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ASU Study Reveals Unusual Adaptation In Gila Monster Hatchlings

By Nicholas Gerbis
Published: Tuesday, July 10, 2018 - 4:59pm
Updated: Wednesday, July 11, 2018 - 8:36am

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Gila monster
Dale DeNardo/Arizona State University
A Gila monster from the Harquahala Mountains in southwestern Arizona.

Some days it just doesn't pay to get out of bed. Now, an Arizona State University study shows how newborn Gila monsters (Heloderma suspectum) take that sentiment to a new level.

The findings appear in Proceedings of the Royal Society B.

Animals are not always born at the optimum time for survival. To cope, nature offers some creatures "snooze buttons" — putting off leaving the nest, delayed egg hatching and embryonic diapause, which "pauses" embryonic development.

In many cases, these strategies allow animals to "overwinter" — pass through hard times and emerge when conditions improve and supplies grow more plentiful. Bears hibernate, and many insects overwinter as adults, pupae or eggs.

But, among hatchlings, nest overwintering was once found only in aquatic turtles facing frigid winters.

The new study suggests baby Gila monsters overwinter and then some, remaining in their nests for 7-10 months after hatching.

Lead author Dale DeNardo of Arizona State University's School of Life Sciences says the reason could come down to food.

baby Gila monsters
Dale DeNardo/Arizona State University
Five of the baby Gila monsters inadvertently excavated from their nest at a construction site. The hatchlings were saved by concerned citizens and incorporated into the study.

"There are no small lizards laying eggs in the winter or even in the early spring, and so the value of coming to the surface, looking for food — therefore wasting energy and putting yourself vulnerable to predators — there isn't a value of doing that," DeNardo said.

The largest lizards in the U.S., Gila monsters typically lay eggs during the onset of the summer monsoon in July, to help keep them from drying out.

Based on observed incubation times, these eggs ought to hatch in the autumn. But newborn Gila monsters do not appear until late April through early August — the hottest time of year, when adult Gila monsters reduce their activity.

Skipping fall and winter makes sense.

Gila monsters raid nests for a living; they feed on the offspring of other vertebrates, mainly small lizards, which don't breed in the Sonoran Desert at that time. In spring, birds and small mammals breed, and their nests contain prey too large for hatchling Gila monsters to eat. That leaves summer.

The paper also suggests night-time temperature could contribute to the late emergence of hatchlings. As summer heats up, adult Gila monsters become chiefly nocturnal, and hatchlings might benefit from the cover of darkness and the warmth of summer nights.

Determining the role each factor plays will require further research. Either way, a summer emergence, however hot, might offer Gila monster hatchlings the best chance of survival.