Male and female black howler monkeys are colored quite differently. Mature males are black, while the young and females are blond. (This difference between male and female is called sexual dimorphism.) Males are also larger than females, weighing about 16.5 pounds as opposed to females at 12.5 pounds. Howlers have greatly enlarged lower jaws which accommodate egg-shaped resonating chambers. These chambers enable howlers to make their unique reverberating calls. The calls warn nearby troops to keep away from feeding areas. Howlers have prehensile tails which are used mostly for balance or a sense of security.