This is a quote from Kerry Hewitt from the National aquarium in Napier in reference to the breeding of Reeves Turtles in reply to an email I sent. " You are correct in thinking temperature can affect the sex determination in reptiles but there are a few reptiles which this does not apply and the sex determination is more genetic than environmental, the reeves turtles are one of these so you will find a mix of sexes from one clutch of eggs. For sexing the males have a longer tail and the cloaca is sited further from the body.
The reeves being supplied from the breeder in Napier are actually from a small population of semi related reeves he has, but is unable to determine the actual genealogy of the babies being sent out, He does have several reeves which are of unrepresented gene stock which he is keeping separate and using for breeding in conjunction with the aquarium and others to maintain a strong and healthy gene pool to work with, there is a stud book being kept to ensure the right animals are being breed together.
If this gene pool was introduced in to the public area we would lose the ability to maintain an accurate record and risk breeding related problems."