our data the frequency of the male song correlated with the survival rate of the male's progeny from egg to adulthood (indirect benefit for the female), but not with the fecundity of his mating partner (no direct benefit for the female). Male wing centroid asymmetry did not correlate with male wing song characters, nor with female egg production nor the fitness of her progeny, suggesting that fluctuating asymmetry in male wings does not play a major role in sexual signalling. The fact that the male song gives the female information on the male's condition/genetic quality in D. montana suggests that in this species the evolution of female preferences for male song characters could have evolved through condition-dependent viability selection presented in some 'good genes' models.

Masculinized females in a population of Leptodiaptomus minutus (Copepoda, Calanoida)

In a population of the common freshwater copepod Leptodiaptomus minutus Lilljeborg, 7.3% of adult females exhibited male secondary sexual characteristics. Although these masculinized or intersex females had the internal morphology of normal females, they possessed one or rarely two normal geniculate male antennules, and in a few instances male fifth legs. The sex ratio of normal adult males to females in the population was 1:1. No differences in body size among normal males, normal females, and masculinized females were found. Antennule lengths of males and masculinized females were not significantly different from one another; however, normal females had longer antennules than either males or masculinized females. Mating experiments indicated that masculinized females were functionally female and they were able to mate with normal males to produce offspring that were normal or masculinized. The occurrence of intersex copepods in lakes of North America may be more widespread than we now recognize because they can be easily overlooked or mistaken for normal adults.