During the last few years, there has been a growing concern amongst breeders about the effects of inbreeding on captive herp populations. While in many cases this concern is justified, it must be tempered by an understanding of the effects of inbreeding - both positive and negative. Many of the questions we get regarding this topic show an almost myopic fear and misunderstanding. So we've decided to share nearly three decades of of experience and thought about inbreeding in hopes of giving our customers a better insight on the subject.
What is Inbreeding?
Simply put, inbreeding is the process of breeding together individuals which are related, often quite closely. Inbreeding has a single consequence, it tends to reduce the number of alleles (genes) available within the population. Such concentration of alleles can be viewed as positive, if they are good traits, or negative if they are bad.
Why would you do this?
Well, there's a number of reasons, all of which can be justified in spite of the dangers involved.
In many very rare species, there may simply be a lack of available breeding stock to utilize in breeding programs. This the case with many of the Australian species. Many of these species have never been legally imported into the United States, and current stocks consist mainly of confiscated specimens held at various zoological institutions. Few of these institutions place much value on captive reproduction and fewer still will allow such captive progeny into the hands of private keepers. As a result, there are few unrelated specimens available to the private sector. So captive breeders are faced with the excruciating choice of either inbreeding their stocks or watching these species vanish from captive collections.
A more common reason for inbreeding is the reproduction of various mutations, such as albinism. Such mutations can represent a large financial investment, as well as being of general interest to many, and there is much incentive to reproduce such individuals. There is absolutely no way to produce additional specimens of these rarities without inbreeding them to some degree. That's simply the reality of it. Each and every albino Burmese Python is in some way related to every other albino Burmese Python. That's just fact.
But inbreeding is in fact, a matter of degrees. In general, the more closely related two specimens are, the less desirable they are as partners for reproduction. In fact, there are laws in most countries regarding the marriage of related persons. You cannot marry your sister, or your cousin, but marrying you fourth cousin twice removed is legal (although if you've heard any jokes about Arkansas hillbillies, you'll know it's not desirable). As I said, it's a matter of degrees.
So What's So Bad About It?
As we discussed above, inbreeding tends to reduce the number of alleles present. While this is good if the allele has desirable effect, such as albinism, it is bad if the allele has a negative effect such as a spinal deformity. There are many potential negative effects and many are well-known in captive reptile populations already: Bloodred Cornsnakes are a line-bred (euphemism for inbred) line of Cornsnakes notorious for producing small hatchings which are difficult to start feeding. Patternless Leopard Geckos are well known for a lack of fertility in males. These are just two well known examples.
Perhaps more common is simply a general lack of vitality which increases with each successive generation. This is exactly what happened with the Bloodred Cornsnakes mentioned earlier. Early in the development of this beautiful morph, breeders selected stocks based on appearance through successive generations. This had the desired effect of producing more and more attractive snakes by reducing the number of alleles which had a counterproductive effect on appearance and increasing the number which had a positive effect. Sadly, this same concentration of alleles had the opposite effect on the hatchlings' desire to accept rodents as first meals.
What Can We Do About It?
Any conscientious breeder will make every attempt to minimize the negative effects of inbreeding through a variety of methods.
A method of primary importance is to utilize out-crossing wherever practicable. Out-crossing involves taking your cherished albino specimen and breeding it to two unrelated wild specimens. You'll now have two groups of heterozygous for albino specimens which are only half-related to breed together. This is much better than breeding the albino directly to his sister, as many new alleles will have been introduced into the population using this method.
Where possible, breeders should attempt to acquire specimens which are unrelated, or close to it, for use as breeders. The use of large groups of unrelated breeders will allow production of many unrelated or partially related offspring by making different pairings each year.
Accurate records should be maintained with regard to lineage. This will allow breeders of future generations to determine exact genetic relationships when selecting specimens for reproduction.
In each and every case, the breeder should attempt to weed out specimens exhibiting undesirable effects. Specimens exhibiting deformities, or which lack vitality should be euthanized or placed with individuals as pets - they should never be used in breeding programs.
The Bottom Line
Hopefully, this discussion has provided some insight into the subject of inbreeding. We hope that the reader will realize that inbreeding should be viewed as just another valuable tool to be used by knowledgeable breeders, but that its use must be tempered by good judgment and caution. Without it, none of the beautiful mutations we enjoy in our hobby today would be available.