The crowns of neighboring trees of similar height do not interdigitate but rather are generally separated by spaces called "crown shyness" gaps. In a black mangrove (Avicennia germinans) forest in Costa Rica, the width of crown shyness gaps was positively correlated with the distance pairs of trees or branches adjacent to the gap swayed in the wind (n = 22; p< .01). Abrasion of buds, leaves and branches due to trees knocking into one another seems to create and maintain the spaces around each tree crown. INTRODUCTION Though recent studies of forest structure and dynamics have focused on large gaps caused by single and multiple treefalls, most openings in forest canopies are extremely small (e.g., Hartshorn, 1978; Runkle, 1982). Small gaps usually remain open for a short time, being temporarily closed by branch movements during winds and permanently closed by lateral growth (e.g., Runkle, 1982). However, in some forests light penetrating into the canopy forms a persistent and striking border around individual crowns that are visible from the ground (Fig. 1). The term "crown shyness" (from Lane-Poole, 1927-1944, cited in Jacobs, 1955) refers to these leafless regions between adjacent crowns of similar height. Much of the light reaching the understory as sunflecks passes through crown shyness gaps. These openings may also be important insofar as they restrict intercrown movements of arboreal animals and vines (Putz, 1982). A similar sort of defense by mechanical abrasion of competitors and dislodgement of herbivores has been observed in wave-swept algae (Velimirov and Griffiths, 1979). Two theories have been proposed to explain how crown shyness gaps are formed and maintained. One view is that abrasion of buds on wind-blown branches leads to open spaces between tree crowns (Tarbox and Reed, 1924; Jacobs, 1955; Richards et al., 1962). Ng (1977), on the other hand, proposed that crown shyness in Malaysian dipterocarps (Dryobalanops aromatica and various species of Shorea) is caused by reduction in lateral growth due to mutual shading. We tested the hypothesis that crown shyness is caused and maintained by the mechanical abrasion of branches swaying in the wind. Specifically, we predicted that if mechanical abrasion causes crown shyness, then flexible trees and branches should be more widely separated than rigid trees and branches. STUDY SITE AND METHODS We studied crown shyness in an approximately 50-ha stand dominated by Avicennia germinans in the Parque Nacional de Santa Rosa, Guanacaste Province, Costa Rica (10035 'N, 85085 'W). The forest floor is inundated with sea water only when the tides