Photo by Charles Willey
Male moose are generally larger than female moose -- as is
the case with this pair, clearly exhibiting a mutual attraction
It may sound politically incorrect, but let’s face it: males and females are different. Some human animals strive to bridge those differences, but in most of the animal kingdom, differences between males and females (aside from the one obvious difference) are drivers of evolution. These differences – in color, size, shape, adornments, smell, vocalizations, or behavior – are what set the sexiest individuals apart from the masses. And by choosing a sexy mate, an animal (usually the female of a species) is doing her best to ensure sexy offspring, which in turn helps secure the survival of her genetic lineage.
One of the most common differences between males and females is in their size; called sexual size dimorphism (SSD), it’s common in mammals, even in humans. In male-biased SSD, males are larger than females (in fishes and insects, though, the females are often the larger of the two sexes). These large males are healthy and robust and therefore exceedingly attractive to most female mammals of the same species. Indeed, females choosing larger males as mates (called sexual selection for size), resulting in larger male offspring, is the most common explanation for SSD.
Recently, a team of scientists studying 14 populations of moose in Norway attempted to determine what sorts of factors besides sexual selection – from population density to food resources to hunting pressure – influence SSD; they began with three hypotheses that they were then able to test by examining data on 24,420 individual moose.
First, they hypothesized that as population density rises, SSD decreases, because less food is available for everyone. Male mammals of more-sexually dimorphic species, like moose, are known to be highly sensitive to food limitation; when food is limited, these males are only able to eat enough to maintain their bodies, not to grow them excessively.
The second hypothesis proposes that seasonal bursts of food availability increase SSD. As latitude and altitude increase, the quality of food – especially the availability of fast-growing, highly nutritious and digestible plants – tends to increase (to a point), and so animal body mass also increases. This burst of rich food has the potential to increase male body size more than female body size, since males invest extra calories in growing their bodies as large as they can, as quickly as they can, for the purpose of mating, rather than bettering the condition of their bodies for bearing young, as females do.
Third, greater hunting pressure decreases SSD. The scientists proposed that since hunting reduces the proportion of large males – the trophy specimens – in a given population, younger males, who would otherwise be too immature to mate, tend to rut prematurely; they invest energy obtained from food into rutting, and they often don’t replenish their reserves. Death is also premature, since younger males are more likely to die early in a rut they’re physically unprepared for, and they forfeit their longevity by rutting, rather than growing, in their youth. With the older males being selectively harvested and fewer younger males reaching maturity, moose populations would ultimately exhibit low adult male-to-female ratios and a correspondingly small SSD.
The scientists discovered that the second and third hypotheses bore out in the populations of moose studied. In these 14 populations, whose males were as much as 24 percent larger than females at adulthood, SSD did not change with the density of the moose population; it did, however, increase in areas with short growing seasons (associated with higher latitudes and altitudes and a seasonal burst of nutritious food). Also, SSD was lower in populations with more females per male, a demographic trait common to hunted populations. The results support the idea that ecological variables, not just female preference, contribute to sexual size dimorphism.