Viviparity is associated with larger female size and higher sexual size dimorphism in a reproductively bimodal lizard

Squamate reptiles are central for studying phenotypic correlates of
evolutionary transitions from oviparity to viviparity because these
transitions are numerous, with many of them being recent. Several models
of life-history theory predict that viviparity is associated with
increased female size, and thus more female-biased sexual size dimorphism
(SSD). Yet the corresponding empirical evidence is overall weak and
inconsistent. The lizard Zootoca vivipara, which occupies a major part of
Northern Eurasia and includes four viviparous and two non-sister oviparous
lineages, represents an excellent model for testing these
predictions.  We analysed how sex-specific body size and SSD is
associated with parity mode, using body length data for nearly 14,000
adult individuals from 97 geographically distinct populations, which cover
almost the entire species’ range and represent all six lineages. Our
analyses controlled for lineage identity, climatic seasonality (the
strongest predictor of geographic body size variation in previous studies
of this species), and several aspects of data heterogeneity. 
Parity mode, lineage, and seasonality are significantly associated with
female size and SSD; the first two predictors accounted for 14–26% of the
total variation each, while seasonality explained 5–7%. Viviparous
populations exhibited a larger female size than oviparous populations,
with no concomitant differences in male size. Variation of male size was
overall low and poorly explained by our predictors.  Albeit fully
expected from theory, the strong female bias of the body size differences
between oviparous and viviparous populations found in Z. vivipara is not
evident from available data on three other lizard systems of closely
related lineages differing in parity mode. We confront this pattern with
the data on female reproductive traits in the considered systems and the
frequencies of evolutionary changes of parity mode in the corresponding
lizard families and speculate why the life-history correlates of
live-bearing in Z. vivipara are distinct.  Comparing conspecific
populations, our study provides the most direct evidence for the predicted
effect of parity mode on adult body size but also demonstrates that the
revealed pattern may not be general. This might explain why across
squamates, viviparity is only weakly associated with larger size.