Lambert, S. M. and J. J. Wiens. 2013. Evolution of viviparity: A phylogenetic test of the cold-climate hypothesis in phrynosomatid lizards. Evolution 67: 2614–2630.

Viviparity is estimated to have originated more than 100 times in squamate reptiles, making this an excellent group for the study of environmental conditions that favor live birth in ectothermic vertebrates. Fence lizards (Sceloporus) are a large, diverse clade that exhibits both oviparous and viviparous modes of reproduction. The “cold-climate” hypothesis suggests that viviparity is more likely to evolve in cooler environments. If this hypothesis is correct, then lizards in cooler environments (such as at higher elevations) should be more likely to evolve viviparity. Researchers at the University of Arizona tested this hypothesis using a combination of temperature records and phylogenetic comparative methods.

They constructed a phylogenetic tree that included (1) a time-calibrated phylogeny of Sceloporus species, (2) reproductive mode of Sceloporus species on the tips of the tree (solid = oviparous, open = viviparous), (3) node labels reflecting reproductive mode according to the likelihood of reconstructed ancestral states, and (4) color-coded lineages reflecting mean temperature at the warmest quarter of the year, since this is when lizards typically produce eggs or offspring.

Questions

Question 1

In Sceloporus lizards, there is a general trend toward viviparity occurring in colder (blue lineage) environments, with a few exceptions, lending support for the cold-climate hypothesis. Viviparity is advantageous in cold environments because mothers can behaviorally maintain higher temperatures for developing embryos. In addition, increased maternal investment leads to larger offspring among viviparous lizards, so the young are more likely to survive the shorter growing season of cold environments.

Question 2

Advantages of viviparity: It can keep eggs safe. Behavioral thermoregulation can keep embryos at an optimal temperature for development. Increased maternal investment results in larger offspring that are more likely to survive the shorter growing seasons of cooler environments. Disadvantages of viviparity: If the mother dies during the gestation, all the eggs die with her. Each female produces fewer offspring. Females are encumbered with the developing embryos, which can make escape from predators harder. Advantages of oviparity: Females are freed from the weight of eggs, so they can reproduce more often and escape faster in response to predation attempts. They can also produce larger clutches of offspring. Disadvantages of oviparity: Developmental temperature of eggs is dependent on the nest site, which limits the temperature of development in colder environments. Females cannot continue to contribute resources to developing eggs, which results in smaller offspring.

Question 3

In mammals, the evolution of viviparity occurred only once. In squamates, viviparity is estimated to have originated more than 100 times, so there is a larger sample size of independent events for testing hypotheses. The transition in mammals occurred more than 100 million years ago. Many of the transitions to viviparity occurred much more recently in squamates (see the Sceloporus phylogeny), making it easier to reconstruct the probable environmental conditions of the time.

Question 4

(Answers will vary.) An appropriate study design should test or demonstrate that (1) females of viviparous species behaviorally thermoregulate, and internal body temperature is different from nest temperature of oviparous species, and (2) differences in incubation temperature affect the fitness of offspring. An example study design was used by Ji et al. (2007), who submitted females of the common sun skink (Mabuya multifasciata) to five different thermal regimes throughout their gestation period and compared female body temperatures across the thermal regimes and to those of nongravid females. The researchers also measured phenotypic and performance traits of the offspring to assess the relationship between fitness and developmental temperature.

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