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Introduction

The tucuxi and Guiana dolphins are perhaps two of the lesser known or popularized species of dolphin. Formerly collectively the same species, the tucuxi and Guiana dolphin are overwhelmingly similar species with only a few notable characteristics defining them, such as some physiological differences and ranges.

Beginning with the tucuxi, this dolphin is found in the Amazon River and its tributaries (though limited to various uncrossable waterfalls), overlapping much of its range with the Amazon River dolphin (AKA the boto). The species appears remarkably similar to the well-known bottlenose dolphin, with similar grey to bluish grey coloration and defined rostrum. However, the tucuxi is much smaller, only growing to maximum of 4.9 feet and 121 lbs and sporting a more triangular dorsal fin, sometimes hooked at the tip.

The Guiana dolphin on the other hand has the same coloration with additional white streaks on the peduncle and growing up a larger 7.2 feet, 176 lbs. This species also has more teeth than the tucuxi, but overall is a very similar species. Though it can be found in some Northeastern South American Rivers, it is mainly found along the coast, venturing no further than reported cases of 40 miles offshore of Brazil.

Illustration showing the slight differences between the tucuxi dolphin (top) and the Guiana dolphin (bottom). Illustration by Uko Gorter.

Behavior & Diet

The tucuxi and Guiana dolphins (Sotalia sp.) have a lot of similarities in terms of diet and behavior. They generally are seen in small groups from 1-6, but can be found in larger groups up to 20 (tucuxi) and 50-60 (Guiana). Sometimes Guiana dolphins are seen cooperatively feeding in groups of 100-200! Groups of Sotalia sp. are very cohesive and social. They will do full aerial leaps, somersaults, fluke ups, spy hopping, surface rolling and porpoising; in fact, the tucuxi is the only river dolphin that jumps like marine dolphins do. Although they do not bow ride, they will surf the wakes of passing boats along the coast. There is even documentation of a lone Guiana dolphin that swimming with dogs! The most common behaviors seen in coastal areas are feeding and travel, and resting/milling are rare; they are busy little social dolphins.

The daily lives of these dolphins are still not well understood. It is likely that group size varies with time of day and activity, but this may vary between populations. One thing that is known is the correlation with vocalizations and group/surface behavior. For both species, behavior and group size influenced whistle variation – so they say different things in different contexts! Their vocalizations are particularly interesting because they have some of the highest pitched whistles of any dolphin. This is linked to their body size: the smaller the size, the higher the pitch.

These dolphins like where they live. They are thought to have very small home ranges (possibly the smallest of known dolphin species). Guiana dolphins have been sighted in the same are for up to 10 years. Where they go within those small ranges is likely influenced by the rivers, and the changing water levels, they are associated with (especially the tucuxi). For example, tucuxi avoid fast moving water and flooded forests (unlike the Amazon river dolphins they share the waters with). Flooded forests may be a riskier habitat, as a dolphin could get stuck when the water levels go down.

A Guiana dolphin mid-leap. Photo: Instituto Boto Cinza.

Wherever they go, they will be looking for prey! The two species eat many of the same things: ray-finned fishes (including herring, drums, croakers, tetras, catfishes), squid, octopus, sometimes shrimp, crab and flounder. It is thought that young fish may be an important part of their diet. They make short dives, 30 seconds to 1 minute for tucuxi and 30 seconds to 2 minutes for Guiana dolphins. These dolphins’ small size means smaller fish will be easier to catch and swallow. The best place for a fish buffet are confluences – where two rivers meet. The pH there is favorable for plankton growth, which feeds small fish, which feed the dolphins. As with other cetacean species, birds will often take advantage of the hard work dolphins do chasing prey. The brown booby, terns, frigates and kelp gulls are all known associates of the Guiana dolphins, and terns will occasionally associate with tucuxi. Free meal for the birds if they can steal it!

These small dolphins can live up to 35 years, and become sexually mature at 5-6yrs. Little is known about their mating behavior, though along the coast it is thought that males herd females while traveling. Their gestation is 11-12 months and they have calves about every 2 years. Calving occurs year round for Guiana dolphins, but for tucuxi calving occurs in September through November, coinciding with the low water season in the Amazon. Hybrid calves have been observed between bottlenose dolphins and tucuxi, where mixed species groups are often social. For Guiana dolphins, mating behavior with bottlenose dolphins has been observed, but usually the interactions are aggressive, with bottlenose being the aggressor and the Guiana dolphins often fleeing. Interspecies interactions can be complex, and there is still so much to learn about these small dolphins and their intra and interspecific relationships.

Status

Tucuxi dolphin showing off its acrobatic nature! Photo: Whale and Dolphin Conservation USA.

The current population count for both tucuxi and Guiana dolphins is unknown, although both seem to be fairly common across their range. The IUCN currently lists tucuxi dolphins as Endangered (as of 2020), and the Guiana dolphin as Near Threatened. Part of the difficulty in determining population size is accessibility: many parts of these dolphins’ range are difficult to get to, and even harder to spot dolphins in!

Threats

  • Accidental Bycatch: Both tucuxi and Guiana dolphins are prone to accidental bycatch in fishing nets. They are particularly susceptible to bycatch in gillnets and seine fisheries, and the Guiana dolphins can also be caught in bottom-set lobster nets. In some parts of its range the tucuxi is the most commonly bycaught cetacean, while in parts of South America it is reported that hundreds of Guiana dolphins are bycaught each year.
  • Intentional Hunting: Although these species are not commercially exploited frequently, they are occasionally hunted for both their meat and blubber (which is used as shark bait). Certain body parts are also sold as talismans or love charms, including the genitals of the tucuxi. It is thought that the myths and cultural legends surrounding river dolphins have prevented the tuxuci and Guiana dolphins from being hunted more vigorously in these regions. Which is just as well, as in most parts of their range they are not protected by law, and in areas where they are legally protected enforcement is extremely difficult!
  • Habitat Destruction: This is one of the most worrying threats facing both species. Encroachment by humans throughout their range has resulted in increasing heavy metal pollution (Mercury, for example, is produced as a byproduct of gold mining), noise pollution, boat traffic, introduction of marine culture farms, and pesticide input (many of which include banned substances). The construction of hydroelectric dams is also a concern for both tucuxi and Guiana dolphins. In addition to the pollution and noise caused during construction, these dams fragment their home ranges even further, reduce fish movement and numbers, resulting in less food for the dolphins and a smaller area in which to reside.

Fun Facts

  • You might be wondering where the common names for these species originated? Well, the tucuxi’s name comes from “tucuchi-una” after the Tupi language of the Mayanas Indians from the Amazon region of Brazil, In this area the tuxuci dolphin is often called boto-tucuxi , boto-cinza , or simply boto. The Guiana dolphin likely takes its name from the location.
  • Their Latin names are also intriguing: the genus Sotalia has no known origin, however the species name for the tucuxi, fluviatilis, means “of a river” – makes sense, right? The Guiana dolphin’s species name is guianensis, again likely in reference to its location.
  • Both species have fairly good eyesight both above and below the water.
  • The tucuxi has one of the largest brain sizes in relation to its body size of all cetaceans!

cindy.elliser@pacmam.org

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