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Why Has Our Tortoise Turned Aggressive?

By: Dr Gareth Evans - Updated: 22 Jun 2015 | comments*Discuss
Tortoise Spur-thighed Tortoise Sexing


We have two female spur thighed tortoises. Until recently they have got on very well. Although in the last week the smaller of the two has become very agressive towards the larger one. She is also trying to mount the other tortoise! Is there a problem or is this normal behaviour?

(N.T, 24 April 2009)


Well, that rather depends on your point of view! Although it’s impossible to be 100 per cent sure from a distance, it sounds like perfectly normal behaviour – but for a true pair of tortoises, rather than a couple of females (though I suppose all things are possible).

If what you’re seeing is the smaller one repeatedly butting and battering the larger one, with a bit of biting going on, especially of her legs, then I suspect that what you have is a male who’s beginning to develop an understanding of the reptile equivalent of that story about the “birds and the bees”!

I suppose the $64,000 question is, just how sure are you about the sex of your tortoises?

Sexing tortoises is problematic at the best of times, especially with youngsters and although the text book guidelines – males have longer, rounder tails and usually a concave plastron – are useful general pointers, it can be a notoriously difficult decision to make. Even really experienced experts who have been around tortoises all their lives can get fooled just looking at the outside appearance – so it’s no wonder the rest of us struggle!

A Boy or a Girl?

The whole question of sex determination in tortoises is a fascinating one, since unlike many creatures, they don’t have sex chromosomes, the sex of the offspring being decided by the temperature at which the eggs are incubated. It’s a trait shared with a few of their other reptilian relatives – including crocodilians, most terrapins and sea turtles, New Zealand’s wonderfully unique living fossil, the Tuatara and a few kinds of snakes and lizards.

In tortoises generally, low temperatures (20-27 degrees C) produce males, while higher temperatures – usually above 30 degrees C – produce females; temperatures between these two points tend to give fairly even numbers of both sexes, although ambiguous inter-sex individuals can also sometimes be produced too.

This effect has actually been well investigated in the species you keep, the Spur-thighed tortoise (Testudo graeca) and it seems that the temperature threshold is a very fine one. Studies have shown that eggs incubated up to 29.5 degrees C result in males and above 31.5 degrees produce females, with just a single degree C making all the difference.

Unless you’re absolutely sure that your tortoises really are both female, I don’t think you’re seeing anything other than a rather happy chappy deciding to try his luck – and who knows, he might just succeed. Incubating tortoise eggs isn’t always the most successful of tasks, but if you do get a clutch, it’s certainly worth a try – so best of luck!

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