Population dynamics of three lizard species from the genus Sceloporus:short-term changes in demographic parameters

Most demographic studies focus on numerical changes that occur within populations across years.However,typically studies at an interannual scale do not provide information on the particular times of the year(particu­lar months or seasons)when rates of survival,recruitment,or migration increase or decrease due to physiolog­ical,behavioral or ecological processes.These monthly or seasonal changes in demographic parameters may lead to substantial variations in population abundance.In this study,we collected capture-mark-recapture data on 3 species of lizards of the genus Sceloporus(Sceloporus torquatus,Sceloporus grammicus and Sceloporus megalepidurus)found in ecologically similar habitats to examine potential changes in demographic rates among 3 different climatic seasons:rainy,cold-dry and warm-dry seasons.We tested different hypotheses about the ef­fect of these seasons on survival,recruitment of new adults,and temporary emigration.We found that during the season with severe thermal constraints,the cold-dry season,survival of S.torquatus decreased markedly.We also detected a considerable increase in the recruitment rate of S.grammicus during the rainy season,when these lizards are establishing territories and finding mates.In contrast,we found no evidence of intra-annual changes in the rate of temporary emigration.In addition,we calculated abundance and population growth rates for each species and for each season.Our study represents a significant contribution to the understanding of in­tra-annual demographic variation in lizards.

Habitat use in eight populations of Sceloporus grammicus(Squamata:Phrynosomatidae)from the Mexican Plateau

Studies on habitat use have often helped explain observed variation in morphology,behavior and reproductive characteristics among populations within a single species.Here we analyze morphological and ecological characteristics of individuals from the Sceloporus grammicus species complex from 7 different localities(CER,El Cerezo;PAC,Pachuca;HUI,Huichapan;EZA,Emiliano Zapata;SMR,San Miguel Regla;LMJ,La Mojonera;and LMZ,La Manzana)in the state of Hidalgo,and one locality(Cahuacán)in the State of Mexico.A canonical correspondence analysis(CCA)showed that females from PAC,EZA,LMZ,HUI,SMR and CAH populations use similar microhabitats characterized mostly by bare soil,in females from LMJ and CER use microhabitats characterized primarily by vegetation and rocks.Females were observed using 12 different types of perches.With regard to perch height use,the CCA showed that females from PAC,LMJ,LMZ,SMR,CER and CAH populations were correlated with height to nearest perch(HNP),in the rest of the females were not related to any perch use variable.In contrast,the CCA showed that males from PAC,LMJ and CAH were characterized by microhabitats with higher vegetal coverage,while males from LMZ and CER used microhabitats composed of bare soil,but males from HUI and SMR populations used microhabitats composed chiefly of bare soil and rocks.With respect to perch height use,the CCA showed that males from PAC,LMJ,EZA and LMZ were correlated with distance to the nearest perch,but the rest of the males were not correlated with any perch use variables.Males were observed in 9 different perch types.The males were larger than the females in all morphological variables analyzed.Moreover,in both sexes the snout-vent length is positively correlated with all morphological variables,and although both the slope and ordinate of the origin of all morphological variables were larger in males than females,the analysis of covariance indicated that there is no increase in the morphological variables with increasing SVL between sexes.Our results suggest that variation in habitat use and morphology among populations is an adaptive response(phenotypic plasticity)to the environmental conditions where these populations of Sceloporus grammicus occur.

Endocrine stress response of Eastern Fence Lizards in fire-disturbed landscapes

Landscape disturbances can alter habitat structure and resource availability,often inducing physiological responses by organisms to cope with the changing conditions.Quantifying the endocrine stress response through measurement of glucocorticoids has become an increasingly common method for determining how organisms physiologically respond to challenges imposed by their environment.We tested the hypothesis that Eastern Fence Lizards cope with fire disturbance effects by modulating their secretion of corticosterone(CORT).We measured the baseline and stress-induced plasma CORT of male Eastern Fence Lizards in a chronosequence of fire-altered habitats(recently burned,recovering from burn,and unburned).Although habitat use by lizards differed among burn treatments,including differences in use of canopy cover,leaf litter,and vegetation composition,we did not detect a significant effect of fire-induced habitat alteration on plasma CORT concentration or on body condition.In addition,we found no effect of blood draw treatment(baseline or stress-induced),body temperature,body condition,or time taken to collect blood samples on concentration of plasma CORT.Low intensity burns,which are typical of prescribed fire,may not be a sufficient stressor to alter CORT secretion in Eastern Fence Lizards(at least during the breeding season).Instead,lizards may avoid allostatic overload using behavioral responses and by selecting microsites within their environment that permit thermoregulatory opportunities necessary for optimal performance and energy assimilation.

Oxygen supply did not affect how lizards responded to thermal stress

Zoologists rely on mechanistic niche models of behavioral thermoregulation to understand how animals respond to climate change.These models predict that species will need to disperse to higher altitudes to persist in a warmer world.However,thermal stress and,thus,thermoregulatory behavior may depend on atmospheric oxygen as well as environmental temperatures.Severe hypoxia causes animals to prefer lower body temperatures,which could be interpreted as evidence that oxygen supply limits heat tolerance.Such a constraint could prevent animals from successfully dispersing to high elevations during climate change.Still,an effect of oxygen supply on preferred body temperature has only been observed when oxygen concentrations fall far below levels experienced in nature.To see whether animals perceive greater thermal stress at an ecologically relevant level of hypoxia,we studied the thermoregulatory behavior of lizards(Sceloporus tristichus)exposed to oxygen concentrations of 13%and 21%(equivalent to PO2 at 4000 m and 0 m,respectively).In addition,we exposed lizards to 29%oxygen to see whether they would accept a higher body temperature at hyperoxia than at normoxia.At each oxygen level,we measured a behavioral response to heat stress known as the voluntary thermal maximum:the temperature at which a warming animal sought a cool refuge.Oxygen concentration had no discernable effect on the voluntary thermal maximum,suggesting that lizards experience thermal stress similarly at all 3 levels of oxygen(13%,12%and 29%).Future research should focus on thermoregulatory behaviors under ecologically relevant levels of hypoxia.

Resting metabolic rates increase with elevation in a mountain-dwelling lizard

ndividuals that inhabit broad elevational ranges may experience unique environmental challenges.Because temperature decreases with increased elevation,the ectotherms living at high elevations have to manage limited activity time and high thermoregulatory effort.The resting metabolic rate(RMR)of a postabsorptive animal is related to its total energy requirements as well as many other fitness traits.Mesquite lizards(Sceloporus grammicus)living on La Malinche Volcano,Mexico,inhabit a wide elevational range with some populations apparently thriving above the tree line.We measured the RMR of lizards from different elevations(i.e.,2,600,3,200,and 4,100 m)at four ecologically relevant temperatures(i.e.,15,25,30,and 35°C)and found that RMR of mesquite lizards increased with temperature and body mass.More importantly,lizards from the high-elevation population had mass specific RMR that was higher at all temperatures.While the higher RMRs of high-elevation populations imply higher metabolic costs at a given temperature these lizards were also smaller.Both of these traits may allow these high elevation populations to thrive in the face of the thermal challenges imposed by their environment.

Would behavioral thermoregulation enable pregnant viviparous tropical lizards to cope with a warmer world?

Sceloporus lizards depend on external heat to achieve their preferred temperature(Tsel)for performing physiological processes.Evidence both in the field and laboratory indicates that pregnant females of this Genus select body temperatures(Tb)lower than 34℃ as higher temperatures may be lethal to embryos.Therefore,thermoregulation is crucial for successful embryo development.Given the increase in global air temperature,it is expected that the first compensatory response of species that inhabit tropical climates will be behavioral thermoregulation.We tested whether viviparous Sceloporus formosus group lizards in the wild exhibited differences in thermoregulatory behavior to achieve the known Tsel for developing embryos regardless of local thermal conditions.We quantified field active body temperature,thermoregulatory behavior mechanisms(time of sighting,microhabitat used and basking time)and available microhabitat thermal conditions(i.e.operative temperature)for 10 lizard species during gestation,distributed along an altitudinal gradient.We applied both conventional and phylogenic analyses to explore whether Tb or behavioral thermoregulation could be regulated in response to different thermal conditions.These species showed no significant differences in field Tb during gestation regardless of local thermal conditions.In contrast,they exhibited significant differences in their behavioral thermoregulation associated with local environmental conditions.Based on these observations,the differences in thermoregulatory behavior identified are interpreted as compensatory adjustments to local thermal conditions.We conclude that these species may deal with higher temperatures predicted for the tropics by modulating their thermoregulatory behavior.