From Costa Rica Travel Guide: Vacation and Travel tips
The fer-de-lance is not only one of the most feared snakes in Latin America, but one of the most feared animals in general. This is a very large pit viper, reaching lengths up to 97 inches, including the non-prehensile tail. When seen from above, the fer-de-lance has a slender, triangular head, which is noticeably bigger than its neck. Between the eye and the nostril on each side of its head is a deep heat-sensitive pit. The fer-de-lance has moderately large eyes and the pupils are vertically elliptical.
The fer-de-lance is found in abundance in the tropical rainforests of both the Caribbean and Pacific regions of Costa Rica, but there are distinct differences between individuals from the opposite sides of the country. Not only in the area of chemical composition of the venom and in scalation characteristics, but also in the length of the females. The Caribbean fer-de-lance is on average 10 inches longer than its Pacific counterpart. A larger body also means the females on the Caribbean side give birth to more young at one time. Gestation for all fer-de-lances is between 6 to 8 months and the litter size can range from 5 to some 86 babies.
Juveniles normally feed on frogs and lizards and the adults dine on birds and mammals and will take prey up to the size of opossums or rabbits. Spending most of the day coiled and concealed in vegetation, they become active at dusk, when they make their way to their feeding sites, frequently at the side of a road or edge of a trail. The fer-de-lance will ambush its prey and their exceptional camouflage makes them particularly hard to see. When threatened they will remain coiled but after a few minutes will retreat to dense vegetation with amazing speed.
Fer-de-lances can be found in a variety of dense vegetation such as tall grass, or scrub, to virgin rainforest and they also seem to adapt well to habitat changes. They are very common around human settlements and have caused many snakebite incidents. This snake vibrates its tail when threatened, producing an unmistakably loud buzzing sound. If further provoked the fer-de-lance will strike with tremendous speed and this coupled with its excitable disposition and strong venom truly make this an exceptionally dangerous snake. The fer-de-lance is responsible for many deaths in Costa Rica, but now treatment for the bite is much more successful, so with proper treatment, most victims survive their encounter.