The length of growing season and adult sex ratio affect sexual size dimorphism in moose

Laboratoire de Biométrie et Biologie Evolutive (UMR 5558), Centre National de la Recherche Scientifique (CNRS), Université Lyon 1, 43 boulevard du 11 novembre, 69622, Villeurbanne, France.
Ecology (Impact Factor: 4.66). 04/2006; 87(3):745-58. DOI: 10.1890/05-0584
Source: PubMed

ABSTRACT While factors affecting body growth have been extensively studied, very little is known about the factors likely to affect the sexual size dimorphism (SSD) in polygynous mammals. Based on the carcass mass of 24420 male and female moose recorded in 14 Norwegian populations, we examine three hypotheses to explain geographical variation in SSD. First, SSD is expected to decrease when the relative density of animals (for a given habitat quality) increases, because resource limitation at high population densities is assumed to affect body growth of males more than females. Second, because males are selected to invest in growth more than females, environmental seasonality and related improvement of the forage quality during the short and intense growing season are expected to increase SSD. Third, by decreasing the proportion of adult males in the population, resulting in start of rutting earlier in life, hunting may decrease the SSD by increasing the reproductive cost of young males. We found that males grew faster and for a longer time of their life than did females and thus were heavier (-24%) when they reached adulthood. Sexual size dimorphism was independent of density but was higher in areas with short growing seasons. The low SSD in populations with largely adult female-biased sex ratios (males per female) shows that male body growth decreases with a decreasing proportion of adult males in the population. Our results indicate that geographical variation in moose SSD is influenced by divergent responses in the sexes to ecological factors affecting body growth.

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Available from: Ivar Herfindal, Sep 27, 2015
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    • "The degree of SSD in Norwegian moose (Alces alces) populations varied with ecological factors affecting body growth. Males are, on average, larger than females, although the degree of dimorphism was lowest in populations with adult female-biased sex ratios (Garel et al., 2006). Consistent with one of the suggestions of Owen-Smith (1993) for other ungulates , it was proposed that the decreased SSD observed was a result of a smaller proportion of adult males in those populations and its effect on mate competition during the rut. "
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    ABSTRACT: The attainment of sexual maturity has been shown to affect measures of sexual size dimorphism (SSD) and adult sex ratios in several groups of vertebrates. Using data for turtles, we tested the model that sex ratios are expected to be male-biased when females are larger than males and female-biased when males are larger than females because of the relationship of each with the attainment of maturity. Our model is based on the premise that the earlier-maturing sex remains smaller, on average throughout life, and predominates numerically unless the sexes are strongly affected by differential mortality, differential emigration, and immigration, or biased primary sex ratios. Based on data for 24 species in seven families, SSD and sex ratios were significantly negatively correlated for most analyses, even after the effect of phylogenetic bias was removed. The analyses provide support for the model that SSD and adult sex ratios are correlated in turtles as a result of simultaneous correlation of each with sexual differences in attainment of maturity (bimaturism). Environmental sex determination provides a possible mechanism for the phenomenon in turtles and some other organisms. © 2014 The Authors. Biological Journal of the Linnean Society published by John Wiley & Sons Ltd on behalf of The Linnean Society of London, Biological Journal of the Linnean Society, 2014, ●●, ●●–●●.
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    • "The areas cover representative parts of the distributional range of moose in Norway. We used data only from calves and yearlings, as their body masses are not affected by regional differences in reproductive tactics (Garel et al. 2006). During the period 1980–2009, data on 44,926 calves and yearlings were collected within the study area. "
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    • "Recent studies showed that NDVI has been successfully used to link plant phenology (as a proxy of vegetation quality) and life history traits in various species of ungulates in open habitats (Pettorelli et al. 2005, 2007; Garel et al. 2011). Again, limited information is available regarding the ability of NDVI to forecast resource quality in forested areas [but see Garel et al. (2006) "
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