Manipulation of offspring sex ratio by second-mated female house wrens

In 1973, Trivers and Willard proposed that offspring sex ratio should be associated with the quality of parental care likely to be provided to the offspring. We tested this hypothesis by comparing fledgling sex ratios in nests of first- and second-mated female house wrens (Troglodytes aedon). In our Wyoming population, second-mated females typically receive little or no male parental assistance and fledge fewer and lower-quality young compared with first-mated females. Assuming that being of lower quality has stronger negative effects on the future reproductive success of males than that of females in this polygynous population, we predicted that fledgling sex ratios in the nests of second-mated females would be female-biased compared with the fledgling sex ratios of first-mated females. Additionally, we asked whether any sex bias at fledging could have resulted from male-biased nestling mortality caused by sex-biased parental provisioning. As predicted, mean fledgling sex ratios in nests of second-mated females were more female-biased than fledgling sex ratios in nests of first-mated females. However, we found no evidence of either sex-biased nestling mortality or sex-biased parental provisioning. These findings suggest that females are responding to their status as second-mated females and to the associated low-quality parental care that their young are likely to receive by producing female-biased clutches rather than manipulating the offspring sex ratio through sex-biased nestling mortality.     

Behavioral and physiological female responses to male sex ratio bias in a pond-breeding amphibian

Introduction

The phenomenon of sexual conflict has been well documented, and in populations with biased operational sex ratios the consequences for the rarer sex can be severe. Females are typically a limited resource and males often evolve aggressive mating behaviors, which can improve individual fitness for the male while negatively impacting female condition and fitness. In response, females can adjust their behavior to minimize exposure to aggressive mating tactics or minimize the costs of mating harassment. While male-male competition is common in amphibian mating systems, little is known about the consequences or responses of females. The red-spotted newt (Notophthalmus viridescens) is a common pond-breeding amphibian with a complex, well-studied mating system where males aggressively court females. Breeding populations across much of its range have male-biased sex ratios and we predicted that female newts would have behavioral mechanisms to mitigate mating pressure from males. We conducted four experiments examining the costs and behavioral responses of female N. viridescens exposed to a male-biased environment.

Results

In field enclosures, we found that female newts exposed to a male-biased environment during the five-month breeding season ended with lower body condition compared to those in a female-biased environment. Shorter-term exposure to a male-biased environment for five weeks caused a decrease in circulating total leukocyte and lymphocyte abundance in blood, which suggests females experienced physiological stress. In behavioral experiments, we found that females were more agitated in the presence of male chemical cues and females in a male-biased environment spent more time in refuge than those in a female-biased environment.

Conclusions

Our results indicate that male-biased conditions can incur costs to females of decreased condition and potentially increased risk of infection. However, we found that females can also alter their behavior and microhabitat use under a male-biased sex ratio. Consistent with surveys showing reduced detection probabilities for females, our research suggests that females avoid male encounters using edge and substrate habitat. Our work illustrates the integrated suite of impacts that sexual conflict can have on the structure and ecology of a population.     

Female mating behavior in the field cricket,Gryllus pennsylvanicus (Orthoptera: Gryllidae) at different operational sex ratios

The influence of operational sex ratio on the mating behavior of female field crickets,Gryllus pennsylvanicus, was investigated. Females were predicted to be more discriminating under conditions of high mate availability and show less selectivity when males were rare. Such selectivity was indicated in this study with the proportion of courtships leading to a mating changing with sex ratio. Females accepted almost 70% of all courtships at the female-biased sex ratio, but only about half of all courtships were successful at even or male-biased sex ratios. Females moved least at the female-biased sex ratio. There was also a trend for females to be guarded more under male-biased conditions. Female weight did not influence any of the behaviors examined.     

The effect of nest aggregation on the reproductive behaviour of the peacock blenny Salaria pavo

The effect of nest aggregation in courtship behaviour was tested experimentally in an ecologically constrained, sex-role reversed population of the peacock blenny Salaria pavo . Mixed sex groups of eight males and eight females were tested in experimental tanks, containing eight potential nests either aggregated or dispersed. In the aggregated treatment, males spent more time inside their nests and monopolized other potential nests, causing a female-biased operational sex ratio (OSR). In the aggregated treatment, females also expressed more courtship behaviour. The results in general support the prediction that the aggregation of nests promotes male monopolization of potential nests, resulting in fewer nest-holding males and therefore a female-biased OSR that leads to the reversal of sex roles.     

EFFECT OF DENSITY ON SECONDARY SEX CHARACTERISTICS AND SEX RATIO IN SILENE ALBA (CARYOPHYLLACEAE)

A field survey of plant and flower sex ratio and secondary sex characteristics was made in Silene alba. Female-biased plant sex ratios were found, as seems typical for the species. Sex ratio distribution correlated with a gradient of soil moisture (with the more moist area having a more female-biased ratio) and with changes in the density of Silene (intermediate and higher density areas having greater female bias). The floral sex ratio was significantly female-biased only at the site that was most female-biased in terms of plant sex ratio. Otherwise the population of flowers was significantly male-biased. Male and female plants harvested from the field differed in secondary sexual characteristics. Males had more flowers and invested proportionately more biomass in leaf, but less in root, stem and reproductive tissue than did females. Although both males and females were larger in terms of total dry weight at the moist site, males produced more flowers at the driest (high density) site. Here the female bias in plant sex ratio was intermediate, but the floral sex ratio was significantly male-biased. A glasshouse experiment was performed in which plants were grown at four densities. Density significantly influenced plant survivorship and the probability of flowering, and increased female bias in the pots, but it did not affect patterns of biomass allocation in flowering plants. Patterns of male and female biomass allocation did not differ in the experiment, except in terms of reproductive allocation (greater in females) and allocation to leaf, greater in males, but only at the lowest density. This work urges caution in interpreting differences between males and females in the field as secondary sex characteristics, since we find such properties to be overlapping under experimental conditions. It supports the idea that males and females of a species may sustain different reproductive output under differing conditions.     

The influence of deer browsing on the reproductive biology of Canada yew (Taxus canadensis marsh.)

Summary Browsed Canada yew (Taxus canadensis) populations have a higher proportion of males and a lower proportion of monoecious plants than unbrowsed yew populations. The proportion of monoecious plants increases with time following protection from browsing suggesting that deer browsing causes male-biased sex expression in Canada yew. In contrast, results from comparing browsed and unbrowsed populations, exclosure studies, and browse simulation experiments indicate that strobilus ratios and phenotypic gender of browsed yews may be female-biased. In part, these results correspond to the influence of size on sex expression in Canada yew; small yews tend to be male, but if monoecious, have female-biased strobilus ratios. Large yews are monoecious, but have male-biased strobilus ratios. There is, however, no consistent relationship between size and gender in Canada yew, suggesting that in some circumstances, yews shift allocation to female function in response to browsing.     

The effects of sex ratio on sexual competition in the European lobster

During the breeding season an individual's access to mates may be affected by operational sex ratios, causing strong variation in mating success. We manipulated adult sex ratios of the European lobster, Homarus gammarus, to test the predictions of models that relate sexual competition to (1) the sex ratio, (2) the time that an individual is not available to mate and (3) 'collateral investment', whereby two males contribute to a single clutch. The model predictions proved to be relatively insensitive to collateral investment. Male-male competition predominated in the male-biased but not in the female-biased sex ratio. This matches the predictions of one model that incorporates an extended period of female receptivity because the time that a male was unavailable to mate was small compared to the time spent by females in cohabitation and parental care. Although females increased their competitiveness when males were in the minority, male competition remained high. The insensitivity of male-male competition to sex ratios may be due to an upper limit to the costs that males can afford when there is a serious risk of injury, preventing males from increasing their aggression when females are in short supply. Copyright 1999 The Association for the Study of Animal Behaviour.     

Sex in the morning or in the evening? Females adjust daily mating patterns to the intensity of sexual harassment

Selection on males to mate at a higher rate than females often results in male harassment of females and counteracting female responses. When the reproductive value of copulation changes over time, these mating strategies are expected to be time dependent. Here, we demonstrate that variation in the intensity of male harassment leads to drastic changes in female daily mating patterns. In feral populations of fowl Gallus gallus domesticus, male harassment is intense, particularly in the evening when inseminations are most likely to result in fertilization. We experimentally manipulated the intensity of male harassment through similar-sized groups of different sex ratios. Male mating propensity was always higher than females', particularly in male-biased groups and in the evening, when males were closer to and more likely to approach females. Females counteracted male harassment by escalating resistance to mating and--crucially--by shifting their daily mating pattern: in strongly female-biased groups with relaxed sexual harassment, females solicited sex in the evening, while in male-biased groups, they solicited sex in the morning, thus avoiding harassment in the evening. Together, these results indicate that intersexual conflict may occur not only over mating rates but also over when in the day to copulate.     

Female-biased Sex Ratios in the Neotropical Treehopper Umbonia ataliba (Homoptera: Membracidae)

The authors report and explain female-biased sex ratios in the neotropical treehopper Umbonia ataliba Homoptera: Membracidae at Monteverde, Costa Rica. Umbonia ataliba mothers semelparously oviposit egg masses into host-plant branches, make feeding holes, and guard the eggs and the nymphs until the young moult to become adults. At adulthood, offspring sex ratios are female-biased, with families having, on average, one male per 3.17 females (SD = 0.149, n = 48). The female bias does not appear to be explained by the hypothesis that males are more difficult to raise to independence: males are smaller than females, males have a shorter development time, males do not require disproportionately more feeding holes, and males do not experience higher mortality in families that are unprotected from parasites and predators, rather, females die more often in protected families. Thus females, not males, may be more difficult to raise to independence. The authors investigated whether increases in the size of males and females increased the fitness of either sex disproportionately, but found no relationship between size and fitness for either sex. We found evidence that local-mate competition conditions and inbreeding occur. Mating occurs at the natal site and nearly all copulations take place between siblings (99.3 %, n = 153 copulations). Most females (mean proportion of females = 0.65, SD = 0.33, n = 7 families) copulate with their male siblings prior to dispersing; whether the unmated proportion copulates later is unknown. This paper suggests that the numerical bias reflects an investment bias favoured under selection by inbreeding and local-mate competition conditions.     

Experimental evolution exposes female and male responses to sexual selection and conflict in Tribolium castaneum

Between-individual variance in potential reproductive rate theoretically creates a load in reproducing populations by driving sexual selection of male traits for winning competitions, and female traits for resisting the costs of multiple mating. Here, using replicated experimental evolution under divergent operational sex ratios (OSR, 9:1 or 1:6 ♀:♂) we empirically identified the parallel reproductive fitness consequences for females and males in the promiscuous flour beetle Tribolium castaneum. Our results revealed clear evidence that sexual conflict resides within the T. castaneum mating system. After 20 generations of selection, females from female-biased OSRs became vulnerable to multiple mating, and showed a steep decrease in reproductive fitness with an increasing number of control males. In contrast, females from male-biased OSRs showed no change in reproductive fitness, irrespective of male numbers. The divergence in reproductive output was not explained by variation in female mortality. Parallel assays revealed that males also responded to experimental evolution: individuals from male-biased OSRs obtained 27% greater reproductive success across 7-day competition for females with a control male rival, compared to males from the female-biased lines. Subsequent assays suggest that these differences were not due to postcopulatory sperm competitiveness, but to precopulatory/copulatory competitive male mating behavior.     

Social environment affects juvenile dispersal in great tits (Parus major)

1.?Habitat selection can affect individual fitness, and therefore, individuals are expected to assess habitat quality of potential breeding sites before settlement. 2.?We investigated the role of social environment on juvenile dispersal behaviour in the great tit (Parus major). Two main contradictory hypotheses can be formulated regarding social effects on juvenile dispersal as follows: (i) High fledgling density and sex ratio may enhance the intensity of local (kin) competition and, therefore, reduce individual survival chance, enhance emigration and reduce settlement ('repulsion' hypothesis) (ii) Alternatively, high fledgling density and sex ratio may signal high-quality habitat or lead to aggregation and thus increase individual survival chance, reduce emigration and enhance settlement ('attraction' hypothesis). 3.?To disentangle positive from negative effects of high density and male-biased sex ratio on dispersal, we manipulated the social composition of the fledgling population in 12 semi-isolated nest-box areas (plots) via a change of fledgling density (low/high) as well as fledgling sex ratio (female-biased/balanced/male-biased) across 3?years. We then tested whether experimental variation in male and female fledgling densities affected variation in local survival, emigration and settlement of juveniles, and whether social effects on survival and dispersal support the 'repulsion' or 'attraction' hypothesis. 4.?We found no experimental effects on local survival and emigration probabilities. However, consistent with the 'attraction' hypothesis, settlement was significantly and positively affected by local experimental sex ratio in each of the study years: both male and female juveniles avoided female-biased plots and settled more in plots that were balanced and male-biased the previous year. 5.?Our study provides unprecedented experimental evidence that local sex ratio plays a causal role in habitat selection. We suggest that settlers avoid female-biased plots because a high proportion of females may reflect the absence or the low quality of local resources in the habitat. Alternatively, male territory acquisition may be facilitated by a high local density of 'candidate' males, and therefore, juveniles were less successful in settling in female-biased plots.     

Bumblebee sex ratios: why do bumblebees produce so many males?

Sex investment ratios in populations of bumblebees are male biased, which contradicts theoretical predictions. Male-biased investment ratios in eusocial Hymenoptera are assumed to be non-stable for both the queen and her workers. In this paper, we show that male-biased sex allocation does not necessarily decrease fitness in the bumblebee Bombus terrestris. A male-biased investment ratio can be the result of an optimal allocation of resources when resources are scarce if (i) there is a large cost difference between male and female production, (ii) there is uncertainty about the amount of resources a colony can invest, and (iii) only a proportion of the investment made in an individual can be reused. This resource allocation then leads to split sex ratios depending on the amount of resources available to a bumblebee colony: colonies under low resource conditions will show a male-biased investment ratio, whereas colonies under high resource conditions allocate more resources towards females. However, the extent to which bumblebee populations show a male-biased sex allocation cannot be explained by cost differences between male and female production alone. In a recent paper, A. F. G. Bourke argued that male-biased investment ratios in bumblebee populations are a by-product of the occurrence of protandry (males emerge before females). Here we will extend Bourke''s argument and show that within a protandrous population, both protandrous and protogynous (females emerge before males) colonies exist. The existence of protandrous and protogynous colonies results in split sex ratios in time, because protogynous colonies rely on males produced by protandrous colonies (partial protandry).     

Complex sex allocation in the laughing kookaburra

In groups of the cooperatively breeding laughing kookaburra(Dacelo novaeguineae), offspring sex varied with the type ofsocial group and with hatch rank. Groups with female helpers,especially if all helpers were female, had male-biased clutchand fledging sex ratios. Groups without female helpers (unassistedpairs or male-only helpers) had female-biased clutch and fledgingsex ratios. Breeding females responded facultatively to increasesin the number of female helpers in their group by producingmore male eggs. These biases may occur if breeding femalestry to limit the number of daughters recruited into their groupbecause unlike male helpers, female helpers depress the breedingsuccess of their parents. Across all nests, two-thirds of first-hatchedyoung were male, two-thirds of second-hatched young were female, and the sex ratio of third-hatched young was even. Hatch ranksex ratios also varied dramatically between different typesof social groups, from 16.7% for second-hatched nestlings ofunassisted pairs to 100% for first-hatched nestlings of groupswith only female helpers. A corollary of the relationship betweenhatch rank and sex was that hatching sex sequences were distributed nonrandomly: all groups avoided hatching a daughter first followedby a son (FM). Sibling competition is aggressive and sometimesfatal. Since females grow to be 15% larger than males the hatchingsequence of sexes could affect nestling growth and mortality.However, an exhaustive analysis found little evidence thatgrowth or survival of males was compromised if hatched aftera sister. The small number of FM sequences may only have occurredin nests that were able to ameliorate any negative consequences.Alternatively, when clutch size is small and fledging successunpredictable because of brood reduction, the preferred broodsex ratio may be contingent on the number of fledged young,making it advantageous to order the sexes in the brood.     

Forest fragmentation is associated with primary brood sex ratio in the treecreeper (Certhia familiaris)

We studied the primary brood sex ratio of an old-growth forest passerine, the Eurasian treecreeper (Certhia familiaris), along a gradient of forest fragmentation. We found evidence that male nestlings were more costly to produce, since they suffered twofold higher nestling mortality and were larger in body size than females. Furthermore, the proportion of males in the brood was positively associated with the provisioning rate and the amount of food delivered to the nestlings. During the first broods, a high edge density and a high proportion of pine forests around the nests were related to a decreased production of males. The densities of spiders, the main food of the treecreeper, were 38% higher on spruce trunks than on pine trunks. This suggests that pine-dominated territories with female-biased broods may have contained less food during the first broods. The observation was further supported by the fact that the feeding frequencies were lower in territories with high proportions of pines. In the second broods, territories with a high forest patch density produced female-biased broods, whereas high-quality territories with a large amount of deciduous trees and mixed forests produced male-biased broods. Our results suggest that habitat quality as measured by habitat characteristics is associated with sex allocation in free-living birds.     

Facultative sex ratio shifts by a herbivorous insect in response to variation in host plant quality

Summary We tested predictions of sex allocation theory with a series of field experiments on sex allocation in an herbivorous, haplodiploid, sawfly, Euura lasiolepis. Our experiments demonstrated the following points. 1) Adult females allocated progeny sex in response to plant growth. 2) Population sex ratios varied in response to plant quality, being male-biased where plant growth was slow and female-biased where plant growth was rapid. 3) Family sex ratios varied in response to plant quality, being male-biased on slow-growing plants and female-biased on rapidly-growing plants. 4) Female fitness increased more rapidly as the result of developing on more rapidly-growing plants than male mass. We conclude from these results that there are unequal returns on investment in male and female progeny. This results in facultatively biased sawfly sex ratios as an adaptive response to variation in plant quality.     

Intersexual competition as an explanation for sex-ratio and dispersal biases in polygynous species

In polygynous mammals, it is commonly observed that both sex ratios at birth and dispersal are male biased. This has been interpreted as resulting from low female dispersal causing high female local resource competition, which would select for male-biased sex ratios. However, a female-biased sex ratio can be selected despite lower female than male-biased dispersal. This will occur if the low female dispersal is close to the optimal dispersal rate, while the male dispersal is not close to the optimal dispersal rate. The actual outcome depends on the joint evolution of sex-biased dispersal and sex ratio. Earlier analyses of joint evolution imply that there will be no sex-ratio nor dispersal biases at the joint evolutionarily stable strategy, thus they do not explain the data. However, these earlier analyses assume no intersexual competition for resources. Here, we show that when males and females compete with each other for access to resources, male-biased dispersal will be associated with male-biased birth sex ratio, as is commonly observed. A trend toward male-biased birth sex ratios is also expected if there is intersexual local resource competition and if birth sex ratio is constrained so that it cannot depart from balanced sex ratio.     

An excess of males: skewed sex ratios in bat flies (Diptera: Streblidae)

Ectoparasitic insects often exhibit female-biased sex ratios, a pattern usually explained by greater female longevity and the likelihood that smaller, more active males will disperse or be groomed off the host. Theory predicts that unbalanced sex ratios should favor males when resources are abundant and predictable, and when males are the dispersing sex. Sex ratios of streblid bat flies were evaluated based on a large biodiversity survey in Venezuela–more than 25,000 bats representing 130 species were searched for flies, yielding more than 36,500 bat flies of 116 species. These samples allowed us to analyze sex ratios in 112 bat fly metapopulations. Our results indicate that fully one-third of these metapopulations were significantly male-biased. Traditional explanations for sex-ratio bias, such as sampling effects, unequal longevity between the sexes, and differential dispersal capability are refuted for bat flies in favor of an alternative hypothesis—selective host grooming. Because host grooming is the principal cause of mortality for these slow-reproducing parasites, and because females are larger than males and gravid for a significant portion of their adult life, host grooming activity is more likely to kill or remove females than males. Incomplete understanding of population dynamics, such as mating behavior, dispersal, and reproductive success, cloud applications of male-biased sex ratios in bat flies to support or refute theoretical predictions. Population studies of mating competition and sex-related dispersal dynamics of this male-dominated group should yield important insights into sex ratio theory.     

Burrow ornamentation in the fiddler crab (Uca leptodactyla): female mate choice and male–male competition

Non-biological ornamentation is found in the nests and burrows of different kinds of animals. We evaluated here whether sand hoods constructed by male fiddler crabs (Uca leptodactyla) are one of the signals used by males to attract females during courtship. We observed females when they were walking among the males, and we quantified the proportion of females that visited male burrows with and without ornamentation and the choice to stay in a male’s burrow. Females visited more burrows with hoods than burrows without hoods, and they chose significantly more builder males. Male investment in ornamentation nevertheless decreased when the proportion of females increased in the area. Male investment was not correlated with the proportion of non-builder males nearby, but was positively correlated with overall density. The density sex ratio, however, was more male-biased in high-density than in low-density areas suggesting that even if building attracts females, the function could be related to male competition for mates.     

Operational sex ratio affects nest tending and aggression in male flagfish Jordanella floridae Goode & Bean

Operational sex ratio (OSR, the ratio of sexually active males to fertilizable females at a given time and location) affected male behaviour in the flagfish Jordanella floridae . When OSR was male biased, males spent (1) more time at their nests and (2) more time fanning prior to receiving eggs. Pre-mating fanning has previously been correlated with male mating success and is hypothesized to be used in female choice in this population. Thus, these results suggest that on average, male flagfish invest more time in behaviour associated with female choice when there are relatively more male competitors. The OSR also affected the frequency of male aggression, and specifically male aggression towards females was more frequent at female-biased OSR treatments. The observed patterns were dependent upon the direction of OSR bias ( i.e. unbiased, male biased and female biased), and in some cases the intensity of the OSR bias affected the patterns of behaviour. These findings suggest that experimentally detecting effects of OSR is sensitive to the specific OSR values considered, and highlight the importance of considering a range of OSR values in future studies.     

Conflict over multiple-partner mating between males and females of the polygynandrous common lizards

The optimal number of mate partners for females rarely coincides with that for males, leading to a potential sexual conflict over multiple-partner mating. This suggests that the population sex ratio may affect multiple-partner mating and thus multiple paternity. We investigate the relationship between multiple paternity and the population sex ratio in the polygynandrous common lizard (Lacerta vivipara). In six populations the adult sex ratio was biased toward males, and in another six populations the adult sex ratio was biased toward females, the latter corresponding to the average adult sex ratio encountered in natural populations. In males the frequency and the degree of polygyny were lower in male-biased populations, as expected if competition among males determines polygyny. In females the frequency of polyandry was not different between treatments, and polyandrous females produced larger clutches, suggesting that polyandry might be adaptive. However, in male-biased populations females suffered from reduced reproductive success compared to female-biased populations, and the number of mate partners increased with female body size in polyandrous females. Polyandrous females of male-biased populations showed disproportionately more mating scars, indicating that polyandrous females of male-biased populations had more interactions with males and suggesting that the degree of multiple paternity is controlled by male sexual harassment. Our results thus imply that polyandry may be hierarchically controlled, with females controlling when to mate with multiple partners and male sexual harassment being a proximate determinant of the degree of multiple paternity. The results are also consistent with a sexual conflict in which male behaviors are harmful to females.