Can a tortoise mate with siblings? -

Can Tortoises Mate With Siblings?

If you have two tortoises from the same clutch of differing sexes, you might be wondering whether you can safely house them together or whether it’s best to keep them apart. The big question is really – will they mate together?

Can tortoises mate with siblings? Tortoises can mate with siblings but they really should not. While inbreeding in reptiles is not as problematic as in mammals and other animals, nature shows us that there are some serious potential consequences. To increase the chances for healthy offspring, tortoise owners should not allow their tortoises to mate with siblings.

Inbreeding between tortoises is quite rare in the wild too, and there are some very clever natural mechanisms that keep it that way. Let’s look at the fascinating details.

Can Tortoises Mate With Their Own Siblings?

Tortoises aren’t people and they don’t come with a moral code and a good understanding of the world around them. They are generally only interested in eating, basking, sleeping, burrowing, hibernating and mating.

One thing that is clear is that tortoises aren’t particularly fussy when it comes to mates. They are solitary creatures when they’re not getting together with members of the opposite sex, so, they tend to seize any opportunity to mate, metaphorically speaking, with all four paws.

Sadly, this indiscriminate attitude means that a tortoise will, happily, mate with a brother or sister because they don’t know about incest and the unfortunate potential effects that it may have.

This means that it’s always best to separate tortoises by sex when they are born and to supply tortoises of only one sex to another tortoise owner, therefore, you remove all the opportunities for tortoises to develop incestuous relationships in their lives.

Do Tortoises Mate With Siblings In The Wild?

It is unlikely that tortoises would mate with siblings in the wild, though, it’s not impossible. In general terms, a female tortoise tends to disappear pretty much as soon as her children hatch, and they are then left to look after themselves.

Being tortoises, the siblings won’t want to spend too much time around each other and they will disperse over whatever geographic area that they can get to. This, assuming that there are other tortoises (unrelated ones) around and about the same area, will normally ensure that they are distributed far enough apart from each other that incest isn’t particularly likely.

However, a wild tortoise is no more discerning in its mating habits than a pet tortoise and thus, it  is theoretically possible that if two related tortoises were to cross paths when mating urges were upon them both that they might get together and produce offspring.

Is It Likely That Tortoises Will Mate With Siblings?

Mother Nature doesn’t tend to encourage characteristics in animals that are harmful to their future development and while it’s fair to say that, as we will see, some incestuous reproduction in reptiles appears to be less damaging than it is in mammals, it is not entirely a good idea, either.

Thus, the biological control against incest appears to be that, in most cases, a clutch of eggs will usually only produce members of one sex or the other. This means that you may well find that there are no opportunities for incest of a problematic kind.

Though it might be possible for tortoises to be homosexual, tortoise obviously cannot become pregnant as a result of their union. If you didn’t know that tortoises are capable of homosexual relationships, have a look at Jonathan the 186+ year old tortoise who is quite definitely gay. Jonathan is also the world’s oldest tortoise, so he’s quite a remarkable creature.

What Problems Are Likely To Come About If Tortoises Mate With Their Siblings?

This is where things become a little more problematic. You see it would be quite unethical for someone to experiment in this area by inbreeding tortoises to see what went awry, animals may not receive quite the same protections under law as people, but this would be a step too far.

So, we are left with two areas of research. The first is the outcomes of incest in some other reptiles and then more general research on the outcomes of incest in all species.

Sand Lizards And Incest

So, let’s take a look at the first. Bererhi et al in their paper, “Inconsistent inbreeding effects during lizard ontogeny” took a look at some sand lizards that were inbreeding and tried to work out what the impact of this inbreeding was on the species.

Charles Darwin, back in 1868, had noted that this was an important thing to study given that inbreeding could result in a negative impact on the overall “fitness” (it may help to remember here that evolution is considered to be the “survival of the fittest” – Darwin is referring to the chance of the animal surviving not its general level of physical health) of any given creature.

It had long been expected thanks to the research efforts of some of Darwin’s successors that animals would avoid inbreeding wherever possible because of this and, indeed, some trials were conducted where this appeared to be the case.

Not So Problematic As Expected

However, Bererhi’s research found something quite differently entirely. They found that the results of inbreeding might lead to negative result in terms of viable eggs (fewer eggs hatched when the parents were incestuous) but they also found that reptiles, which did hatch, that were the product of incest – were no less likely to live out their first year than any other reptile.

In short, their research suggests that incest (at least short-term, single generational incest) in reptiles may not lead to as severe an outcome as it does in other species.

We need to stress at this point that a tortoise is not a sand lizard. While this data is interesting, and it might even apply to tortoises – there is no way to guarantee that this is true. There are huge levels of variety between reptile species and some may be more suited to incestuous breeding than others.

Reptiles Magazine also weighs in on this saying that in their opinion, “The fear of harmful results from inbreeding animals is generally exaggerated, coming more from a few misunderstood problems in human genetics. Many reptile populations are tightly inbred already.”

Incest and The Problem In Other Species

There is, of course, good reason that we find incest so distasteful as human beings. It’s not some arbitrary judgement criteria – it’s because inbreeding can result in harmful genetic mutations.

Now, you may not think that this is particularly important to human beings but there are quite a few countries around the world which had to introduce genetic tests for couples considering marriage. In these societies it’s not always clear who is related to whom and thus, the rate of birth deformities was running out of control

Why does this kind of breeding result in deformities? Well, it’s because when you take two genetically similar (very similar but not necessarily identical) animals and breed them together – you risk the weaknesses found in their genetic inheritance becoming magnified in the next generation.

This means that there is a risk of birth deformity with the product of incest from human beings and that in every incestuous generation that follows – there will be a larger degree of deformity.

This has been proven to be true in most species. So, given that tortoises will, probably, at some point or another carry out incestuous behavior in the wild – why can’t we find deformed tortoises?

Well, that’s fairly obvious when you think about it, a deformed tortoise is unlikely to survive very long after hatching. They will be easier prey for animals, more prone to diseases, etc. and thus, they’re going to die young. This is Mother Nature’s last and most harsh measure to discourage incest in animal populations.

What’s The Easiest Way To Prevent These Problems?

With pet tortoises these problems are fairly easy to prevent. You need to separate tortoises by sex as they are hatched. This shouldn’t present too much of a problem because, as we’ve seen – most clutches only contain tortoises of one sec.

In fact, the best prevention of all is the one that your tortoise is likely to prefer the most – only keep one tortoise and don’t let them mix with any other tortoises. They can’t have an incestuous relationship by themselves.

Finally, if you can’t do either of the things above – consider having the male tortoises castrated. That way they can’t make any babies.


Can tortoises mate with siblings? Tortoises can mate with siblings and while it’s a rarity in the wild – they certainly do so. This, of course, is a risky behavior which might have severe implications for any offspring, so as a responsible tortoise owner – you should always take measures to stop them from doing so.

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