On the afternoon of 2 July 2005, during a field excursion in the center of the Brazilian Pantanal (18°59′S, 56°39′W), we had the opportunity to witness and photograph the unusual presence of four giant anteaters in the same two-hectare patch of open scrub grassland. All four animals were adult-sized but of unknown sex. Two of them were foraging separately, walking in parallel some 50 m apart; throughout the encounter they ignored each other and the other two anteaters (Fig. 1a). These latter two, however, became involved in an agonistic encounter, which followed the pattern of an injury-producing fight as described by Shaw et al. (1987). While foraging, their paths drew near to each other; when they were approximately 10 m apart, one of them apparently detected the other by scent and walked directly towards it (Fig. 1b), producing a long, deep “harrrr” sound. Both animals began to circle one another with tails raised (Fig. 1c), and after about a minute the aggressor began striking with its forepaw at the other animal's face (Figs. 1d and 1e). This lasted only a few seconds. When the attacked animal fled, the aggressor chased it for over 100 m with its tail raised and piloerected (Fig. 1f), and then resumed foraging nearby.
Although Shaw et al. (1987) frequently observed agonistic behavior among free-ranging giant anteaters in Serra da Canastra National Park in central Brazil, to our knowledge it has not been reported in field studies elsewhere.
CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) provided a scholarship to one of the authors (FLR) during this study. We thank Arnaud Desbiez for his comments and English revision of the manuscript.
 Fabiana L. Rocha Programa de Pós-Graduação em Ecologia e Conservação, Dept. Ecologia, Universidade Federal de Mato Grosso do Sul, Brazil
 Fabiana L. Rocha and Guilherme Mourão, Embrapa Pantanal, Rua 21 de Setembro, 1880, C. P. 109, Corumbá 79320-900, Mato Grosso do Sul, Brazil, e-mail: <firstname.lastname@example.org>.