Natural history of the black-chested spiny-tailed Iguanas, Ctenosaura Melanosterna (Iguanidae), on Cayo Cochnino Menor, Honduras.
Two populations exist within the Cayos Cochinos Archipelago Natural Marine Reserve operated by the nongovernmental Honduran Coral Reef Foundation. The population on Cayo Cochino Menor (CC Menor) is protected by the presence of a research establishment of the Foundation; however, the second population on Cayo Cochino Mayor (CC Mayor), although legally protected, remains susceptible to poaching due to limited presence of the Foundation on the island, leaving this population of C. melanosterna increasingly vulnerable. The vulnerability of the population on CC Mayor is heightened by the affinity of the species for human settlements (Wilson and Cruz Diaz, 1993), which may make the iguanas more susceptible to poaching. Exploitation and destruction of habitat on CC Mayor leaves the population on CC Menor as the most stable of the species. However, continued threats, including poaching, potential for invasion by feral mammals, and competition with the increasing population of green iguanas (Iguana iguana) continue to put C. melanosterna at risk on CC Menor.
The objective of our study was to examine the basic ecology and life history of C. melanosterna on CC Menor, including aspects of morphology, reproduction, diet, and use of habitat. Such information is important for establishing strategies of management for the conservation of C. melanosterna, an evolutionary significant unit, on the Cayos Cochinos.
The Cayos Cochinos are a group of small islands ca. 17 km north of the mainland town of Nueva Armenia, Department of Atbintida, Honduras, and part of the Department of Islas de la Bahia, Honduras (Fig. 1; Wilson and Cruz Diaz, 1993). The archipelago consists of two main islands and several smaller cays, with a total of 2.28 [km.sup.2] of land (Wilson and Cruz Diaz, 1993). The smaller of the two main islands, CC Menor is ca. 0.64 [km.sup.2], and the larger, CC Mayor, is ca. 1.55 [km.sup.2] in area (Wilson and Cruz Diaz, 1993). With the exception of a small research station operated by the Honduran Coral Reef Foundation, CC Menor is uninhabited. The primary habitats of CC Menor include hill forest, dominated mainly by tropical lowland oak (Quercus oleoides), and wind-swept forest, dominated by Q. oleoides, prostrate sea grapes (Coccoloba uvifera), or a mixture of the two (Wilson and Cruz Diaz, 1993). Other habitats on the islands include palm forest, beach scrub, rocky promontory, and mangrove forest (Wilson and Cruz Diaz, 1993; Bermingham et al., 1998; Montgomery et al., 2007). Ctenosaura melanosterna is known to inhabit palm groves, rocky promontories, hill forest, and wind scrub and to have an affinity for human settlements (Wilson and Cruz Diaz, 1993; Shaw, 2005). Based on climatic data collected at the station on CC Menor, rainfall on the island is lowest in April, May, and June (43.2, 53.9, and 81.0 mm, respectively) and highest in October, November, December, and January (400.2, 460.0, 425.5, and 331.4 mm, respectively). Mean monthly temperatures ranged from 26-29[degrees]C between 1988 and 2007, with the hottest temperatures during the dry season.
We conducted periodic fieldwork across seasons from 2007-2012: 12 May-24 August 2007; 29 December 2007-1 January 2008; 14 March-21 August 2008; 16-26 October 2008; 31 December 2008-3 January 2009; 7-9 March 2011; 14 May-9 August 2011; 25 June-26 July 2012. We captured lizards in the field using a combination of noosing, live box-traps, nets, and pitfall traps. We processed all captured lizards in the field or at the Honduras Coral Reef Foundation Research Station and released them within 24 h. At the site of capture, we recorded date, time, and location in Universal Transvers Mercator coordinates (WGS84) using a Garmin GPS II Plus (Garmin International, Olathe, Kansas). During processing, we measured whole body mass, snout-vent length, and tail length. We measured mass to the nearest gram using Pesola spring scales or an electronic balance, and length to the nearest millimeter (error rate [+ or -] 1.0% of total length) using a measuring tape. In addition, we recorded sex (by external morphology or cloacal probing) and stage class (adult, juvenile, or young-of-the-year). We categorized individuals as adult ([greater than or equal to] 15.0 cm), juvenile (7.0-15.0 cm) or hatchling (<7.0 cm) according to reports in the literature (Gutsche and Streich, 2009; Pasachnik et al., 2012a). We permanently marked each adult lizard by injection of a small (11.0 x 2.1 mm) polymer-encased microchip (passive integrated transponder, PIT, tag; Biomark, Inc., Boise, Idaho) intraperitoneally or subcutaneously in the left lateral abdominal region (Camper and Dixon, 1988) and a unique bead tag combination in the nape of the neck (Rodda et al., 1998). We marked the smallest individuals by toe-clipping. In addition, we temporarily marked each lizard with a nontoxic paint-mark on the left and right lateral surface of the body so that recent captures could be identified from a distance.
We made observations on the timing of various reproductive behaviors, including courtship, male-male combat, mating, and nesting. We examined eight (two recently laid and three hatched) nests to determine characteristics of nests and size of clutches. We examined characteristics of hatchlings and time of emergence by capturing newly hatched individuals. We examined broad patterns of use of habitat by conducting line-transects through all representative habitats on CC Menor. We examined dietary breadth through observations of foraging. We also examined predatory attempts by other taxa, primarily Boa constrictor.
We captured 583 individual lizards (90 males, 117 females, and 376 juveniles) during the study. Adult males (24.28 [+ or -] 0.51 cm in snout-vent length) were significantly larger than adult females (21.50 [+ or -] 0.31 cm in snout-vent length; t-test, P < 0.0001), as is common among other iguanids (e.g., Wikelski and Trillmich, 1997; Beovides-Cases and Mancina, 2006), and specifically other Ctenosaura (Pasachnik et al., 2012b; Pasachnik, 2013). We captured significantly more hatchlings and juveniles than adults; however, there is a large bias towards hatchlings and juveniles because of techniques for capture. The sex ratio of the adults was slightly female-biased, which is commonly observed in iguanids from stable populations (Fitch and Henderson, 1977; Munoz et al., 2003; Pasachnik, 2013).
As with other lizards, size of territory seemed to be dependent on food resources based on our observations of marked individuals (Krekorian, 1976; Christian and Tracy, 1985). Larger males occupied smaller territories around the restaurant at the research station, where discarded food-scraps were plentiful. Away from the station, lizards occurred at much lower densities, and adult males seemed to have larger territories. Adult males were observed defending territories aggressively from other adult males, while females and juvenile males moved freely between and among male territories.
We observed courtship and mating behavior during spring from 6 March through 30 June. Nesting occurred from 29 April through 15 June. Females dug subterranean nests in loose, sandy substrates, with little canopy cover in beach, low vegetation, and palm forest across the island. Nests have an entrance that leads to a chamber that averages 34.6 cm (25-57 cm; n = 11) below the surface. We did observe shallow, empty burrows, indicating females initially dig test burrows prior to nesting, possibly to locate nest-sites with optimal conditions. Following deposition of eggs, females back-filled and closed the burrow. Eleven nests containing eggs were discovered between 1 June and 19 August. Clutches consisted of 16-38 eggs (n = 11; average = 19.4 [+ or -] 9.6), which is within the range of sizes reported in captivity (11-41; www.iucnredlist.org). Size of clutches increased later in the season (Least Squares Regression, [r.sup.2] = 0.82; P < 0.05; Fig. 2), which may indicate that larger females were nesting later in the season. Timing of captures indicates that hatchlings emerge from mid-June to mid-September (Fig. 3). Later nesting results in emergence of hatchlings at the onset of the wet season, which may provide hatchlings with greater availability of resources (Snell and Tracy, 1985; van Marken Lichtenbelt and Albers, 1993).
Observations of foraging events and examination of scats indicate that C. melanosterna are omnivores, consuming vegetation (flowers, fruits, and leaves), invertebrates (insects, mollusks, and crustaceans), and vertebrates (reptiles and birds; Table 1). In addition, C. melanosterna will consume food-scraps and carrion opportunistically. Ctenosaura and other iguanids have been reported to have omnivorous diets (Christian et al., 1984; Troyer, 1984; Rand et al., 1990; Durtsche, 2000; Pasachnik and Chavarria, 2011; Pasachnik and Corneil, 2012), and many show an ontogenetic shift from a more carnivorous diet to a more herbivorous diet with increased age (Durtsche, 2000).
To examine use of macrohabitat, we established 10 line-transects (50-200 m in length) within the six dominant macrohabitats across CC Menor, including oak forest, wind-swept oak forest, palm forest, low beach-vegetation, gumbolimbo forest, and beach (Fig. 4). We surveyed each transect twice per week during summer 2011. We recorded each individual encountered along each transect and indicated if the animal was a new or previously marked individual. Considering these data, as well as opportunistic observations and captures, C. melanosterna has an affinity for habitats with open canopy, including beach, beach scrub, wind-swept forest, and trail edge (Fig. 4). Those individuals encountered in closed-canopy oak-hill forest were often associated with tree-fall gaps. Data from line-transects and opportunistic encounters and captures demonstrate that C. melanosterna is active throughout the year; however, activity declines during the wet season, likely due to lower temperatures and reduced solar radiation.
The most observed source of mortality for the species is predation by Boa constrictor (Reed et al., 2006). Boa constrictor on CC Menor occur at high density presumably due to availability of abundant food and the absence of mammalian predators. Additional predators included the boat-tailed grackle (Quiscalus major) which was observed consuming hatchlings and juveniles. The Honduras Coral Reef Foundation also had guard dogs periodically during the study. Some of these dogs were observed capturing and killing adult C. melanosterna. After consultation with the Foundation, these dogs were kept in pens during the day to reduce their impact on the population of iguanas. Another potential threat to survival of C. melanosterna, directly and indirectly, is the recent observed increase in the population of the green iguana (Iguana iguana) on CC Menor. Green iguanas appear to be native to Cayos Cochinos, but, until recently, they have appeared to be at relatively low density (Shaw, 2005). Green iguanas may compete with C. melanosterna for food, directly and indirectly, as well as for nesting and basking sites (JAF, pers. observ.; Krysko et al., 2007).
The population of C. melanosterna on CC Menor appears to be more stable than the population on CC Mayor and the mainland due to the presence of the Honduras Coral Reef Foundation, which limits the opportunities for exploitation by humans. The higher relative density of C. melanosterna near the research station also makes this population susceptible to increased stress due to encounters with humans (French et al., 2010). These potential threats must be considered when making decisions about management of the species and the area as a whole. Continued monitoring and assessment of recruitment and mortality of the population on CC Menor are crucial for assessing the effects of potential threats and protecting the stability of this population.
Submitted 17 April 2013. Acceptance recommended by Associate Editor Charles M. Watson 14 February 2014.
We thank M. Logan, S. Waddell, G. Demare, A. Beese, A. Dalecki, C. Noelle, J. Owens, and M. Howard for assistance in the field with collection of data. We thank J. Sheppard for assistance with construction of the map. We also thank C. Sempertegui for assistance with translation of the abstract into Spanish. We thank the Honduran Coral Reef Fund and Operation Wallacea for logistical support. Truman State University (Kirksville, Missouri), International Iguana Foundation, and United States Fish and Wildlife Service Wildlife Without Borders provided financial support. All research was complied in accordance with Institutional Animal Care and Use Committee guidelines of Truman State University. Research permits were granted by the Honduras Coral Reef Foundation and the Republic of Honduras Instituto Nacional de Conservacion y Desarrollo Forestal.
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CHAD E. MONTGOMERY, * STESHA A. PASACHNIK, LESLIE E. RUYLE, JULIUS A. FRAZIER, AND STEVEN E. W. GREEN
Biology Department, Truman State University, Kirksville, MO 63501 (CEM)
Bay Islands Foundation, Honduras (SAP)
Odum School of Ecology, University of Georgia, Athens, GA 30602 (LER)
Biology Department, California Polytechnic State University, San Luis Obispo, CA 93401 (JAF)
Operation Wallacea, Wallace House, Old Bolingbroke, Lincolnshire, PE23 4EX, United Kingdom (SEWG)
Present address of SAP: Institute for Conservation Research, San Diego Zoo Global, 15600 San Pasqual Valley Road, Escondido, CA 92027
Present address of LER: Applied Biodiversity Science Program, Texas A&M University, College Station, TX 77843
* Correspondent: email@example.com
TABLE 1--Dietary items observed being ingested or in fecal samples of Ctenosaura melanosterna on Cayo Cochino Menor, Honduras. Dietary item Common name Taxonomic classification Leaves Hibiscus Hibiscus rosa-sinensis Guacamayo Andira inermis Fruits Mango Mangifera indica Sea grape Coccoloba uvifera Guacamayo Andira inermis Palm Thrinax Beach almond Terminalia catappa Flowers Hibiscus Hibiscus rosa-sinensis invertebrates Cricket Gryliidae Ant Formicidae Wood roach Polyphagidae Land crab Ocypodidae Vertebrates Black-chested Ctenosaura melanosterna spiny-tailed iguana Green iguana Iguana iguana Allison's anole Anolis allisoni Yucatan vireo Vireo magister Fork-tailed emerald (a) Chlorostilbon canivetti (a) Observed attempted predation.
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|Author:||Montgomery, Chad E.; Pasachnik, Stesha A.; Ruyle, Leslie E.; Frazier, Julius A.; Green, Steven E.W.|
|Date:||Jun 1, 2014|
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