In a recent publication in Global Ecology and Biogeography we (Anna and many others) examined the selective forces that potentially drive the evolution of viviparity (live-bearing) in squamates.
Vertebrates are known for their versatility in reproductive strategies and features. Those, in turn, facilitated their successful expansion across various types of environments worldwide. For instance, the evolution of shelled eggs promoted the expansion of tetrapods into terrestrial habitats, and the retention of eggs inside the parent’s body significantly improved embryo survivability in harsh environmental conditions. Live-bearing (henceforth ‘viviparity’) evolved across all major vertebrate groups, except birds, crocodilians and turtles. Whereas it evolved only once in the mammal history, it is thought to evolve over 100 times independently in squamates (lizards and snakes), with about 20% of their species being viviparous. The prevalence of both egg-laying (oviparity) and viviparity in many squamate clades, and the multiple origins of viviparity, make squamates an excellent model to study the selective forces behind the evolution and biogeography of reproductive modes.
In this study, we aimed to examine most of the common selective forces hypothesized to drive the evolution of viviparity, and the relationship of reproductive mode with body size. Specifically, we tested the predictions that viviparity will be associated with (1) cold climates, (2) unpredictable climates, (3) high elevations (a proxy for hypoxic conditions), and (4) large adult body sizes. In order to do that, we collated a dataset for over 9,000 squamate species (about 80% of non-marine living species), making it the largest-scale study on the subject. Furthermore, because the factors mentioned above may be associated both directly and indirectly (i.e., through another factor) with reproductive mode, we used methods, such as path analysis, that enable to detect and account for such complex relationships.
Our main finding was that viviparity was strongly associated with cold climates, in line with earlier studies. In fact, there are relatively more viviparous than oviparous species in colder climates, and some of the coldest regions occupied by squamates do not include oviparous species at all. Notably, although some warm regions harbor many viviparous species (much more than the coldest climates), such regions usually include even more egg-laying species. The roles of climatic variation and of elevation were found to be less important and not straightforward. Even though the proportions of viviparity at high elevations are higher, elevation probably exerts various selective pressures and influences the prevalence of viviparity primarily through its effect on temperature. Our findings highlight the complexity of processes potentially underlying the evolution of viviparity, but they also provide clear support for low temperatures as selecting for viviparity in squamates.

Author: Anna Zimin

In a recent publication in Global Ecology and Biogeography GARDians explored the global diversity and distribution of lizard clutch sizes.

We tested the geographic factors that affect clutch sizes in across nearly 4000 lizard species. We found similar patterns to those that have long been known in birds but were never seriously studied in other groups of organisms: lizards lay large clutches at high latitudes and at highly seasonal regions. We postulate that high latitudes with their short, pronounced productivity peals both allow the production of large clutches and promote putting all the eggs in one basket – because the window of opportunity is short in highly seasonal regions. We hypothesize that this may further be a factor preventing taxa with fixed clutch sizes from colonizing high latitudes.

Authors: Shai Meiri, Uri Roll

In a recent publication in the Journal of Animal Ecology we show that the fundamental changes to the mode of life that viviparity brings to squamate females, were surprisingly not reflected in either the number of offspring produced at a single reproductive event (birth, clutch), or their size, or the total mass of offspring produced relative to the size of their mother. The distributions of all these traits in viviparous squamates are remarkably similar to those of oviparous ones. Incidentally we have found that the mass of a recently hatched squamate is (on average, despite much variation) similar to the mass of the egg its mother laid.

In a recently published paper in the Biological Journal of the Linnean Society Shai has shown that the spread of ratios of hatchling or neonate masses to adult masses is very similar across the three classes of amniotes (mammals, birds and, of course, reptiles). This suggests that relatively large offspring are the ancestral and dominant mode of amniotes and have not evolved in response to the elaborate parental care of endotherms

In a recent publication in Global Ecology and Biogeograpy, I present a vast dataset of over 20 body size, ecological, thermal biology, geographic, phylogenetic and life history traits for global lizards.

I hope these data will facilitate more study into the biology of these most fascinating of creatures, and that the database publication will encourage others to add yet more data and to correct errors I surely have made when assembling them.

Author: Shai Meiri

In a recent publication in the Journal of Biogeography we show that Insular lizards with variable clutch sizes follow the predictions of the island syndrome, while lizards with fixed clutches do not.

Author: Rachel Schwarz