DOI: 10.5061/dryad.866t1g1kz
title: Data from: Viviparity does not affect the numbers and sizes of reptile offspring

description: Viviparity (live-bearing) has independently evolved from oviparity
 (egg-laying) in more than 100 lineages of squamates (lizards and snakes).
 We might expect consequent shifts in selective forces to affect per-brood
 reproductive investment (RI = total mass of offspring relative to maternal
 mass) and in the way in which that output is partitioned (number versus
 size of offspring per brood). Based on the assumption that newly-born
 offspring are heavier than eggs, we predicted that live-bearing must
 entail either increased reproductive investment or a reduction in
 offspring size and/or fecundity. However, our phylogenetically-controlled
 analysis of data on 1,259 squamate species revealed no significant
 differences in mean offspring size, clutch size or RI between oviparous
 and viviparous squamates. We attribute this paradoxical result to (1)
 strong selection on optimal offspring sizes, unaffected by parity mode,
 (2) the lack of a larval stage in amniotes, favouring large eggs even in
 the ancestral oviparous mode, and (3) the ability of viviparous females to
 decrease the mass of uterine embryos by reducing extra-embryonic water
 stores. Our analysis shows that squamate eggs (when laid) weigh about the
 same as the hatchlings that emerge from them (despite a many-fold increase
 in embryo mass during incubation). Most of the egg mass is due to
 components (such as water stores and the eggshell) not required for
 oviductal incubation.  That repackaging enables live-born
 offspring to be accommodated within the mother’s body without increasing
 total litter mass. The consequent stasis in reproductive burden during the
 evolutionary transition from oviparity to viviparity may have facilitated
 frequent shifts in parity modes.
description: To compare oviparous and viviparous species with respect to
 fecundity (clutch vs. litter size), offspring size (hatchling vs. neonate
 mass), and reproductive investment (RI), we collected data on squamate
 life history traits, body sizes, and geographic ranges from the
 literature, museum databases and our own observations in the field, of
 captive reptiles and in museum collections. We define
 all live-bearing species (including ovoviviparous taxa) as “viviparous”
 because our interest lies not in the physiology of gas and material
 transfer between the embryo and the air or uterus, but with the effects of
 carrying developing embryos inside the body cavity of the mother.  We
 excluded from the dataset all records of squamates that lay fixed clutch
 sizes of one or two eggs (Schwarz & Meiri, 2017). These are
 defined here by phylogenetic criteria, as members of the Gekkota (geckos
 and pygopodids), Dactyloidae (anoles) and Gymnophthalmidae. Because the
 number of eggs per clutch is tightly constrained in these taxa, their
 responses to the evolution of viviparity may differ from those of other
 squamates. In practice, excluding these taxa has little impact because, as
 far as is known, all anoles and gymnophthalmids and ~98% of gekkotans lay
 eggs (Meiri, 2018). Members of other lineages with very small brood sizes
 (e.g., 1 or, less often, 2 in the Solomon Island skink, <i>Corucia
 zebrata</i>; Honegger, 1985) were included because their small
 broods presumably reflect adaptations rather than constraints.
 Data on clutch and litter (henceforth: brood) sizes and on body
 sizes of hatchlings, neonates and adult females are means. In cases when
 multiple mean values were available, we used the average between the
 largest and smallest reported means. If data on means were unavailable, we
 used the average between the largest and smallest reported numbers or
 sizes. To compare among animals that vary greatly in shape (e.g., heavyset
 lizards vs slender snakes) we converted data on snout vent lengths (SVL)
 (for all lizards and some snakes) or total lengths (TL, for most snakes)
 to masses using clade-specific equations, taking lizard leg reduction into
 account (Meiri, 2010, Feldman & Meiri, 2013, Feldman, Sabath,
 Pyron, Mayrose & Meiri 2016). For four species (<i>Uromastyx
 thomasi, Lerista arenicola, Varanus timorensis </i>and<i> V.
 dumerilii</i>) we could only obtain data on female mass, but not
 length, so we used mass data (of non-gravid females) directly. To
 calculate RIs we multiplied the mean brood size by mean hatchling/neonate
 mass and divided this by female mass. We only used species for which we
 had data for all parameters (mode of reproduction, hatchling/neonate size,
 brood size, female mass) as well as phylogenetic information.