Chris,

I don't want to state the obvious but lizards being cold blooded, have their cell cycle rates determined by temperature.  The other two animals are homeotherms and have a fixed temperature, thus a fixed rate for the cell cycle.  Of course this is all dependent on the cell type (eg; neuron vs epidermal), as they are all different as well.

Lee

Reptile red blood cell lifespan has been studied extensively and is much longer than mammals and birds. A review can be found in Biology of the Reptilia.