In most mammals, males tend to be at least slightly larger than females; whereas, in most invertebrates, in many fishes, amphibians, reptiles and some birds, females are larger than males. A surprising number of mammalian species exhibit extreme dimorphism in body size, whereby one sex is on average 1.6 times heavier than the other sex. To begin with, extreme sexual dimorphism in body size in favor of females is uncommon in mammals, occurring only in our local family of bats (vespertilionids). Two reasons have been proposed for the big-mother hypothesis in these bats. First, a larger female may be at an advantage by being able to produce more offspring than a smaller female. Second, and probably more importantly, larger body size in females enables them to produce more milk; greater milk production would allow young bats to gain body weight quicker.

This is adaptive because bats have a limited amount of time in spring and summer to raise young to adult size, and young must gain weight rapidly in preparation for winter hibernation.

Conversely, extreme sexual dimorphism in favor of males is found in large aquatic species (e.g., sperm whale, northern elephant seal, northern fur seal and walrus), large terrestrial species (e.g., great red kangaroo, orangutan, gorilla and African elephant) and small weasels. Speaking of elephants, do you know the real reason why elephants never forget? -- No one ever tells them anything. The evolution of large males seems to be associated with adult body size and the type of mating system (monogamy versus polygamy, or more appropriately, polygyny), at least in the larger mammalian species.

For instance, the male harbor seal is only slightly larger than the female, and it is monogamous. On the other hand, the northern elephant seal is sexually dimorphic and polygamous, with males mating with more than one female and investing no time in care of young. For some unexplained reason, body size and type of mating system are not related in small mammals. In these mammals, extreme sexual dimorphism is absent, yet these small mammals are polygamous. Why is extreme sexual dimorphism in favor of males adaptive? Larger body size may better enable a male to mate and subsequently defend his harem and young from rival males, as in northern elephant seals and gorillas. In another species, the walrus, harem defense is not the issue because copulation takes place in the water; instead, large body size better ensures that large, dominant males occupy the best position on land within large groups along the shore. These males then are located in safer, central positions, whereas the smaller, subordinate males are positioned along the edge of the colony where they are more susceptible to predators, such as killer whales (orca). The elongated weasels give us some additional insight into the value of extreme sexual dimorphism in favor of males.

Ermine (or short-tailed weasel), for instance, rely heavily on small mammals as prey. The smaller female ermine can follow mice in tunnels, but the larger male cannot fit into these areas and relies more on somewhat larger prey. Thus, differential body size enables male and female weasels to better partition food.

In addition, the larger, more mobile male weasel has a much larger home range than the less mobile female. Because of a relatively large territory size, males probably encounter several mates. When males encounter each other, the largest of the two is likely to be more dominant over the smaller individual; therefore, larger body size allow a male to better defend his territory or an estrous female from a smaller rival male. The size advantage of a male weasel over a female weasel also enables him to physically dominate her prior to mating.