The southernmost subspecies of the cantil, Agkistrodon bilineatus howardgloydi, occurs in almost all terrestrial habitats of the Sector Santa Rosa, in Guanacaste Conservation Area, Costa Rica. We obtained 21 specimens during several visits to the park from 1993 to 1996. Neonates were observed only during late May through July, despite searching in other months, suggesting that birth in this subspecies occurs at the beginning of the rainy season. Fecal analyses revealed differences in diet composition between juveniles and adults. Juveniles prey primarily on lizards and various species of frogs, whereas adults eat rodents. This ontogenetic change in diet does not seem to trigger any corresponding change in venom composition with snake age. Venom of this subspecies is highly toxic (LD50 = 1.25 μg/g), and has elevated hemorrhagic, hemolytic, and myotoxic activities. Extreme deforestation of dry forest habitats in the Pacific lowlands of Central America has made A. bilineatus a rare species over most of its range. Sector Santa Rosa in Costa Rica is the only area within its distributional range where the species still can be regularly observed.
