2Talamanca Dolphin foundation, Manzanillo, Talamanca, Costa Rica

A small (<20 individuals) population of tucuxi dolphins (Sotalia fluviatilis) off Costa Rica’s Caribbean coast was observed for 90 days from most months of the years 1997-1999. The primary area of sighting was approximately 8 km north of the Panama border, in a partial bay fed by a number of rivers and creeks (9°37’N, 82°37’W).

Tucuxi were observed with bottlenose dolphins (Tursiops truncates) on more than half our sightings. Tucuxi were usually observed over an extended shallow (>10 fathoms) sand shoal .5 km. Offshore a small creek. Bottlenose were observed and followed more widely between Manzanillo and the Sixaola River. We observed a pronounced difference in activity of tucuxi when alone, than when with bottlenose. When alone tucuxi were elusive and sedentary in behavior, except while feeding in early morning. When with bottlenose, tucuxi were more active: leaping, spy-hopping, and tail-slapping.

Mixed species groups included sub-groups with two – four bottlenose and one tucuxi. Mixed sub-groups moved erratically and exhibited a variety of high energy behaviors. On four occasions bottlenose observed a male bottlenose achieve intromission with a female tucuxi swimming above.

Based on body size, dorsal fin scars, frequency of extruded penises, and behavior we conclude that groups of male bottlenose approached and herded female tucuxi in attempts to mate. We have not yet documented evidence of successful hybridization. Similarity in body shape and coloration of both species limit ability to detect obvious hybrids. We have observed regularly two large tursiops-sized animals with triangular dorsal fins more characteristic of tucuxi than bottlenose. Our observations indicate inter-species mating is a regular occurrence among these two groups. Continued effort to photo-identify animals and collect skin biopsies may provide conclusive evidence of hybridization.