c$.$ Pets who procreate can create external costs due to problems with stray animals. Strays are extremely common on the streets of poor countries. Sterilization can solve the problem, but is a tax$/$subsidy or command and control a better method to encourage sterilization? Does the best solution depend on the sex of the animal?

A $$ system might work better with male pets, but a $$ system might work better with female pets.