Too Many Men, Too-Short Lives: The Effect of the Male-Biased Sex Ratio on Mortality

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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Female biased sex-ratio in Schistosoma mansoni after exposure to an allopatric intermediate host strain of Biomphalaria glabrata

Experimental Parasitology ◽

10.1016/j.exppara.2013.07.023 ◽

2013 ◽

Vol 135 (2) ◽

pp. 350-356 ◽

Cited By ~ 5

Author(s):

Julie M.J. Lepesant ◽

Jérôme Boissier ◽

Déborah Climent ◽

Céline Cosseau ◽

Christoph Grunau

Keyword(s):

Sex Ratio ◽

Schistosoma Mansoni ◽

Intermediate Host ◽

Biomphalaria Glabrata ◽

Host Strain ◽

Biased Sex Ratio

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Infection by a vertically-transmitted microsporidian parasite is associated with a female-biased sex ratio and survival advantage in the amphipod Gammarus roeseli

Parasitology ◽

10.1017/s0031182007002715 ◽

2007 ◽

Vol 134 (10) ◽

pp. 1363-1367 ◽

Cited By ~ 26

Author(s):

E. R. HAINE ◽

S. MOTREUIL ◽

T. RIGAUD

Keyword(s):

Sex Ratio ◽

Species Interactions ◽

Negative Effects ◽

Microsporidian Parasite ◽

Host Fitness ◽

Gammarus Roeseli ◽

Ratio Distortion ◽

Biased Sex Ratio ◽

Host Parasite ◽

Specific Virulence

SUMMARYVertically transmitted parasites may have positive, neutral or negative effects on host fitness, and are also predicted to exhibit sex-specific virulence to increase the proportion or fitness of the transmitting sex. We investigated these predictions in a study on the survival and sex ratio of offspring of the amphipod Gammarus roeseli from females infected by the vertically transmitted microsporidia Nosema granulosis. We found, to our knowledge, the first evidence for a positive relationship between N. granulosis infection and host survival. Infection was associated with sex ratio distortion, not by male-killing, but probably by parasite-induced feminization of putative G. roeseli males. This microsporidia also feminizes another amphipod host, Gammarus duebeni, which is phylogenetically and biogeographically distant from G. roeseli. Our study suggests that the reproductive system of gammarids is easily exploited by these vertically-transmitted parasites, although the effects of infections on host fitness may depend on specific host-parasite species interactions.

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The spatial pattern of a Juniperus communis var. depressa population on a continental dune in subarctic Québec, Canada

Canadian Journal of Forest Research ◽

10.1139/x99-018 ◽

1999 ◽

Vol 29 (4) ◽

pp. 446-450 ◽

Cited By ~ 7

Author(s):

Gilles Houle ◽

Mario Duchesne

Keyword(s):

Sex Ratio ◽

Nearest Neighbor ◽

Dispersion Pattern ◽

Neighbor Distance ◽

Juniperus Communis ◽

Biased Sex Ratio ◽

Males And Females ◽

Nearest Neighbor Distance ◽

Population Dispersion ◽

Crown Size

We performed a nearest-neighbor analysis to determine the population dispersion pattern and the association between males and females in a Juniperus communis L. var. depressa Pursh population occupying a continental dune in subarctic Quebec, Canada. The overall dispersion pattern was contagious, and males (or females) were proportionately as likely to have a male as they were to have a female nearest neighbor. Crown size was positively related to nearest-neighbor distance for the male-male comparison only, suggesting a somewhat stronger intrasex competition between males. Nearest-neighbor distance increased with crown size (significantly related to age) suggesting a change in the intensity of aggregation with age possibly related to self-thinning. Higher mortality as a result of stronger male-male competition could explain the female-biased sex ratio and the absence of spatial segregation between sexes. The overall contagious dispersion pattern in the population may be related to the fact that most seed cones fall directly underneath the mother plant. Birds can eat the cones of J. communis and thus disperse seeds. However, these seeds are deposited in clumps, a process that may also explain contagion within the population.

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Determination of the Female-Biased Sex Ratio in Some Young-of-the-Year and Spawner Samples of the Threespine Stickleback Gasterosteus aculeatus by Environmental, Not Genetic, Factors

Biology Bulletin ◽

10.1134/s1062359021050034 ◽

2021 ◽

Vol 48 (5) ◽

pp. 577-587

Author(s):

V. S. Artamonova ◽

N. V. Bardukov ◽

P. V. Golovin ◽

T. S. Ivanova ◽

M. V. Ivanov ◽

...

Keyword(s):

Sex Ratio ◽

Gasterosteus Aculeatus ◽

Genetic Factors ◽

Threespine Stickleback ◽

Biased Sex Ratio ◽

Young Of The Year

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Fledgling sex-ratio is biased towards the helping sex in a Neotropical cooperative breeder, the brown-and-yellow marshbird (Pseudoleistes virescens)

Behaviour ◽

10.1163/1568539x-bja10061 ◽

2021 ◽

pp. 1-26

Author(s):

Myriam E. Mermoz ◽

Cecilia Villarruel ◽

Alicia de la Colina ◽

Bettina Mahler

Keyword(s):

Sex Ratio ◽

Breeding Success ◽

Population Level ◽

Adaptive Strategy ◽

Individual Level ◽

Cooperatively Breeding ◽

Nestling Survival ◽

Biased Sex Ratio ◽

Communal Areas ◽

Breeding Seasons

Abstract In many cooperatively breeding species, helpers increase the breeding success of their parents. The repayment hypothesis predicts a skewed sex-ratio towards the helping sex at population level; at individual level bias would increase in broods attended by a smaller number of helpers. We studied a brown-and-yellow marshbird (Pseudoleistes virescens) population during 11 breeding seasons. We found that 90% of helpers were males and that they increased nestling survival, although this effect disappeared in presence of parasitic shiny cowbirds. Helpers sometimes helped at nests of adults other than their parents. Population sex-ratio of fledglings was highly skewed towards males (1.4:1). At individual level, male-biased sex-ratio of fledglings was more pronounced early in the season and increased with brood losses but was not affected by number of helpers. Marshbirds feed at communal areas so retaining helpers would not be costly. Therefore, a general skew towards males might be the best adaptive strategy.

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