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ECOLOGY OF THE IGUANID LIZARD, UROSAURUS
GRACIOSUS, IN ARIZONA
Item type text; Dissertation-Reproduction (electronic)
Authors Gates, Gerald Otis, 1938-
Publisher The University of Arizona.
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Downloaded 14-Nov-2016 06:55:54 Link to item http://hdl.handle.net/10150/284509 This d i s s e r t a t i o n has been 64-4763 microfilmed exactly as r e c e i v e d GATES, G e r a l d O t i s, 1 9 3 9 -
ECOLOGY O F THE IGUANID LIZARD
UROSAURUS GRACIOSUS IN ARIZONA.University of Arizona, P h. D., 1963 Zoology University Microfilms, Inc., Ann Arbor, Michigan
ECOLOGY OF THE IGUANID LIZARD UROSAURUS
GRACIOSUS IN ARIZONAby Gerald Otis Gates A Thesis Submitted to the Faculty of the
DEPARTMENT OF ZOOLOGYIn Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY
THE UNIVERSITY OF ARIZONA1963
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGEI hereby recommend that this dissertation prepared under my direction byfloridfi.fla*«« entitled Ecology of t h e Iguanld Lizard Urosaurua iciosus i n Arizona be accepted as fulfilling the dissertation requirement of the degree of Doctor of Philosophy £-- /S.-^^ isertation Direct Dissertation Director Date After inspection of the dissertation, the following members of the Final Examination Committee concur in its approval and
recommend its acceptance:*
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s H- ^i •7*r ?- / * • - *?
S~•12-- 6,3, mau- M-6* *This approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examination. The inclusion of this sheet bound into the library copy of the dissertation is evidence of satisfactory performance at the final examination.
STATEMENT BY AUTHORThis thesis has been submitted In partial fulfill- ment of requirements for an advanced degree at the Univer- sity of Arizona and is deposited In The University Library to be made available to borrowers under rules of the library.
Brief quotations from this dissertation are allow-, able without speclallpermission, provided that accurate acknowledgement of source is made. Requests for permission for extended quotation frnmfirreproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgement the proposed use of the material is In the interests of scholarship. In all other Instances, however, permission must be obtained from the author.
Field studies were conducted in an area of central Arizona in which two species of Iguanid lizards (Urosaurus graclosus and U. ornatus) occur in sympatry. The results of these investigations have been reported in three parts.
Part I. Comparative Ecology of Urosaurus graclosus and U. ornatus in an Area of Sympatry.—A comparative study of these two species demonstrated that although they occupy the same geographical region in the study area, they are isolated into two distinct micro-environments which differ, in part, in the amount and type of plant cover. The basis for the ecological separation of these species is complex and includes differences in required thermal thresholds and in behavior.
During the period of study, climatic and environmental conditions changed markedly within the study area and subsequently there were changes in the spatial distributions of the two species, with the virtual decimation of one (U. ornatus). As the distribution of U. ornatus diminished, that of the other species (U. graclosus) expanded accordingly. The data of these investigations are in support of Gause's rule. It is concluded further that there is no direct evidence that competition exists between reptilian species; the nature of the present data itself may permit quite different interpretations.
iii Part II. Demographic Aspects of the Ecology of Urosaurus graclosus.—As the distribution of U. graclosus expanded within the study area, there was a decrease in population-density. As a result, several density-dependent functions were investigated in addition to usual studies of natural history. It was found that the age (and size) at which males reach effective maturity and the sex ratio were density-dependent functions. Both of these were related to the ontogenetic change in throat color. The hypothesis is presented that social pressures trigger a physiological response that inhibits the attainment of effective sexual maturity (and a change in throat color); consequently, the rate of increase of the population is not directly controlled by the number of males.
Part III. Thermoregulation in Urosaurus graclosus.— lavestigations of physiological and behavioral thermoregulation of U. graclosus demonstrated several seasonal and dally differences in thermal thresholds and in the methods of regulation. Physiological responses involved thermal over-compensation (short term) and seasonal acclimation (long term).
Behavioral responses consisted of regulation of the time of activity periods, height above ground, and angle of incidence with solar radiation. The hypothesis is presented that differential activity of certain phenotypes may also play an important role in thermoregulation.
The author is indebted to Dr. Charles H. Lowe, Jr.
for direction of the problem. Others who have made contributions to the preparation of this paper are Drs. H. K. Gloyd W. G. Heath, W. B. Heed, and J. W. Tremor, and Mr. P. J.
Lardner. The suggestions and help of these people are deeply appreciated•
Three outstanding fields of biology, in terms of understanding evolutionary mechanisms, are genetics, ecology, and demography. Although all three of these disciplines have most commonly been treated separately, the need for studies relating these areas to each other and to older fields of biology has long been recognized. Such a trend is evident in the recent development of, and emphasis upon, such fields as population genetics and physiological ecology.
Although reptiles as a group have not lent themselves favorably to the field of genetics, they do appear to be extremely suitable for enriching our knowledge In the fields of ecology and demography. Studies of reptilian ecology, in terms of adaptations, stem mostly from the recent work of Raymond B. Cowles on thermoregulation.
Concomitant with this study, and also subsequent to it, several other studies have been undertaken, with excellent success, In terms of understanding ^certain mechanisms of reptilian adjustment and adaptation to different / varying environmental conditions.1 More recently several workers have also concentrated on using reptiles for population studies,2 and although several of the physiological aspects of demography are not, as of yet, clearly delineated, reptiles appear to offer remarkable tools for these areas of study, partly because of the ease with which changes in their populations can be followed, and also because of their relatively sedentary habits, easily observable behavior, and rather simple social structure.
An integral segment of both ecology and demography involves those problems associated with interspecific "competition" and the associative ecological isolating mechanisms. Reptiles have been the subjects of many papers concerned with such problems.3
1. Attsatt, 1939? Barden, 19^2; Bogert, 19^9a, 19**9b, 1953a, 1953b, 1959; Cole, 19^35 Cook,.19^9; Cowles, 19*fl; Cowles and Bogert, 1 9 ^ ; Dawson, I960; Dawson and Bartholomew, 1956; Gelineo and Gelineo, 1955; Ihger, 1959;
Larson, 1961; Lowe and Vance, 1955; Lueth, 19^1; Mayhew, 1961; Mazek-Fialla, 19*+1; Mosauer, 1936; Pearson, 1951*;
Rogers, 1953; Ruibal, 1961; St. Girons and St. Girons, 1956; Stebbins, 1958, I960; Strelnikov, 1 9 ^ ; Templeton, I960; Tremor, 1962s Ushakov and Deverskii, 1959; Vance, 1953; Wilhoft, 1958a, 1958b, 1961; and Wilhoft and Anderson, i960.
2. Blair, I960; Fitch, 19*K, 195S 195 6 ; Sexton.
Heatwole, and Meseth, 1963; and Tinkle, 19ola, 1961b, 1962.
3. Bogert, 19^9a, l$W9b$ Cagle. 19^5 Carpenter, 1952, 1962, and elsewhere; Hunsaker, i960, 19o2; Inger5 1959; Laughlln, 1958, I960; Milstead, 1957a, 1957b, 19ol;
Rogers, 1953; Ruibal, 1961; Tinkle, 1958; Ushakov and Deverskii, 1959; and Wilhoft, 1958, 1961.
3 An area in central Arizona (10 miles southeast of Wickenburg, Maricopa County) in which two closely related species of iguanid lizards, Urosaurus graciosus Hallowell, l85*f, and Urosaurus ornatus Balrd and Girard, 1852, occur together, has afforded an opportunity to study several aspects of reptilian biology in terms of their adaptations, ecology, and demography.
The present study was originally planned to provide an exhaustive comparison of many features of the biology of these forms, but changing climate and environment led to such drastic changes in the relative abundance of the species (with the local decimation of one, U. ornatus) that many of these studies could not be completed. Yet, these same conditions have enabled observation of the direct effect of climate-induced environmental changes upon the distributions of the two forms; furthermore, they have enabled a rather extensive study of certain effects of population-density upon one of the species (U. graciosus).
The first part of this report is concerned with the original outline, and is a report of a synecological study of the two species. The second and third parts, which are more limited in scope, are concerned with certain demographic and thermoregulatory aspects, respectively, of one of these species (U. graclosus).
Although all three parts of this study were conducted simultaneously, the particular methods for each will be reported separately in conjunction with the results of each part. In general, however, all three have several methodological aspects in common.
These investigations were conducted from August, 1957 to October, 1962, in the immediate vicinity of the Otis Gates Ranch, 2.9 miles by road west of Morristown, Maricopa County, Arizona (10 miles southeast of Wickenburg). The individuals within the populations of Urosaurus graclosus and U. ornatus in this area were marked by toe clipping.
To avoid confusion of marked animals with those that had lost toes naturally, no less than two toes were clipped from each individual; no more than four toes were clipped from any one individual and of these no more than two were ever taken from the same foot. Toes considered to be of greater use to the animals in terms of locomotion, etc. were not removed. These Included the first and fifth digit of the fore-feet and the fourth and fifth digit of the hind-feet. Observations and marking of the population were continued at both regular and irregular intervals throughout the duration of the study. Care 5 was generally taken to avoid recapture of the same individual within two or three weeks of its preceding capture to avoid or reduce learning. Evidence of such learning was noted in the few individuals that were collected during shorter periods and has also been shown for other iguanids (Stebbins, 1 9 ^ ; and Blair, I960).
Upon capture, temperatures of body, substratum, and air were generally recorded along with several other data including notes on coloration, behavior, growth, etc.
The animals were released immediately (within 2 or 3 minutes) after capture at the exact spot where they had been obtained, except in a few instances where they were retained for slightly longer periods (up to 15-20 minutes) for certain thermal measurements.
Collecting and observations in the study area were conducted during all seasons of activity (from March to October), and attempts were also made to secure representative samples of individuals from different parts of the same day.
The territory (or the locus) of each individual in the study area was marked so that movements of individuals within the area could be traced.
Standard statistical methods as outlined by Simpson, Roe, and Lewontin (I960) and Snedecor (1956) 6 were used in the process of data reduction, and the levels of significance are based upon the tables presented by Fisher and Yates (1953).
Descriptions of the study area and of the two species are included in the section on results and discussion in Part I.
Part I. Comparative Ecology of Urosaurus graciosus and Urosaurus ornatus in an Area of Sympatry
The Interactions of similar species in sympatry has been a subject of much interest among biologists for several years. Darwin (1859) was one of the first to bring attention to the general problem when he stated that "It is the closely allied forms—varieties of the same species and species of the same and related genera—which, from having nearly the same structure, constitution and habits9 generally come into the severest competition with each other; consequently each new variety or species, during the progress of its formation, will generally press hardest on its nearest kindred, and tend to exterminate them."
This concept was later stated, in somewhat different In 1931*, Gause supplied the terms by several biologists.
Crombie, 19^7; D'ancona, 1951*-* Forbes and h.
Richardson, 1908; Gause, 193*+, 1935, 1937; Grinnell, 190^, 1917, 1928; KotBtitzin, 1937; Lodka, 1932; Volterra, 1931;
and Winsor, 193^; to mention only a few.