Humans can use condoms, birth control ect. As far as I know, S/N is the only form of Birth control for animals.
These people that are overpopulating in other countries, do it out of choice, animals however, cannot control themselves when another animal is in heat. They are 2 VERY different subjects, and quite frankly i think they should be kept as 2 different subjects.
I see more need of animals being S/N, then humans, because, no matter how populated the earth gets, humans wil never be "humanely" Euthanized, like perfectly healthy animals are everyday.
Keely - Yorkie
Haley - German Shepherd
Casey - version 2.0 - Black lab
Jasper - White cat
R.I.P Casey #1.
Gone but never ever forgotten.