Are you able to identify any key differences in the behavior/body structure of neutered vs unaltered male dogs?
Early neutering (most often defined as being <6 months of age) alters the way the body develops because it removes a large portion of testosterone during the main growth and development stage of dogs.
Testosterone is important in the development of growing dogs (and any animal, really) because it “tells” the body when to close growth plates and stop growing.
When a dog is neutered early, those growth plates don’t close properly (normally around 1-3 years of age, depending on size and breed), and the dogs retain puppy-like body proportions.
In some dogs like Afghans, they will develop a cottony puppy-like coat instead of the long, silky fur of an intact, adult Afghan hound. I have also read that this can occur in other breeds like Collies, Chinese Cresteds, and Setters. ( http://news.vin.com/vinnews.aspx?articleId=27205 )
Due to the growth plates not closing at the proper time, the legs grow longer than usual and neutered dogs tend to have a gangly, leggy look to them.
Early neutering can cause the genitals to develop improperly, and in females this can cause a recessed vulva (as well as the health issues that come with it). Neutered males tend to have a sheath that looks like a bellybutton (similar to what puppies have), and intact males don’t.
Testosterone is important for muscle-building, and early neutering can cause dogs to grow up with a lack of muscle-tone. Testosterone is important for keeping weight off and this is why so many dogs become overweight after being neutered.
Growth plates in large breeds don’t close until 2-3 years of age, so typical body and head shapes of certain breeds don’t set in until maturity.
In dogs like Rottweilers, the males have wider heads than the females, and dogs neutered early can have more bitch-like heads. The first year for large breeds is growing in height, and the second and third year is width and muscle tone. If the growth plates development is hindered early on, they tend to retain that immature, teenage dog look instead of wide, robust, “blocky” appearance typical of that breed and sex.
I can usually tell a neutered dog in person versus an intact one from a distance. A lack of muscle tone, legginess, thin body and head type in comparison to the standard of his breed and sex, obesity, and a “bellybutton” are all giveaways to me.
Of course there’s always going to be variables with breeds. I know that weirdly brachycephalic Rottweilers are a trend that’s on the rise, to the point that FCI/AKC/UKC standard Rottweilers are called “American type” now.
Studies done on neutered dogs also showed that dogs who were neutered early tend to be more anxious, more fearful, aggressive, insecure, and harder to train with a shorter attention span. This isn’t really something I use as an identifier.