Partitioned parturition: birthing asynchrony in cordylid lizards

Hatching/birthing asynchrony, when siblings emerge at least 12 h apart, is thought to be a significant driver of phenotypic variation and group cohesion that is commonly reported in invertebrates and birds, but rarely in squamates. We examined birthing asynchrony in African cordylid lizards (Cordylidae), a clade characterized by a wide range of sociality (a hypothesized evolutionary driver of this unique phenomenon). We monitored parturition from wild-caught mothers from four species, which vary in their conspecific grouping behaviour. In two species, most litters were born asynchronously, over a maximum of 3–4 days respectively. The other two cordylids also exhibited asynchronous birth in all litters with more than one offspring, although this was not applicable for most litters because there was a prevalence of singleton litters. Our study uncovered birthing asynchrony in a novel taxonomic group, which suggests it evolved convergently in at least two social lizard clades from different continents. Furthermore, the function of birthing asynchrony and limiting litter size to a single offspring may be similar in social animals. We discuss the potential significance of this rare phenomenon in this disparate taxon, and compare it with other more well-studied taxa, in order to guide future research directions.