Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
There is not much scientific literature on the subject concerning optimum age for neutering dogs and cats [the term neuter is applied in this text as the surgical removal of the ovaries and uterus in females (spay) or the testicles in males (castration)]. Probably the most obvious advantage for early neutering is to avoid unwanted pregnancies. The number of homeless and healthy pets euthanized annually has reached 20 million. Many animal shelters have neuter contracts at the time of processing adoptions but as many as 70% fail to return for the surgery. For this reason the animal shelters would like to implement a policy of sterilization prior to releasing the animal. Many of the animals being adopted are puppies and kittens so the subject of early neutering is very valid in their concern in controlling pet over-population.
Nagakura, A. R. and Clark, T. L.
"Positive Effects of Prepubertal Neutering in Dogs and Cats,"
Iowa State University Veterinarian: Vol. 54
, Article 12.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol54/iss1/12