THE POPULATION AND FEEDING ECOLOGY OF TORTOISES AND FERAL BURROS ON VOLCAN ALCEDO, GALAPAGOS ISLANDS By LYNN ELIZABETH FOWLER A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1983 ACKNOWLEDGEMENTS I would like to thank the scientists and personnel of the Charles Darwin Research Station and Parque Nacional Galapagos for providing advice and logistical support during the course of my field research. Special thanks are extended to former CDRS director Hendrik Hoeck and SPNG Superintendent Miguel Cifuentes. I thank Ulrike Eberhardt and Henk van der Werff for their help with botanical identifications, and Marsha Cox at the Smithsonian Institution for straightening out my confused accounts. Without the generous financial assistance of friends and folks that I met while in the Islands, this study would not have been possible. I thank these con- tributors; Mrs. Vera Stangl in particular receives my gratitude for her support. I am grateful for the friendship of the DeRoy and Moore families who provided me with a home in the Islands, and of the captains and marineros who trans- ported me to and from Isabela Island and brought me my mail. I want to express my deepest appreciation to assistants John Roe and my sister, Ada Fowler, who pro- vided invaluable help and moral support in the field. Thanks go also to Dr. Patti Moehlman for her advice in the field and afterwards. I have greatly appreciated the guidance and encour- agement that my chairman, Dr. John H. Kaufmann, has given during my field work and since my return to the States. Thanks are also extended to my committee members, Dr. Carmine Lanciani and Dr. George Tanner for their advice and review of this manuscript. Paloma Ibarra did my illustrations. Dr. Mark K. Johnson, Louisiana State University, kindly aided me with diet quantification and Gary Matson's lab in Montana analyzed the burro teeth I col- lected. Dr. Frank Martin and Dr. John Cornell helped with statistics. Finally, I would like to express my deepest thanks to my mother, Margaret Fogg, Eduardo and Ella Neira and my family for their patience, loving support and con- tinuing confidence throughout my student years. This research was supported, in part, by funds from Sigma Xi, the National Geographic Society and the Explorer' s Club. TABLE OF CONTENTS PAGE ACKNOWLEDGEMENTS ii LIST OF TABLES vi LIST OF FIGURES ix ABSTRACT X CHAPTER ONE INTRODUCTION 1 TWO THE STUDY SITE 5 Volcan Alcedo, Isabela Island 5 Climate 10 Vegetation 18 THREE TORTOISE AND FERAL BURRO POPULATION SIZES AND DISTRIBUTIONS 23 Methods 23 Rim Census and Count Results 28 Burro Birth Season and Group Size/Composition Results 47 Discussion 59 FOUR BURRO MORTALITY 68 Methods 69 Results 71 Discussion 75 FIVE THE EMERGENCE SUCCESS OF TORTOISE NESTS AND THE EFFECT OF BURROS ON NEST SUCCESS . . 82 Methods 83 Results 85 Discussion 92 CHAPTER PAGE SIX FEEDING ECOLOGY OF TORTOISES AND BURROS ON ALCEDO 101 Methods 103 Results 106 Discussion 119 SEVEN TORTOISE DAILY TIME BUDGETS 122 Methods 123 Results 125 Discussion 130 EIGHT CONCLUSIONS 135 LITERATURE CITED 142 BIOGRAPHIC SKETCH 150 LIST OF TABLES TABLE PAGE 2-1 Monthly Rainfall 11 2-2 Daily Air Temperatures Isabela Island 12 2-3 Monthly Frequency of Garua Days Rim Camp, Alcedo 15 2-4 Average Daily Garua Catch Measurements ... 17 3-1 Burro and Tortoise Censuses and Counts ... 27 3-2 Average Number of Burros on Rim Censuses by Month and Area 31 3-3 Average Number of Tortoises on Rim Censuses by Month and Area 31 3-4 Rim Camp Censuses 37 3-5 Southeast Slope Censuses 38 3-6 South Floor Censuses 40 3-7 Sulfur Slope Counts 40 3-8 Midcamp Censuses 46 3-9 North Plateau Counts 46 3-10 Seasonal Distribution of Young and Pregnant Burros 50 3-11 Percent of Burro Groups of Different Sizes by Season 51 3-12 Percent of Burro Groups of Different Sizes by Area 52 TABLE PAGE 4-1 Burro Sex and Age at Death 74 4-2 Young Burro Age at Death 76 4-3 Years Since Death Based on Weathering of Burro Bones 76 5-1 Success of South Caldera Floor Nests .... 86 5-2 Success of North Caldera Floor 1979/1980 Nests 87 5-3 Fates of South Caldera Floor Nests ..... 89 5-4 Fates of Eggs in Burro Damaged Nests South Caldera Floor 89 5-5 Fates of North Caldera Floor Nests 91 5-6 Success of Undisturbed Nests South and North Floor 91 5-7 A Comparison of Fertility, Hatching and Emergence Success of Undisturbed Natural Nests of Geochelone elephantopus porteri, ephippium and vandenburghi .... 93 6-1 Numbers of Burro and Tortoise Feeding Plots Examined 105 6-2 Numbers of Burro and Tortoise Fecal Samples Collected 105 6-3 Plant Species Eaten by Burros in Feeding Plots 107 6-4 Common Plant Species Eaten by Tortoises in Feeding Plots 109 6-5 Uncommon Plant Species Eaten by Tortoises in Feeding Plots 110 6-6 Plant Species Eaten by Burros and Tortoises, Volcan Alcedo 111 6-7 Percent Relative Density of Plant Fragments in Burro and Tortoise Feces, Volcan Alcedo 115 TABLE PAGE 7-1 Summary of Tortoise Activity 126 7-2 Seasonal Comparison of Daily Time Budgets of Alcedo Tortoises 128 LIST OF FIGURES FIGURE PAGE 2-1 Galapagos Islands 7 2-2 Volcan Alcedo Study Sites 9 3-1 Around-the-Rim Censuses of Burros and Tortoises 29 3-2 Burros on Around-the-Rim, South Floor and Midcamp Censuses 33 3-3 Tortoises on Around-the-Rim and South Floor Censuses 34 3-4 Distribution of Small Tortoises on Alcedo 42 3-5 Distribution of Medium Tortoises on Alcedo 43 3-6 Distribution of Large Tortoises on Alcedo 44 3-7 Reproductive Periodicity of Burros 48 3-8 Seasonal Changes in Burro Group Composition 54 3-9 Burro Group Composition on Different Areas 57 4-1 Months in Which Burros Died 73 Abstract of Dissertation Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy THE POPULATION AND FEEDING ECOLOGY OF TORTOISES AND FERAL BURROS ON VOLCAN ALCEDO, GALAPAGOS ISLANDS By LYNN ELIZABETH FOWLER April 1983 Chairman: John H. Kaufmann Major Department: Zoology Feral burros ( Equus asinus) were introduced to the Galapagos Archipelago in the 1830s. Volcan Alcedo, Isabela Island, harbours 500 to 700 burros in addition to the largest remaining population of Galapagos tortoises, Geochelone elephantopus vandenburghi. Burro and tortoise population and feeding ecologies were studied on Alcedo to investigate the possible impact of burros on tortoises. There is no permanent source of fresh water on Alcedo; during the wet season (January to June) rain water collected in pools and was readily available. Peak burro natality coincided with the rainy season, as did tortoise breeding. During the dry season water was occasionally avail- able in drip-puddles along the southeastern crater rim. Temporary water availability influenced distributions and behavior of burros and tortoises. Both species showed a tendency to congregate along the moist southeastern section in the dry season. Apparently water shortages result in an unusually high level of mortality among young, sexually mature burros. Forty percent of 136 burro carcasses and skele- tons were of animals between three and six years old. Water shortages probably limit burro population growth. Burro and tortoise diets were investigated using direct observation of feeding animals and fecal analysis techniques. Seventy-two percent of 92 plant species con- sumed by burros and/or tortoises were eaten by both animals. Burro and tortoise wet season and early dry season diets were different, but in the late dry season both animals consumed Sida and competition for food is a possibility. An investigation of seasonal tortoise feeding behavior demonstrated, however, that in late dry season months tortoises spent little time feeding. Late in the dry season feeding occupied only nine percent of tortoise daily activity time. In wet and early dry season months tortoises fed during 40 percent of their active hours. Even during the dry season competition for food may be insignificant because tortoises scarcely feed. Burros trampled some tortoise nests; eighteen percent of 88 monitored nests were damaged by burros. Entire clutches were destroyed in 4.5 percent of the nests. Natural emergence success for Geochelone elephantopus vandenburghi was 64.9 percent. CHAPTER ONE INTRODUCTION About twelve thousand years ago man began to domes- ticate selected animal species. As he spread across the globe, he took his domesticated animals with him. In time, domestic animals escaped or were abandoned. Some successfully established feral breeding populations in their new homelands. The problems created by feral mammals are diverse and widespread. Island ecosystems are particularly frag- ile and vulnerable to the ecological disturbances that are created when domestic animals become feral. Goats, cattle, pigs, and sheep are among the destructive, large herbivores that have been widely introduced onto islands across the world and have been the subject of much research (on goats, Yocom [1967], Sykes [1969], Williams and Rudge [1969], Spatz and Mueller-Dombois [1973, 1975a], Coblentz [1976], Bullock [1977], Rudge and Campbell [1977], Wardel et al. [1978]; on sheep and cattle, Wilson and Orwin [1964], Taylor [1971]; on pigs, Taylor [1971], Spatz and Mueller-Dombois [1975b]). The Galapagos Islands, 960 kilometers off the coast of Ecuador, are unique in their flora and fauna and in their historical role in the origin of Darwin's theory of evolution by natural selection. As on many of the world's island systems, various endemic Galapagos species are threatened by populations of exotic plants and animals. Feral horses, burros, cattle, goats, pigs, dogs, cats, rats and mice inhabit the Islands. Research projects are being conducted to investigate the ecologi- cal impact of these introduced species and recommenda- tions are being made concerning methods of control or eradication. In an effort to determine the effect that feral burros have on the endangered Galapagos tortoise, Geochelone elephantopus , I studied feral burro and tor- toise feeding ecologies, population distributions and interactions on Volcan Alcedo, Isabela Island. Research began in October 1979 and was completed in December 1980. Feral burros occur on all five of the major islands in the Galapagos Archipelago. The exact date of intro- duction is not known. Colonists first settled on Isla Floreana in the 1830s and brought with them a variety of domestic animals. Since burros are utilized by farmers on the mainland and are preadapted to arid climates, they were probably among the first animals taken to the Galapagos by early settlers. Burros were soon dispersed to even the uninhabited regions by oil seekers (R. H. Beck in Van Denburgh, 1914) who used them to trans- port kegs of tortoise oil to ships and settlements, and by miners who were in search of sulfur in the deposits around the volcanic craters. Estimates of the feral burro populations on the major islands are as follows: 300 on San Cristobal, 200-300 on Santa Cruz, 500-700 on Santiago (Lucho Calvopina, pers. coram.)/ and 2,000-4,000 on Floreana (Tina Beach and Felipe Cruz, pers. coram. ) . In addition, burros occur on three of the five volcanoes that make up the largest island, Isabela. Volcan Cerro Azul and Sierra Negra on southern Isabela have relatively small burro populations; Volcan Alcedo, to the north, has a population of between 500 and 700 animals (this study). Feral burros had become established on Isabela by the 1860s (S. Habel, 1868 in Salvin, 1876). By the 1880s, they were very numerous on Isabela as well as on San Cristobal, Floreana and Santa Cruz Islands (T. Wolf in Baur, 1891). Old literature makes no specific mention of exactly how and when burros arrived on Volcan Alcedo. Because Alcedo had both a large tortoise population and sulfur deposits, however, oil seekers and sulfur miners with pack burros surely visited its slopes. The Charles Darwin Research Station (CDRS) and Galapagos National Park personnel have long feared that feral burros damage the flora and fauna of Alcedo and the other islands where they occur. Thorton (1971), Wiggins and Porter (1971), MacFarland et al. (1974a), and van der Werff (1978) expressed these fears. Prior to this study, however, no investigation of the impact of feral burros in the Galapagos had been undertaken. Similar studies have been completed of feral burros in the southwestern United States and their impact on flora, their competi- tion with bighorn sheep, and their effect on birds and small mammals (Moehlman, 1974, 1979; Woodward, 1976; Woodward and Ohmart, 1976; USDI, 1977; Hanley and Brady, 1977 a, b; Norment and Douglas, 1977; Seegmiller and Ohmart, 1981). In addition to a large burro population, Volcan Alcedo has the largest remaining population of Galapagos Geochelone tortoises. In the past, the Galapagos giant tortoises were heavily exploited; first by pirates, sealers and whalers in the 1600-1 800s, then by colonists and oil seekers, and during the early 1900s, by scien- tific collecting expeditions. Well over 100,000 tor- toises were taken from the Galapagos Archipelago (Townsend, 1925). Of the original fourteen races of Geochelone elephantopus only ten races remain; seven of these are severely threatened due to decreased populations and introduced mammalian competitors and predators (MacFarland et al., 1974a, b) . Alcedo' s Geochelone elephantopus vandenburghi population is the least en- dangered of the tortoise races. However, introduced rats, cats, and burros on Alcedo pose a potential threat from both predation and competition. CHAPTER TWO THE STUDY SITE Volcan Alcedo, Isabela Island Isabela is by far the largest island in the Galapagos group. Its land surface area of 4,670 square kilometers (Wiggins and Porter, 1971) includes more than half of the total land area of all the islands in the Archipelago combined. Six volcanoes, connected by exten- sive lava flows, form this J-shaped island (Figure 2-1). Volcan Alcedo, in the middle of Isabela and 1128 meters high, has a large central caldera which is between seven and eight kilometers wide (Eanfield et al., 1956 and Parque Nacional Galapagos, 1980). The Galapagos volcanoes are typical, gently sloping shield volcanoes; several of them are active. Sierra Negra, to the south of Alcedo, erupted as recently as 1979. Volcan Wolf erupted in September, 1982. Alcedo last erupted in 1954, from a small fissure on its outer southeastern flank (Thorton, 1971). There is an active fumarole on the inner southeastern wall of the crater which, until 1969, was surrounded by a bubbling pool of mineral-laden water. The pool has since dried, but the fumarole remains and emits hot sulfur steam o a to a 0) U 3 Oi ■H fa 8 continuously. Other small sulfur vents dot the inner south and southwestern slopes of the caldera, testimony to the incessant activity underground. To reach Alcedo's crater, one lands on a small beach towards the northeastern side of the volcano. From there, a burro/tourist trail leads ten kilometers up its flank to the base of the crater rim. My "Midcamp" study site was located in this area, at the eastern foot of the volcano (Figure 2-2). The rim of Alcedo rises abruptly from its outer slopes. A few hundred meters high and rel- atively flat-topped in some places and hilly in others, the rim varies greatly in width. My "Rim Camp" study site was approximately five kilometers southeast of the ascent path from outer flanks to rim. Six kilometers further south along the rim was a descent path to the inner wall fumarole. From the fumarole, a path led to the caldera floor and my "South Floor" study site. One of the main tortoise nesting zones was located in this area. During the rainy season several large pools of water formed on the south floor. These were heavily used and of great importance to animals since there is no permanent source of fresh water on Alcedo. The "North Plateau" study site was located on the widest, northern section of the rim. Directly below the North Plateau, on the caldera floor, was the north floor nesting area, where many tortoises nested. V. DARWIN Figure 2-2 Volcan Alcedo Study Sites 10 To facilitate data collection, I divided the caldera rim into four areas of roughly equal length. Area two encompassed the wide North Plateau and area four included Rim Camp study site. Areas one and three were relatively narrow sections of the rim with vegetation somewhat similar to Midcamp and North Plateau, respec- tively . The Climate The Galapagos Islands, though they straddle the equator, are not typically "tropical" in their climate, which is strongly influenced by the sea surrounding them. The Humboldt current, sweeping up the western coast of South America from Antarctica, turns west at the equator and bathes the Islands in its chilling waters. Sea water temperatures range from 19 to 23 degrees C during the warmer months of Janftary through June and are usually several degrees cooler during the remainder of the year (Wellington, 1975). There are two seasons in the Galapagos: a "warm season" (also referred to as "wet season") from January through June and a "cool, garua season" (or "dry season") from July through December. Rains, though they primarily fall during the warm, wet season, vary considerably from one location to the next. During the dry season, coastal areas may be several months without precipitation. High- land areas, however, are often blanketed in a fine wet 11 ■H CO T— rH >J 1 ro a; 1-H rH S x: — -U c 0 s ■K E c 0 03 (0 T3 rH o O OJ a> o H a n •r— 0 rH 03 *— > < 03 H N 3 e OJ 03 U H C CJ O T! 3 • CM (1) J 4-> V£> £ CO 03 O co rH 6 -U 03 (I) c 6 XI o 03 0 03 lD co H CO H m ■H 0 ■H 03 4J e rH Vj m a> e OJ ■H X! 3 rH 03 »£> 04 -H CO > H N 3 CO U e OS u Q • v£> u CO c 0) 0 T T-r-i— vorr in 'tfin P"» O C ID O vr> tt o r- CM CM t- CM I VO I | 00 | CM voooo«— r^^i'CMcriTrOTj'oo CMP-oocNrocoaoooocMioao voo ctiocmvoidvooidid CM CM r- «— ■*t cm o o -*r o O CO ON CO ID NOIO'J'- «— O <— ID CM ID CM CM ID CO «— «— CM r~ C o«— f oi m mt r- CM VO CO >1 OJ Vj Sj >i Vj XI VJ oj j 03 -U g OJ X! X! 03 3 JZ H CO OJ X! g g 3 -J O'H 0) >i 3 JJ 0 0) 0) C XI )j Vj >, c rH XT- CX -U > O 03 d) 03 CX 03 3 3 3 OJ O 0 0) 1-3 to S i c Sj o 03 E 3 C a; 03 x: T) 4J U-l U-l 0 0 CO U-l >1 rH 03 03 T3 JC OJ OJ > c •H c U-l 4-1 -U CO CO Vj 03 -H • rH U-| a e OJ OJ 03 .C sz O ■p HI E >1 >1 ■H rH rH OS c c 0 0 J-l CO J-i u 03 0 0 OJ U-l u-j £ 4J 03 m 3 JJ 4J o 03 03 CO a O * ■K ■K 12 Table 2-2 Daily Air Temperatures (°C) Isabela Island, 1980 Location Pto.Villamil Santo Tomas V.Alcedo* Altitude 6 m 350 m 1100 m Max. Min. Max. Min. Max. Min. January 26.4 24.3 27.8 18.6 20.6 15.0 February 29.6 26.5 29.2 19.0 • 24.2 15. 1 March 27.6 24.0 31.4 19.4 26.5 16.1 April 27.6 24.2 30.1 20.1 26.3 17.0 May 26.1 24.0 26.5 19.4 25.0 16.7 June 25.3 23.3 25.0 17.3 - - July 24.1 21. 1 23.0 16.4 21.9 12.0 Auqust 23.9 19.4 23.2 16.2 26.7 11 .4 September 23.7 19.0 23.4 16.0 — — October 23.1 19.6 23.2 16.5 24.5 13.0 November 23.1 20.0 24.1 16.5 23.7 12. 1 December 23.1 20.8 25.0 16.8 23.3 13.1 ♦Southeast Rim Camp. 13 mist, "garua," which condenses on the vegetation. The prevailing winds are from the southeast, hence the southern, windward slopes of islands are wetter than their northern slopes. Wiggins and Porter (1971) and van der Werff (1978) provide more detailed discussions of the climate of the Galapagos Islands. Meteorological data for 1980 on Volcan Alcedo is presented in Tables 2-1 and 2-2. Monthly rainfall and daily temperatures from Alcedo (Rim Camp) are compared with data from other CDRS weather stations in the Islands. As mentioned previously, rainfall is quite var- iable and often localized. Areas of higher altitude tend to receive more precipitation, but not necessarily during the same months of the year, or as a result of the same storms. During 1980, rainfall on Alcedo was greatest between January and May; March was unusually dry. From August through early November, very little rain fell. Light rains began again in mid-November. Temperatures at 1100 meters on Alcedo were predictably lower than temper- atures recorded on the southern coast of Isabela at Puerto Villamil or on the slope of Sierra Negra at Santo Tomas. Sporadic rainfall and temperature records from the South Floor and Midcamp study sites indicate that these areas generally received less rainfall than did Rim Camp. Likewise, at both sites, maximum and minimum temperatures were usually several degrees higher than at Rim Camp. 14 Rim Camp, situated on the southeastern section of Alcedo's rim, was in the path of the wet prevailing winds. Consequently this area received a great deal of precipitation in the form of garua, even during the cool, dry season. Frequently, when the rest of the volcano was dry and warm, and bathed in sunshine, Rim Camp was wet, cold and windy, under a thick blanket of garua. Heavy garua, condensing on the vegetation along the south- eastern rim, would often form drip-pools under the moss covered trees. Many of these pools had become quite deep and enlarged after years of tortoise and burro use. Daily weather profiles were kept for 121 days while I was camped at Rim Camp (Table 2-3). On 92.6 percent of these days there was garua. Rarely was the rim clear at sunrise, and only 7.4 percent of the days were garua-free from sunup to sundown. Most common were days with garua at dawn, followed by some sunshine in the early after- noon. Often, just before sunset, the garua clouds would roll in again. Seeking to compare the amount of moisture received by the various sections of the volcano, I set up several "garua catches." Four locations along the crater rim were chosen and at each site two wire window-screens cut into squares measuring 50 centimeters by 50 centimeters were erected. Garua condensed on the screens, dripped down into a slanted pipe-trough along the lower edge and was collected in a holding container until measured. 15 Table 2-3 Monthly Frequency of Garua Days Rim Camp, Alcedo 1980 January February March 3 5 3 April 3 May July August October 8 7 0 1 November 6 December 11 Total 47 Percentage 38.8% Garua/ Rain Garua and Sun Entire Day Sun Entire Day 2 0 9 0 5 3 11 0 2 0 6 0 6 3 5 1 14 2 _5 0 65 9 53.7% 7.4% 16 I positioned the two screens at each rim location at slightly different angles, to insure that at least one would be perpendicular to the direction of the winds for maximum garua collection. Later, where one screen was obviously more correctly oriented to the winds, I dis- carded the data from the poorly positioned screen. Or, if the two screens collected approximately equal amounts of water and neither was consistently more efficient, I averaged the amounts collected. It was difficult to suspend containers for water holding more than 3.8 liters, due to their weight when filled, and to the destructive curiosity of tortoises. Tortoises destroyed my garua catch number three. I was forced to attach other screens high or over rockpiles where tortoises could not reach. Unfortunately, because I could only infrequently check the garua catches north and south of Rim Camp, on extremely wet days the 3.8 liter containers overflowed. Therefore the data pre- sented in Table 2-4 are an underestimate of garua precip- itation collected by catches number one/two and four. Catches five and seven never overflowed. As seen from Table 2-4, the amount of garua mois- ture condensing on the southeastern section of Alcedo was much greater than the amount condensing on the eastern and northeastern rim. I found that differences in amount of moisture strongly influenced the distribution of both 17 ■""< i rH £ ■rliJ^ 32 Vj r- en 0 *- th 4J 2 ra 1— CN CN o CN CO D T3 o O O *— *"■ rO CU (0 V£> .J Vj CU CO o 0 -u ii • *4-i m c fa CU iH ZQd< 4J (OH CO •H -H >, VJ « fO 00 "* 3 Vj -U o o CN >43 o O E-t — o o «— o T— to En in vo vo ■p jj ^-»- o c 4J C II • 0) fO CU c E U (D » w U w <: 3 — WCN CO m co >1 1) u E * s cu ■H —13 *r t— ^r CN CN ■K 4J CJ CU1* t— in *"■ CM in ■tC x: a> *~ O E w m m *— *- ,— *- T— «* 4J -H CO U II • 1 (0 4J c *"" (N u c O (1) 00 4J cu BJUm c «H 3\ — CU X) u w o 03 (0 Vj w Eh U cu 4J tH rH CU — Q CU CN Uj ■H >, CU 0 Ul <0 •H -rH > Vj TJ CO IT) V£> m CU > < c 0 •H 4J — (TJ Sj o cu 0 X! Vj u >J E U 0) cu Vj 3 4J cu X5 XI cu JC 2 CO XI E £ XI O >1 3 0 CU cu E 4J c rH cr 4-1 > u cu rtJ o 3 3 a 0 cu o CJ -H ID < o 2 c cu a) JJ C7*l 9 fC -H re E l D W Vj O >i Vj 0 CU SH (0 04 > pH 3 o — < E TJ , 4-1 rH c ra •h x: x: w 4J 3 f0 x: CU 4J T3 CO C 3 - (0 flj O z (- o o 1 CO en U) C e> e 3 •H Dj < « 6 i eo _l A Ti 3 ~3 o 00 1 S 0) C 'd 1 ^ 3 G 0 (C H < u 0 > G 0 < 0 r-i 2 Cm 0 A u_ U -P LL M 3 > O Q UJ C 1 cc III UJ a ■S X ui UJ u_ Q UJ 1- rr UJ a Ul X UJ S0NI1H0IS JO A0N3n03ad 3AI1VT3H 34 o UJ t- u UJ a. X UJ Q UJ o UJ UJ cr o cr UJ UJ UJ tr o (- u UJ CL X UJ UJ UJ cr o (- o UJ 0. X 2 u. 2 UJ u. 2 UJ Q UJ t- or O uj UJ » X UJ UJ u. rcs c (0 E ■H PCS CD £ +J I CO rd a. < 3 UJ w < ± < UJ CC < < < 5 < °- _ z a Q n UJ UJ iii (- cc H cc i— o UJ n UJ UJ U UJ (1 UJ UJ cr o u UJ a X UJ o X UJ X IU u. 2 UJ u. 2 Ui ^ 2 o °° -p o A < H — r CO UJ Q. X 2 UJ u. 2 UJ u. UJ -l H 4-1 < en •H C Q 0 I rn d) S-l ■H fa S0NI1H0IS dO A0N3n03UJ 3AI1V13U 45 one and two in addition to the South Floor. Medium tor- toises were more abundant than expected at uniform on the South Floor and large tortoises were common on area four. Counts made at the Midcamp and North Plateau study sites were taken at infrequent intervals, whenever I could travel to these areas (Tables 3-8 and 3-9). Burro numbers were significantly different between months at both Midcamp and North Plateau (x2=50.2 for Midcamp burros, x2=49«5 for North Plateau burros, P<.001 for both). A larger number of burros than expected was tallied at Midcamp in February and July and fewer burros than expected were counted there in November and December (Figure 3-2). Burros on the North Plateau were most num- erous in July, and least abundant in November. Tortoise Midcamp census totals did not vary significantly during the months of 1980. Using data from the various censuses and counts reported above, and knowledge of Volcan Alcedo following a year of extensive hiking and camping there, I estimated the Alcedo feral burro population to be between 500 and 700 animals. I estimated the tortoise population to be around 3,000 animals. MacFarland et al. (1974a) esti- mated 3,000-5,000 tortoises on Alcedo but I have been unable to find out on what procedures their estimate was based. As mentioned earlier, I based my estimates on the number of burros or tortoises generally observed in a 46 Table 3-8 Midcamp Censuses, 1980 Month No. of Counts Average Average No. No. Burros Tortoises February 6 March 2 April and May 3 July 3 October 1 November 3 December 1 36.0 29.5 16.0 32.3 25.0 6.0 2.0 9.0 11.7 4.3 0 2.7 6.0 Table 3-9 North Plateau Counts, 1980 No. of Average Month Counts No. Burros May 2 73.5 July 1 106.0 August 1 77 .0 November 1 24. 0 47 particular habitat on Alcedo and the extent of that habitat on the volcano. Burro Foaling Season and Group Size/Composition Results Very young burros were recorded on Alcedo during all months of 1980. Yet there was a definite peak in the number of births during the rainy season (Figure 3-7). Between March and July, burros from newborn to five months old were observed more often than young animals of other age classes. However, very few newborn to five month olds were seen near the end of the year, in November and December. Graphs of the frequency of sight- ings of juvenile burros (six to nine months old) and adolescents (ten months to over a year old, but not fullgrown) follow the pattern established in the graph of infant sightings. By August through December, the rainy season infant cohort was classified as juvenile; there was a corresponding increase in sightings of six to nine month old burros during these months. Further evidence demonstrating that foals were born between February and April or May comes from my record of sightings of pregnant females in their last months before giving birth. I noted the vast majority of these females between January and April. The distributions of pregnant burros ana burros with young were not uniform among areas on Alcedo (Table 3-10). Chi-square tests show significant 43 40-i CO -I < PREGNANT FEMALES 60- 40- w< g^ 0- |p ADOLESCENTS 10 MONTHS TO ONE YEAR + JUVENILES 6 TO 9 MONTHS JAN MAR MAY AUG NOV FEB APR JUL OCT DEC 1980 ^_5, INFANTS 1 XI o o o . o in — vc « §1 |l 8 "3 3 B 52 u < A to + „, o G 0 + ■H > -P ■H en o > e 0 + u O z o »- o CO CI o O Q. 0 u + i u \> O o 0 0. £ ZD •H o en ()+ cc

•H O E O U o u u o n 0) ■H 57 < til a < i — r Q UJ I- Ul u IK 2 uj i — r o UJ I Q. 'I Q UI ►- ui a a: ui SdHOUO dO A0N3n03Ud 3AI1VT3U 58 groups of more than three animals were recorded. Female groups of up to four or five and even more may exist on Alcedo; it was more difficult to quickly sex females than males from afar. The largest mixed sex and age burro groups observed were of 15 animals, including three young burros. The largest male and female groups were of six animals; one, made up of one male and five females and another of three males and three females. Other larger mixed sex groups no doubt existed but I could never rapidly sex all the animals in the largest groups before they fled. Sixty- four percent of the male and female groups contained either one male and one female or one male and two females. I never saw more than five young burros in a group (a group of one male, five females and five young) and groups of up to seven females with three young were observed. Seventy-five percent of the female with young groups were of a single mother and offspring. Eleven percent were made up of a mother and her offspring plus another female, six percent were of two females with two young and four percent were of three females with one young. All other female and young groups were seen less than one percent of the time. 59 Discussion Burro and Tortoise Distribution Studies of feral burros in North America have shown that burros use a wide range of habitats and frequently have seasonally distinct habitat preferences (Moehlman, 1974; Woodward, 1976; USDI, 1977; O'Farrell, 1978; Seegmiller and Ohmart, 1981). Summer burro distributions in the southwestern United States were found to be strongly influenced by water availability. Burros were generally concentrated within three or four kilometers of water sources during the hottest months of the year. In summer, Moehlman (1974) observed that adult burros watered every 24 hours in Death Valley, while females with young foals watered several times a day. During the cooler winter months, burros ranged up to 1 0 or 1 3 kilo- meters from water. In winter in the Chemehuevi Mountains of California, burros watered every three days (Woodward, 1976). Water availability apparently also influences the distribution of burros on Alcedo. During the early half of 1980, the puddles which formed after heavy rains pro- vided ample water for both burros and tortoises on all sections of the volcano. At this time of the year, burros were most common on the South Floor, the North Plateau and Midcamp areas. They avoided the Rim Camp area which was often cool and foggy, particularly in the mornings. On Alcedo, unlike the southwestern U.S., the 60 hottest months of the year are also the wettest. Hence, when Alcedo's weather was hot, burros usually had plenty of water available to them. There was abundant forage on all of the volcano during the warm wet season. In mid-August, burros began leaving the wide North Plateau and Midcamp areas where grasses and ephemeral plant species were desiccating. By late in the year, large numbers of burros were concentrated on the moist Rim Camp section of Alcedo. This was particularly true in November and December of 1979. In 1980, light rains had begun to fall on Alcedo by mid-November but 1979 was dry until late December; this explains the greater number of burros at Rim Camp at the end of that year. There burros could obtain moisture from garua dampened grasses and occasionally find water in garua drip-pools under trees. Burros are known to travel for several kilometers to reach water (Woodward, 1976). Alcedo burros appar- ently traveled regularly from other areas in search of water near Rim Camp during the dry season. On wet garua nights, particularly on wet nights following several con- secutive dry days on the southeast rim, burros were active and noisy during the night. All night long, caravans of burros would pass my camp. I could hear burros just ten meters from my tent, stomping and slurp- ing in the mud under a tree from which garua dripped and formed a favorite drinking hole. In the mornings, I 61 could hide and watch as single file burro trains passed me, going from one muddy puddle to the next in search of moisture. I once followed a lone male burro for an hour as he investigated over a dozen garua drip-holes, wading and putting his muzzle in the mud. Alcedo burros behave differently around water than do the feral burros of California; Woodward (1976) reports that she never saw a burro so much as put its hoof in the mud or water of the Colorado River. Twice I saw burros licking water droplets off mosses and leaves. The shortage of water in the dry months of the year on Alcedo also seemingly influenced the distribution of pregnant female burros and females with offspring. By the end of the dry season, in October to December, pregnant females and females with young were concentrated on area four at Rim Camp. Since lactating females provide fluids for themselves and their young, their water requirements are greater than those of the average adult female burro. The lack of a permanent source of water on Volcan Alcedo is a relatively new situation on that volcano. Until the late 1960s, a pool of water surrounding the southeastern inner caldera wall fumarole provided year round water for burros and tortoises. When the pool mys- teriously dried up, several hundred burros died. The current burro population on Alcedo may be much smaller than it once was (MacFarland, pers. comm. ) 62 Burros are well adapted to life in arid environ- ments and are able to withstand a water loss of up to 30 percent of their body weight (Maloiy, 1970). On Alcedo, however, they must be under water stress during the dry season when they may go for several months without the chance to drink fully. Both their behavior at times of some garua-water availability and the changes in their distribution as the dry season progressed provide evi- dence that water shortages do exist on Alcedo and may strongly influence the feral burro population. Tortoises can go for months without food or water. This fact led to the over-exploitation of the Galapagos tortoises for oil and meat by pirates, sealers, whalers and other seamen in the seventeenth and eighteenth cen- turies. Tortoises were stored alive aboard ships for several months. One tortoise, lost aboard the ship Niger out of New Bedford, was found alive in the lower holds after two years (Townsend, 1925). In spite of their ability to withstand long periods without water, tortoises too seemed to be distributed on Alcedo in response to water, and possibly to food avail- ability. And, like the burros, tortoises were very intent on searching for water under dripping trees fol- lowing a wet garua night on the southeastern rim. On mornings in the dry season when there was only a very small amount of water in garua drip-puddles, tortoises 63 would spend hours going from puddle to puddle nosing in the mud (see Chapter Seven for details). Tortoise distribution was actually more straight- forwardly related to moisture availability than was burro distribution. This was probably because tortoises, unlike burros, are not capable of rapid mobility. While burros could easily travel several kilometers to reach the southeast rim during a wet night, drink and leave, all in the span of a few hours, tortoises travel too slowly to do this. They apparently had more regular migrations to and from the various sections of Volcan Alcedo. During the rainy months, many tortoises were con- centrated on the South Floor where the largest pools for drinking and wallowing formed. Tortoises were also com- mon on areas one and two along the rim, where food and water were plentiful during the wet season. Rim area four, Rim Camp, with its many rainy season puddles and abundant forage, had few tortoises, again perhaps because of the cool foggy days that were common there yet rare on other sections of Alcedo. Data on both burro and tortoise use of the Rim Camp area confirm that tortoises moved into that section of the volcano in June or July after the rains had ended and remained there until the end of the dry season. Burros, however, were moving in and out of the Rim Camp area. The two species were not identically distributed on 64 Alcedo, although their patterns of distributor! are similar. Both burro and tortoise distributions may be the result of the combined influences of water and food availability. Food availability on Alcedo will be dis- cussed in Chapter Six. Burro Foaling Season and Group Size/Composition A peak in burro natality occurs on Alcedo during the rainy months, when ample food and water resources are available. Infant burros were seen during all months of the year on Alcedo, however. Moehlman (1974) likewise found year round reproduction in the burros of Death Valley, with a peak in births occurring when forage was abundant, between May and July. Foaling in the burro herds of the Grand Canyon was restricted to the months of March to July (USDI, 1977). Woodward (1976) reported no peak foaling season for the Colorado River feral burro herds and proposed that the mild winters there may not exert selective pressures towards the development of a distinct breeding season. Water is available to these burros all year long. Indeed, it seems likely that peak birth seasons are selected for only in populations where foals produced out of season have a decreased chance of survival. This may well be the case on Alcedo; giving birth in the dry season may be selected against since females with foals would be burdened with an added fluid stress. 65 Studies of the feral Equus asinus populations in North America have revealed that feral burros exhibit a range of social organizations and behaviors. In the southwestern United States, adult male burros are typi- cally solitary and some are territorial. Temporary groups of mixed ages and sexes are common and the basic stable unit is the mother and offspring pair (Woodward, 1979). Koehler (1974) is alone in reporting the occur- rence of stable groups in the Southwest of four to six animals existing for several months on the periphery of the feral burro range at Bandelier, New Mexico. However, on the lush humid island of Ossabaw, Georgia, stable burro groups are the rule, rather than the exception (Moehlman, 1979 and McCort , 1979). On Ossabaw, the occurrence of stable harem groups and of high sociability (greeting, mutual grooming, social play in foals, etc.) among these groups may be in response to an environment with near optimal conditions (Moehlman, 1979). The social organization of the wild African ass (Equus asinus) has been described by Klingel (1972, 1977) as a form of territoriality where the only stable groups are mother and offspring pairs. Fowler et al., (in prep.) found that during the wet season on Alcedo, stable harem burro groups with specific home ranges occurred. During the dry season, when food and water resources are not abundant, these groups may disband and the individ- uals then disperse. Evidently burro social organization 66 is extremely plastic and is strongly influenced by various environmental factors. In the Alcedo population, 34.6 percent of all burro sightings were of single animals, mostly males. This falls into the range of percentages of solitary animals recorded in other feral burro studies; 23.9 percent soli- tary males reported by Moehlman in Death Valley and 50 percent by O'Farrell (1973) in one of his study herds in Nevada-Arizona. For Death Valley burros, 60 percent of all groups contained two to four individuals; 51 per- cent of the groups were of two to four animals on Alcedo. Larger groups of up to 20-29 burros were reported in most of the studies from the southwestern United States. These large herds were usually associated with a scarce resource such as water or an estrous female. I saw simi- lar herds on Volcan Alcedo; 'many of the largest were gathered around shade trees and favorite dust-bathing localities. Other large, predominantly male groups were undoubtedly temporarily attracted together by an estrous female. On Alcedo, only four percent of all burro groups contained eight or more animals; similarly in Death Valley, three percent of the burro groups sighted were of 8-21 individuals (Moehlman, 1979). Solitary female burros and all-female groups appear to be more common on Alcedo than in other study areas; other researchers have seldom noted females alone. 67 Young burros (those still with their mothers and not yet full grown) make up 9.3 percent of the Alcedo burro population. Seegmiller and Ohmart (1981) found that the age structure in the Bill Williams Mountains was 64.4 percent adult, 16.7 percent yearlings and 18.9 per- cent foals. Woodward (1976) has comparable data from the burro herds in southeastern California. According to her, the Chemehuevi Mountain burros epitomize a success- ful colonizing exotic species in a habitat without limi- tations; they show precocious sexual maturity and have a high reproductive rate. Twenty-three percent of that population is made up of young burros. The Alcedo population, by comparison, may be near the carrying capa- city of its environment and restricted by some limiting resource (water or food), hence, the relatively low reproductive rate. CHAPTER FOUR BURRO MORTALITY The feral burros on Volcan Alcedo live completely free from predators. The endemic and introduced pred- ators of Alcedo (hawks, owls and cats) are all too small to prey on burros. Packs of feral dogs may hunt young and weak burros elsewhere in the Archipelago, but there are no feral dogs on Alcedo. And, because theirs is an isolated population which never comes into contact with other large mammalian species, the burros of Alcedo rarely encounter disease. Hence I was intrigued, when I arrived on Alcedo in October 1979, to find fresh car- casses of numerous adult and juvenile burros. Mortality in the herds of feral burros of the southwestern United States has been mentioned by several researchers. In the populations studied, the observed natural adult mortality rates were uniformly low, with juvenile mortality somewhat higher and more variable (Moehlman, 1974; Norment and Douglas, 1977; USDI, 1977; Seegmiller and Ohmart, 1981). To investigate the appar- ently high level of burro mortality on Volcan Alcedo, I examined burro carcasses and collected teeth from the skulls. 68 69 Methods Recently dead burros were easily located by smell. The bleached bones of older skeletons also could be readily detected, particularly in the dry season when plant growth was minimal. With the bi-monthly around- the-rim censuses, frequent study site censuses and counts, and trips to the landing beach every fifteen days for food supplies, I had ample opportunity to search much of Alcedo for dead burros. There were, of course, sections of the volcano that I did not visit frequently, and some areas that I never explored. I did not discover all the burros that died on Alcedo in 1980. But I certainly found a large percentage of the animals that died along the crater rim and in my four study site areas. Beginning in January 1980, I searched for all dead burros that were detected by scent. In only four in- stances was I unable to locate or to reach a carcass. In addition to investigating fresh carcasses, I searched the crater rim and my four study site areas for older skele- tons. Dead burros were sexed and aged whenever possible. Young burros could be aged based on knowledge of Equus tooth eruption timing and sequence, but adults could not be aged in the field. Young burro carcasses could be sexed but skeletons of juvenile animals could not be. Animals older than four years could always be sexed; males have large canine teeth, while in females the 70 canines are absent or rudimentary (Simpson, 1951). The first incisor, first premolar and first molar of upper and lower jaws were collected from the skulls of adult burros. Based on the extent of decay of carcasses, I estimated the approximate date of death for each animal. Carcasses were examined for clues to the cause of death. Older skeletons were classified as to length of time since death of the animal according to the weathered appearance of the bones. I was later able to use bone weathering information from Behrensmeyer (1978) to translate these classifications into approximate estimates of years since death. Equus spp. have traditionally been aged based on tooth wear. Wear is related to diet, soil conditions, and in addition, varies from individual to individual. More recently, mammalogists have been using an aging technique in which tooth cementum layers are analyzed. Cementum is produced throughout a mammal's life by cementoblast cells on the outer surface of the tooth roots. Dark cementum bands are formed when there is a change in cementoblast activity. In North American, dark bands are thought to be formed during winter; in the tropics, they apparently coincide with the dry season (Matson, 1981). Matson's Commercial Microtechniques Lab in Milltown, Montana specializes in tooth sectioning and aging by cementum analysis. I sent incisors from dead 71 burros I had located on Alcedo to Matson's for process- ing. Decalcified teeth were sectioned longitudinally at 14 microns, stained with Giemsa and permanently mounted on microscope slides for aging (see Humason, 1972 for a description of standard paraffin preparation method). Cementum band patterns vary among species (Matson, 1981). For aging burro teeth, Matson's assumed that tooth erup- tion occurred before the age of three years; hence the first major dark cememtum band on a tooth section marked the third year of life. Matson's can only handle burro incisors; premolars and molars are too large to fit into their trimming and sectioning equipment. But as skulls deteriorate in the field, the first teeth to become loose, and therefore lost, are the incisors. Hence for some of the oldest skeletons found on Alcedo, I was unable to collect incisors. Ages of animals for which only premolars and molars were collected were based on wear criteria. Molars were measured according to Joubert (1972) and then, using linear regression, correlated to the molar measurements of animals aged by cementum analysis (correlation coefficient of lower molar length to age = .90 for females, .75 for males and .80 combined). Results Thirty recently dead burros were found on Alcedo in 1980. For 22 of these, I made notes in the field 72 concerning the stage of tooth eruption or collected teeth for laboratory aging. Four carcasses of adult burros were too fresh to extract teeth when I first discovered them and I could not locate them later. I could smell but was unable to find or reach another four dead animals. Over half (56.7%) of the dead burros I discovered had died in January 1980, at the very end of the dry sea- son (Figure 4-1). All the others, except for one which died during the rainy month of March, died between September and December of 1979 or of 1980, which were dry months on Alcedo. Twelve of the 30 burro carcasses were of adults and fourteen were of animals under two years of age. Table 4-1 summarizes the sexes and ages of the 126 burro skeletons and carcasses examined. Aging was done by the three methods previously described. The adult male to female ratio was 1 : . 95. Other researchers also have reported sex ratios approaching one-to-one in some of the feral burro populations of the southwestern United States (Moehlman, 1974; Woodward, 1976; USDI, 1977; Norment and Douglas, 1977). Burros in the age classes of three to five years, six to nine years and ten to 14 years were found at almost equal frequencies. Each class comprised between 17 and 23 percent of the total number of dead burros. Fewer dead animals (7.1%) that were older than 15 years were found. More burros of 73 •IS DC £«« OQ O < U 10 a u. ° 6 GC UJ OQ 2 2 3 J □ ADULT ^ 2 YEARS OLD OR LESS UNKNOWN Wr i ■ i n r< Y^y^ ON D J FMAMJJAJ5 0 N D 1979 1980 Figure 4-1 Months in Which Burros Died (n=30) 74 Table 4-1 Burro Sex and Age at Death (n=126*) Aging Method 0-2 Not Sexed Sex and 3-5 a ? ? Age in Years 6-9 10-14 d* ? ?