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.
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Male common lizard basking.
© James Lindsey/Wikimedia
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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.
Literature
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.
Labels: cold climate hypothesis, evolution, maternal manipulation hypothesis, squamata