Some reptile species give birth to live young, but turtles have never evolved to do so — perhaps because of low oxygen levels in their egg-laying tubes, or oviducts.

Anthony Rafferty at Monash University in Clayton, Australia, and his group show that oxygen diffused more slowly in secretions from the oviducts of four species of turtle than in saline solution. When the turtles' eggs were incubated at low oxygen levels they stopped developing, whereas those kept at ambient conditions developed normally. The low oxygen levels in the oviducts could explain how turtles are able to store eggs in a state of arrested development until they can lay them on land.

Am. Nat. http://dx.doi.org/10.1086/668827 (2013)