Live bearing promotes the evolution of sociality in reptiles

Identifying factors responsible for the emergence and evolution of social complexity is an outstanding challenge in evolutionary biology. Here we report results from a phylogenetic comparative analysis of over 1000 species of squamate reptile, nearly 100 of which exhibit facultative forms of group living, including prolonged parent–offspring associations. We show that the evolution of social groupings among adults and juveniles is overwhelmingly preceded by the evolution of live birth across multiple independent origins of both traits. Furthermore, the results suggest that live bearing has facilitated the emergence of social groups that remain stable across years, similar to forms of sociality observed in other vertebrates. These results suggest that live bearing has been a fundamentally important precursor in the evolutionary origins of group living in the squamates.


Supplementary
. Correlated evolution of viviparity and stable social grouping among squamate reptiles. (a) Ancestral state reconstructions of parity mode and stable social grouping by stochastic character mapping. Phylogeny restricted to species in the conservative data set (n = 324, see below). Branch colours represent posterior probability densities of edge states based on 1000 stochastic character maps of each reconstruction. (b) Distribution of similarity scores between stochastic character map sets (n = 1000) based on separate ancestral character state reconstructions of egg attendance and social grouping. The grey line represents the null expectation of similarity between map sets if the two traits being compared show no evolutionarily correlation during reconstruction. The red line represents the mean similarity between map sets based on our reconstructions. Figure 3. Number of independent transitions to stable social grouping from viviparity and oviparity. Dots represent the mean (± one standard deviation) number of transitions from a background of each parity mode based on 1000 stochastic character maps of the joint reconstruction of parity mode and stable social grouping.

Egg Manipulation Defence
Neonatal Assistance In test models, rates of transition toward social grouping from a background of oviparity and viviparity have been constrained to be equal to perform a likelihood ratio test comparing differences in model fit when transition rates toward social grouping from each parity mode are allowed to vary. Transition rates toward social grouping are always higher from a background of viviparity (q34) than oviparity (q12) whether parity reversal are allowed or prohibited. Significant P values in bold. N equals the number of species included in each analysis. The 'signal in response (s2)' parameter is derived from a model fit with no predictor variables and provides an estimate of phylogenetic structure of social grouping from each data set. Model estimates are reported ±SE. Confidence intervals of parameter estimates by parametric bootstrapping are derived from PGLMMs performed on simulated data (1000 and 500 simulations of the conservative and relaxed data sets, respectively; see Methods for details). Bias values represent the difference between parameter estimates from each real data model and the mean of the posterior of parameter estimates from simulations. Significant terms are shown in bold.

Care Mode Definition
Nesting Behaviour Construction of a nest space in which to receive eggs. This includes species that excavate nests and those that construct nests from leaf litter and debris. Does not include communal nesting.

Egg Manipulation
Any behavior involving parental contact or manipulation of the eggs. This includes brooding behavior, relocation of eggs during incubation and removal or consumption of non-viable eggs.

Defense
Any behavior in which parents are observed to respond defensively to potential predators approaching the eggs or nest.

Neonatal Assistance
Reports of parents assisting hatchlings/neonates out of their shell/birthing membrane during hatching/parturition.

Parent-Offspring Association
Sustained association or tolerance between parent(s) and offspring after hatching or birth. Includes active or passive defense of neonates, crevice or burrow sharing, and postponed dispersal in which offspring remain within the parental territory beyond the birthing/hatching period. Table 6. Parameter estimates and likelihoods for MuSSE models exploring the alternative causal explanation that social grouping promotes the evolution of viviparity. State dependent speciation (λ) and extinction (μ) rates are allowed for all states (1: oviparous, no SG; 2: oviparous, SG; 3: viviparous, no SG; 4: viviparous, SG). Light grey cells indicate transition rates that have been fixed to be equal. Black cells indicate transition rates that have been constrained to zero (parameters representing simultaneous double transitions (e.g. q14) constrained to zero but not shown). In test models, rates of transition toward viviparity from a background of social grouping and no social grouping have been constrained to be equal to perform a likelihood ratio test comparing differences in model fit when transition rates toward viviparity from each mode of sociality are allowed to vary. Transition rates toward viviparity from a background of no social grouping (q13) are equal to or higher than from a background of social grouping (q24) in four out of eight models. Furthermore, transition toward social grouping from a background of viviparity (q34) was higher than transitions to viviparity from a background of social grouping (q24) in all but the two least conservative models (relaxed data set, parity reversals allowed