Stephen C. Adolph

Temperature, Activity, and Lizard Life Histories

Lizard life-history characteristics vary widely among species and populations. Most authors seek adaptive or phylogenetic explanations for life-history patterns, which are usually presumed to reflect genetic differences. However, lizard life histories are often phenotypically plastic, varying in response to temperature, food availability, and other environmental factors. Despite the importance of temperature to lizard ecology and physiology, its effects on life histories have received relatively little attention. We present a theoretical model predicting the proximate consequences of the thermal environment for lizard life histories. Temperature, by affecting activity times, can cause variation in annual survival rate and fecundity, leading to a negative correlation between survival rate and fecundity among populations in different thermal environments. Thus, physiological and evolutionary models predict the same qualitative pattern of life-history variation in lizards. We tested our model with published life-history data from field studies of the lizard Sceloporus undulatus, using climate and geographical data to reconstruct estimated annual activity seasons. Among populations, annual activity times were negatively correlated with annual survival rate and positively correlated with annual fecundity. Proximate effects of temperature may confound comparative analyses of lizard life-history variation and should be included in future evolutionary models.

Plastic inducible morphologies are not always adaptive: the importance of time delays in a stochastic environment

SummaryWe present a mathematical model for predicting the expected fitness of phenotypically plastic organisms experiencing a variable environment. We assume that individuals experience two discrete environments probabilistically in time (as a Markov process) and that there are two different phenotypic states, each yielding the highest fitness in one of the two environments. We compare the expected fitness of a phenotypically fixed individual to that of an individual whose phenotype is induced to produce the better phenotype in each environment with a time lag between experiencing a new environment and realization of the new phenotype. Such time lags are common in organisms where phenotypically plastic, inducible traits have been documented. We find that although plasticity is generally adaptive when time lags are short (relative to the time scale of environmental variability), plasticity can be disadvantageous for longer lag times. Asymmetries in environmental change probabilities and/or the relative fitnesses of each phenotype strongly influence whether plasticity is favoured. In contrast to other models, our model does not require costs for plasticity to be disadvantageous; costs affect the results quantitatively, not qualitatively.

Growth Plasticity and Thermal Opportunity in 'Sceloporus' Lizards

We studied the relationship between daily active time and growth rate in hatchling sagebrush lizards (Sceloporus graciosus), a mainly montane species, and western fence lizards (Sceloporus occidentalis), which inhabit a broader range of thermal environments. The study involved two populations of each species along in altitudinal gradient in southern California and one population of S. occidentalis from Oregon. In the field daily activity time varied seasonally: activity periods were short during spring and fall and longer during the summer. Activity patterns also varied geographically: in montane and high latitude environments, hatchlings experienced reduced opportunity for growth both daily and seasonally, because fewer, shorter days are available before the end of the activity season. We conducted laboratory experiments to measure the effect of the thermal environment on hatching growth rates. Laboratory—incubated hatchlings had access to radiant energy for part of each day and were kept at 15°C (too lo...

Growth, seasonality, and lizard life histories: age and size at maturity

Temperature influences the activity seasons, reproductive phenology, survival rates, and growth rates of lizards. We present a model of lizard growth that predicts phenotypic patterns of age and size at reproductive maturity in different thermal environments (i.e. different activity seasons). The model predicts a threshold in length of activity season: above this threshold (long season), lizards can mature one year earlier, but at a smaller size, compared to populations with activity seasons below the threshold. This environmentally imposed pattern reflects the proximate consequences of temperature. together with simple rules about the timing of maturation. A key prediction of the model is that age and size at maturity can vary non-linearly with the length of the activity season, and with the timing and duration of egg laying and hatching. We tested these predictions with published data from field studies of the phrynosomatid lizard Sceloporus undulatus, which is geographically widespread and occupies a range of thermal environments. We estimated activity seasons for each population by modeling the links between climates. microclimates and lizard body temperatures using heat-transfer principles. Female age at maturity showed the predicted threshold in length of activity season. whereas female size at maturity did not show the predicted threshold, but instead was negatively correlated with length of activity season. Two prairie populations were exceptions to this pattern: females matured in one year despite their short activity seasons, and consequently matured at an unusually small size. Prairie populations may have evolved differences in growth response and reproductive timing. The thermal environment appears to be an important correlate of life history variation among populations of Sceloporus undulatus.

Estimating maximum performance: effects of intraindividual variation

SUMMARY Researchers often estimate the performance capabilities of animals using a small number of trials per individual. This procedure inevitably underestimates maximum performance, but few studies have examined the magnitude of this effect. In this study we explored the effects of intraindividual variation and individual sample size on the estimation of locomotor performance parameters. We measured sprint speed of the lizard Sceloporus occidentalis at two temperatures (20°C and 35°C), obtaining 20 measurements per individual. Speed did not vary temporally, indicating no training or fatigue effects. About 50% of the overall variation in speed at each temperature was due to intraindividual variation. While speed was repeatable, repeatability decreased slightly with increasing separation between trials. Speeds at 20°C and 35°C were positively correlated, indicating repeatability across temperatures as well. We performed statistical sampling experiments in which we randomly drew a subset of each individuals full set of 20 trials. As expected, the samples maximum speed increased with the number of trials per individual; for example, five trials yielded an estimate averaging 89% of the true maximum. The number of trials also influenced the sample correlation between mean speeds at 20°C and 35°C; for example, five trials yielded a correlation coefficient averaging 90% of the true correlation. Therefore, intraindividual variation caused underestimation of maximal speed and the correlation between speeds across temperatures. These biases declined as the number of trials per individual increased, and depended on the magnitude of intraindividual variation, as illustrated by running sampling experiments that used modified data sets.

Predicting the consequences of dreissenid mussels on a pelagic food web

Although abundant in marine systems, hard substratum fouling organisms with high fecundity and large dispersal capability are rare in freshwater ecosystems. A noteworthy exception is a recent invader of North American lakes and rivers, the zebra mussel, Dreissena polymorpha. Consequences of this novel ecological type in these systems are not fully understood. Using computer simulations we modelled the potential consequences of Dreissena abundance for lake planktonic community structure along a nutrient gradient, using Green Bay of Lake Michigan as an example. We constructed a model of food-web interactions that accurately reproduces abundances of phytoplankton and zooplankton along this nutrient gradient. To this model we added consumption and filtration rates of Dreissena at varying densities. Model results indicate that Dreissena have strong negative effects on large phytoplankton, with chlorophyll reduced up to 80% as compared to the same community without zebra mussels. Results also indicate that Dreissena have relatively small effects on nanoplankton and herbivorous zooplankton. Effects are qualitatively similar along the nutrient gradient, and are strongest at the most eutrophic site. The effects of zebra mussels on the abundance of small phytoplankton and zooplankton were far less than the effects of nutrient levels along the gradient. The effects of zebra mussels on the large phytoplankton, however, were in the same order of magnitude as the effects of the nutrient gradient.

LATITUDINAL AND CLIMATIC VARIATION IN BODY SIZE AND DORSAL SCALE COUNTS IN SCELOPORUS LIZARDS: A PHYLOGENETIC PERSPECTIVE

Squamates often follow an inverse Bergmanns rule, with larger‐bodied animals occurring in warmer areas or at lower latitudes. The size of dorsal scales in lizards has also been proposed to vary along climatic gradients, with species in warmer areas exhibiting larger scales, putatively to reduce heat load. We tested for these patterns in the diverse and widespread lizard genus Sceloporus. Among 106 species or populations, body size was associated positively with maximum temperature (consistent with the inverse of Bergmanns rule) and aridity, but did not covary with latitude. Scale size (inferred from the inverse relation with numbers of scales) was positively related to body size. Controlling for body size via multiple regression, scale size was associated negatively with latitude (best predictor), positively with minimum temperature, and negatively with aridity (similar results were obtained using scores from a principal components analysis of latitude and climatic indicators). Thus, lizards with larger scales are not necessarily found in areas with higher temperatures. Univariate analyses indicated phylogenetic signal for body size, scale counts, latitude, and all climate indicators. In all cases, phylogenetic regression models fit the data significantly better than nonphylogenetic models; thus, residuals for log10 number of dorsal scale rows exhibited phylogenetic signal.

Effect of activity duration on recovery and metabolic costs in the desert iguana (Dipsosaurus dorsalis).

The majority of elevated O(2) consumption associated with short and vigorous activity occurs during recovery, thus an assessment of associated metabolic costs should also examine the excess post-exercise oxygen consumption (EPOC). This study examined O(2) uptake during exercise, EPOC and distance traveled during 5-, 15-, 60- and 300-s sprints at maximal treadmill intensity in Dipsosaurus (N=10; 74.3+/-2.1 g). EPOC (0.08, 0.14, 0.23 and 0.18 ml O(2) g(-1), respectively) was large (80-99% of total elevated O(2) consumption) and increased significantly between 5 and 60 s. The cost of activity (C(act); ml O(2) g(-1) x km(-1)), intended to reflect the total net costs associated with the activity, was calculated as the total elevated O(2) consumption per unit distance traveled. C(act) decreased with activity duration due to proportionally larger increases in distance traveled relative to EPOC volume, and is predicted by the equation C(act)=14.7 x activity duration (s)(-0.24). The inclusion of EPOC costs provides an ecologically relevant estimate of the total metabolic cost of locomotor activity. C(act) exceeds standard transport costs at all durations examined due to the addition of obligate recovery costs. The differences are large enough to impact energy budget analyses for ectotherms.

Methods for estimating peak physiological performance and correlating performance measures

Estimates of animal performance often use the maximum of a small number of laboratory trials, a method which has several statistical disadvantages. Sample maxima always underestimate the true maximum performance, and the degree of the bias depends on sample size. Here, we suggest an alternative approach that involves estimating a specific performance quantile (e.g., the 0.90 quantile). We use the information on within-individual variation in performance to obtain a sampling distribution for the residual performance measures; we use this distribution to estimate a desired performance quantile for each individual. We illustrate our approach using simulations and with data on sprint speed in lizards. The quantile method has several advantages over the sample maximum: it reduces or eliminates bias, it uses all of the data from each individual, and its accuracy is independent of sample size. Additionally, we address the estimation of correlations between two different performance measures, such as sample maxima, quantiles, or means. In particular, because of sampling variability, we propose that the correlation of sample means does a better job estimating the correlation of population maxima than the estimator which is the correlation of sample maxima.

Mathematical biology at an undergraduate liberal arts college.

Since 2002 we have offered an undergraduate major in Mathematical Biology at Harvey Mudd College. The major was developed and is administered jointly by the mathematics and biology faculty. In this paper we describe the major, courses, and faculty and student research and discuss some of the challenges and opportunities we have experienced.