The platypus is a venom-producing mammal. Adult males have a spur on their hind legs that they use to inject poison. The venom seems to be associated with the breeding season and competition for mates, although it is also used against would-be predators.
Platypus venom is produced by an organ in the thigh called the crural gland. This is a feature they share with their relative the echidna, another egg-laying Australian mammal. Males inject the poison by gripping tightly with their hind legs and forcing their heels against their opponent. Flexing their thighs squeezes the gland and injects the poison. The venom is very painful to humans and has long-lasting effects, but it is not lethal. However, smaller animals, including dogs, are more susceptible to being fatally poisoned.