South American rattlesnake

Identification

The South American rattlesnake is a heavy-bodied pit viper with a triangular head that is clearly wider than the neck, keeled scales, vertical pupils and a heat-sensing pit on each side of the face^2,7. Its back carries a row of dark diamonds edged with pale scales, and a pair of dark stripes runs along the neck². The tail ends in a rattle, which is the easiest way to recognise it¹⁴.

In north-eastern Brazil, mature rattlesnakes averaged about 1.1 m from snout to vent, and the largest measured 1.47 m, not counting the tail⁴. Males and females there were about the same size⁴.

Distribution

The Reptile Database lists the South American rattlesnake from Brazil, Venezuela, Colombia, Guyana, Suriname, French Guiana, Peru, Bolivia, Paraguay, northern Argentina and Uruguay¹. It is patchily distributed, living in open country north and south of the Amazon rainforest and in isolated patches of savanna².

Rattlesnakes from Mexico and Central America were once included in this species but are now treated as separate species, such as Crotalus simus^1,11. The Aruba Island rattlesnake is also now a separate species, Crotalus unicolor¹.

Habitat

In Brazil, the South American rattlesnake lives mainly in open habitats, including the Cerrado, Caatinga, Pampa, Pantanal and Araucaria forest, and in open areas within the Amazon and Atlantic Forest⁷. It also lives in pastures, crop fields and forest edges², but it is rarely found on the coast or in marshes³. It can colonise land cleared of forest, which has helped it spread into new areas^2,6.

Behaviour

This rattlesnake is most active from dusk until around midnight, and it rests by day in burrows or among grass and shrubs². In a Cerrado study it was active mainly on the surface and was often found under bushes⁶. When a person approaches it usually flees, but it may also coil, turn its head towards them and shake its rattle, usually only when the person is within about a metre². Males sometimes fight each other in ritual combat².

Diet

The South American rattlesnake hunts mainly by waiting in ambush on the ground². In a study of 452 rattlesnakes from central Brazil, about three-quarters of the prey found were rodents, and the rest were marsupials, other mammals and reptiles⁵. Lizards are taken in smaller numbers, and birds have been recorded in north-eastern Brazil².

Reproduction

The South American rattlesnake gives birth to live young^1,2. Litters average about 12 in south-eastern Brazil and range from 12 to 33 in north-eastern Brazil⁴. Females store sperm through the winter, and the young are born in summer and early autumn⁴. Newborns measure about 30 cm from snout to vent⁴.

Venom and medical significance

The South American rattlesnake is dangerously venomous, and the WHO places it in Category 1, the highest medical importance, in Brazil and most other countries where it lives¹¹. The bite site often shows little pain or swelling, only tingling, but within hours the venom can cause drooping eyelids, blurred or double vision and a sleepy-looking face, followed by muscle pain and dark urine as muscles break down^3,8. Blood clotting can also be affected, and acute kidney injury is the main complication³.

In Brazil in 2023, rattlesnakes caused 8% of reported snakebites but 17% of snakebite deaths, and a bite was more than twice as likely to be fatal as a bite by a lancehead⁷. It is treated with anticrotalic antivenom, given in hospital⁹. Anyone bitten should go to hospital immediately.

Conservation

The South American rattlesnake is listed as Least Concern on the IUCN Red List (published 2021)¹². It has a wide range and is able to live in farmland and cleared areas^2,6.

Similar species

In Brazil, the rattle is the clearest sign of a rattlesnake¹⁴. The rattle can break or be lost, however¹⁰. A rattlesnake without its rattle could be mistaken for a lancehead such as the jararaca (Bothrops jararaca), which has dark triangular or trapezoid marks along its sides¹³. The rattlesnake has dark diamonds along the back and a pair of dark stripes on the neck². These differences are not a safe basis for handling any snake. Keep your distance from every snake you cannot identify.

In captivity

The South American rattlesnake is a dangerously venomous snake and is not suitable as a pet. In Brazil, keeping venomous snakes is allowed only in research centres or for the pharmaceutical industry¹⁰. Its rattle grows a new segment each time it sheds, so the number of segments does not tell its age, and segments can break off¹⁰.