Optimal Foraging of a Herbivorous Lizard, Green Iguana, in a Seasonal Environment
Wouter D. van Marken Lichtenbelt, cologia (1993) 95:246-256
Excerpted by Melissa Kaplan
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.
availability and nutrient analyses
availability and intake
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.
of food classes
and protein requirements for females
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.
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.
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.
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