|Short-Horned Lizard (Phrynosoma douglassii)|
DESCRIPTION: L=3" (7.6cm) including tail. Overall color varies with habitat from tan to gray. Flattened body is fringed with single row of soft spines. The horns on the very back of the head are short (thus its name) compared to the Regal Horned Lizard (see bottom image left).
NATURAL HISTORY: Carnivorous, feeding on arthropods and other small invertebrates. Diurnal. Short-horned Lizards are relatively cold-tolerant, thus occur at higher elevations than other horned lizards. One adaptation they have to living at higher elevations is to retain their eggs within their body and to give birth to live young (see To Lay or Not to Lay Eggs below).
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lay eggs? So that you don't have to haul them around with you.
the option? Retain the eggs within your body (ovoviviparous versus oviparous)
so that you can keep them safer and use behavioral thermoregulation (shuttling
in and out of the sun, orienting body toward sun, etc) to keep them warmer
so that they develop faster and hatch sooner.
Solution. All birds lay eggs (weight, after all, is critical to flight). But not all reptiles lay eggs. Around Tucson, lizards that live in the warmer desert habitats (e.g., Regal Horned Lizard and Desert Spiny Lizard) lay eggs (dump the weight and space) while the lizards in the higher elevations (e.g., Short-horned Lizard and Mountain Spiny Lizard) give birth to live young (to ensure the eggs stay warm enough).