Behaviors that allow desert lizards to avoid scorching environments ensure survival, but the trade-off can be the loss of time for feeding and finding mates. A four-year study on two sand-dwelling lizard species in Argentina’s Monte Desert reveals the hidden costs of behavioral thermoregulation. The researchers tracked 376 adult lizards and found that environmental temperatures fell within the lizards’ preferred range only 3.2% to 10.8% of the time.
Despite these harsh conditions, both species maintained their body temperatures with remarkable precision. However, during the hottest periods, they were forced to retreat into the shade or burrow beneath the sand. This restricted their potential activity time by 18.3% to 50.0%; the endangered species Liolaemus scapularis lost half of its potential activity time during the wet season. This ecological cost is often overlooked when focusing solely on the success metric of “effective body temperature regulation.”
30-Second Summary
- The researchers measured field body temperatures in 376 lizards: 235 L. scapularis and 141 L. riojanus.
- Experiments on preferred temperature were conducted on 65 individuals, and experiments on critical thermal maximum (CTmax) were conducted on 37 individuals.
- Out of 3,927 environmental model records, only 3.2–10.8% fell within the preferred temperature range.
- Although both species exhibited high thermoregulatory efficiency, they lost 18.3–50.0% of their potential activity time due to heat.
- The observations reflect current behavioral buffering; the study did not directly measure future population decline rates.
Comparing two related species across two desert seasons
The study focused on sister species inhabiting sandy environments in northwestern Argentina: Liolaemus scapularis and L. riojanus. The habitats of the two populations are approximately 700 km apart, yet they share the same broad climatic context of the Monte Desert, characterized by annual precipitation of less than 200 mm. Between 2018 and 2022, the research team combined field surveys with laboratory-based thermal gradient experiments conducted during both dry and wet seasons.
| Item | L. scapularis | L. riojanus |
|---|---|---|
| Common name (English) | Shoulder Tree Iguana | Riojan Tree Iguana |
| Classification | Order Squamata, Family Liolaemidae | |
| Distribution | Catamarca, Tucumán, and Salta, Argentina | La Rioja and San Juan regions, Argentina |
| IUCN | Endangered (EN), 2016 | Least Concern (LC), 2016 |
| Field body temperature samples | 235 individuals | 141 individuals |
What was measured and how?
Active adult lizards were captured, and their cloacal body temperatures were measured within 20 seconds; they were then marked to prevent recapture and released back into the wild. Simultaneously, thermal models mimicking the size and shape of the lizards were placed in sunny areas, light shade, and deep shade to record the environmental temperatures (Te) available to the animals. After adjusting for microhabitat bias, 3,927 records remained for analysis.
For a separate group of 65 lizards, preferred temperature ranges (Tset) were calculated by repeatedly measuring the body temperatures they chose for themselves across a 20–60°C thermal gradient. Among them, 37 individuals were used to determine the critical thermal maximum (CTmax) by raising their body temperature at a rate of 0.8°C per minute and observing the point at which they could no longer right themselves after being turned over. This allowed for a simultaneous comparison of field body temperatures, available environmental temperatures, preferred temperatures, and physiological limits.
Body temperatures were maintained despite unfavorable environmental conditions.
Environmental models fell within the preferred temperature range for L. scapularis 3.2% of the time in the dry season and 10.8% in the wet season; for L. riojanus, the figures were 6.1% and 6.6%, respectively. Nevertheless, the proportion of actual lizard body temperatures falling within the preferred range was approximately 20–25%. The act of selecting specific shade and times of day resulted in thermal conditions that were 2.3 to 7.8 times more favorable than the ambient environment.
The thermoregulation efficiency index (E) exceeded 0.75 on average for both species. However, the hottest environmental temperatures at the sites were approximately 14.5–14.9°C higher than the average CTmax, resulting in a negative calculated “warming margin.” This does not imply that the lizards could survive just anywhere; rather, it indicates that they maintained their observed body temperatures only by actively avoiding dangerous times and locations.
The price of the ‘thermal shield’ was activity time.
Activity restriction in L. scapularis jumped from 18.3% in the dry season to 50.0% in the wet season; the corresponding figures for L. riojanus were 31.8% and 38.5%. L. scapularis exhibited seasonal plasticity by raising its preferred temperature by 1.6°C during the dry season; however, in the wet season, even shaded areas remained hot for extended periods, diminishing the cooling effect of refuges. Researchers viewed this behavioral avoidance as a ‘shield’ but interpreted the trade-off as a loss of time otherwise available for foraging, reproduction, and territorial defense.
Why it matters
When assessing climate vulnerability, focusing solely on physiological thermal limits overlooks an animal’s ability to behaviorally avoid danger. Conversely, focusing only on the ability to maintain a stable body temperature in the wild overlooks the cost of spending half the day in hiding. This study demonstrates that thermal heterogeneity—such as shade in microhabitats, sand depth, and vegetation structure—is not merely a convenience but a resource that preserves activity time.
These figures should not be directly applied to captive environments. The preferred temperatures and behaviors of these two wild species do not serve as a universal temperature chart for other species in captivity. The only applicable principle is the general guideline that ectotherms require access to temperature gradients and refuges where they can make their own choices.
Limitations and Points to Avoid Overinterpreting
- Body temperatures were measured only for individuals found while active; thus, individuals that were completely inactive underground were excluded. The research team identified this as ‘active lizard bias.’
- The activity restriction rate represents the proportion of time that modeled environmental temperatures exceeded the upper thermal preference limit, rather than a direct measurement of actual food intake or reproductive success.
- The study sites represent limited populations of each species, and the findings cannot be generalized to either species as a whole or to the entire Monte Desert.
- This study measured current behavioral buffering; it did not directly predict the extent of future population decline due to warming.
Four Key Takeaways for Readers
- While the 376 individuals successfully maintained appropriate body temperatures, the suitability of the environment itself was only 3.2–10.8%.
- Behavioral thermoregulation was effective, though it reduced activity time by 18.3–50.0%.
- Endangered species L. scapularis faced a 50% reduction in potential activity time during the wet season.
- Conservation efforts must address not only average temperatures but also the duration of actually cool microhabitats.
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