Incubation temperature and phenotypic traits of Sceloporus undulatus: implications for the northern limits of distribution |
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Authors: | Scott L Parker Robin M Andrews |
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Institution: | (1) Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0406, USA;(2) School of Biological Sciences, University of Sydney, Heydon-Laurence Building (A08), Sydney, NSW, 2006, Australia |
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Abstract: | Cold environmental temperature is detrimental to reproduction by oviparous squamate reptiles by prolonging incubation period,
negatively affecting embryonic developmental processes, and by killing embryos in eggs directly. Because low soil temperature
may prevent successful development of embryos in eggs in nests, the geographic distributions of oviparous species may be influenced
by the thermal requirements of embryos. In the present study, we tested the hypothesis that low incubation temperature determines
the northern distributional limit of the oviparous lizard Sceloporus undulatus. To compare the effects of incubation temperature on incubation length, egg and hatchling survival, and hatchling phenotypic
traits, we incubated eggs of S. undulatus under temperature treatments that simulated the thermal environment that eggs would experience if located in nests within
their geographic range at 37°N and north of the species’ present geographic range at latitudes of 44 and 42°N. After hatching,
snout–vent length (SVL), mass, tail length, body condition (SVL relative to mass), locomotor performance, and growth rate
were measured for each hatchling. Hatchlings were released at a field site to evaluate growth and survival under natural conditions.
Incubation at temperatures simulating those of nests at 44°N prolonged incubation and resulted in hatchlings with shorter
SVL relative to mass, shorter tails, shorter hind limb span, slower growth, and lower survival than hatchlings from eggs incubated
at temperatures simulating those of nests at 37 and 42°N. We also evaluated the association between environmental temperature
and the northern distribution of S. undulatus. We predicted that the northernmost distributional limit of S. undulatus would be associated with locations that provide the minimum heat sum (∼495 degree-days) required to complete embryonic development.
Based on air and soil temperatures, the predicted northern latitudinal limit of S. undulatus would lie at ∼40.5–41.5°N. Our predicted value closely corresponds to the observed latitudinal limit in the eastern United
States of ∼40°N. Our results suggest that soil temperatures at northern latitudes are not warm enough for a sufficient length
of time to permit successful incubation of S. undulatus embryos. These results are consistent with the hypothesis that incubation temperature is an important factor limiting the
geographic distributions of oviparous reptile species at high latitudes and elevations. |
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Keywords: | Sceloporus undulatus Distributional limits Cold climate hypothesis Degree-days Egg incubation |
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