The Diet of a Generalized Folivore: Iguana iguana in Panama
AS Rand, BA Dugan, H Monteza and D Vianda. Journal of Herpetology 24(2):211-214, 1990
Though many extinct archosaurs were herbivores, herbivory in modern reptiles is limited to turtles and several lineages of lizards (Rand, 1978), particularly the iguanines (lverson, 1982). These large lizards are widely distributed and in some places economically important (Fitch et al., 1982), but few detailed studies of their diet have been published: Auffenberg (1982) on Cyclura carinata and Sylber (1988) on Sauromalus varius and S. hispidus are outstanding exceptions. Here we present diet information on Iguana iguana, a much more widespread iguanine. We describe the diet of adult iguanas on three small islands in Panama Bay and compare the plants eaten with a survey of the plants present in the habitat.
Iguana iguana occurs from Mexico to Brazil (Etheridge, 1982), and is largely arboreal. It has been reported to eat a wide variety of foods including carrion (Loftin and Tyson, 1965) and insects (Hirth, 1963), but it is primarily an herbivore eating mostly leaves (Rand, 1978; McBee and McBee, 1982). Only two studies (Hirth, 1963; Van Devender, 1982) have reported the contents of a number of iguana stomachs. They found primarily vegetable material, predominantly leaves, but gave little information on the species of plants eaten. However, not all plant material, or even all leaves, are nutritionally equal (Milton, 1979), and folivores may be highly specialized as to the species or the age of the leaves they eat.
We analyzed the stomach contents of 31 iguanas collected in the course of a study on the social behavior of Iguana iguana (Dugan, 1980, 1982) and the foraging data from focal animal observations made during the same study. The study was conducted on Flamenco, Perico, and Naos Islands, at the Pacific entrance to the Panama Canal between October 1978 and January 1979. These islands are connected to one another and to the mainland by a 3 km causeway. The islands are steep-sided, covered with second-growth vegetation, and have a seasonal climate with a marked dry season from mid-December to mid-April (Dugan, 1980, 1982).
Stomach contents of 31 adult iguanas were examined. Twenty-eight were taken from October 1978-January 1979, from Perico (21), Flamenco (4), Naos (1) and the nearby mainland (2). Iguanas were captured during the day and killed within three hours of capture. Two road-killed iguanas found in February 1978 and one found in July 1978 were also examined. Stomach (but not intestinal) contents were preserved in formalin or a formalin-acetic acid-alcohol mixture. When the volume was very large, only a sample of the stomach contents was saved.
To identify the stomach contents we made a floristic survey of Flamenco and Perico Islands in February 1980. Herbarium specimens and a set of reference slides of leaf epidermis were deposited as voucher specimens at the Smithsonian Tropical Research Institute. Because iguanas swallow fruit, flowers and leaves whole or in very large pieces and little digestion occurs in the stomach (McBee and McBee, 1982), stomach contents usually could be identified by comparison with herbarium sheets. When necessary, slides were made from the leaves in the stomach and compared with the reference slides. Only a small amount of material in one stomach could not be identified.
Each of the 31 stomachs examined contained only plant material (Table 1). Food passes rapidly through an iguana stomach (11.2 hrs, Troyer, 1982) so our samples probably represent only one day's feeding. Twenty-four stomachs contained only leaves, two only fruit and one only flowers; four contained both leaves and flowers. Twenty-six species of plants were represented in the 31 iguana stomachs. Sixteen stomachs contained only a single plant species, seven had two, seven had three, and one had four.
Stomach contents analysis showed that iguanas ate mostly, but not entirely, abundant and common plant species. During the floristic survey of the islands, 95 species of plants were subjectively categorized as either abundant (17 species collected, 9 eaten), common (32 collected, 12 eaten), scarce (26 collected, 2 eaten) or rare (20 collected, 1 eaten). The one rare species eaten (Cissus sicyoides) is represented in 5 stomachs. Two species found in stomachs were not recorded as to abundance. The over-representation of the abundant and common species in the stomachs is significant at the 99% level (x2 = 18.01, df = 4, .01 < P < 0.001, scarce and rare categories combined).
Since Iguana iguana is highly arboreal it is not surprising that most of the species eaten are trees (7) and vines (14). The seven abundant plant species not eaten include 2 grasses (ChIoris radiata and Lasiacis scabriori), 1 herb (Cyperus papyrus), 2 bushes (Cnidosculos urens and Malvaviscus arboreus), 1 vine (Mucuna rostrata) and 1 tree (Cestrum racemosum). Some of the diversity in the stomach contents sample probably reflects local differences in vegetation at various collection localities on Perico and Flamenco. Iguanas have small home ranges (Dugan, 1982) and, because even on these small islands the vegetation is not completely hornogeneous, different individuals may live among different species of plants.
Though most of the leaves eaten came from the common species of trees and vines among which these arboreal lizards lived, the less common plants that were eaten may be important nutritionally. We frequently saw an iguana travel from one tree to another, feed there, and then return to the first tree, ignoring many leaves in passing. This suggests that iguanas do not find all leaves equally desirable and expend energy to feed on some leaves rather than others.
More than half of the iguana stomachs had only a single plant species; iguanas were often observed to feed in the same area for a number of days, each day taking essentially the same foods. If an iguana must vary its diet to secure a nutritional balance, or to detoxify defensive compounds, as Freeland and Janzen (1974) suggested for mammals, it does so over a period longer than a few hours.
Focal animal observations were made on Flamenco Island in February and August, 1978. Among focal animals observed, males and females were represented in roughly the same proportions as in the population (Dugan, 1980). One hour sampling periods were distributed uniformly between 0600 and 1900. Whenever an iguana in the same tree as a focal animal began to feed, the food item and, if possible, the duration of the feeding bout (time spent feeding uninterruptedly) were recorded for this animal as well.
Focal animal sampling revealed tha!t feeding bouts were least frequent in the early morning and late afternoon and most common in late morning and early afternoon (Dugan, 19-80). Though iguanas probably feed every day, they spend very little time feeding. During 378 h of focal sampling, 53 feeding bouts totaling 145 min were observed (Table 2), during which 8 species of plants were eaten. Timed bouts varied from 3 sec-17 min with most lasting 1-5 min. Of 11 bouts lasting longer than five minutes, seven involved iguanas eating foods that were only seasonally available (Cecropia flowers and new leaves and Sapium new leaves), while three involved iguanas eating the intermittently available new leaves of Genipa. Three of the 8 species of plants eaten during the focal animal observations (Genipa americana, Ficus sp., and Ipomea sp.) are missing in the stomach contents samples. This difference might reflect local differences between the places where the samples were taken or possibly a seasonal shift in diet.
Our observations on diet of Iguana iguana can be compared with two detailed studies on other iguanine lizards, by Auffenberg (1982), and Sylber (1988). Auffenberg (1982) studied the diet of Cyclura carinata on Caicos island in the Bahamas using direct observation and analysis of stomach contents and feces. Sylber (1988) reported the diets of Sauromalus varius and S. hispidus in the Gulf of California on the basis of fecal pellet analysis.
Iguana iguana is almost entirely vegetarian, feeding primarily on leaves of trees and vines. Cyclura carinata is primarily vegetarian but takes about 5% animal material. It eats many more leaves than fruit but the volumes are about equal. The Sauromalus species are strictly vegetarian. They feed mostly on bush and tree parts and take greater proportions of fruits, seeds, flowers and stems than does Iguana iguana. Sand and gravel were commonly found in the Sauromaius fecal samples and Sylber suggests that gravel at least is ingested intentionally. Though Sokol (1971) reports geophagy in Iguana iguana, we found no sand or gravel in the stomachs we examined.
The arboreal Iguana iguana sometimes comes to the ground for food while the primarily terrestrial C. carinata and Sauromalus spp. sometimes climb trees and bushes to feed. Iguana iguana eats a variety of plant species, mostly common species, but less common species also are eaten and an individual may travel substantial distances to feed. Similarly, C. carinata eats a variety of common and uncommon plant species. Auffénberg (1982) argues that there are complex food sampling patterns which result in a varied diet producing an optimal nutrient mix rather than a maximizing of caloric content. Some individual plants are browsed more heavily by C. carinata than others of the same species even when they are equally accessible. The data for Sauromalus spp. suggest that some plants that are common in the environment are not eaten, and that individual lizards may travel substantial distances to feed.
The predominance of mature leaves in the diet of these four iguanine lizards is probably because they all use hindgut fermentation of structural carbohydrates such as cellulose, as has been demonstrated in Iguana iguana (Iverson, 1982; McBee and McBee, 1982; Troyer, 1984). It is not clear how much of the difference in diet between Iguana iguana and these other iguanines are due to the much more xeric fioras in which Cyclura and Sauromalus occur, how much is related to their terrestrial foraging and the essential absence of predators on adults in the small islands where they occur, and how much is due to yet unrecognized differences in ecology or physiology. Some of these questions could be resolved by data on diets of mainland Sauromalus and of Iguana iguana populations that live on small isolated islands in the Caribbean (Lazell, 1973).
A diversity of plants is eaten by each of these species. Auffenberg (1982) suggests for Cyclura that this variety is necessary to provide the necessary nutritional mix. The observation for iguana that individual stomachs contain only material from one or two plant species argues that if an individual must balance its diet it does so over a period of days, not of hours. Experimental evidence on the effects of different natural diets will resolve this question.
The ability to subsist on mature leaves of a wide variety of plants, a diet exploited in the neotropics by only a few other arboreal herbivorous vertebrates such as sloths and howler monkeys (Montgomery, 1978), may be an important factor explaining the large geographic range and wide variety of habitats occupied by Iguana iguana.
B=Bush; H=Herb; T=Tree, V=Vine.
? = abundance in habitat not recorded
Also of interest is comparing the green iguana to the chuckwalla (Sauromalus sp.). Chuckawallas live in an arid/desert environment compared to even the semi-arid environment in which the iguanas studied by van Marken Lichtenbelt studied on Curacçao. There, van Marken Lichtenbelt observed that during the dry season, the green iguanas consumed more flowers and fruit--which contained more water--than leaves. Presumably, that is what the Sauromalus are doing as well. Studies of green iguana gut contents from iguanas living and feeding in undisturbed areas of their native range consistently turn up no evidence of selective omnivory. In this Rand et al. paper, the authors allude to omnivory ("Iguana iguana is almost entirely vegetarian, feeding primarily on leaves of trees and vines") but no mention is found of any direct observations of selection of invertebrate or other prey, or of carrion, nor is there any mention of finding any in their examination of the stomach contents.
The issue of geophagy is, I feel, similarly in question. The Sauromalus live in a rocky, sandy, gravelly desert environment; green iguanas live semi-arid to wet forest areas with dense vegetation. Sauromalus typically feed on the ground; green iguanas typically feed up in trees or shrubs. Thus, the opportunities of inadvertent ingestion of small stones and gravel is far more likely in Sauromalus than in the green iguana.
The authors themselves raise questions on these issues. This highlights the need to not base species diet composition, range and other behavioral considerations solely or primarily on the findings of studies done on very small man-made island animal populations.
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