It takes a lot to survive the extreme temperatures of the desert, but the lizards of the genus Acanthodactylus have managed it pretty well. I used to see them darting between spindly bushes quite frequently in the afternoons and early mornings in the desert. Stay out much later and they’d pretty much evaporate on the spot in the Middle Eastern sun.

The Acanthodactylus genus is widespread from the Iberian peninsula through Africa to the Middle East. Schmidt’s fringe-toed lizard inhabits the Arabian peninsula, as well as southern Iran and Iraq, and is common throughout arid areas here. These lizards are said to be the most beautifully coloured of the genus, tan in colour with small white spots along the flanks. They are listed as ‘Least Concern’ by the IUCN.

Schmidt’s fringe toed lizard narrowing its eyes against a small sandstorm.

The fourth toe on the hind foot is enlarged and has extended scales giving rise to a comb like ‘fringe’. Like many other desert dwelling reptiles, these lizards use the burrows as a refuge from predators and from temperatures too high or too low for their activity on the sand surface. They normally make their burrows around the roots of desert shrubs and other vegetation.

Fringe like scales on the toes are just visible.

The lizards regulate their activity patterns with what is known as a circadian rhythm – a daily cycle of activity around a 24-hour period, influenced by regular variations in the environment. Human sleep cycles are also dictated by a circadian rhythm. Some desert species emerge from their burrows and return at around the same time each morning and night using an internal clock, as temperatures will vary widely at these times across the year. Other species emerge and retire at specific temperatures, so their times of activity may vary from one day to the next depending upon thermal conditions or seasons.

Eyeing me up from within one of small shrubs where they tend to make burrows.

Fringe toed lizards have been shown to have rhythms more in line with differences in temperature, as the winters in Arabia can actually be (relatively) quite cold. There is likely a selective advantage of temperature-sensitive emergence as heading out at the same time year round would be sub-optimal for basking in winter, in turn reducing thermoregulation efficiency.

Desert habitat (left) and the tracks (right) of the fringe toed lizard.

Thermoregulation is important in reptiles as they are cold blooded and so processes such as metabolism (related to body temperature) are influenced strongly by environmental conditions. Juveniles and sub-adults of this species were found to have higher rates of oxygen consumption (a proxy for metabolism) than adult lizards. This is due mostly to higher growth rates in these groups as well as the general rule that smaller animals have higher metabolic rates than larger ones.

I guess they aren’t too different from us really, we do the same thing and head for our burrows when its too cold at night or too hot to be out in the sun!

Quick facts:

Native to the Arabian peninsula.
Gets its name from the enlarged comb like scales on its feet which help locomotion on sand.
Circadian rhythm is attuned mostly to temperature rather than daylight differences.

Further reading:

Constantinou, C., & Cloudsley‐Thompson, J. L. (1985). The circadian rhythm of locomotory activity in the desert lizard Acanthodactylus schmidti. Biological Rhythm Research, 16(2), 107-111.
Focal paper of the tale. Here they use an aktograph in order to study the activity patterns of the lizards. They had a treatment of 12 hours on and 12 off for light, with fluctuating temperatures and with a ‘square wave’ of temperature change (12h of 16oC and 12h of 31oC). The paper is a bit weird and very short which actually makes it a bit difficult to understand. There is barely any research with this species though!

Al-Sadoon, M. K., & Abdo, N. M. (1991). Temperature and body mass effects on the metabolic rate of Acanthodactylus schmidti Weigmann (Reptilia: Lacertidae). Journal of arid environments, 21(3), 351-361.
Briefly mentioned in the tale, this paper uses wild caught lizards and measures their Oxygen consumption to establish metabolic rates. As mentioned they found sub adult and juvenile lizards have higher metabolic rates than the adults. They took the metabolic rates in 5oC intervals from 10-35oC.

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