Patrick Moldowan, Cristian Peñafiel
Among Galápagos giant tortoises, antagonism is particularly well documented in adults of saddleback taxa (Fritts 1983;Schafer and Krekorian 1983;Nieuwolt 1989) but less is known for domed taxa.We herein describe agonistic behaviour among wild juvenile Eastern Santa Cruz Giant Tortoises (Chelonoidis donfaustoi), a domed taxon.We also discuss wounding in the context of conspecific behavioral signalling, and provide morphological descriptions of head and neck coloration among species of Galápagos giant tortoises. MethodsMonitoring the ecology and reproductive biology of several species of Galápagos giant tortoises has been ongoing since 2009 via the Galápagos Tortoise Movement Ecology Program (GTMEP).To date, studies of the GTMEP have involved four species across three islands: Western Santa Cruz Giant Tortoise (C.porteri) and Eastern Santa Cruz Giant Tortoise (Chelonoidis donfaustoi) on Santa Cruz Island, Española Giant Tortoise (C.hoodensis) on Española Island, and the Alcedo Volcano Giant Tortoise (C.vandenburghi) of the Alcedo Volcano, Isabela Island.During 2023-2025, detailed reproductive monitoring of free-living C. donfaustoi (Donovan et al., in press) provided opportunities for behavioral observations.During fieldwork, we opportunistically recorded observations of juvenile aggression and wounding.PDM also conducted observational stud-ies of juveniles of several species -C.donfaustoi, C. hoodensis, Santiago Giant Tortoise (C.darwini), and individuals of Floreana Giant Tortoise (C.niger) ancestry -at the Fausto Llerena Tortoise Breeding Center, Puerto Ayora, Santa Cruz Island, Galápagos, and noted body sizes, morphology, and wounding. ResultsObservation 1.-At 0734 h local time (UTC-6) on 22 September 2023, we encountered two combative juvenile C. donfaustoi in the vicinity of Cerro Fatal on eastern Santa Cruz Island, Galápagos (0.6, -90.3;WGS 84).We observed interactions from about 15 m for approximately 10 min, of which 6 min and 50 sec were recorded (Video S1: https:// youtu.be/TCJuJY9yyIc).The smaller juvenile, measuring 40 cm in curved midline carapace length (CCL; Method E in Iverson and Lewis 2018), asserted itself as the aggressor by confronting, biting, and trailing the larger juvenile (47 cm CCL).The video recordings documented approximately 31 biting attempts by the smaller individual, of which at least 19 bites (61%) contacted the larger individual.Of landed bites, 13 were delivered to the left forelimb and six to the left hindlimb of the larger individual.The larger individual did not contact the small individual during any of five bite attempts.In addition, the smaller individual displaced the larger individual approximately one meter during a bout of pushing (beginning 1:10 of Video S1).The two individuals repeatedly elevated their bodies, fully extended their necks, hissed, and gaped throughout the observation period.The smaller juvenile emerged as the apparent victor in all of the brief contests, often landing bites, physically displacing, and forcing retreat of the larger individual.Observations ceased when the tortoises moved into vegetation.White scarring around the jowls of both juveniles was visible at a distance and was confirmed by subsequent field observations (Fig. 1A shows the larger, subordinate juvenile).Two Galápagos