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Author |
Cameron, E.Z. |
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Title  |
Facultative adjustment of mammalian sex ratios in support of the Trivers-Willard hypothesis: evidence for a mechanism |
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Year |
2004 |
Publication |
Proceedings. Biological sciences / The Royal Society |
Abbreviated Journal |
Proc Biol Sci |
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Volume |
271 |
Issue |
1549 |
Pages |
1723-1728 |
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Keywords |
Age Factors; Animals; Body Constitution; *Evolution; Female; Glucose/metabolism/physiology; Litter Size; Male; Mammals/*physiology; *Models, Biological; Reproduction/physiology; Seasons; Sex Factors; *Sex Ratio; Time Factors |
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Abstract |
Evolutionary theory predicts that mothers of different condition should adjust the birth sex ratio of their offspring in relation to future reproductive benefits. Published studies addressing variation in mammalian sex ratios have produced surprisingly contradictory results. Explaining the source of such variation has been a challenge for sex-ratio theory, not least because no mechanism for sex-ratio adjustment is known. I conducted a meta-analysis of previous mammalian sex-ratio studies to determine if there are any overall patterns in sex-ratio variation. The contradictory nature of previous results was confirmed. However, studies that investigated indices of condition around conception show almost unanimous support for the prediction that mothers in good condition bias their litters towards sons. Recent research on the role of glucose in reproductive functioning have shown that excess glucose favours the development of male blastocysts, providing a potential mechanism for sex-ratio variation in relation to maternal condition around conception. Furthermore, many of the conflicting results from studies on sex-ratio adjustment would be explained if glucose levels in utero during early cell division contributed to the determination of offspring sex ratios. |
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Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa. ezcameron@zoology.up.ac.za |
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0962-8452 |
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PMID:15306293 |
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413 |
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Clutton-Brock, T.H.; Russell, A.F.; Sharpe, L.L.; Brotherton, P.N.; McIlrath, G.M.; White, S.; Cameron, E.Z. |
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Title |
Effects of helpers on juvenile development and survival in meerkats |
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Journal Article |
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Year |
2001 |
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Science (New York, N.Y.) |
Abbreviated Journal |
Science |
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Volume |
293 |
Issue |
5539 |
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2446-2449 |
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Keywords |
Animals; Breeding; Carnivora/growth & development/*physiology; *Cooperative Behavior; Feeding Behavior; Female; Male; Survival Rate; *Weight Gain |
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Abstract |
Although breeding success is known to increase with group size in several cooperative mammals, the mechanisms underlying these relationships are uncertain. We show that in wild groups of cooperative meerkats, Suricata suricatta, reductions in the ratio of helpers to pups depress the daily weight gain and growth of pups and the daily weight gain of helpers. Increases in the daily weight gain of pups are associated with heavier weights at independence and at 1 year of age, as well as with improved foraging success as juveniles and higher survival rates through the first year of life. These results suggest that the effects of helpers on the fitness of pups extend beyond weaning and that helpers may gain direct as well as indirect benefits by feeding pups. |
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Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK. thcb@hermes.cam.ac.uk |
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0036-8075 |
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PMID:11577235 |
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414 |
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Linklater, W.L.; Cameron, E.Z.; Stafford, K.J.; Austin, T. |
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Title |
Chemical immobilisation and temporary confinement of two Kaimanawa feral stallions |
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Year |
1998 |
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New Zealand veterinary journal |
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N Z Vet J |
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46 |
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3 |
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117-118 |
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0048-0169 |
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PMID:16032032 |
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412 |
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Cameron, E.Z.; Linklater, W.L.; Stafford, K.J.; Minot, E.O. |
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Title |
Aging and improving reproductive success in horses: declining residual reproductive value or just older and wiser? |
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Journal Article |
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Year |
2000 |
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Behavioral Ecology and Sociobiology |
Abbreviated Journal |
Behav. Ecol. Sociobiol. |
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47 |
Issue |
4 |
Pages |
243-249 |
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Keywords |
Maternal investment – Equidae – Equus caballus |
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In many mammalian species, female success in raising offspring improves as they age. The residual reproductive value hypothesis predicts that each individual offspring will be more valuable to the mother as she ages because there is less conflict between the current and potential future offspring. Therefore, as mothers age, their investment into individual offspring should increase. Empirical evidence for an influence of declining residual reproductive value on maternal investment is unconvincing. Older mothers may not invest more, but may be more successful due to greater experience, allowing them to target their investment more appropriately (targeted reproductive effort hypothesis). Most studies do not preclude either hypothesis. Mare age significantly influenced maternal investment in feral horses living on the North Island of New Zealand. Older mares, that were more successful at raising foals, were more protective for the first 20 days of life, but less diligent thereafter. Total maternal input by older mothers did not seem to be any greater, but was better targeted at the most critical period for foal survival and a similar pattern was observed in mares that had lost a foal in the previous year. In addition, older mothers were more likely to foal in consecutive years, supporting the hypothesis that they are investing less than younger mares in individual offspring. Therefore, older mothers seem to become more successful by targeting their investment better due to experience, not by investing more in their offspring. |
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