Oecologia (Berlin) (1983) 60:122-126 Oecologia 9 Springer-Verlag 1983 Passive aggression: An alternative hypothesis for the Piper-Pheidole association D.K. Letourneau University of California/Berkeley, Division of Biological Control, 1050 San Pablo Avenue, Albany, CA 94706, USA Summary. Myrmecophytic Piper spp. were re-examined in view of what has become a common expectation in studies of mutualistic ant@ant associations: overtly aggressive be- havioral traits in ants that function in anti-herbivore de- fense. Piper plants provide food and shelter for the Pheidole ant inhabitants. The ants, small and "sluggish", were pre- viously thought to serve in nutrient procurement for the plant even though their foraging is probably restricted to the plant surface and most of the ants' food is produced by the plant itself. An alternative hypothesis, that Pheidole ants function in anti-herbivore defense by disrupting herbivores as eggs or early instars occurring on Piper foliage, was tested using the following lines of reasoning: (1) If the youngest, tender leaves are more vulnerable to herbivore attack, or simply more valuable to the plant, then ants may "patrol" these leaves preferentially. More ants were found on young leaves (2=2.00 ants/leaf) than on mature leaves (J?=0.51). (2) If the ants are functioning in anti-herbivore defense, herbivory should be lower on occupied plants than on plants without ants. Estimates of mean percent foliovore damage per sap- ling showed that for samples with similar average heights (and presumably, ages), the unoccupied plants had signifi- cantly greater damage. A comparison of newest leaves showed a significant trend for damage to decrease with increased ant activity. (3) If ants are attacking hervibores at early stages of development, baiting with eggs should demonstrate this activity. Over 75% of all egg baits on young and mature leaves were taken up by an ant during observation periods of 30 or 60 rain. Baits were found sig- nificantly faster on young than on mature leaves. In over half of the baiting trials, the egg was taken to the edge of the leaf and dropped to the ground rather than seques- tered as a nutrient source. These data suggest that Pheidole inhabiting Piper plants are important in plant defense from herbivores and that adherance to a classical notion of aggression in similar stu: dies might bias the initial question-asking stages of an inves- tigation. Introduction Protocooperative and mutualistic relationships between plants and ants have been described and evaluated empiri- cally (Belt 1874; Miiller 1876; Wheeler 1910), experimen- tally (Janzen 1967; Bentley 1976; Inouye and Taylor 1977; Koptur 1979), and theoretically (Heithaus etal. 1980; Keeler 1981). In most cases, the plant has been shown to serve as a source of shelter and/or nutrients for one or many ant species. Ants may protect the plant from en- croaching vegetation (a biotic form of allelopathy) and/or herbivores (Janzen 1966; Janzen 1972; Bentley 1977b; Deuth 1977; Keeler 1980) or ants can function as a source of nutritional input (Janzen 1974; Huxley 1978). Ants that protect plants often exhibit pugnacious displays, aggrega- tions, biting, and stinging; these have become part of a suite of "aggressive" characters expected in ants that de- fend their plant hosts. Consequently, the lack of these be- haviors by Pheidote bicornis Forel on 3 species of Piper plants (Piperaceae) in Costa Rica was taken as evidence for a nutrient procurement function (see Risch et al. 1977). This paper reconsiders the notion of aggression and pro- vides evidence for an anti-herbivore function of P. bicornis dwelling within Piper plants. I determined the distribution of ants on the foliage, estimated foliovore damage on occu- pied and unoccupied plants, and conducted baiting experi- ments to assess whether the small, non-stinging, sluggish P. bicornis may provide protection from foliovores through a subtle form of aggressive behavior, that of removing eggs and early instar nymphs and larvae from the foliage. Piper-Pheidole interaction: Biology and earlier hypotheses A P. bicornis queen colonizes a young Piper and she and her brood occupy an expanded, hollow, sheathing, leaf-base which affords some protection from predation and the de- structive potential of daily rains (Risch et al. 1977). As the colony grows, the workers gradually hollow out stem pith tissue and the entire plant becomes a domicile. The inner surface of the expanded petioles produces opalescent food bodies, rich in lipids, which the ants harvest and consume. Risch et al. (1977) proposed that these ants probably do not protect the plant from herbivores since live insects re- leased on the foliage elicited no reaction from the ants. Rather, Pheidole may provide a nutritional input both from decaying frass and body parts, absorbed in the form of aqueous ammonia through the soft, inner tissues of the stems and by increasing the carbon dioxide concentration within the plant. A secondary hypothesized benefit is some degree of protection from encroaching vines due to cleaning behavior by ants. The ants, however, have only been seen foraging on the plant itself; thus the quantity of nutritional input to