by Elodie Briefer
M. Rughetti & M. Festa-Bianchet, 2011, J. Zoology, 284:257–264
Sexual Size Dimorphism (SSD) refers to differences in body size between males and females. Among mammals, this phenomenon is common; males have been selected to be larger than females in order to compete with other males and gain access to females. This is especially true in polygynous species, such as most ungulates, in which competition to mate is very strong. However, gaining and maintaining a large body size is very costly for males.
Rughetti and Fiesta-Bianchet investigated the secret behind chamois SSD, by measuring both skeletal size and body mass of males at different times of the year. The costs associated with these two measures are quite different, skeletal size depending on growth rate during the first years of life, and body mass relying on muscle and fat accumulation in adult males. This study thus differs with most previous studies on SSD, which did not try to tease apart these two measures. Interestingly, the authors found that, even if males were 40% heavier than females at the beginning of the rut, this difference decreased to 4% after the rut. SSD in spring was mainly attributable to differences in skeletal size (5%).
These results clearly show that SSD in chamois results from a greater accumulation of muscles and fat in males than females before the reproductive season. Body mass increased quickly in males before the rut and decreased during this period, which is associated with an increase in energy expenditure and a concomitant decrease in foraging time in many mammals. Seasonal decreases in body mass seem to be common among ungulates, and goes up to 20-30% loss in fallow deer, bighorn sheep and mountain goats. However, chamois appear to be unique in that SSD during the rut results entirely from an accumulation of body fat and muscles tissues in males. Such seasonal SSD appears as a good strategy, because it prevents males from having to fulfil the high energetic requirements of maintaining a large body size over the winter.
Similar studies on other ungulates, in which variations throughout the year in both skeletal size and body mass are measured, could help us understanding the different reproductive strategies observed among ungulates and the costs associated with sexual dimorphism.