Fetal Sex Ratios in Caribou: Maternal Age and Condition Effects

Thomas, Dixon; Barry, Samuel J.; Kiliaan, Hendrick P.

doi:10.2307/3809582

Cited by 45 publications

(27 citation statements)

References 11 publications

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“…Kruuk et al (1999) found the proportion of red deer (Cervus elaphus) calves born each year that were males declined with increasing population density and winter rainfall, both environmental variables associated with maternal nutritional stress during pregnancy. Thomas et al (1989) also found support for the TW model among caribou (Rangifer tarandus pearyi and R. t. groenlandicus) with differential maternal condition. Conversely, several other studies of ungulates found female-biased offspring sex ratios when maternal nutrition was high (Hoefs and Nowlan 1994;Pederson and Harper 1984;Sachdeva et al 1973;Verme 1969).…”

Section: Discussionmentioning

confidence: 77%

North Pacific Climate Mediates Offspring Sex Ratio in Northern Elephant Seals

Lee¹,

Sydeman²

2009

Journal of Mammalogy

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Determinants of sex ratios in animal populations have been of general interest to ecologists for decades. We tested the hypothesis that offspring sex ratio in a population of northern elephant seals (Mirounga angustirostris) was related to large-scale environmental indices reflecting ocean climate and productivity that affected the condition of reproductive females over 31 years . More males were produced during years of weaker pressure differences and warmer sea surface temperature anomalies in the northeastern Pacific, conditions that reduce or disperse prey resources for gestating females in the North Pacific Ocean. For this species, sexually distinct forage resources exist, so competition for food resources occurs predominately within sex. These results are consistent with the resource competition model for facultative adjustments of offspring sex ratio. Anthropogenic global warming is predicted to warm the North Pacific. This could result in altered basin-scale ocean productivity, increased nutritional stress, and an overabundance of males that may adversely affect this and other similarly regulated mammalian populations.Key words: climate change, elephant seal, Farallon Islands, Mirounga, Northern Oscillation Index, Pacific Decadal Oscillation, Phocidae, sex ratio Evolutionary theory predicts that parents should invest in offspring in ways that enhance their own lifetime reproductive success (Fisher 1930). Darwin (1871) thought that natural selection controls sex ratio of animals. Selection theoretically should result in an equal offspring sex ratio (probability of a son ¼ 0.5), but vertebrates often deviate from this ideal (Clutton-Brock and Iason 1986; Darwin 1871). Hypotheses concerning adaptive (or facultative) parental adjustment of offspring sex ratio in mammals focus on differences in the expected future reproductive success of the 2 sexes.The 2 major hypotheses explaining offspring sex-ratio adjustment in mammals are the Trivers-Willard (TW) model (Trivers and Willard 1973) and the resource competition (RC) model (Clark 1978;Silk 1983). Trivers and Willard (1973) postulated that when parental investment affects offspring reproductive success, mothers who can afford to invest heavily (i.e., mothers in good condition) should invest more in the sex with greater variance in reproductive success. In polygynous, sexually dimorphic species, females in good condition could produce larger male offspring that would have a disproportionately higher mating success, therefore the TW model predicts that the sex ratio of offspring should be male-biased during years of abundant food resources (Trivers and Willard 1973). Mothers in poor condition would gain more reproductively from producing daughters than sons because the survival and reproductive success of daughters is usually less affected by low levels of preweaning investment (Clutton-Brock et al. 1988). The RC model (Clark 1978), a generalization of the local mate competition model of Hamilton (1967), proposes that competition for resources between mothers a...

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Section: Discussionmentioning

confidence: 77%

North Pacific Climate Mediates Offspring Sex Ratio in Northern Elephant Seals

Lee¹,

Sydeman²

2009

Journal of Mammalogy

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“…In sheep (Ovis aries) and sea otters (Enhydra lutris), younger females produce more female offspring than older animals (Kent 1992;Bodkin et al 1993). The picture is less clear for different populations of bison (Bison bison) (Rutberg 1986;Wolff 1988;Green and Rothstein 1991), reindeer (Rangifer tarandus) (Skogland 1986;Kojola and Eloranta 1989;Thomas et al 1989) and white-tailed deer (Odocoileus virginianus) (Verme 1969(Verme , 1983Caley and Nudds 1987). In red deer (Cervus elaphus), dominance was related to an increased proportion of male offspring, and dominance and body mass were positively related , 1984.…”

Section: Discussionmentioning

confidence: 99%

Offspring sex ratio in relation to female size in southern elephant seals, Mirounga leonina

Arnbom

Fedak²,

Rothery

1994

Behav Ecol Sociobiol

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Southern elephant seals Mirounga leonina display extreme sexual dimorphism. In addition females show great variation in size and stored resources at parturition. Therefore they present an excellent opportunity for examination of responses of sex ratio to resource availability. We studied the relationships between the size of southern elephant seal females at parturition and the size and sex of their pups at South Georgia over tbur breeding seasons. We found a large individual variation in maternal post-partum mass (range 296-977 kg, n=151). Larger mothers gave birth to larger pups, irrespective of the sex of their pup. Male pups were on average 14% larger than females at birth and consequently more costly to bring to parturition. Our results suggest that female southern elephant seals must weigh more than 300 kg if they are to breed at all, and more than 380 kg if they are to give birth to a male pup. Above this threshold the proportion of males among offspring rapidly increases with maternal mass, and stabilizes at a level not significantly different from parity. These results show that smaller females of southern elephant seals vary offspring sex ratio in a way that is consistent with theories on adaptive offspring sex ratio. A smaller mother with a male foetus may benefit from terminating her pregnancy and allocating the resources she saves to her own growth. She could then give birth to and raise a larger pup in the subsequent season.Y. Arnbom (~)

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“…First, old females have lower reproductive potential and a higher mortality rate than young females (Thomas and Barry, 1990a, b). Second, from 1980 through 1987, 61% of calves produced by females one to three years old in the Beverly herd were female calves, whereas only 33% of calves produced by old females were females (Thomas et al, 1990). Those young and old age classes represented about 33.4% and 9.4% of females older than calves, and they produced 38.2% and 7.3% of sampled female calves (Thomas and Barry, 1990b).…”

Section: Discussionmentioning

confidence: 99%

Antler Mass of Barren-Ground Caribou Relative to Body Condition and Pregnancy Rate

Thomas¹,

Barry²

2010

ARCTIC

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ABSTRACT. Antlers of caribou (Rangifer tarandus) likely contribute to breeding success of males, defence of feeding sites by females, and recognition of the hierarchical status of conspecifics. However, few data are available on how antler mass is related to age, body mass, fat reserves, other physical indices, and reproduction. Mass of antlers was recorded for 958 caribou obtained in early and late winter, 1982 through 1987, from the Beverly herd of barren-ground caribou (R. t. groenlandicus) in north-central Canada. In females, antler mass increased with age to 14 years, whereas mean body mass was stable from 7 through 13 years. Female antler mass correlated with body mass, body size, fat indices, and pregnancy rate both among and within age classes. In males, antler mass correlated highly with lean body mass to age 4 years. Antler mass varied among years in concert with fatness in females and with changes in body mass in both sexes. We conclude that antler mass is an index of body mass, body size, condition, and pregnancy rate, but high variation makes it inferior to other indices.Key words: antler and body mass, Arctic, Subarctic, fat indices, pregnancy rate, Rangifer tarandus RÉSUMÉ. La ramure du caribou (Rangifer tarandus) contribue probablement au succès de la reproduction chez le mâle, à la défense des aires d'alimentation chez la femelle et à la reconnaissance du statut hiérarchique parmi les congénères. On possède toutefois peu de données sur la façon dont le poids de la ramure est relié à l'âge, à la masse corporelle, aux réserves de gras, à d'autres indices physiques et à la reproduction. On a consigné le poids des ramures pour 958 caribous prélevés au sein de la harde Beverly de caribous des toundras (R. t. groenlandicus) dans le centre-nord du Canada, au début et à la fin de l'hiver, de 1982 à 1987. Chez la femelle, le poids de la ramure augmentait avec l'âge jusqu'à 14 ans, alors que de 7 à 13 ans, la moyenne de la masse corporelle était stable. Chez la femelle, le poids de la ramure était en corrélation avec la masse corporelle, la taille du corps, les indices de gras et le taux de gestation entre les groupes d'âge et au sein de ces groupes. Chez le mâle, le poids de la ramure était fortement corrélé avec la masse corporelle maigre jusqu'à l'âge de 4 ans. Le poids de la ramure variait d'une année à une autre de concert avec l'adiposité chez la femelle, et de concert avec les changements dans la masse corporelle chez les deux sexes. On conclut que le poids de la ramure constitue un indice de la masse corporelle, de la taille du corps, de l'état corporel et du taux de gestation, mais que sa grande variation en fait un indice inférieur à d'autres.Mots clés: ramure et masse corporelle, Arctique et Subarctique, indices de gras, taux de gestation, Rangifer tarandus Traduit pour la revue Arctic par Nésida Loyer.

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Fetal Sex Ratios in Caribou: Maternal Age and Condition Effects

Cited by 45 publications

References 11 publications

North Pacific Climate Mediates Offspring Sex Ratio in Northern Elephant Seals

North Pacific Climate Mediates Offspring Sex Ratio in Northern Elephant Seals

Offspring sex ratio in relation to female size in southern elephant seals, Mirounga leonina

Antler Mass of Barren-Ground Caribou Relative to Body Condition and Pregnancy Rate

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