They are great boxers, and they get bulkier with age: Male red kangaroos are the George Foremans of the animal world.
Five-year-old red kangaroo males average about 50 kilograms (110 lbs.) in weight. But 15-year-old males average about 75 kilograms (143 lbs.), and some weigh more than 80 kilograms (175 lbs.). Male kangaroos grow steadily bigger and stronger throughout their lives, although at a decelerating rate as they age. Even their forearms grow longer and more muscular. The big, older males can sport such broad chests, wide shoulders and long brawny forearms that although younger, smaller males may adopt challenging displays, they quickly back down from actual fights.
Older males' increasing size is thought to act as a kind of sexual attractant: It signals to females that the males are long-lived and, therefore, desirable mates.
Red kangaroo males are not alone in growing bigger with age. Male eastern and western grey kangaroos, wallabies, pademelons and swamp wallabies do the same, as do American bison, giraffes, African and Indian elephants, mule deer and white-tailed deer. But the disparity in weight between red kangaroo genders at puberty--the males can weigh five times more than the females--is unrivaled among hoofed herbivores.