Body elongation and decreased reproductive output within a restricted clade of lizards (Reptilia: Scincidae).

Relationships between body shape and relative abdominal size were compared among differentially elongate species within the scincid lizard genus Brachymeles, to investigate how morphological evolution affects the proportion of body volume available to hold eggs and offspring. Relative abdominal size is inversely related to elongation, suggesting that relative clutch mass decreases with addition of abdominal body segments. Shape-volume relationships contradict trends seen in comparisons among distantly related limbed and limbless squamates (lizards and snakes), in which snakes have relatively more abdominal volume. Comparison within a phylogenetically restricted group allows the identification of functional and ontogenetic factors potentially limiting reproductive output. In Bruchymeles, constraining factors include retention of anterior body segments bearing parasternal ribs, which prevents extension of the clutch anteriorly within the body, and reduction of allometry of abdominal segments, which provides extended series of uniformly-sized vertebrae for limbless locomotion, but reduces the relative size of the abdomen. The latter trait is associated with overall size reduction, which affects relative egg-size and packing. Factors constraining abdominal volume in this genus are probably common to other elongate lizards, a morphological group that has been rarely represented in comparative studies of life history. 1994 The Zoological Society of London.