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Van de Walle J, Larue B, Pigeon G, Pelletier F. Different proxies, different stories? Imperfect correlations and different determinants of fitness in bighorn sheep. Ecol Evol 2022; 12:e9582. [PMID: 36514553 PMCID: PMC9731912 DOI: 10.1002/ece3.9582] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2022] [Accepted: 11/16/2022] [Indexed: 12/13/2022] Open
Abstract
Measuring individual fitness empirically is required to assess selective pressures and predicts evolutionary changes in nature. There is, however, little consensus on how fitness should be empirically estimated. As fitness proxies vary in their underlying assumptions, their relative sensitivity to individual, environmental, and demographic factors may also vary. Here, using a long-term study, we aimed at identifying the determinants of individual fitness in bighorn sheep (Ovis canadensis) using seven fitness proxies. Specifically, we compared four-lifetime fitness proxies: lifetime breeding success, lifetime reproductive success, individual growth rate, individual contribution to population growth, and three multi-generational proxies: number of granddaughters, individual descendance in the next generation, and relative genetic contribution to the next generation. We found that all proxies were positively correlated, but the magnitude of the correlations varied substantially. Longevity was the main determinant of most fitness proxies. Individual fitness calculated over more than one generation was also affected by population density and growth rate. Because they are affected by contrasting factors, our study suggests that different fitness proxies should not be used interchangeably as they may convey different information about selective pressures and lead to divergent evolutionary predictions. Uncovering the mechanisms underlying variation in individual fitness and improving our ability to predict evolutionary change might require the use of several, rather than one, the proxy of individual fitness.
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Affiliation(s)
- Joanie Van de Walle
- Biology DepartmentWoods Hole Oceanographic InstitutionWoods HoleMassachusettsUSA
| | - Benjamin Larue
- Département de BiologieUniversité de SherbrookeSherbrookeQuébecCanada
| | - Gabriel Pigeon
- Institut de recherche sur les forêtsUniversité du Québec en Abitibi‐TémiscamingueRouyn‐NorandaQuébecCanada
| | - Fanie Pelletier
- Département de BiologieUniversité de SherbrookeSherbrookeQuébecCanada
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2
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Moreno E, Jebali A, Espeso G, Benzal J. Reintroducing Cuvier’s gazelle. Better than expected from captive-bred founders. Glob Ecol Conserv 2020. [DOI: 10.1016/j.gecco.2020.e01094] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
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3
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Festa‐Bianchet M, Côté SD, Hamel S, Pelletier F. Long‐term studies of bighorn sheep and mountain goats reveal fitness costs of reproduction. J Anim Ecol 2019; 88:1118-1133. [DOI: 10.1111/1365-2656.13002] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2018] [Accepted: 03/20/2019] [Indexed: 11/30/2022]
Affiliation(s)
- Marco Festa‐Bianchet
- Département de biologie Université de Sherbrooke Sherbrooke Québec Canada
- Centre d'études nordiques Québec City Québec Canada
| | - Steeve D. Côté
- Centre d'études nordiques Québec City Québec Canada
- Département de biologie Université Laval Sainte‐Foy Québec Canada
| | - Sandra Hamel
- Department of Arctic and Marine Biology, Faculty of Biosciences, Fisheries, and Economics UiT The Arctic University of Norway Tromsø Norway
| | - Fanie Pelletier
- Département de biologie Université de Sherbrooke Sherbrooke Québec Canada
- Centre d'études nordiques Québec City Québec Canada
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4
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Merkling T, Hatch SA, Leclaire S, Danchin E, Blanchard P. Offspring sex-ratio and environmental conditions in a seabird with sex-specific rearing costs: a long-term experimental approach. Evol Ecol 2019. [DOI: 10.1007/s10682-019-09983-2] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
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5
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Douhard M, Festa-Bianchet M, Pelletier F. Maternal condition and previous reproduction interact to affect offspring sex in a wild mammal. Biol Lett 2017; 12:rsbl.2016.0510. [PMID: 27512136 DOI: 10.1098/rsbl.2016.0510] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2016] [Accepted: 07/18/2016] [Indexed: 11/12/2022] Open
Abstract
Trivers and Willard proposed that offspring sex ratio should vary with maternal condition when condition, meant as maternal capacity to care, has different fitness consequences for sons and daughters. In polygynous and dimorphic species, mothers in good condition should preferentially produce sons, whereas mothers in poor condition should produce more daughters. Despite its logical appeal, support for this hypothesis has been inconsistent. Sex-ratio variation may be influenced by additional factors, such as environmental conditions and previous reproduction, which are often ignored in empirical studies. We analysed 39 years of data on bighorn sheep (Ovis canadensis) that fit all the assumptions of the Trivers-Willard hypothesis. Production of sons increased with maternal condition only for mothers that weaned a son the previous year. This relationship likely reflects a mother's ability to bear the higher reproductive costs of sons. The interaction between maternal condition and previous weaning success on the probability of producing a son was independent of the positive effect of paternal reproductive success. Maternal and paternal effects accounted for similar proportions of the variance in offspring sex. Maternal reproductive history should be considered in addition to current condition in studies of sex allocation.
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Affiliation(s)
- Mathieu Douhard
- Département de Biologie and Centre d'Études Nordiques, Université de Sherbrooke, 2500 boulevard de l'Université, Sherbrooke, Québec, Canada J1 K 2R1
| | - Marco Festa-Bianchet
- Département de Biologie and Centre d'Études Nordiques, Université de Sherbrooke, 2500 boulevard de l'Université, Sherbrooke, Québec, Canada J1 K 2R1
| | - Fanie Pelletier
- Département de Biologie and Centre d'Études Nordiques, Université de Sherbrooke, 2500 boulevard de l'Université, Sherbrooke, Québec, Canada J1 K 2R1
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6
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Douhard M. Offspring sex ratio in mammals and the Trivers-Willard hypothesis: In pursuit of unambiguous evidence. Bioessays 2017; 39. [DOI: 10.1002/bies.201700043] [Citation(s) in RCA: 24] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Affiliation(s)
- Mathieu Douhard
- Département de Biologie; Université de Sherbrooke; Sherbrooke Québec Canada
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7
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Kornél Á, Lanszki J. Pre-, postnatal growth and maternal condition in a free ranging fallow deer population. FOLIA ZOOLOGICA 2017. [DOI: 10.25225/fozo.v66.i1.a1.2017] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/03/2022]
Affiliation(s)
- Ács Kornél
- SEFAG, Forest Management and Wood Industry Share Co., Bajcsy-Zsilinszky Str. 21, 7400 Kaposvár, Hungary
- Kaposvár University, Department of Wildlife Biology and Ethology, P.O. Box 16, 7401 Kaposvár, Hungary
- The study is dedicated to the memory of Kornél Ács (1978–2016)
| | - József Lanszki
- Kaposvár University, Department of Wildlife Biology and Ethology, P.O. Box 16, 7401 Kaposvár, Hungary
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Sex allocation and secondary sex ratio in Cuban boa (Chilabothrus angulifer): mother's body size affects the ratio between sons and daughters. Naturwissenschaften 2016; 103:48. [PMID: 27216175 DOI: 10.1007/s00114-016-1369-9] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2016] [Revised: 04/23/2016] [Accepted: 04/25/2016] [Indexed: 10/21/2022]
Abstract
Secondary sex ratios of animals with genetically determined sex may considerably deviate from equality. These deviations may be attributed to several proximate and ultimate factors. Sex ratio theory explains some of them as strategic decisions of mothers improving their fitness by selective investment in sons or daughters, e.g. local resource competition hypothesis (LRC) suggests that philopatric females tend to produce litters with male-biased sex ratios to avoid future competition with their daughters. Until now, only little attention has been paid to examine predictions of sex ratio theory in snakes possessing genetic sex determination and exhibiting large variance in allocation of maternal investment. Cuban boa is an endemic viviparous snake producing large-bodied newborns (∼200 g). Extremely high maternal investment in each offspring increases importance of sex allocation. In a captive colony, we collected breeding records of 42 mothers, 62 litters and 306 newborns and examined secondary sex ratios (SR) and sexual size dimorphism (SSD) of newborns. None of the examined morphometric traits of neonates appeared sexually dimorphic. The sex ratio was slightly male biased (174 males versus 132 females) and litter sex ratio significantly decreased with female snout-vent length. We interpret this relationship as an additional support for LRC as competition between mothers and daughters increases with similarity of body sizes between competing snakes.
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Douhard M, Festa-Bianchet M, Coltman DW, Pelletier F. Paternal reproductive success drives sex allocation in a wild mammal. Evolution 2016; 70:358-68. [DOI: 10.1111/evo.12860] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2015] [Accepted: 01/10/2016] [Indexed: 01/08/2023]
Affiliation(s)
- Mathieu Douhard
- Département de biologie and Centre d’Études Nordiques; Université de Sherbrooke; 2500 boulevard de l'Université Sherbrooke Québec J1K 2R1 Canada
| | - Marco Festa-Bianchet
- Département de biologie and Centre d’Études Nordiques; Université de Sherbrooke; 2500 boulevard de l'Université Sherbrooke Québec J1K 2R1 Canada
| | - David W. Coltman
- Department of Biological Sciences; University of Alberta; Edmonton Alberta T6G 2E9 Canada
| | - Fanie Pelletier
- Département de biologie and Centre d’Études Nordiques; Université de Sherbrooke; 2500 boulevard de l'Université Sherbrooke Québec J1K 2R1 Canada
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10
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Sex-specific demography and generalization of the Trivers-Willard theory. Nature 2015; 526:249-52. [PMID: 26390152 DOI: 10.1038/nature14968] [Citation(s) in RCA: 63] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2014] [Accepted: 07/22/2015] [Indexed: 11/09/2022]
Abstract
The Trivers-Willard theory proposes that the sex ratio of offspring should vary with maternal condition when it has sex-specific influences on offspring fitness. In particular, mothers in good condition in polygynous and dimorphic species are predicted to produce an excess of sons, whereas mothers in poor condition should do the opposite. Despite the elegance of the theory, support for it has been limited. Here we extend and generalize the Trivers-Willard theory to explain the disparity between predictions and observations of offspring sex ratio. In polygynous species, males typically have higher mortality rates, different age-specific reproductive schedules and more risk-prone life history tactics than females; however, these differences are not currently incorporated into the Trivers-Willard theory. Using two-sex models parameterized with data from free-living mammal populations with contrasting levels of sex differences in demography, we demonstrate how sex differences in life history traits over the entire lifespan can lead to a wide range of sex allocation tactics, and show that correlations between maternal condition and offspring sex ratio alone are insufficient to conclude that mothers adaptively adjust offspring sex ratio.
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11
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Merkling T, Welcker J, Hewison AM, Hatch SA, Kitaysky AS, Speakman JR, Danchin E, Blanchard P. Identifying the selective pressures underlying offspring sex-ratio adjustments: a case study in a wild seabird. Behav Ecol 2015. [DOI: 10.1093/beheco/arv032] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
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12
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13
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Blank D, Yang W. Sex ratio in goitered gazelles (Gazella subgutturosa Guldenstaedt, 1780). ACTA ACUST UNITED AC 2012. [DOI: 10.1007/s13364-012-0097-6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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14
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Merkling T, Leclaire S, Danchin E, Lhuillier E, Wagner RH, White J, Hatch SA, Blanchard P. Food availability and offspring sex in a monogamous seabird: insights from an experimental approach. Behav Ecol 2012. [DOI: 10.1093/beheco/ars023] [Citation(s) in RCA: 37] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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15
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Abstract
Several theories predict the evolution of bias in progeny sex ratio based on variations in maternal or offspring reproductive value. For mammals, however, tests of sex-bias theories have produced inconsistent results, and no clear patterns have emerged. Each theory is based on assumptions that are difficult to satisfy, and empirical tests require large data sets. Using a long-term study on bighorn sheep (Ovis canadensis), we identified several parameters that influence progeny sex ratio according to maternal state. For older females, progeny sex ratio was affected by an interaction between reproductive strategy and environmental conditions. When conditions were good, old females reproduced every year but minimized fitness costs by producing daughters. When conditions were poor, old females produced more sons but did not reproduce every year. Sons of older females were of similar mass to those born to younger females under poor conditions but were smaller and likely disadvantaged under good environmental conditions. For young and prime-aged females, progeny sex ratio was independent of environmental conditions. Environmental conditions and age should be considered when studying sex ratio bias, which appears to be a function of maternal state rather than of maternal condition. We suggest that a conservative reproductive strategy drives progeny sex ratio in older females according to the "cost of reproduction hypothesis." By manipulating offspring sex ratio, older females reduced the cost of reproduction and increased their expected fitness returns.
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Affiliation(s)
- Julien G A Martin
- Département de Biologie, Université de Sherbrooke, Sherbrooke, Québec J1K2R1, Canada.
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16
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ROBERT KA, SCHWANZ LE. Emerging sex allocation research in mammals: marsupials and the pouch advantage. Mamm Rev 2010. [DOI: 10.1111/j.1365-2907.2010.00168.x] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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17
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Schwanz LE, Janzen FJ, Proulx SR. SEX ALLOCATION BASED ON RELATIVE AND ABSOLUTE CONDITION. Evolution 2010; 64:1331-45. [DOI: 10.1111/j.1558-5646.2009.00916.x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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18
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Nováková M, Vašáková B, Kutalová H, Galeštoková K, Průšová K, Šmilauer P, Šumbera R, Frynta D. Secondary sex ratios do not support maternal manipulation: extensive data from laboratory colonies of spiny mice (Muridae: Acomys). Behav Ecol Sociobiol 2009. [DOI: 10.1007/s00265-009-0853-z] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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19
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20
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Sex ratio variation and mixed pairs in roe deer: evidence for control of sex allocation? Oecologia 2008; 158:361-70. [PMID: 18813958 DOI: 10.1007/s00442-008-1142-7] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2007] [Accepted: 08/22/2008] [Indexed: 10/21/2022]
Abstract
Sex allocation has provided rich ground for the development of evolutionary theory. The dominant models in vertebrates have provided predictions of sex ratio based on asymmetry in variance in breeding success between sexes in the breeding system, and the relative effect of local competition. In ungulates, empirical work has provided some support for these models, but has also generated apparently contradictory observations. Recent models have provided some predictions for both individual and population sex ratio, showing that the availability of high-quality habitat patches and dispersal rates can critically affect both population- and individual-level expectations. We explore patterns in offspring sex ratio among a large sample of roe deer, a species with some interesting aspects of the breeding system. We found that in singleton pregnancies (the minority) there was an excess of male embryos, which was more marked in does in good condition at most sites. While the sex ratio among twins was close to parity, and unaffected either by average condition among populations or by individual condition within populations, we observed a clear excess of mixed pregnancies (sub-binomial variance, SBV). The excess was greater where the average condition was high. Within sites, population changes in average condition were associated with changes in SBV: in populations declining in condition, SBV also declined. There was no tendency for mixed twins to be more likely in individuals in better condition within a population. We conclude that condition-dependent allocation of sex does occur in roe deer. We suggest that in these "income breeders", mixed pairs might offer the greatest opportunity to optimize maternal investment during lactation, when conditions will be unpredictable, and that this is increasingly the case when average condition is poor.
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21
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Variation in offspring sex ratio among individual Weddell seal (Leptonychotes weddellii) females of different quality. Behav Ecol Sociobiol 2008. [DOI: 10.1007/s00265-008-0596-2] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
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22
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Grant VJ. Could maternal testosterone levels govern mammalian sex ratio deviations? J Theor Biol 2007; 246:708-19. [PMID: 17379251 DOI: 10.1016/j.jtbi.2007.02.005] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/16/2006] [Revised: 02/06/2007] [Accepted: 02/08/2007] [Indexed: 10/23/2022]
Abstract
Although maternal dominance and good condition are frequently associated with raised offspring sex ratios in mammals, the key factor may be female testosterone, which not only underpins the behavioural indicators but could also provide a pathway to a possible proximate mechanism for sex determination. By taking into account the fact that female testosterone levels rise in response to environmental stressors, it is possible to re-interpret the findings of atypical sex ratios in mammals in a way that reconciles seemingly conflicting results and reveals instead what could be a coherent, adaptive system of sex allocation in mammals.
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Affiliation(s)
- Valerie J Grant
- Psychological Medicine, Faculty of Medical and Health Sciences, University of Auckland, Private Bag 92019, Auckland 1, New Zealand.
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23
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Røed KH, Holand Ø, Mysterud A, Tverdal A, Kumpula J, Nieminen M. Male phenotypic quality influences offspring sex ratio in a polygynous ungulate. Proc Biol Sci 2007; 274:727-33. [PMID: 17254998 PMCID: PMC2197213 DOI: 10.1098/rspb.2006.0214] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Evolutionary models of sex ratio adjustment applied to mammals have ignored that females may gain indirect genetic benefits from their mates. The differential allocation hypothesis (DAH) predicts that females bias the sex ratio of their offspring towards (more costly) males when breeding with an attractive male. We manipulated the number of available males during rut in a polygynous ungulate species, the reindeer (Rangifer tarandus), and found that a doubling of average male mass (and thus male attractiveness) in the breeding herd increased the proportion of male offspring from approximately 40 to 60%. Paternity analysis revealed indeed that males of high phenotypic quality sired more males, consistent with the DAH. This insight has consequences for proper management of large mammal populations. Our study suggests that harvesting, by generating a high proportion of young, small and unattractive mates, affects the secondary sex ratio due to differential allocation effects in females. Sustainable management needs to consider not only the direct demographic changes due to harvest mortality and selection, but also the components related to behavioural ecology and opportunities for female choice.
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Affiliation(s)
- Knut H Røed
- Department of Basic Sciences and Aquatic Medicine, Norwegian School of Veterinary SciencePO Box 8146, Departmental Division, 0033 Oslo, Norway
| | - Øystein Holand
- Department of Animal and Aquacultural Sciences, Norwegian University of Life SciencesPO Box 5025, 1430 Ås, Norway
| | - Atle Mysterud
- Centre for Ecological and Evolutionary Synthesis (CEES), Department of Biology, University of OsloPO Box 1066, Blindern, 0316 Oslo, Norway
- Author for correspondence ()
| | - Aage Tverdal
- Department of Basic Sciences and Aquatic Medicine, Norwegian School of Veterinary SciencePO Box 8146, Departmental Division, 0033 Oslo, Norway
| | - Jouko Kumpula
- Finnish Game and Fisheries Research Institute, Reindeer Research Station99910 Kaamanen, Finland
| | - Mauri Nieminen
- Finnish Game and Fisheries Research Institute, Reindeer Research Station99910 Kaamanen, Finland
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Servanty S, Gaillard JM, Allainé D, Brandt S, Baubet E. Litter size and fetal sex ratio adjustment in a highly polytocous species: the wild boar. Behav Ecol 2007. [DOI: 10.1093/beheco/arl099] [Citation(s) in RCA: 48] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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25
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Blanchard P, Hanuise N, Dano S, Weimerskirch H. Offspring sex ratio in relation to parental structural size and body condition in the long-lived wandering albatross (Diomedea exulans). Behav Ecol Sociobiol 2006. [DOI: 10.1007/s00265-006-0307-9] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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26
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Schwanz LE, Bragg JG, Charnov EL. Maternal condition and facultative sex ratios in populations with overlapping generations. Am Nat 2006; 168:521-30. [PMID: 17004223 DOI: 10.1086/507993] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2005] [Accepted: 06/21/2006] [Indexed: 11/04/2022]
Abstract
Facultative investment in offspring sex is related to maternal condition in many organisms. In mammals, empirical support for condition-dependent sex allocation is equivocal, and there is some doubt as to theoretical expectations. Much theory has been developed to make predictions for condition-dependent sex ratios in populations with discrete generations. However, the extension of these predictions to populations with overlapping generations (OLGs; e.g., mammals) has been limited, leaving doubt as to the specific prediction for maternal-condition-dependent sex ratios in mammals. We develop a population genetics model that incorporates maternal effects on multiple offspring fitness components in a population with OLGs. Using a rare-gene and evolutionarily stable strategy approach, we demonstrate that sex ratio predictions of this model are identical to those for equivalent discrete generations models. We show that the predicted sex ratios depend on the sex-specific ratio of R(o) (offspring lifetime fitness) for offspring of good and poor mothers. This offspring lifetime fitness rule indicates that empirical research on conditional sex ratios should consider all three components of offspring R(o) (juvenile survival, adult life span, and fertility).
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Affiliation(s)
- Lisa E Schwanz
- Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA.
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27
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James WH. Possible constraints on adaptive variation in sex ratio at birth in humans and other primates. J Theor Biol 2005; 238:383-94. [PMID: 16045937 DOI: 10.1016/j.jtbi.2005.05.022] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2005] [Revised: 04/26/2005] [Accepted: 05/24/2005] [Indexed: 10/25/2022]
Abstract
There is general agreement that adaptive variation of sex ratio at birth has not been decisively demonstrated in primates (including human beings). So some workers have questioned whether it actually exists. Others have conjectured that it exists but is subject to as yet unidentified 'constraints' (factors opposing the modifying influences of selection in the phenotype). Meanwhile though most workers have called for research to reveal the proximate causes of sex ratio variation, few (if any) have directed studies toward that end. Here it is argued that hormonal action is responsible both for much adaptive and non-adaptive sex ratio variation, and for constraints on the adaptive variation. My hypothesis proposes that levels of steroid hormones (testosterone and oestrogen) of both parents around the time of conception are positively associated with offspring sex ratio (proportion male at birth) of mammals including man. Testosterone in men and oestrogen in women are also known to be positively associated with the health, attractiveness and fertility of individual human beings. However, high levels of testosterone in women are frequently associated with adverse medical conditions. It is suggested that for these reasons (and contrary to some adaptive theory) some classes of people (particularly women) in suboptimal health ("condition") produce excesses of sons. It seems that gonadal hormones are responsible for adaptive variation; and that maternal adrenal hormones are responsible for maladaptive variation. In evolutionary terms, gonadal hormones precede adrenal hormones.
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Affiliation(s)
- William H James
- The Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK.
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