Offspring sex ratio skew in the sexually monomorphic house martin Delichon urbicum

Sex ratio at conception may be under selection pressure, given that male and female offspring differ in the cost of production or generate different fitness returns under specific conditions. We studied adjustments in the primary, secondary and tertiary sex ratio in house martin Delichon urbicum, which is a sexually monomorphic, socially monogamous, colonial bird. Males of this species engage in extra‐pair copulations with heavy males acquiring the highest fertilization success. We analyzed variation in the sex ratio in relation to clutch size and parental characteristics including body condition, wing length, as well as length and pigmentation of the white rump patch during three breeding seasons. The only variable which significantly explained the variation in the sex ratio was wing length of the social father and mother. The proportion of sons among offspring was positively correlated to wing length of the social father and negatively correlated to mother wing length. Social father wing length positively correlated with mean brood body mass at fledging, which may suggest that females that mated with long‐winged males produced sons, which acquired the highest total fertilization success. Consequently, our results indicate that house martin females may adaptively adjust offspring sex composition at egg laying in relation to the characteristics of their social mate.     

Development of parameters influencing blood oxygen carrying capacity in the welcome swallow and fairy martin

Welcome swallow (Hirundo neoxena) and fairy martin (Petrochelidon ariel) nestlings develop relatively slowly. They exceed adult mass early in development, then lose weight and fledge at about adult mass, i.e. have a mass-overshoot-recession (Mor) growth profile. Development of parameters influencing blood oxygen carrying capacity (O2Cap) was documented to determine if O2Cap also reached a plateau substantially before fledging or increased continuously throughout nestling development. Hematocrit (Hct), erythrocyte count (RBC) and whole blood haemoglobin (Hb) increased 1.8- to 2.8-fold, so that O2Cap doubled during development in both species. Increase in Hct, Hb and RBC was not continuous, peak values occurring well before fledging, in contrast to passerines with standard growth profiles in which the increase occurs throughout nestling development and peak values occur at fledging. However, the timing of O2Cap increase differed from that in some other Mor species (e.g. shearwaters). Mean erythrocyte volume (MCV) decreased linearly throughout development by 30-41%, but mean erythrocyte haemoglobin content (MCH) remained constant, so that mean erythrocyte haemoglobin concentration (MCHC) increased linearly 1.3- to 1.5-fold. Possible reasons for the apparent differences in the timing of O2Cap increase between rapidly and slowly growing altricial species and among MOR species are discussed.     

Mutual mate choice in a female-dominant and sexually monomorphic primate

Sexual dimorphism is common in polygynous species, where intrasexual competition is often thought to drive the evolution of large male body size, and in turn, male behavioral dominance over females. In Madagascar, the entire lemur radiation, which embraces diverse mating systems, lacks sexual dimorphism and exhibits frequent female dominance over males. The evolution of such morphological and behavioral peculiarities, often referred to as "the lemur syndrome," has proven difficult to understand. Among other hypotheses, a potential role of intersexual selection has been repeatedly proposed but hardly ever tested. Here, we investigate whether female choice favors small and compliant males, and whether male choice favors large females in captive gray mouse lemurs (Microcebus murinus). Detailed analysis of a combination of behavioral observations and hormonal data available for both sexes shows that (1) females accept more matings from males with higher fighting abilities, (2) males adjust their investment in intrasexual competition to female fertility, and (3) both male and female strategies are weakly influenced by the body mass of potential partners, in directions contradicting our predictions. These results do not suggest a prominent role of intersexual selection in the evolution and maintenance of the lemur syndrome but rather point to alternative mechanisms relating to male-male competition, specifically highlighting an absence of relationship between male body mass and fighting ability. Finally, our findings add to the growing body of evidence suggesting flexible sex roles, by showing the expression of mutual mate choice in a female-dominant, sexually monomorphic and promiscuous primate.     

Speciation is not necessarily easier in species with sexually monomorphic mating signals

Should we have different expectations regarding the likelihood and pace of speciation by sexual selection when considering species with sexually monomorphic mating signals? Two conditions that can facilitate rapid species divergence are Felsenstein's one‐allele mechanism and a genetic architecture that includes a genetic association between signal and preference loci. In sexually monomorphic species, the former can manifest in the form of mate choice based on phenotype matching. The latter can be promoted by selection acting upon genetic loci for divergent signals and preferences expressed simultaneously in each individual, rather than acting separately on signal loci in males and preference loci in females. Both sexes in the Chrysoperla carnea group of green lacewings (Insecta, Neuroptera, Chrysopidae) produce sexually monomorphic species‐specific mating signals. We hybridized the two species C. agilis and C. carnea to test for evidence of these speciation‐facilitating conditions. Hybrid signals were more complex than the parents and we observed a dominant influence of C. carnea. We found a dominant influence of C. agilis on preferences in the form of hybrid discrimination against C. carnea. Preferences in hybrids followed patterns predicting preference loci that determine mate choice rather than a one‐allele mechanism. The genetic association between signal and preference we detected in the segregating hybrid crosses indicates that speciation in these species with sexually monomorphic mating signals can have occurred rapidly. However, we need additional evidence to determine whether such genetic associations form more readily in sexually monomorphic species compared to dimorphic species and consequently facilitate speciation.     

Sex allocation according to multiple sexually dimorphic traits of both parents in the barn swallow (Hirundo rustica)

Parents should differentially invest in sons or daughters depending on the sex‐specific fitness returns from male and female offspring. In species with sexually selected heritable male characters, highly ornamented fathers should overproduce sons, which will be more sexually attractive than sons of less ornamented fathers. Because of genetic correlations between the sexes, females that express traits which are under selection in males should also overproduce sons. However, sex allocation strategies may consist in reaction norms leading to spatiotemporal variation in the association between offspring sex ratio (SR) and parental phenotype. We analysed offspring SR in barn swallows (Hirundo rustica) over 8 years in relation to two sexually dimorphic traits: tail length and melanin‐based ventral plumage coloration. The proportion of sons increased with maternal plumage darkness and paternal tail length, consistently with sexual dimorphism in these traits. The size of the effect of these parental traits on SR was large compared to other studies of offspring SR in birds. Barn swallows thus manipulate offspring SR to overproduce ‘sexy sons’ and potentially to mitigate the costs of intralocus sexually antagonistic selection. Interannual variation in the relationships between offspring SR and parental traits was observed which may suggest phenotypic plasticity in sex allocation and provides a proximate explanation for inconsistent results of studies of sex allocation in relation to sexual ornamentation in birds.     

Sex allocation in heterostylous plants

This paper examines the heterostylous breeding system in plants from the perspective of sex allocation theory. Models are used to predict the frequencies of the different floral morphs in populations of heterostylous species and the relative amounts of resources allocated to pollen and seeds in each morph. We note that in natural populations morph ratios may deviate from equality and that the morphs may specialize in the production of either pollen or seeds. Our sex allocation models are an attempt to explain why such patterns exist.     

Sex allocation in hermaphroditic plants

Hermaphroditic plants allocate their reproductive resources to different functions: male, female and pollinator attraction. While earlier sex-allocation models considered only male and female functions, more recent ones can divide reproductive resources into multiple functions. The basic predictions derived from these models are similar. While most models predict sex allocation at the fruit stage (pollen and seeds), some have examined allocation at the flower stage (pollen and ovules). Selfing rate, mode of pollination and competition among offspring of the same parent are some of the factors that can influence sex allocation among populations. Although the empirical evidence lags behind the theoretical development, sex-allocation theory has been quite successful at predicting trends among populations.     

雌雄同花植物的性分配

性分配理论假定雌雄功能之间存在着trade-off,对一种性别的投入增多必然会减少对另一性别的投入。雌雄功能投入的适合度曲线的形状决定了哪种繁育系统是进化稳定的。因此,性分配理论可以解释植物繁育系统的进化,尤其被认为是雌雄异株进化的选择机制之一。目前的实验研究分别在物种间、种群间、个体间及花间四个层次上进行:自交率的程度对物种和种群的性分配都有影响;虫媒和风媒植物的性分配是个体大小依赖的;而且花序内花的性分配模式受昆虫访花行为的影响。相对于理论,性分配的实验研究明显滞后,随着分子标记技术的普及,性分配理论将会获得更大的发展。繁殖分配需要进一步与性分配理论结合,尤其在空间尺度上资源分配与繁育系统变化的研究是很有意义的。     

Perineal muscles and motoneurons are sexually monomorphic in the naked mole-rat (Heterocephalus glaber)

Naked mole-rats are eusocial mammals that live in colonies with a single breeding female and one to three breeding males. All other members of the colony, known as subordinates, are nonreproductive and exhibit few sex differences in behavior or genital anatomy. This raises questions about the degree of sexual differentiation in subordinate naked mole-rats. The striated perineal muscles associated with the phallus [the bulbocavernosus (BC), ischiocavernosus (IC), and levator ani (LA) muscles], and their innervating motoneurons, are sexually dimorphic in all rodents examined to date. We therefore asked whether perineal muscles and motoneurons were also sexually dimorphic in subordinate naked mole-rats. Muscles similar to the LA and IC of other rodents were found in naked mole-rats of both sexes. No clear BC muscle was identified, although a large striated muscle associated with the urethra in male and female naked mole-rats may be homologous to the BC of other rodents. There were no sex differences in the volumes of the LA, IC, or the urethral muscles. Motoneurons innervating the perineal muscles were identified by retrograde labeling with cholera-toxin-conjugated horseradish peroxidase. All perineal motoneurons were found in a single cluster in the ventrolateral lateral horn, in a position similar to that of Onuf's nucleus of carnivores and primates. There was no sex difference in the size or number of motoneurons in Onuf's nucleus of naked mole-rats. Thus, unlike findings in any other mammal, neither the perineal muscles nor the perineal motoneurons appear to be sexually differentiated in subordinate naked mole-rats.     

Sex allocation variation in Daphnia pulex

Daphnia (Crustacea: Cladocera) reproduce by cyclical parthenogenesis in which the sex of offspring is environmentally determined. Although numerous studies have demonstrated that factors such as crowding and short-day photoperiod stimulate male production, there is limited information on variation in allocation to male and female offspring for any species of Daphnia . The present study assessed the presence or absence of male production in 96 isofemale lines (clones) from each of eight populations of Daphnia pulex . An average of 37% (range 18–51%) of clones failed to produce males under crowded conditions in the laboratory. A subset of 14 of these non-male-producing clones also failed to produce males under short-day photoperiod (8L:16D). Three male-producing clones were within-clone mated as well as crossed to three non-male-producing clones to study the inheritance of the failure to produce males. The average frequency of non-male-producing F ₁ progeny was significantly higher (58%, N = 486) among the outcrossed progeny than the inbred progeny (5%, N = 86). In addition, when sixteen of the male-producing outcrossed progeny were within-clone mated, only 7% ( N = 106) of the resulting F ₂ progeny failed to produce males. These results are consistent with a genetic basis for the absence of male production. Average survival of the progeny from the nine outcrossed matings was more than twice (67%) that of the inbred progeny from the three within-clone matings (30%), suggesting that within-clone mating would result in significant inbreeding depression. We present a model that suggests that even low levels of inbreeding could allow non-male-producing females to be maintained in a population. The co-occurrence of non-male-producing females and females that produce both males and females in Daphnia pulex bears a similarity to the gynodioecious breeding system found in some plant species.     

寄生蜂性别分配行为

寄生蜂是性比分配行为领域的研究热点对象,其性别决定方式为单双倍型,一般情况下,未受精的单倍型卵发育成雄蜂,受精的二倍型卵发育为雌蜂。局部配偶竞争和近交等因素使得偏雌性比成为这类生物的进化稳定策略;其性比具有可调节性,产卵个体可以根据对产卵环境的判定来调控后代性比,从而获得最大适合度。在此基础上形成的局部配偶竞争理论阐述了寄生蜂性比的这种可调节性,成为进化论的优秀论据。     

Sex allocation in black-capped chickadees Poecile atricapilla

Optimal sex allocation for individuals can be predicted from a number of different hypotheses. Fisherian models of sex allocation predict equal investment in males and females up to the end of parental care and predict brood compositions based on the relative costs of producing males and females. The Trivers-Willard hypothesis predicts that individual females should alter the sex ratio of their broods based on their own condition if it has a differential impact on the lifetime reproductive success of their sons and daughters. The Charnov model of sex allocation predicts that females should alter sex allocation based on paternal attributes that may differentially benefit sons versus daughters. Because females are the heterogametic sex in birds, many recent studies have focussed on primary sex ratio biases. In black-capped chickadees Poecile atricapilla , males are larger than females suggesting they may be more costly to raise than females. Female condition affects competitive ability in contests for mates, and thus may be related to variance in fecundity. Females prefer high-ranking males as both social and extrapair partners. These observations suggest that females might vary the sex ratio of their broods based on the predictions of any of the above models. Here, we report on the results of PCR based sex determination of 1093 nestlings in 175 broods sampled from 1992 to 2001. Population-wide, we found a mean brood sex ratio of 0.525±0.016, with no significant deviation from a predicted binomial distribution. We found no effect of clutch size, female condition, hatch date, parental rank or paternity. Our results reject the idea that female black-capped chickadees systematically vary sex allocation in their broods.     

Sex allocation: At the females' whim.

Experimental studies of wild birds suggest that females have a previously unappreciated ability to control the sex ratio of their offspring in response to variation in sex-specific fitness benefits.     

Perineal muscles and motoneurons are sexually monomorphic in the naked mole‐rat (Heterocephalus glaber)

Naked mole‐rats are eusocial mammals that live in colonies with a single breeding female and one to three breeding males. All other members of the colony, known as subordinates, are nonreproductive and exhibit few sex differences in behavior or genital anatomy. This raises questions about the degree of sexual differentiation in subordinate naked mole‐rats. The striated perineal muscles associated with the phallus [the bulbocavernosus (BC), ischiocavernosus (IC), and levator ani (LA) muscles], and their innervating motoneurons, are sexually dimorphic in all rodents examined to date. We therefore asked whether perineal muscles and motoneurons were also sexually dimorphic in subordinate naked mole‐rats. Muscles similar to the LA and IC of other rodents were found in naked mole‐rats of both sexes. No clear BC muscle was identified, although a large striated muscle associated with the urethra in male and female naked mole‐rats may be homologous to the BC of other rodents. There were no sex differences in the volumes of the LA, IC, or the urethral muscles. Motoneurons innervating the perineal muscles were identified by retrograde labeling with cholera‐toxin‐conjugated horseradish peroxidase. All perineal motoneurons were found in a single cluster in the ventrolateral lateral horn, in a position similar to that of Onuf's nucleus of carnivores and primates. There was no sex difference in the size or number of motoneurons in Onuf's nucleus of naked mole‐rats. Thus, unlike findings in any other mammal, neither the perineal muscles nor the perineal motoneurons appear to be sexually differentiated in subordinate naked mole‐rats. © 2002 Wiley Periodicals, Inc. J Neurobiol 51: 33–42, 2002     

Sex allocation theory aids species conservation

Supplementary feeding is often a key tool in the intensive management of captive and threatened species. Although it can increase such parameters as breeding frequency and individual survival, supplementary feeding may produce undesirable side effects that increase overall extinction risk. Recent attempts to increase breeding frequency and success in the kakapo Strigops habroptilus using supplementary feeding inadvertently resulted in highly male-biased chick sex ratios. Here, we describe how the inclusion of sex allocation theory has remedied this conservation dilemma. Our study is the first to manipulate chick sex ratios in an endangered species by altering maternal condition and highlights the importance of incorporating evolutionary theory into modern conservation practice.