One lump or two? Explaining a major latitudinal transition in reproductive allocation in a viviparous lizard

In viviparous ectotherms, the interval between reproductive bouts is often extended by long gestation times, preventing multiple reproductive events per annum. We assessed the potential roles of physiological adaptation and environmental constraints in driving an unusual case of geographic variation in life history, in the viviparous lizard (Eulamprus quoyii), which has either one or two reproductive bouts per annum, depending on the geographic location of the population. Using dynamic energy budget theory, we developed an integrated model of the energetics of growth and reproduction in this lizard, and applied it in conjunction with biophysical calculations of body temperature and activity time across its geographic range to predict reproductive frequency. Our model indicated that geographic variation in body temperature alone (i.e. environmental constraints) explained the observed pattern of litter frequency, suggesting that differences in energy allocation among populations were unlikely to be a major cause of differences in litter frequency in E.quoyii. It also suggested that natural selection should favour fixation of litter size in the transition zone.