Enter the weird world of the echidna—a mammal in a category all its own

The penile shaft has four heads, each plumbed to deliver semen—but only two heads are used in each erection, and the male alternates which two. That’s possible because echidnas’ erectile tissue anatomy differs from most other mammals’, Australian scientists reported in a 2021 study. The anomaly likely helps males compete for mates: Swapping penis sides allowed one echidna to “ejaculate 10 times without significant pause,” study authors noted.