Although they're known for laying eggs, some snakes and lizards give birth to live young just like mammals do. They supposedly do this to protect their offspring from cold climates. But that hypothesis couldn't explain why some tropical lizards and snakes bear live young. Now, a study of one family of lizards, presented here on Saturday at Evolution 2013, the annual meeting of the American Society of Naturalists, the Society for the Study of Evolution, and the Society of Systematic Biologists, shows that tropical live-bearers hail from high elevations—where it can get cold—and suggests that this reproductive strategy in reptiles may often originate on tropical mountainsides.
The study "is the first [progress] in 30 years on this question," says David Reznick, an evolutionary biologist from the University of California, Riverside, who was not involved in the work.
Shea Lambert became interested in the evolution of live births last year because he was looking for a problem that he could study using a combination of environmental and family tree data. A graduate student at the University of Arizona in Tucson, he scanned the scientific literature for the reproductive styles of 117 Phrynosomatidae lizards. He decided to analyze this group, which includes horned lizards and fence lizards, in part because the species range from cool Canada to warm Central America. Live birth, or viviparity, evolved six times in this group, and Lambert wanted to figure out where and when this change occurred. He arranged all the Phrynosomatidae species on a family tree so that he could determine how they were related to one another and which species were ancestral to the others. Taking information from a global weather database that included GIS information, he marked where each of these species lived—high elevation, lowlands, temperate, or tropical environments—and the local temperature during the breeding season. He also marked where on the family tree viviparity had evolved—more than 40 species are now live-bearing in this group.
Live-bearing makes sense where temperatures dip so low that embryos inside eggs laid in the ground develop slowly or not at all. A female that carries young inside her can move to warmer spots, enabling them to mature faster and with less risk.
The analysis revealed that viviparity originated at high elevations in the lower latitudes, not in temperate regions. For this group, "viviparity is favored in the tropics," Lambert reported at the meeting. Even viviparous lizards now living in temperate climates came from the south, he said to the audience. He also proposed that a lack of gene flow between warm-weather and cold-adapted lizards in the tropics is why live-bearing took hold in the tropics and not in temperate regions.
Tropical lizards that require warm weather year-round and live at low elevations can't cope with cold and rarely survive to breed with their high-mountain relatives. Isolated, those relatives are not held back from shifting to a form of reproduction that protects the young from the cold. Continued mixing with lizards adapted for warmer weather would slow the evolution of live-bearing, if not prevent it altogether, and that could be what's happening in temperate areas, Lambert explained. There, low-altitude lizards are better suited for cold temperatures and thus can survive forays up mountains to breed with lizards that would benefit from having live births.
In the Phrynosomatidae, two live-bearing lizards live in tropical lowlands. But the family tree revealed that these species belong to lines that were once high-altitude dwellers, where they became live-bearing, and then moved to the lowlands more recently, Lambert said.
"I hope that the broad impact of [Lambert's] results is to encourage examination of other warm climate viviparous species" belonging to other lizard and snake families, says Matthew Brandley, an evolutionary biologist at the University of Sydney in Australia, who was not involved with the work.
By solving this mystery, Lambert "has opened the door to other questions," Reznick says. He thinks that Lambert and others should look at what other advantages viviparity might offer besides cold protection. For example, viviparity might enable a female to invest more energy into helping her young grow while they are still inside her or to protect them from predators.
Yarrow's spiny lizard photo © Nature's Archive Photography