How Neutering Tom is Different than Spaying Ally
Let's say you wanted to control or reduce the number of cats in your area for whatever reason. Let's say half the people get on board spaying 50% of the females and neutering 50% of the males. This will give you around a 55% reduction in reproductive capacity. Still, you may not realize how different the effect is in sterilizing the males versus the females.
The hitch is cats are not monogamous and male cats can father multiple litters per year. Spaying 50% of the females gives you a 50% reduction in reproductive capacity without even touching a single male. The effect from females is a line and the effect from males is a curve. Here's why.
Setup. Say you have an isolated island with a cat population of 10 males and 10 females. Each female can have one litter per year. Let's say each male can father 8 litters per year.
Scenario One. In a year the 10 female cats combined can potentially have 10 litters. Meanwhile the 10 male cats combined can potentially father 80 litters. But there are only 10 females, the extra 70 potential impregnations are overcapacity, they can't impregnate females that don't exist. Total number of litters: 10.
Scenario Two. You neuter 8 of the males. Let's say the two fertile males each impregnate 8 females, six in common and two each not in common. This leaves no female without a litter. In this scenario neutering 80% of the males has gained you nothing in terms of population control. Total number of litters: 10.
Scenario Three. Instead you spay 8 of the females. The male potential is for 80 litters. However there are only 2 females to bear litters. The extra 78 potential impregnations are overcapacity. Total number of litters: 2.
Scenario Four. You spay 8 females and neuter 8 males. The male potential is for 16 litters. However there are only 2 females to bear litters. The result is the same as if you didn't neuter any of the males because even with only 2 fertile males you still have an overcapacity to impregnate the available fertile females. Total number of litters: 2.
This is why neutering male cats for population control can be problematic. There is little effect to polygamous animal populations from neutering males until you reach about 70%. Even then the effect is slight for the number neutered, growing ever more effective the closer you get to 100%. At the same time there is a direct one for one reduction in reproductive potential for each female spayed.
If you want a real world example, visit a dairy farm. It only takes one bull to maintain a herd of 100 cows. You could double (minus one) the population of this herd in one year. But if you had a herd of 100 bulls and one cow it would take at least 7 years to double provided every calf born were a female which could bear a calf the following year.
It may be for population control the resources used to neuter males might be more effectively used to spay females. If the time and money spent neutering 50% of the males could be used to spay just 10% more of the females, you'd get more results. Which is not to say this can easily be done, but it's a thought.
Granted, this is strictly a cold calculation about population control. It doesn't address the health and wellfare of cats, feral or otherwise. Whether you think of cats as pets or pests might have you thinking along other lines. Different purposes would have different calculations. But, that's another story.
If you're interested in feral cats as per this issue try Alley Cat Allies.
copyright Terry Colon, 2008