Donald W. Tinkle

The Concept of Reproductive Effort and Its Relation to the Evolution of Life Histories of Lizards

Most aspects of the life histories of individual organisms are explicable in terms of natural selection, a fact often neglected in life history studies, with the result that such studies rarely contain the type of information necessary to test evolutionary hypotheses concerning life history phenomena. Williams (1966a, 1966b) has directed attention to life history phenomena that generally should be explicable from evolutionary considerations. He has made the prediction that a higher per season reproductive effort should characterize small, short-lived rather than larger, long-lived species. He has, as a corollary, cited differences in behavior that might be expected between species with differing demographic properties. Tinkle (1969) presented data on the lizard Uta stansburiana which accord with Williamss hypothesis. One purpose of the present paper is to compare reproductive effort of many species, using criteria suggested by Williams (1966a, 1966b) to determine how well lizard species accord with Williamss hypotheses. A second purpose is to formulate a general theory for the evolution of life history types in lizards.

Environmental Certainty, Trophic Level, and Resource Availability in Life History Evolution

Evolutionary theory has not yet determined the necessary and sufficient environmental factors that can be used to explain the observed diversity of life history patterns in plants and animals. Although recent theoretical treatments of the evolution of life history rely heavily on the concepts of r- and K-selection, we find this framework inadequate to explain life histories of many well-known organisms. Instead, using well-studied examples from the literature, we attempt to identify causal mechanisms in the evolution of their life histories. The density of the population in relation to resources, the trophic and successional position of the population, and predictability of mortality patterns all appear to be important determinants of adaptive strategies. Therefore, consideration of many environmental dimensions seems essential to provide complete understanding of the evolution of life histories.

Sceloporus Undulatus: A Study of the Intraspecific Comparative Demography of a Lizard

Population studies were conducted on the eastern fence lizard in South Carolina, Texas, Ohio, and Colorado. Hatching occurs in early to middle summer and well into the fall in southern populations, but is restricted to late summer and early fall in Colorado and Ohio. The hatchlings in Texas reach a mature size in 3 months and these lizards, as well as those in South Carolina, reproduce before they are a year old. In Colorado and Ohio the lizards do not mature until almost 2 years of age, but at a larger size than those in Texas or South Carolina. Lizards in the four populations differ significantly in average clutch size (7.4—11.8) and in clutch frequency (1—3) and in egg size (.23—.42 g per egg). In all populations there is a significant positive correlation between clutch size and body and the regression lines for these variables differ in slope between populations. Survivorships of eggs, hatchlings, yearlings, and adults differ among the populations and these differences have been related indirectly to increased predation in the southern and western populations. The adult mortality rates are inversely related to population density. Life tables for the populations show large differences among the populations in the contributions made by each age class to the total population replacement rate. The life table characteristics of each population show that the measured parameters are consistent with maintenance of stable population numbers even though the life history strategies are clearly different. Evolutionary explanations are provided for these differences and the relevance of the data to the concept of r— and K—selection is discussed.

Lizard reproductive effort: caloric estimates and comments on its evolution

The energy content of eggs of 10 lizard species was determined and used as a measure of reproductive effort (ratio of clutch calories to body calories). Values for eggs ranged from 5.87 to 7.20 cal/mg ash-free dry weight (x = 6.37). Species differences in calories going into reproduction during a breeding season, however, were primarily due to differences in clutch size and clutch frequency between species. Correlation analysis between three mea- sures of reproductive effort, including the preceding, and a suite of demographic variables revealed only one significant correlation (negative) between clutch calories to body calories ratio and mean annual adult survivorship. The energy allocated to eggs in comparison to total annual energy expenditure was estimated for three of the species investigated. Sceloporus graciosus had the highest reproductive effort, with proportional values for first and older breeders about the same. The proportion of total energy expended on reproduction was slightly lower for Uta stansburiana, with first and older breeders exhibiting the same effort. Scelop- orus jarrovi had the lowest reproductive effort, but showed an increasing effort with increasing age. These data suggest that the demographic environment may not be the primary selec- tive pressure determining reproductive effort, and that reproductive effort may not be posi- tively correlated with age in all species.

Nesting Frequency and Success: Implications for the Demography of Painted Turtles

Nesting ecology and reproduction of painted turtles (Chrysemys picta) in southeast Michigan were intensively studied from 1975 to 1978. The average clutch size of Michigan painted turtles was 7.55, with body size accounting for only 9-13% of the variance. Data on nesting frequency indicate that from 30 to 50% of the females possibly do not reproduce every year and that approx. =6% reproduce twice in a given year. Predation within 48 h of egg-laying is responsible for the failure of 20% of the nests. An additional 12% nest failure is due to various other causes. These data substantially alter the life table previously reported in this population of painted turtles.

Testing hypotheses of aging in long-lived painted turtles (Chrysemys picta)

For 38 of the past 50 years, Painted Turtles were studied on the University of Michigans E.S. George Reserve in southeastern Michigan. We compared age specific body sizes, reproductive traits and survival of Painted Turtles ranging from 9 to 61 years of age to test contrasting predictions of the Relative Reproductive Rate and Senescence Hypotheses of aging. Indeterminate growth (i.e. continued body growth of adults) was important in increasing reproductive output of older turtles; however, growth rate of the oldest age-group was reduced compared to that of younger adults. Although clutch size and among year reproductive frequency did not increase with age, within year reproductive frequency (production of second clutches), egg size, and hatchling size did. Nest predation rates and the proportion of surviving nests that produced hatchlings were similar among age groups, and embryo mortality in nests was not related to age. Survivorship of males was less than that of females, and survivorship of the oldest group of females was not statistically different from that of a younger group of females. No decline in reproductive output or survivorship was detected in the oldest females as predicted by the Senescence Hypothesis. Thus, the majority of data on reproductive traits and survivorship support the Relative Reproductive Rate Hypothesis. We also compared Painted Turtles to Blandings Turtles, another species studied on the E.S. George Reserve. That Painted Turtles exhibit indeterminate growth whereas Blandings Turtles do not, appears to be a primary mechanism for some differences between species in the relationships between reproductive traits and age. An important mechanism for increasing reproductive output in both species was increased reproductive frequency in older females. Painted Turtles also increased offspring quality (egg and hatchling size) with age, whereas Blandings Turtles did not. Compared to younger individuals, there was no reduction in survivorship in the oldest Painted Turtles and survivorship increased in the oldest Blandings Turtles.

Natural selection and the parameters of population growth.

The constant that measures the ability of a population to grow under given conditions is called the intrinsic rate of natural increase r. The Malthusian m which is a measure of genotypic increase on the other hand has the same mathematical formula as r but its biological characteristics are different except in isogenic populations. Genotypes with the highest m under a given set of conditions are favored by selection which may result in the reduction of r for that population. The mechanism of this process is discussed and it is concluded that evolutionary reductions in r are most likely to have occurred under density dependence conditions.(AUTHORS MODIFIED)

Life History and Demographic Variation in the Lizard Sceloporus Graciosus: A Long‐Term Study

An 1 1-yr study of life history and demographic variation in the sagebrush lizard Sceloporus graciosus was carried out on two study areas (Rattlesnake Ridge and Ponderosa Flat) in the Kolob Mesa Section of Zion National Park, Utah. Two primary objectives of this mark-recapture study were to: (1) quantify variation in age structure, age and size at maturity, age-specific survivorship and fecundity, and individual growth rates, and (2) conduct a series of density reduction experiments designed to elucidate the effects of density on growth rates and survival of posthatchling lizards. In addition, we examined the relationships of variation in population density and deviation from long-term average precipitation and temperature to variation in individual growth, reproduction, and demography. At both sites the active season was - 160 d, extending from early April to mid-September. Reproduction occurred during a 50-d period between mid-May and early July. Mean clutch size was 3.7 eggs and most females produced their first clutch in the 2nd yr of life (their third active season) at an age of - 22-24 mo and a minimum snout-vent length of - 50 mm. Most mature females produced two clutches of eggs per year, and there was no statistically significant variation in either mean clutch size or body-size-adjusted clutch size among the 11 yr of study. Clutch size was significantly correlated with body size. Relative clutch mass averaged 0.247 and was not significantly correlated with body size. Since hatchlings first appeared in early to mid-August, their first growing season was -2 mo long. There was no significant sexual dimorphism in growth rate or body size in either population. There was great variation in estimates of egg-yearling survival among years. Egg-yearling survival probability varied from 0.12 to 0.59 with a mean of 0.28. At Ponderosa Flat, the survival of yearling males (0.38) was significantly lower than that of yearling females (0.47). Survival of yearling males (0.45) and females (0.43) at Rattlesnake Ridge was not significantly different. There were no other significant differences in the survival of males and females (X = 0.56 for both sexes) within any age class in any year of the study. However, the survival of yearlings was significantly lower than that of older lizards in both populations. Mean posthatchling survival over all years was 0.45, and there was significant heterogeneity in posthatchling survival among years. Average annual survival of immigrants (0.32) was significantly lower than that of residents (0.44). There was a significant negative linear relatonship between yearling body size in late June and total density of posthatchling lizards. A stepwise linear regression model revealed significant effects of both rainfall (and presumably resource availability) and population density on the growth of yearlings. This model explained 78% of the annual variation in yearling growth. Rank correlation analysis revealed that survivorship of hatchlings was negatively correlated with density of nonspecific lizards. The negative correlation implies direct density dependence of hatchling mortality rates and is a potentially important mechanism of population regulation. Removals of almost all yearling and older age lizards from the study sites resulted in significant increases in growth rates of hatchlings in the year of the removal and yearlings during the following year. Four results from this study combine to suggest substantial resource limitation of S. graciosus on the Kolob Mesa. (1) Snout-vent lengths attained by yearling lizards were positively correlated with deviations from long- term mean rainfall values. (2) Body sizes attained by yearlings were greatest in the years following density reductions. (3) Body size attained by yearlings was negatively correlated with density of conspecifics. And (4), in a year in which a density reduction followed a warm, wet spring, more yearling females reached maturity than in all other years of the study combined. Finally, we compared data from the Kolob Mesa populations to those from other populations of S. graciosus.

Body Size in Island Lizards: A Cautionary Tale

Samples of lizards of the genus Uta were collected from many of the islands in the Gulf of California. Other species of lizards coexisting with Uta were collected simultaneously. These samples provided data for a detailed analysis of body size and degree of sexual dimorphism in each island uta population and the correlation of body size with numerous independent variables such as the number and abundance of potentially competing species. This study tested the null hypothesis that competiting species had no significant influence on the body size or sexual dimorphism of the utas on particular islands. Various corollaries of the general hypothesis were also tested. The results not only supported the null hypothesis of no competitive effect, but demonstrated that independent variables not obviously associated with competition explained at least much of the variance in Uta body size as did variables directly related to competition. Previous studies on the evolution of body size in insular lizards have become paradigms of a presumed causal relationship between both size and intensity of interspecific competition. Our study invites caution to the indirect type of approach to the study of competition used in these previous studies. The weaknesses of past approaches are discussed at better approaches to the study of competition are suggested. Selective pressures that might have at least as great an effect on body size of organisms as does competition from other species are identified.

Reproductive potential and cycles in female Crotalus atrox from Northwestern Texas.

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