Also cultivate plants caterpillars feed on! We won’t have any butterflies if the only food available is only edible by adults.

Estrus in bats - some bloody discharge while in the fertile part of their cycle. Only great apes have menstrual cycles (shedding unused uterine lining at the end of a cycle, NOT fertile when discharging blood).

Physical trauma makes sense for large animals. If you have 50 lab rats that you need to euthanize, a gassing setup can make more sense than individually whacking them.