Why don’t turtles give birth to live young? It may sound like a dumb question, but remember that the only reason a marine turtle ever returns to land is to lay eggs (male turtles never return to land*). After all, viviparity (giving birth to live young) was quite a common reproductive strategy for prehistoric marine reptiles; ichthyosaurs, mosasaurs and plesiosaurs all gave birth to live young, allowing them to adopt a completely aquatic lifestyle.
Viviparity evolved from oviparity (egg-laying), and this transition has occurred independently in vertebrates about 140 times. Rather surprisingly, the vast majority of these events (about 120) occurred within the reptiles, but never in the chelonians (turtles and tortoises). (On an interesting side note, recent research has produced a model of the earliest ancestor of all placental mammals).
A team led by Anthony R. Rafferty of Monash University sought to identify factors which potentially limit chelonian progression towards a viviparous reproductive strategy. Their research, published in American Society of Naturalists, appears to have identified the limiting factor. For an animal to give birth to live young, the mother must retain the young until they’ve reached an advanced developmental stage. Turtles cannot do this because of the hypoxic (low oxygen) conditions present in the mother’s oviducts. Under low oxygen conditions, turtle embryos undergo arrested development and their development essentially pauses. Once the mother lays the eggs, they are exposed to atmospheric oxygen and their development resumes. So in short, turtles cannot adopt a viviparous lifestyle without somehow evolving a mechanism to provide the young with oxygen during the later stages of development.
However, the low oxygen conditions in the oviducts may not be coincidental. The mother secretes a mucus-like substance which retards oxygen diffusion, suggesting that there is a drive to stall embryonic development at a particular stage. This makes sense, because at a later stage in development, the internal membranes fuse and the egg can no longer be disturbed without killing the developing turtle. If the young were allowed to develop to this stage inside the reproductive tract, the very act of egg-laying would kill them.
Research such as this is vital to informing conservation efforts. The more we know about how the eggs develop the better our chances of identifying potential threats. For example, we know that a turtle’s sex is not determined by chromosomes, as in humans, but by incubation temperatures. Cooler incubation temperatures produce male turtles and warmer temperatures produce females. Global warming appears to be skewing turtle sex ratios, with a higher number of females emerging as a result of rising temperatures. Conservationists could potentially use our knowledge of incubation temperatures to their advantage by removing a selection of eggs from a nest and artificially incubating them at cooler temperatures.
While on-going research continues to provide conservationists with useful management tools, we are unfortunately no closer to answering the big question in turtle biology: How much toxic goo must I spill on one to make it an anthropomorphic, crime-fighting, ninja?
* There are exceptions. Sick or injured males may beach themselves, and one population of green turtles does crawl onto beaches to bask.
Photos: Credits as per captions.
Thanks to A. Rafferty, for his input.