Mortality of Fledgling Females Causes Male Bias in the Sex Ratio of Rufous Vangas (Schetba Rufa) in Madagascar

Abstract The Rufous Vanga (Schetba rufa) is a cooperative breeder. Although young males typically remain in natal territories, yearling females do not remain, but disperse to breed for themselves. The male-biased sex ratio likely restricts independent reproduction by young males. We analyzed demographic data from a seven-year study of the Rufous Vanga in Madagascar to estimate the mortality for each sex. We also examined the sex ratio at fledging and at hatching, sexing nestlings with DNA analysis on sex chromosomes. Analyses indicated that the biased sex ratio was due to differential mortality of fledglings rather than the bias at birth. We suggest that the delayed dispersal improved survival of auxiliary males.

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Sex Ratio of Small Hive Beetles: The Role of Pupation and Adult Longevity

Insects ◽

10.3390/insects10050133 ◽

2019 ◽

Vol 10 (5) ◽

pp. 133 ◽

Cited By ~ 1

Author(s):

Anna Papach ◽

Jérémy Gonthier ◽

Geoffrey R. Williams ◽

Peter Neumann

Keyword(s):

Sex Ratio ◽

Field Studies ◽

Differential Mortality ◽

Adult Longevity ◽

Published Data ◽

Larval Feeding ◽

Intraspecific Interactions ◽

Adult Sex Ratio ◽

Biased Sex Ratio ◽

Small Hive Beetles

The sex ratio of sexually reproducing animal species tends to be 1:1, which is known as Fisher’s principle. However, differential mortality and intraspecific competition during pupation can result in a biased adult sex ratio in insects. The female-biased sex ratio of small hive beetles (SHBs) is known from both laboratory and field studies, but the underlying reasons are not well understood. Here, we used laboratory mass and individual pupation to test if differential mortality between sexes and/or intraspecific interactions can explain this sex ratio. The data show a significant female-biased adult sex ratio in both mass and individual rearing, even when assuming that all dead individuals were males. Our results therefore suggest that neither differential mortality during pupation nor intraspecific interactions are likely to explain the female-biased sex ratio of freshly emerged adult SHBs. We regard it as more likely that either competition during the larval feeding stage or genetic mechanisms are involved. In addition, we compared our data with previously published data on the sex ratio of both freshly emerged and field-collected SHBs to investigate possible gender differences in adult longevity. The data show a significantly greater female bias in the sex ratio upon emergence, compared to field-collected SHBs, suggesting that adult females have a shorter longevity.

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Demography, sex ratio, and sexual size dimorphism in a northern population of common musk turtles (Sternotherus odoratus)

Canadian Journal of Zoology ◽

10.1139/z96-105 ◽

1996 ◽

Vol 74 (5) ◽

pp. 918-925 ◽

Cited By ~ 15

Author(s):

Jonathan H. Edmonds ◽

Ronald J. Brooks

Keyword(s):

Sex Ratio ◽

Sexual Size Dimorphism ◽

Differential Mortality ◽

Adult Males ◽

Differential Growth ◽

Size Dimorphism ◽

Georgian Bay ◽

Differential Movement ◽

Sternotherus Odoratus ◽

Biased Sex Ratio

A population of common musk turtles (Sternotherus odoratus) was studied in Georgian Bay, Ontario, near the northern limit of the species' range, during the summers of 1991–1994. A total of 314 (229 adult males, 68 adult females, and 17 juveniles) turtles were captured, marked, measured, and released. Over the geographic range of S. odoratus there was a positive relationship between body size and latitude, the musk turtles of the Georgian Bay population being significantly larger than those in any of the more southerly populations. The sex ratio was significantly male-biased, and the study population was unique in having sexual size dimorphism, with significantly larger males. We have hypothesized that this dimorphism has arisen through sexual selection that is being driven by the male-biased sex ratio. Male–male competition for mates is intensified and thereby selects for larger males. Proximately, biased sex ratios and sexual size dimorphism in turtles have been attributed to differential ages and sizes at sexual maturity. However, we found no support for this hypothesis in our population. We have attributed sexual size dimorphism to either differential growth rates after maturity or differential mortality of adults. The male-biased sex ratio may be due to differential movement patterns, differential mortality of the sexes, or the effects of ambient temperature on sex determination.

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Experimental observations on the sex ratio of adult Schistosoma mansoni, with comments on the natural male bias

Parasitology ◽

10.1017/s0031182099006393 ◽

2000 ◽

Vol 121 (4) ◽

pp. 379-383 ◽

Cited By ~ 34

Author(s):

J. BOISSIER ◽

H. MONÉ

Keyword(s):

Sex Ratio ◽

Schistosoma Mansoni ◽

Sex Ratios ◽

Vertebrate Host ◽

Natural Conditions ◽

Male And Female ◽

Male Bias ◽

Biased Sex Ratio ◽

And Bisexual

The sex ratio of adult worms has been observed biased towards males in Schistosoma mansoni under natural conditions. The origin of this bias is unknown. This paper determines whether males are more infective than females under controlled experimental bisexual conditions, and hence if the sex ratio is male-biased as a consequence of this. The male and female cercarial infectivities in uni- and bisexual vertebrate host infections using a range of controlled cercarial sex ratios were studied. The results showed that, in experimental unisexual infections, male cercariae were more infective than females, and that in experimental bisexual infections, male cercarial infectivity was similar to that of female, irrespective of cercarial sex ratio. Furthermore, cumulative male and female cercarial infectivity was maximal when sex ratio was equilibrated. The unbiased sex ratios obtained in our experimental bisexual infections are discussed in terms of behavioural and/or biochemical male–female interaction. Alternative explanations of the natural biased sex ratio are proposed.

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No room for males in caves: Female‐biased sex ratio in subterranean amphipods of the genus Niphargus

Journal of Evolutionary Biology ◽

10.1111/jeb.13917 ◽

2021 ◽

Author(s):

Ester Premate ◽

Špela Borko ◽

Simona Kralj‐Fišer ◽

Michael Jennions ◽

Žiga Fišer ◽

...

Keyword(s):

Sex Ratio ◽

Biased Sex Ratio

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Fisher vs. The Worms: Extraordinary Sex Ratios in Nematodes and the Mechanisms That Produce Them

Cells ◽

10.3390/cells10071793 ◽

2021 ◽

Vol 10 (7) ◽

pp. 1793

Author(s):

Justin Van Goor ◽

Diane C. Shakes ◽

Eric S. Haag

Keyword(s):

Sex Ratio ◽

Sperm Competition ◽

Sex Chromosomes ◽

Sex Ratios ◽

Environmental Sex Determination ◽

Fitness Advantage ◽

Selection For ◽

Facultative Parthenogenesis ◽

Multicellular Organisms ◽

Inbred Populations

Parker, Baker, and Smith provided the first robust theory explaining why anisogamy evolves in parallel in multicellular organisms. Anisogamy sets the stage for the emergence of separate sexes, and for another phenomenon with which Parker is associated: sperm competition. In outcrossing taxa with separate sexes, Fisher proposed that the sex ratio will tend towards unity in large, randomly mating populations due to a fitness advantage that accrues in individuals of the rarer sex. This creates a vast excess of sperm over that required to fertilize all available eggs, and intense competition as a result. However, small, inbred populations can experience selection for skewed sex ratios. This is widely appreciated in haplodiploid organisms, in which females can control the sex ratio behaviorally. In this review, we discuss recent research in nematodes that has characterized the mechanisms underlying highly skewed sex ratios in fully diploid systems. These include self-fertile hermaphroditism and the adaptive elimination of sperm competition factors, facultative parthenogenesis, non-Mendelian meiotic oddities involving the sex chromosomes, and environmental sex determination. By connecting sex ratio evolution and sperm biology in surprising ways, these phenomena link two “seminal” contributions of G. A. Parker. 

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Male-biased sex ratio in Argulus canadensis Wilson, 1916 (Crustacea: Branchiura) ectoparasitic on sticklebacks

Canadian Journal of Zoology ◽

10.1139/z89-296 ◽

1989 ◽

Vol 67 (8) ◽

pp. 2078-2080 ◽

Cited By ~ 3

Author(s):

Robert Poulin ◽

Gerard J. FitzGerald

Keyword(s):

Salt Marsh ◽

Sex Ratio ◽

Laboratory Experiments ◽

Fish Predation ◽

Reproductive Behaviour ◽

Egg Deposition ◽

Alternative Hypotheses ◽

Biased Sex Ratio ◽

The Difference ◽

Fish Hosts

Females of the ectoparasitic crustacean Argulus canadensis must leave their fish hosts at least temporarily to deposit their eggs on the substrate. To test the hypothesis that this difference in reproductive behaviour between the two sexes could result in male-biased sex ratios on their stickleback hosts, we sampled sticklebacks in tide pools of a Quebec salt marsh from early July to early September 1986. During this period, fish harboured significantly more male than female A. canadensis. Laboratory experiments were done to test two alternative hypotheses offered to explain this biased sex ratio. The first hypothesis was that male A. canadensis were more successful than females in attacking their stickleback hosts; however, we found no differences in attack success on their hosts between the two parasite sexes. The second hypothesis was that sticklebacks ate more female than male A. canadensis. Although males were less vulnerable to fish predation than females, the difference was not significant. We conclude that sexual differences in reproductive behaviour, i.e., egg deposition behaviour of females, can account for the male-biased sex ratio of A. canadensis on sticklebacks.

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Female-Biased Sex Ratio at Hatching in the Green Woodhoopoe

The Auk ◽

10.2307/4088009 ◽

1990 ◽

Vol 107 (4) ◽

pp. 765-771 ◽

Cited By ~ 35

Author(s):

J. David Ligon ◽

Sandra H. Ligon

Keyword(s):

Sex Ratio ◽

Biased Sex Ratio

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Variably male-biased sex ratio in a marine bird with females larger than males

Oecologia ◽

10.1007/s004420050698 ◽

1999 ◽

Vol 118 (1) ◽

pp. 16-22 ◽

Cited By ~ 54

Author(s):

Roxana Torres ◽

Hugh Drummond

Keyword(s):

Sex Ratio ◽

Marine Bird ◽

Biased Sex Ratio

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Effects of phytoseiid predators on the sex ratio of the spider mite Panonychus ulmi

Canadian Journal of Zoology ◽

10.1139/z91-031 ◽

1991 ◽

Vol 69 (1) ◽

pp. 208-212 ◽

Cited By ~ 3

Author(s):

Dan L. Johnson ◽

Heather C. Proctor

Keyword(s):

Population Density ◽

Sex Ratio ◽

Spider Mite ◽

Total Population ◽

Panonychus Ulmi ◽

Encounter Rate ◽

Male Mortality ◽

Adult Sex Ratio ◽

Biased Sex Ratio ◽

Male Activity

The effect of predator presence on the adult sex ratio of a spider mite (Panonychus ulmi) was examined in a field experiment. Phytoseiid predators (chiefly Typhlodromus occidentalis) were removed from 32 trees harboring P. ulmi populations, and allowed to remain at natural levels on 32 other trees. Both total population density and proportion of males in the prey population were significantly higher in predator-free trees. Mechanisms that could explain the increase in the proportion of males are examined. The most probable is that greater male activity results in a higher encounter rate between predator and prey, and that subsequent higher male mortality when predators are present exaggerates the female-biased sex ratio. The theoretical effects of sex-biased predation on diplo-diploid and haplo-diploid organisms are discussed.

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