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Last updated January 1, 2014

Optimal Foraging of a Herbivorous Lizard, Green Iguana, in a Seasonal Environment

Wouter D. van Marken Lichtenbelt, Œcologia (1993) 95:246-256

Excerpted by Melissa Kaplan

Abstract
Food selection was studied in free living green iguanas (Iguana iguana) throughout the year in a semi-arid environment, Curaçao (Netherlands Antilles). Food intake was determined by direct observations and converted in biomass intake. Comparison between intake and biomass availability of the various food items revealed that the lizards were selective and that changes in seasonal food availability led to periodic switching of food plants. The extent to which nutrient constraints determine iguana feeding ecology was investigated. Potential constraints were the requirements for water, digestible crude protein, and metabolizable energy. By using a linear programming model that incorporates characteristics of the food (chemical composition, energy content, item size) and requirements and constraints of the green iguana (nutrient and energy requirements, digestive tract capacity, feeding rate) it was possible to identify which factors determine food choice over the year.

During the dry period, when the iguanas had no access to drinking water they consumed flowers to increase water intake, though the amount of flowers consumed was too low to cover the maintenance requirements for either energy or protein. After the young leaf flush, following the early rains in May, the biomass increased, free surface water was available during showers, and the linear programming solutions indicate that food selection conformed to the protein maximization criterion.

Reproduction in green iguanas shows an annual cycle in which oviposition takes place at the end of ht. dry season, when intake is below maintenance levels. Females show an 8-10 month gap between acquisition of most of the protein required for egg synthesis and the act of laying. Thus, as in avian and mammalian herbivores, food availability during a period prior to the energy and protein demanding reproductive season of iguanas determines reproductive success. Timing of the reproductive cycle has presumably evolved to maximize reproductive value. Reproduction, however, need not necessarily coincide with high food abundance. Annual maxima food availability may allow the extraction of more energy from the environment to cover the increased demand or to store energy, or alternatively reduce the cost of foraging and maintenance, and hence allow a reallocation of energy.

Generalist herbivores face a wider variety of food types than carnivores and food availability, both in quantity and quality may vary during the course of the year. Contrary to earlier models of optimal diets that were based on the supposition that energy yield per unit foraging time should be maximized (MacArthur and Pianka 1966; Emlen 1966; Schoener 1969), more recent models describe optimal diets in terms of a mixture of nutrients within a given amount of food in relation to the momentary needs of the animal. This study aims to explain the seasonal variation in the diet of a generalist herbivore reptile, the green iguana (Iguana iguana) on Curaçao (Netherlands Antilles).

The green iguana is one of the few species of lizards living as a herbivore throughout its life (Rand 1978; Troyer 1984b; White 1985). The reproductive cycle of iguanas is adapted to seasonal environments (Rand and Greene 1982). Food choice of herbivorous reptiles has been studied only recently in the natural environment and food selection by iguanines (mainly large herbivorous lizards) is restricted to a few studies (Auffenberg 1982; Christian et al. 1984; Troyer 1984a; Mautz and Nagy 1987). Only Auffenberg's (1982) study of Cyclua carinata included both food intake and food availability throughout the year.

The digestive and nutritional physiology of the green iguana (a reptile with hindgut fermentation) differs importantly from that of avian and mammalian herbivores, and hence in how nutrient constraints might affect feeding ecology. Green iguanas are ectotherms with comparatively lower metabolic rates and longer transit time of food (compare: small [< 3 kg] herbivorous mammals - less than 10 h; green iguana - 2-8 d (Troyer 1984b; van Marken Lichtenbelt 1992]. Moreover, green iguanas have a more varied diet (leaves, flowers and fruits) than most endothermic herbivores. For a large part of the year, the green iguanas (on Curaçao) have no access to drinking water and can only achieve water balance through consumption of vegetation. In the present study, food availability and food intake of green iguanas on Curaçao were studied throughout the annual cycle. Information on nutritional value, grazing behavior, digestive physiology, nutrient requirements, and food availability are used to predict the general patterns of diet makeup of the green iguana throughout the year using a linear programming model.

Methods

Study Site
The field work was conducted from July 1985-April 1988 at Sta. Barbara on Curaçao (Netherlands Antilles). The site covered some 0.7 ha [1.73 acres] of limestone boulders covered with shrubs, cactuses and small trees. The intervening areas were densely vegetated with trees, shrubs and climbing plants. The climate is semiarid with strong spatial and seasonal variation in rainfall. Mean annual rainfall amounts to 570mm [22.4 inches] to which the October-January rains contribute about 64%. Mean air temperature is 27.5°C [81.5°F], with strong northeast trade winds.

Feeding Observations
Feeding habits of free-living green iguanas were studied by continuous observation through telescopes and binoculars. Observations were carried out from blinds by two to three persons who communicated by radio. Because the green iguanas tend to forage in the top layer of the trees, foraging behavior could be observed in detail. Data on grazing were taking on a minute to minute basis, recording the number of bites and steps. For each bite, plant species, part of plant, estimated size and number of items was recorded. A distinction is made between "complete observation days", those days on which individual animals were followed continuously throughout the day and bites recorded for more than 90% of their total foraging time, and food intake observed over shorter periods of time. Complete days (n=203) were used to calculate total daily intake. Metabolizable energy intake and digestible crude protein intake were calculated. Data of all recorded meals were used to study food selection.

Feces Analysis
During the first part of the study (January-May 1986) diet composition was determined by fecal analysis. Most food components were distinguishable in the droppings. Leaves were often excreted completely or in large fragments. Most species were determined microscopically. [After drying the feces, other computations were made to determine more information relative to plant and plant part selection and quantities.

Biomass availability and nutrient analyses
A vegetation map of the study area was compiled. Every four to six weeks biomass of all abundant plant species was determined in addition to observations and counts made on lizard observation days. Using fresh and dried samples, the energy content, nitrogen content, and cell wall contents were calculated.

Results

Food availability and intake
Of the 57 plant species in the area, of which 23 were abundant, the green iguana utilized 22 species. [Table 1] During the dry season (February-May), intake was dominated by Acacia tortuosa and Haematoxilon brasiletto flowers, and Bourreria succulenta berries, as overall leaf production and availability in the area declined. During the early rains in May caused a flush of young leaves in June. From June-September, young leaf intake increased up to 78% in July. August-September the diet was more of a mixture of leaves and flowers. During the rainy season (October-January) the availability of mature leaves increased; iguanas shifted to eating mature leaves rather than the young leaves and flowers selected in August-September. During relatively dry years, the diet was dominated by mature leaves.

Food selection
Diet composition was significantly different from biomass availability throughout the year indicating that food selection occurred in all phases of the annual cycle. In most periods, plant items consumed and not consumed did not differ in quality as far as cell wall constituents (NDF, ADF, lignin and cutin) and energy content were concerned. Of the cell wall components, however, ADF was nearly always lower in the diet (range:16-20%) than in the available food (range: 20-25%). Mean crude protein (CP) content of the food items in the diet was significantly higher than in the available biomass, except for the late wet and early dry seasons, due to the high amount of berries or mature leaves in the diet. Mean water content was higher in the diet than in the available biomass throughout the year but the difference was significant in the dry season only.

Daily dry matter intake (DMI), metabolizable energy intake and digestible crude protein intake reached lowest values in the dry season, showing an increase after the early rain and a decrease from November onwards.

Properties of food classes
In general, young leaves have higher protein in comparison to mature leaves, flowers and berries. Flowers and berries had the most water content. Flowers had low lignin content while berries are high in lignin. Digestibility and transit time varied: berries 3.9 days; mature leaves 8.5 days; young leaves 6.6 days; flowers 4.8 days. Digestive tract capacity (size of the stomach and colon) also plays a role in feeding and processing food, along with the relative digestibility of food, rate of water absorption, etc.

Energy requirements
Daily energy requirements are set by the daily energy expenditure. The DMI depends on the food specific metabolizable energy content and the costs of foraging (distance traveled, size of bite, distance covered per bite).

Energy and protein requirements for females
Female that produce eggs need more energy and protein for maintenance only. Energy needs for clutch production amounts to 15% of the annual daily energy expenditure (DEE).

Discussion
This study provides data on food selection in the green iguana based on direct observations of food intake and availability, including seasonal changes. Using a linear programming model that incorporates characteristics of the food (chemical composition, energy content, item size), and requirements (energy, nutrients) and constraints (digestive tract capacity, feeding rate) of the animals revealed that different criteria explain food choice, depending on the time of the year. There is a clear shift from selection conforming to the criterion of water maximization in the dry period towards protein maximization during the onset of the rainy period.

Foraging theory attempts to find general rules about what animals feed on. The most frequently used model for diet selection is the contingency model of Pulliam (1974) and Charnov (1976) which assumes that an animal maximizes the rate of ingestion of energy in food, or instead another currency, e.g. protein.

The advantage of LP modeling is the simultaneous treatment of energy and other requirements. Plant material varies widely with respect to digestibility. Moreover, transit time through the intestinal tract differs among foods as a result of differences in digestibility and absorption as well as selective retention in the gut as documented for ruminants and marsupials. Models have rarely been applied that incorporate both ingestion rate in combination with digestibility and transit time. The LP model presented here combines the ingestion of energy, protein and water with the indirectly determined rate of absorption by the digestive system as expressed in the constraint formula for the digestive tract capacity. ... The study described here shows that the same pattern holds in the herbivorous reptile. It seems to be a general principle that reproductive success in herbivores is dependent of previously stored body reserves.

Table 1. Abundant plant species at the main study site.

Type: T =tree, C =cactus, Cl =climber, S =shrub.

Place: D =between rocks, R =on rocks.

Consumed parts: + =consumed, - =not consumed, @ =no fruits or flowers observed.

			Consumed Parts
Species	Type	Place	Mature Leaf	Young Leaf	Flower	Fruit
Acacia tortuosa	T	D	+	+	+	+
Bourreria succulenta	T	D	-	+	+	+
Bursera bonairensis	T	D	+	+	-	-
Caesalpinia coriaria	T	D	+	+	+	+
Capparis flexuosa	T	D	-	+	+	@
Coccoloba swartzii	T	R	-	-	-	-
Condalia henriquezii	T	D	+	+	-	+
Cordia alba	T	D	+	+	+	+
Guapira spec.	T	D	-	+	+	@
Haematoxilon brasiletto	T	DR	-	+	+	+
Machaonia otonis	T	D	+	+	-	@
Malpighia emarginata	T	D	+	+	+	+
Metopium brownei	T	R	-	+	+	@
Randia aculeata	T	D	+	+	+	+
Trichilia trifolia	T	D	+	+	-	-
Antirhea acutata	S	R	-	-	-	+
Capparis odoratissima	S	R	-	+	+	@
Commicarpus scandens	S	D	+	+	@	@
Cordia currassavica	S	R	-	+	-	-
Erythalis fructicosa	S	R	-	-	+	+
Serjania curassavica	CL	DR	-	+	-	-
Cephalocereus lanuginosus	C	R			+	-
Cereus repandus	C	D			+	-
Lemairocereus griseus	C	R			+	-

The full article is 11 pages in length, including references. For those who want to read the entire article, it is available by email as a 1.2 MB PDF file.