The length of growing season and adult sex ratio affect sexual size dimorphism in moose.
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.
Garel
Mathieu
M
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. mgarel@biomserv.univ-lyon1.fr
Solberg
Erling Johan
EJ
Saether
Bernt-Erik
BE
Herfindal
Ivar
I
Høgda
Kjell-Arild
KA
D000821:Animal Feed; D000818:Animals; D001835:Body Weight / Q000502:physiology*; D003670:Deer / Q000033:anatomy & histology / Q000254:growth & development / Q000502:physiology*; D005260:Female; D008297:Male; D009664:Norway; D011156:Population Density; D012621:Seasons; D012727:Sex Characteristics; D017678:Sex Distribution; D012726:Sexual Behavior, Animal / Q000502:physiology; D013997:Time Factors