Why reduce clutch size to one or two eggs?: Reproductive allometries reveal different evolutionary causes of invariant clutch size in lizards

1. Large clutch size is unambiguously an ancestral state in reptiles. However, females of several lizard lineages lay just one or two eggs per clutch, called invariant clutch size. Selective forces leading to a dramatic drop in fecundity during a single reproductive bout are poorly understood. 2. We compared interspecific egg and clutch mass allometries in four lizard lineages with contrasting ways of reproduction. Lacertids and sceloporines have ancestral variable clutch size, while anoles and eublepharid geckos independently evolved invariant clutch size. To reconstruct the anoles' ancestral situation more precisely, we included data from a member of the closely related genus Polychrus, which is also arboreal but possesses variable clutch size. 3. We found the relative mass of the eublepharid double-egg clutch is comparable with the relative mass of the whole clutch in lizards with variable clutch size. Clutch mass in eublepharids and lizards with variable clutch size increases proportionally to female size. However, single-egg anole clutches show similar negative allometry as the sole egg of lizards with variable clutch size. 4. It appears that invariant clutch size evolved under (and is maintained by) selection on offspring enlargement in geckos, but selection on reducing female reproductive burden in anoles, i.e. the phenomenon of invariant clutch size apparently evolved in these individual groups for different reasons. 5. Besides understanding lizard life history, our study illustrates how singular events in life-history evolution can be reconstructed by comparing the ancestral and evolved reproductive allometries.