Abstract
Live-bearing reproduction (viviparity) has evolved from egg-laying (oviparity) independently many times and most abundantly in squamate reptiles. Studying life-history trade-offs between the two reproductive modes is an inherently difficult task, as most transitions to viviparity are evolutionarily old and/or are confounded by environmental effects. The common lizard (Zootoca vivipara) is one of very few known reproductively bimodal species, in which some populations are oviparous and others viviparous. Oviparous and viviparous populations can occur in sympatry in the same environment, making this a unique system for investigating alternative life-history trade-offs between oviparous and viviparous reproduction. We find that viviparous females exhibit larger body size, smaller clutch sizes, a larger reproductive investment, and a higher hatching success rate than oviparous females. We find that offspring size and weight from viviparous females was lower compared to offspring from oviparous females, which may reflect space constraints during pregnancy. We suggest that viviparity in common lizards is associated with increased reproductive burden for viviparous females and speculate that this promoted the evolution of larger body size to create more physical space for developing embryos. In the context of life-history trade-offs in the evolution of viviparity, we suggest that the extent of correlation between reproductive traits, or differences between reproductive modes, may also depend on the time since the transition occurred.
Highlights
Live-bearing reproduction is one of the most ubiquitous life-history transitions across the animal kingdom (Sites et al 2011)
Viviparous females were significantly larger than oviparous females, as measured by body length (N = 428, F = 98.2, η2 = 0.19, P < 0.0001; Fig. 2a)
Clutches laid by viviparous females had on average almost one offspring fewer (∆ = 0.92) than clutches laid by oviparous females (N = 436, F = 40.9, η2 = 0.05, P < 0.0001; Table 1; Fig. 3a; Table S1)
Summary
Live-bearing reproduction is one of the most ubiquitous life-history transitions across the animal kingdom (Sites et al 2011). It has evolved independently from egg-laying more than 150 times across all vertebrates (Shine 2005; Blackburn 2006) and numerous times among invertebrates. Experimental case studies of squamates support this hypothesis (Rodríguez-Díaz and Braña 2012), though many examples exist of tropical viviparous species (Tinkle and Gibbons 1977; Vitt and Blackburn 1983; Webb et al 2006), which suggests other life-history trade-offs are important (Webb et al 2006). The causes are even less understood, mainly due to the limited number of transitions and the difficulty to separate correlative variables from causative factors (Wourms and Lombardi 1992; but see Bassar et al 2014)
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