Watching a female squamate giving birth or lay eggs is similar in many ways, peristaltic waves move the embryo or the egg to the cloaca and the opening expands to allow the next generation to enter the world. Egg laying is an ancestral trait in squamates - but live birth has evolved numerous times in many different clades. Over the years I have had occasion to incubate Bullsnake, Fox Snake, and Green Snake eggs. In the first two species incubation time was at least eight weeks, while the Green Snake eggs, much to my surprise, hatched within about two weeks of being laid. Female Green Snakes hold their eggs for longer periods of time, the embryo is at a more advanced state of development, before the female deposits them in a nest – a trait thought to be one step in the transition between being oviparous and viviparous.
Transitioning between laying eggs and giving live birth has been considered the result of gradual changes, but it is well known that populations of some egg-laying species have live-birth so gradual may not be the correct adjective. The Viviparous Lizard is such a species. It is widely distributed in Eurasian, from Spain, Italy, Serbia, Bulgaria and Macedonia in the south it ranges northward into the Arctic Circle. In the Alps it can be found as high as 3000 m above sea level. The more southerly populations lay eggs, while populations at higher latitudes have live birth.
Male common lizard basking.
© James Lindsey/Wikimedia
In two recent papers Tania Rodríguez-Díaz of the Universidad de Oviedo, Spain and colleagues have examined the impact of incubation temperature on the young of the Viviparous Lizard, Zootoca vivipara, and the impact of egg retention on the timing of egg laying.
Rodríguez-Díaz et al. (2010) studied variations in the temperature selected by gravid females compared with those selected by males and non-gravid females of Zootoca vivipara of Northern Spain as well as the impact of incubation temperature on the hatchlings. They found cloacal temperatures of gravid females active in the field were lower than those of males and non-gravid females, as well as the temperatures selected in a thermal gradient created in the laboratory (32°C for gravid females; 34°C for males and non-gravid females). Effects of temperature were assessed by incubating eggs at five constant temperatures between 21 and 34°C. The incubation temperatures affected the hatchlings’ morphology and survival rates. Hatchlings incubated at 34 °C had shorter heads than those from other temperatures; and they had lower survival (58%), significantly lower than at the other temperature treatments (mean 93%). Gravid females select lower body temperatures, as might be expected based on the predictions made by the maternal manipulation hypothesis. The shift in preferred temperature by pregnant females would result in only a very short delay, if any, of hatching time and, because the temperature selected by pregnant females is much higher than average temperatures recorded in natural nests. Retaining eggs shortens incubation time, according to predictions of the cold-climate hypothesis and the authors’ found their experiments with the Viviparous Lizard were in agreement with both the maternal manipulation hypothesis and the cold climate hypothesis.
In a second paper, Rodríguez-Díaz and Braña (2011) investigated the Viviparous Lizard’s ability to retain eggs. Female Z. vivipara were forced to retain their eggs by keeping them on dry substrates. They then assessed the effects on embryonic development, hatching success, offspring phenotype, and locomotor performance. Forced egg retention for the additional week affected the developmental stage of embryos at egg laying, as well as hatchling robustness, and locomotor performance. Embryos from forced clutch retention treatment reached one level of development beyond control embryos at oviposition time. Embryos from control eggs were more developed than embryos from experimental eggs after approximately the same period of external incubation, showing that embryonic development is retarded during the period of extended egg retention, despite the high temperature inside the mother's body. The experimental group with forced egg retention had lower hatching success (21.1%) than in the control group (95.4%).
The results suggest retaining eggs interferes with development, and that simply retaining eggs for longer periods of time does not represent a clear advantage to offspring (or their mothers). However, females that can retain eggs during unfavorable climatic conditions can be successful and doing so may be an important step in the transition to live birth.
Rodríguez-Díaz, T., F. González, X. Ji and F. Braña. 2010. Effects of incubation temperature on hatchling phenotypes in an oviparous lizard with prolonged egg retention: are the two main hypotheses on the evolution of viviparity compatible? Zoology 113:233-38.
Rodríguez-Díaz, T., F. and F. Braña. 2011. Plasticity and limitations of extended egg retention in oviparous Zootoca vivipara (Reptilia: Lacertidae). Biological Journal of the Linnean Society 102:75–82.
For an earlier post on this topic go to http://squamates.blogspot.com/2010/09/changing-from-egg-laying-to-live-birth.html.