Marking Techniques
Iguanas can be marked for research purposes either temporarily or permanently using a variety of methodologies. Some techniques allow for individual identity, while others indicate only whether the animal has been sampled or not. Further, while some methods require recapture to determine individual identity, several techniques allow recognition of individuals without the need for recapture. The importance of choosing an appropriate marking technique is illustrated by the experience of workers at the Hope Zoo in Jamaica (Hudson 1994). Captive-hatched Jamaican iguanas were initially marked with paint and photos were taken of head scalation patterns. Over time, however, the paint markings vanished and the scalation patterns proved unreliable. As a result, the identities of individuals had to be verified later by genetic analyses.
Selection of marking method depends on the nature of the study, and should take into consideration the number of times animals will be sampled, whether data from individuals are important, and constraints of time and money. Also important is whether a particular marking system might compromise collection of certain data. For survivorship studies, conspicuous marks could increase risk of predation, particularly for juveniles. In addition, the regulating agency granting permission for the work may object to or favor certain techniques, especially when endangered taxa are involved.
Temporary marks
Painting and tagging are commonly used techniques which permit recognition of previously sampled individuals over the short term. Moreover, they can be used for individual recognition without the need for recapture. For example, J. Iverson (personal communication) and C. Knapp (1995) applied fingernail polish or enamel paint to the dorsum of Allen's Cay iguanas and Exuma Island iguanas in the field to recognize and avoid recapturing animals. Wiewandt (1977), using a plastic dart gun, and Iverson (1979), employing a squirt gun-like syringe, painted the dorsum of iguanas from a distance to individually mark and study the behavior of Mona Island iguanas and Turks and Caicos iguanas without the need for disruptive capture and handling. Alphanumeric characters painted on the dorsum have been used in the field to recognize individuals of Exuma Island iguanas (Coenen 1995), desert iguanas (Glinski and Krekorian 1985), chuckwallas (Smits 1985) and marine iguanas (Laurie 1989; Wikelski and Trillmich 1994). In general, enamel paint will remain on the integument of iguanas for months, usually until the animal sheds. Henderson (1974) used a felt-tip pen to mark numbers on the dorsum of common iguanas but the marks faded after several weeks. As a general rule, red marks should probably be avoided because iguanas are highly attracted to this color and will bite at it persistently (J. Lazell and N. Mitchell, personal observation).
Tags of various sorts can also be employed for marking purposes. For example, Minnich and Shoemaker (1970) marked desert iguanas with colored cloth tape around the base of the tail, Henderson (1974) tied small bells to the necks of common iguanas with fishing line, and Rao and Rajabai (1972) tagged agamid lizards with different shapes of colored aluminum rings placed around the thigh. Leg bands are used routinely for marking birds and could also work well for iguanas.
Permanent marks
Although many permanent marking systems require recapture of study animals to establish individual identity, some allow for recognition of free-ranging individuals without the need for recapture. Toe-clipping is the most widely used technique for permanently marking lizards (Ferner 1979), and has been employed in long-term field studies of Allen's Cay iguanas (J. Iverson, personal communication). Using a coded system, the number of individuals that can be uniquely marked is substantial (Ferner 1979). While concerns about pain and potential harmful effects resulting from missing toes exist, toes are frequently lost naturally and no studies to date have shown that toe loss compromises survival of lizards (Dodd 1993). When toe-clipping is the only method used, animals must be recaptured to confirm individual identities, and confusion may result if additional toes are lost subsequent to marking. Branding the integument of animals with unique marks can be accomplished by heat or freezing (Ferner 1979; Honneger 1979). M. Wikelski and D. Werner (personal communication) have used heat branding to mark alphanumeric codes on marine and common iguanas with no ill effects. These brands can be read only at close distances, but can be combined with more visible but temporary paint marks.
Affixing colored glass beads to the nuchal crest has recently become popular among investigators studying large iguanas (Rodda et al. 1988; van Marken Lichtenbelt et al. 1993; G. Gerber and A. Alberts, personal communication; W. Hayes and R. Carter, unpublished studies). This method, developed for iguanas by Rodda et al. (1988), can be applied not only to adult iguanas but also to juveniles, as the number and sizes of beads affixed to the crest can be adjusted for animal size. Pain and ill effects associated with the procedure appear to be minimal. Occasional loss of the beads has been noted (Rodda et al. 1988; W. Hayes and R. Carter, unpublished observations), most likely resulting from degradation of beads (especially if plastic) and suture material (usually monofilament line), or biting of the beads by conspecifics. Glass beads not only retain their color better than plastic beads, but are also superior in resistance to crushing. Suture material should be selected for resistance to degradation by ultraviolet light. The beads are fairly visible from a distance, especially with the aid of binoculars, making this technique valuable for mark-recapture studies.
A potential drawback to color-coded beads is that their conspicuousness may render marked animals more visible to predators. This evidently was not a problem for common iguanas on the mainland (Rodda et al.1988), and the adults of large insular taxa such as rock iguanas are generally under reduced predation pressure. Because iguanas are inclined to taste or tongue-touch brightly colored objects, juveniles marked by this method could potentially be injured or killed by curious adults. Conceivably, the brightly colored beads could also influence mate choice, as has been demonstrated for colored leg bands in certain birds (Harvey 1986). Finally, visitors to natural iguana populations may find the color markers to be of interest, as evidence that someone cares enough to study the animals, or as an unattractive distraction. More study of the consequences of colored bead marking is needed to adequately evaluate their effectiveness.
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Color-coded glass beads attached to the crest are being used increasingly for permanent marking of iguanas. |
Passive integrated transponder (PIT) tags have received attention recently as a means of permanently marking animals for unambiguous individual identification. PIT tags contain a unique magnetic signature packaged within a tiny glass capsule, and a virtually unlimited number of codes are available. They are usually inserted by syringe just beneath the surface of the skin. For larger animals, insertion into the thigh is recommended, while for hatchlings and smaller juveniles, the inguinal region is preferred. Upon recapture, a hand-held, battery-powered transponder is passed within a few centimeters of the animal to obtain a reading of the tag's unique identification code. Although this technique has primarily been used in captive recovery programs (e.g., Grand Cayman and Jamaican iguanas, R. Hudson, personal communication), J. Iverson, F. Burton, P. Vogel, and A. Alberts have begun to use the method in the field. PIT tags have the added advantage of offering law enforcement officials the possibility of proving smuggling charges should animals having implants appear in the international wildlife trade. All field investigators of endangered taxa should consider adopting this technique, at least as a secondary marking system. Unfortunately, some of the more sophisticated animal smugglers and dealers have become alert to the possible presence of PIT tags, and purchase their own receivers to detect and remove them.
Radiotelemetry
Radiotelemetry has proven to be a useful tool in evaluating aspects of West Indian iguana ecology and management strategy. It has provided information about home range and social structure for iguanas on Anegada Island and in the Lesser Antilles, and is being used to monitor the reintroduction of headstarted Jamaican iguanas, released as subadults into the Hellshire Hills region.
Two-stage external transmitters have been most effective for studies of wild iguanas because they can support relatively large antennas (e.g., 1/4l whips) which give greater range of reception. This is important for species with large home ranges or those using underground refugia. External tags may also be fitted with larger batteries than internal tags, which decreases the frequency of battery replacement and handling of animals. Externally mounted tags, however, are more apt to be damaged by sun or battered in rocky habitats than internal tags.
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A shoulder harness attachment for radiotransmitters has been used successfully in the Hellshire Hills, Jamaica. |
Transmitters have been mounted successfully on neck collars (M. Day, personal communication), waist belts (N. Mitchell, 1999), and shoulder harnesses (R. Hudson, personal communication). For more approachable animals, two-stage tags with small wire loop antennas (originally designed for rats) have been imbedded in paraffin, coated by a thin layer of dental acrylic, wrapped in chicken skin, and placed in locations where iguanas will encounter and ingest them. This technique has yielded four to seven days of data after which the tag is expelled with the feces (Goodyear and Lazell 1994). An advantage of this method is that the animal need not be handled. Animal positions have been determined by triangulating remotely using receivers at fixed telemetry stations or by directly approaching the animal and recording its location using a global positioning system. Collapsible yagi antennas used with small receivers work well in the dense scrub and forest habitats occupied by many iguanas.
