Data from: Squamate hatchling size and the evolutionary causes of negative offspring size allometry

Although fecundity selection is ubiquitous, in an overwhelming majority of
animal lineages, small species produce smaller number of offspring per
clutch. In this context, egg, hatchling and neonate sizes are absolutely
larger, but smaller relative to adult body size in larger species. The
evolutionary causes of this widespread phenomenon are not fully explored.
The negative offspring size allometry can result from processes limiting
maximal egg/offspring size forcing larger species to produce relatively
smaller offspring (‘upper limit’), or from a limit on minimal
egg/offspring size forcing smaller species to produce relatively larger
offspring (‘lower limit’). Several reptile lineages have invariant clutch
sizes, where females always lay either one or two eggs per clutch. These
lineages offer an interesting perspective on the general evolutionary
forces driving negative offspring size allometry, because an important
selective factor, fecundity selection in a single clutch, is eliminated
here. Under the upper limit hypotheses, large offspring should be selected
against in lineages with invariant clutch sizes as well, and these
lineages should therefore exhibit the same, or shallower, offspring size
allometry as lineages with variable clutch size. On the other hand, the
lower limit hypotheses would allow lineages with invariant clutch sizes to
have steeper offspring size allometries. Using an extensive data set on
the hatchling and female sizes of > 1800 species of squamates, we
document that negative offspring size allometry is widespread in lizards
and snakes with variable clutch sizes and that some lineages with
invariant clutch sizes have unusually steep offspring size allometries.
These findings suggest that the negative offspring size allometry is
driven by a constraint on minimal offspring size, which scales with a
negative allometry.