Donald W. Tinkle’s research while affiliated with University of Akron and other places

For 38 of the past 50 years, Painted Turtles were studied on the University of Michigan's 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 Blanding's Turtles, another species studied on the E.S. George Reserve. That Painted Turtles exhibit indeterminate growth whereas Blanding's 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 Blanding's Turtles did not. Compared to younger individuals, there was no reduction in survivorship in the oldest Painted Turtles and survivorship increased in the oldest Blanding's Turtles.

Potential costs and benefits of tail autotomy in lizards have been inferred almost exclusively from experimental study in semi-natural enclosures and from indirect comparative evidence from natural populations. We present complementary evidence of the costs of tail autotomy to the lizard Uta stansburiana from detailed demographic study of a natural population. On initial capture, we broke the tails of a large sample of free-ranging hatchlings (560) and left the tails of another large sample (455) intact, and then followed subsequent hatchling growth and survival over a 3-year period. Surprisingly, in 1 out of the 3 years of study, survival of female hatchlings with broken tails exceeded that of female hatchlings with intact tails. Furthermore, no effects of tail loss on survivorship were detected for male hatchlings. However, in 2 years when recaptures were very frequent (1961, 1962). growth rates of hatchlings with broken tails were significantly slower than those of their counterparts with intact tails. We discuss our results in the broader context of estimating the relative costs and benefits of tail autotomy in natural populations, and suggest that long-term demographic studies will provide the best opportunity to assess realized fitness costs and benefits with minimum bias. We also describe how experimentally induced tail autotomy can be used as a technique to complement experimental manipulation of reproductive investment in the study of life-history trade-offs.

An 11-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 relationship 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 conspecific 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.

Plastron lengths of reproductive females varied from 159-235 mm. Mean clutch size over 6 yr was 27.9 eggs (range 12-41) and showed a significant positive linear realtionship with body size of females. Females produced only 1 clutch per year. The youngest known-age, reproductive female was 12 yr old. Duration of the nesting season varied from 13-31 days; initiation date varied by 22 days (22 May-12 June). The beginning of nesting activity each year was significantly correlated with the amount of heat available during March, April and May. Daily nesting activity was essentially bimodal with a major peak occurring between 0600-1100 h and a lesser peak of activity between 2000-2300 H. Nest construction averaged 111 min. Nests averaged 183 m straight-line distance from the nearest relatively permanent water; no significant difference was found between distances from water of nests destroyed by predators and those escaping predation. Females nested in open areas that were adjacent to their marsh of residence or adjacent to other bodies of water. Females moved as far as 1625 m (straight-line distance) in preparation for nesting. Some females that were observed nesting in more than 1 yr constructed nests within 5 m of a previous nest; other females changed nesting areas and constructed nests up to 1000 m apart. Predation rates on nests averaged 70% and ranged from 30-100%. Nests preyed upon by foxes Vulpes fulva were significantly older and further from water than nests destroyed by raccoons Procyon lotor. An average of 4.14 eggs or developing embryos died in nests that escaped predation. Weekly mean temperatures in exposed and shaded nests ranged from 17.2-23.3°C during the entire incubation period. Shaded nests do not provide enough heat to allow complete development. Dates of hatchling emergence ranged from late August to early October, with most emergence in September. All sources of mortality resulted in a probability of 0.22 of surviving from age zero to age 1 (c90 day period from egg laying to hatchling emergence from the nest). -from Authors

Life history and demographic variation of populations of the sceloporine lizards Sceloporus undulatus and S. clarki were studied in a diverse lizard community in central Arizona from 1971-1977. At this locality S. undulatus was active from March into Nov., while S. clarki was active from April into Oct. Female S. undulatus matured at about 60 mm SVL and an age of about 11 months. S. clarki females were mature at an SVL of 90 mm and an age of 22 months. The reproductive season of S. undulatus extended from mid-April through mid-July and that of S. clarki from late May through mid-July. Duration of the reproductive season was sufficient for the production of at least three clutches (averaging 8.3 eggs per clutch) by S. undulatus, whereas most S. clarki produce only a single clutch (averaging 19.6 eggs) per season. Size-specific growth rates varied seasonally in both species with the most rapid growth occurring in the fall and spring following hatching. Age-specific survivorship and population density varied seasonally and among years in both species. The annual survivorship of adult female S. clarki was high (averaging 50%), whereas that of the adult female S. undulatus was considerably lower (averaging 13%). Life tables based on average age-specific mortality and fecundity rates are presented for both species and compared. Life history and demographic data from 10 other populations of S. undulatus are summarized and compared with those from the population studied here. Differences are discussed in light of current life history theory.

The life history and demography of a population of the tree lizard, Urosaurus ornatus, were studied in a diverse lizard community in central Arizona (Maricopa Co.) from 1971-1977. At this locality tree lizards are active from mid-April through Oct. The majority of females begin vitellogenesis by late May, and development of the initial clutch requires about 30 days. Females remain reproductive until late Aug. Duration of the reproductive season is about 90 days, sufficient for production of three clutches. Individual females produce multiple clutches averaging 7.1 eggs per clutch. Both sexes attain sexual maturity at about 45 mm SVL by May-June of the year following hatching. Periods of most rapid growth are in the fall and spring following hatching. Survivorship of yearlings and adults varies among years with average adult survivorship being 11%. Life tables based on average age-specific fecundity and mortality rates result in net replacement rates ( R0) of less than 1.0. Comparative demographic data from three other detailed studies of tree lizard populations are presented and discussed.

A study of three species of sceloporine lizards (Sceloporus clarki, S. undulatus, and Urosaurus ornatus) was begun in 1971 and continued into 1977 in a very diverse (at least 11 species) lizard community in central Arizona, USA. The present paper reports the results of a density manipulation experiment carried out with these three lizards to ascertain whether direct evidence for interspecific competition could be found in a community of lizards in which circumstantial evidence suggested its importance. In May 1975 the study area was divided along a natural constriction into an experimental area and a control area and an attempt was made to remove all U. ornatus and S. clarki from the experimental areas. This experimental treatment was maintained until the termination of the experiment in 1977. Patterns of habitat utilization, population density, survivorship, and individual body size of S. undulatus on experimental and control areas both before and after the removals were analyzed for evidence of competitive interactions. There were no detectable effect of the removal of S. clarki and U. ornatus on habitat selection, perch height, survivorship, population density, or individual body size of S. undulatus inhabiting the removal area. Implications of these results for studies of lizard community structure are discussed.