Testing Central Place Foraging Theory in the Ant Genus Pogonomyrmex: a Review of the Literature from 1977 Through 2008

Testing Central Place Foraging Theory in the Ant Genus Pogonomyrmex: a review of the literature from 1977 through 2008 A.B. Mayo Animal Population Ecology Dr. Miriam Benabib December 9, 2009 1 INTRODUCTION Foraging strategy is one of the most important and defining traits for any animal species because it relates obtaining energy to ecological situation. Thus, foraging behaviors have a direct impact on fitness and selection is expected to be fairly strong resulting in optimal patterns (Schoener 1971; Pyke 1984). Although morphological and physiological characteristics also play significant roles in foraging, optimal foraging theory focuses on behavioral components which are related to fitness through a currency and involve a time component (Pyke 1984). Optimal foraging theory attempts to predict the optimal diet based on maximizing benefit to cost of the currency. (See Schoener 1971; Pyke, Pulliam and Charnov 1977; Pyke 1984; Schoener 1983). The original optimal foraging models predicted diet choice for animals that forage individually and eat basically where they capture or find their prey. Problems in using these models quickly become apparent when applied to social insects which forage from a central place to which they return with the forage and eat communally. Central place foraging theory was developed to account for costs associated with the distance to and from the food resource, additional handling of prey, and transporting the prey (Orians and Pearson 1979; Schoener 1979; Olsson, Brown, and Helf 2008). Optimal foraging models usually assume that, in the long-term, net energy gain is maximized, that animals have perfect knowledge of patches, and that animals have simultaneous detection of available prey. However, some social insects maximize their net energy efficiency (Houston, Schmid-Hempel, Kacelnik 1988) and ants probably detect prey primarily by physical encounter, not necessarily taking the first item. Thus, ants make foraging decisions based on encounter rates through sequential assessment of forage (Davidson 1978). 2 Models of ant foraging are further complicated by eusociality, a variety of foraging strategies, possible variation at the individual forager as well as colony level, and that ants cache forage (Traniello 1989). Besides variation in behavior, some ant species exhibit worker polymorphism which may result in size matching between ant morphology and forage (Traniello 1989; Morehead and Feener 1998). Ant foraging strategies also display a continuum from individual forms (in which foragers leave the nest in random directions) to collective forms mediated by pheromones (for instance, ants that follow a trunk trail to a foraging area). Keeping these concerns in mind, this review will focus on the application of central place foraging theory to seed harvesting ants in the genus Pogonomyrmex. METHODS The general foraging literature was searched for reviews, explanations, and tests of optimal and central place foraging with an emphasis on foraging in ants for background on the theories in general. The literate included in this review was restricted specifically to field based tests of optimal and central place foraging in the seed harvesting ant genus Pogonomyrmex (Hymenoptera: Formicidae: Myrmicinae) and includes papers from 1977 through 2008. LITERATURE REVIEW Background Seed harvesting ants of the genus Pogonomyrmex, are very similar in morphology and ecological niche. Workers are monomorphic and worker castes typically differ by age with foragers being oldest (Gordon 1999). Though Pogonomyrmex ants live in a diversity of habitats, many of these ants are prominent species in arid habitats which are severely seed limited 3 (Bernstein 1974). For studied species, the diet is 60-100% seeds and when Pogonomyrmex species co-occur, there can be intense competition for food resources. These ants also forage on frass, vegetative and floral parts of plants, and animal parts. However, consideration of these non-seed items has been excluded from tests of optimal foraging probably because they are a small portion of the diet and animal parts may be foraged for proteins and not carbohydrates (energy), which may complicate the choice of currency in the model. A more detailed understanding of ant diet is needed (Kay 2002). Despite the importance of foraging to colony structure and therefore, its impact on social organization and fitness, there are few studies that specifically test central place foraging in Pogonomyrmex ants. The studies done focus on different assumptions, use different techniques or variable ranges, and assume that all Pogonomyrmex species are similar enough that the findings from one study apply to other species. Of the 40+ species in the genus, those studied include P. barbatus, P. californicus, P. desertorum, P. maricopa, P. occidentalis, and P. rugosus, all of which are arid habitat dwellers. Despite morphological and niche similarities, these species show variability in foraging strategy (trunk trail foraging, individual foraging, or mixed strategies) which alters the basic concerns of the theory such as distance, energy and time costs on the individual forager level, among colonies, and among Pogonomyrmex species. Because several of these species co-occur, a consideration of differences in foraging strategies among species may help explain how they co-exist (Whitford 1978; Morehead and Feener 1998). For testing central place foraging models in Pogonomyrmex, a very basic model for optimal foraging is: ( ) 4 with e, the net energy gain from the forage item; B, the benefit (gross) of the item foraged; and C, the cost of the foraging the item; and the subscript i, designating the particular item. The costs of foraging include energy expended in travel to the patch ( ) , in search within the patch ( ), in handling the forage ( ), and in the return travel carrying the forage ( ) (based on Taylor 1977, 1978 and other articles in this review). This model assumes that energy (ei) is the currency. If the ants are foraging optimally, then they will select seeds that maximize the net energy intake such that, as the costs change, so do the seeds preferred. The tests which are reviewed here were field manipulations in which different variables were altered and the resulting choice of forage, rate change, or efficiency were assessed. Summary of Results From the empirical tests of optimal and central place foraging in the Pogonomyrmex ants, certain patterns are shown across the species studied. While generally, the foraging patterns fit central place foraging models, some important differences were found (Bailey and Polis 1987). It is lamentable that, as Taylor pointed out, many of the parameters that are needed to fully test central place foraging models in ants are very difficult to obtain (Taylor 1977, 1978). These missing measurements were assumed constant or assumed controlled for across colonies. The primary concern in these studies has been to establish the currency, the important time costs, how forage items are ranked, and how these concerns vary among species and might influence co-existence. Although foraging theories focus on behavioral traits, behaviors are shaped and often constrained by morphological features. Since Pogonomyrmex ants carry seeds in their mandibles and these species vary in size, several morphological features of the head, thorax, and leg were tested for their importance in reducing the cost of foraging (Morehead and Feener 1998). None 5 of the morphological traits had a significant impact on seed selection. This test also refutes the idea that there might be seed to body size matching. The currency for foraging in these ants is the net energy gain (Taylor 1977; Bailey and Polis 1987; Fewell 1988; Weier and Feener 1995; Morehead and Feener 1998). Taylor (1977) was able to rule out minimization models and Fewell (1988) found that P. occidentalis preferred low plant cover trails which yielded the same net energy gain as more dense cover trails, even though the use of low cover trails was less energy efficient. The cost of foraging is nearly negligible (Weier and Feener 1995) with an estimate for P. occidentalis of < 0.1% of the energy from the forage (Fewell 1988). Weier and Feener (1995) found that, despite the employment of different foraging strategies by P. rugosus and P. maricopa, energy gain per trip, about 100 times the cost of these trips, did not differ significantly. This result led Weier and Feener to suggest that the cost of foraging could be “safely ignored in future studies.” In earlier studies, handling and return time had been assumed constant but some Pogonomyrmex ants do remove various extra-floral and other seed parts (Whitford 1978). Contrary to these studies, Bailey and Polis (1987) found that both handling time, including travel time back to the nest, and success rate of securing a seed increased with seed size. From Pyke’s review (1977), Pogonomyrmex ants are expected to rank seed types based on expectations of energy intake for each seed type (Davidson 1978). The ants did optimize net energy gain in terms of seed size and ranked seeds consistently according to size with respect to changes in distance from the nest, abundance of seeds, and size of the seed patch (Taylor 1977). While Taylor (1977) and Davidson (1978) found evidence to support this ranking of seeds by size, the ants did not narrow the size range of seeds as distance increased in Taylor’s work but did in Davidson’s study, though the ants never took the smallest seeds offered. Davidson 6 suggested that Taylor did not use a great enough distance to show this effect. However, Davidson did find partial preferences which also challenges optimal foraging. The partial preferences probably reflect individual forager variation as well as the need for foragers to continually encounter seeds in order to consistently rank them. Continual encounter cannot be assumed for any forager since individual foragers may make few trips in any one day. This brings us to a particular concern of ants: recruitment. Recruitment to a patch may be achieved by an increase in the total number of ants recruited to that area or an increase in the rate of a particular forager’s return to that patch.

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