DIAZ, J.A. 1993. Breeding coloration, mating opportunities, activity, and survival in the lacertid lizard Psammodromus algirus. Canadian Journal of Zoology. 71: 1104-1110

I studied the activity, spacing patterns, courtship behavior and survival of males (classified into two groups according to the development of the head's sexual coloration) and females from a population of the lacertid lizard Psammodromus algirus. Brightly colored (C+) males were significantly larger (and probably older) than dully colored (C-) males, but the results related to behavioral aspects differed between study years. In 1989, C+ males had larger home ranges, were more active, overlapped more females and courted them more frequently than C- males. In 1990, none of these traits differed between the two male categories. The variables associated with survival were different in the two sexes. Larger and more active males survived less well than smaller and less active ones, whereas for females survival was associated with later observation times during the day. Mortality could be due to a higher predation risk affecting animals that were active more days or under less favorable conditions. I postulate that the increased activity of males (mainly C+ category in years favoring stronger selection) would increase their short-term reproductive success but negatively affect their survival.

DIAZ, J.A. y CARRASCAL, L.M. 1993. Variation in the effect of profitability on prey size selection by the lacertid lizard Psammodromus algirus. Oecologia 94: 23-29.

Maximizing the average rate of energy intake (profitability) may not always be the optimal foraging strategy for ectotherms with relatively low energy requirements. To test this hypothesis, we studied the feeding behaviour of captive insectivorous lizards from a population of Psammodromus algirus, and we obtained experimental estimates of prey mass, handling time, profitability, and attack distance for several types of prey. Handling time increased linearly with prey mass and differed significantly among prey types when controlling for prey size differences, and mean profitabilities differed among prey taxa, but profitability was independent of prey size. The attack distance increased with prey length and with the mobility of prey, but it was unrelated to profitability. Thus, lizards did not seem to care about the rate of energy intake per second as a proximate cue eliciting predatory behavior. This information was combined with pitfall trap based censuses of prey (in late April, mid June, and late July) that allowed us to compare the mass of the prey captured in the environment with that of the arthropods found in the stomachs of sacrificed free-living lizards. In April, when food abundance was low and lizards were reproducing, profitability had a pronounced effect on size selection and lizards selected prey larger than average from all taxa except the less profitable ones. As the activity season progressed, and with a higher availability of food, the number of prey per stomach decreased and their mean size increased. The effect of profitability on size selection decreased (June) and eventually vanished (July-August). This variation could probably be related to seasonal changes in the ecology of lizards, e.g. time minimization in the breeding season as a means of saving time for nonforaging activities vs. movement minimization by selecting fewer (but larger) prey in the postbreeding season. Thus, the hypothesis that maximizing profitability could be just an optional strategy for a terrestrial ectothermic vertebrate was supported by our data.

BELLIURE, J., CARRASCAL, L. M: y DIAZ, J. A. 1996. Covariation of thermal biology and foraging mode in two Mediterranean lacertid lizards. Ecology 77: 1163-1173.

Body temperatures, heat exchange rates, behavioral thermoregulation, and movement behavior (as an index of foraging mode) were studied in two widely distributed, medium sized lacertid lizards (Acanthodactylus erythrurus and Psammodromus algirus). Psammodromus algirus mainly inhabits broad-leaved forests, while A. erythrurus prefers open sandy areas with sparsely distributed vegetation. These habitat preferences parallel differences between the areas in which both genera presumably originated: Eurosaharian xeric steppes with high operative temperatures (Te’s) for Acanthodactylus, and Mediterranean open forests with lower Te’s for Psammodromus.

Field observations showed that percentage of time spent basking and basking rate (no. of basks per minute) were negatively related to Te, although average bask duration was not. Percentage of time spent moving, moving rate (no. of moves per minute), and the average duration of individual moves were inversely related to Te, and were higher in P. algirus. The percentage of total locomotion time that was spent moving in the shade was also higher in P. algirus. Behavioral thermoregulation strategies differed between both species in a laboratory thermogradient where P. algirus basked more often and for shorter periods, and selected warmer patches, than A. erythrurus. Selected body temperatures (Tb’s) in a laboratory thermogradient were significantly higher in A. erythrurus than in P. algirus. Shade Seeking Tb was higher in A. erythrurus, while Resume Basking Tb did not differ significantly between the two species. Heating and cooling rates were also different in the two species: A. erythrurus warmed more slowly, and cooled faster, than P. algirus.

Our data support the existence of a complex syndrome which combines aspects of the behavior, physiology, and ecology of both species, so that the thermal consequences of inhabiting a certain type of habitat can be counterbalanced by behavioral and physiological means that, in turn, affect movement (and hence foraging) behavior. Thus, the more active species (P. algirus) heated faster, cooled more slowly, and basked more often but for shorter periods and at warmer patches than the less active one (A. erythrurus).

DIAZ, J. A., BAUWENS, D. y ASENSIO, B. 1996. A comparative study of the relation between heating rates and ambient temperatures in lacertid lizards. Physiological Zoology 69: 1359-1383.

(Pulse aquí para ver las diapositivas y notas de una presentación de este trabajo en el III Congreso Mundial de Herpetología, Praga 1997).

It is well known that lizard species from thermally diverse areas show differ in their use of thermoregulatory behaviors as a means of making short-term adjustments to differences in ambient conditions.. In contrast, the extent of long-term adjustments in thermal physiology is poorly documented and still under debate. In this study we report a clear relationship between heating rates and environmental temperatures among eight species in a homogeneous clade of lacertid lizards. Phylogenetically-based analyses of covariance demonstrates that species with a northern or montane distribution warm at a faster mass-specific rate than lacertid species from a southern (Mediterranean) climatic zone. Correlational analyses using independent contrasts confirm that mass-specific heating rates increase with environmental temperatures at the lizards' capture site. A reduction of the time spent warming, associated with higher heating rates, should be especially advantageous for lizards from cool climates, which bask for large amounts of time. Thus, we suggest that the relation between heating rate and climatic conditions could be a result of adaptive evolution. The behaviorally-selected preferred temperatures were lower, but not significantly so, in the northern/montane species set; interspecific correlations with ambient temperatures were positive, but not significant. However, the divergence of heating rates and the statistically nonsignificant difference in preferred temperatures have similar impacts in the estimated amount of time spent heating. Thus, differences in both heating rate, a physiological trait, and preferred temperature, a behavioural characteristic, contribute equally to variation in heating times..

DIAZ, J. A. 1997. Ecological correlates of the thermal quality of an ectotherm's habitat: a comparison between two temperate lizard populations. Functional Ecology 11: 79-89.

1. Although thermal factors are of paramount importance to the quality of the habitats occupied by small ectotherms (e.g. lizards), the hypothesis that the relative abundance of squamate populations is related to the thermal quality of their habitats remains untested. If differences in the availability of thermally suitable microhabitats was the primary determinant of the overall quality of a lizard’s habitat, population density in a habitat should be proportional to its thermal quality.

2. I compared the influence of the thermal quality of a habitat on thermoregulatory behaviour, body temperatures, expected physiological performance, and relative abundance in two populations of the temperate lizard Psammodromus algirus separated by 700 m altitude in central Spain. Habitat thermal quality was estimated with an index of how closely the available operative temperatures in a habitat match the range of body temperatures that lizards attempt to maintain in a laboratory thermogradient (Hertz, Huey & Stevenson 1993).

3. Operative temperatures, measured using copper models, were higher at the lowland site, where a large proportion of readings fell within or above the preferred range, especially in the early morning. Thermal habitat quality was also higher at the low altitude site, with smaller absolute deviations from the preferred range throughout most of the morning hours. Moreover, lizard body temperatures, the accuracy of thermoregulation, and the predicted running performance of lizards were somewhat higher at the lowland site, although differences were smaller than expected on the basis of operative temperatures. A log-linear analysis showed that lizard selectivity for basking sites was more pronounced at the thermally inferior montane site. Thus, the observed differences in the thermal quality of habitats appeared to influence the behaviour of lizards.

4. Nevertheless, the relative abundance of lizards, as estimated by repeated transect counts, was higher at the montane site. This unexpected result suggests that thermal constraints may not be the most important factor determining lizard population densities on a regional scale. Alternatively, food availability and/or predation pressure might have a greater impact on the overall quality of a habitat for temperate lizards.