Sotalia guianensis (P. -J. van Beneden, 1864)

Russell A. Mittermeier & Don E. Wilson, 2014, Delphinidae, Handbook of the Mammals of the World – Volume 4 Sea Mammals, Barcelona: Lynx Edicions, pp. 410-526 : 504-505

14.

Guiana Dolphin

Sotalia guianensis

French: Dauphin costero / German: Kiisten-Sotalia / Spanish: Delfin costero

Other common names: Costero, Guiana River Dolphin

Taxonomy. Delphinus guianensis Van Beneden, 1864,

Maroni or Marowijne River on the eastern border of Suriname.

Taxonomy of this genus has been in flux for some time, and prior to 2007, only one species (S. fluviatilis ) and two subspecies, a riverine type ( fluviatilis ) and a coastal type ( guianensis ), were recognized. An accumulation of genetic evidence, in addition to ecological and morphological differences, supports separate species status for these two types. Monotypic.

Distribution. Coasts of E Central America and NE South America from c.16° N in Honduras to ¢.27° S in S Brazil. Range into the Orinoco River system in Venezuela is mapped for this species, but it is uncertain if this population belongs to the Guiana Dolphin or the Tucuxi (S. fluviatilis ).

Descriptive notes. Total length up to 170 cm (males) and up to 190 cm (females); weight up to 40 kg. Neonates are 80-115 cm long. The Guiana Dolphin has robust body shape similar to bottlenose dolphins (7ursiops spp.), and it is larger in size than its riverine counterpart, the Tucuxi. Like the Tucuxi, the Guiana Dolphin has long, narrow beak, broad flippers, and short, triangular dorsal fin with a wide base. Visible crease separating melon and beak is characteristically absent in this genus. Skin is dark gray with a brown or blue tinge dorsally that fades to pale gray on belly. Belly tends to be darker in the Guiana Dolphin than in the Tucuxi, and dark-gray band connecting eye and base of flipper is broader and less well defined in the Guiana Dolphin. Individuals may have a pale-gray streak on sides of tailstock. Flippers and flukes are dark gray on both sides. There are 30-36 pairs of small, conical teeth in upperjaw and 28-34 pairs in lower jaw.

Habitat. Inshore coastal waters, particularly protected areas in estuaries and bays. The Guiana Dolphin has a patchy, discontinuous distribution. Its southern limit along the Brazilian coast likely reflects a natural temperature boundary created by the colder Malvinas Current in the area. Near Rio de Janeiro, they prefer deeper channels more than 6 m deep, but elsewhere they prefer shallower habitats less than 5 m deep. Individual Guiana Dolphins may be observed up to 70 km offshore, but most sightings are within 100 m of the shore.

Food and Feeding. More than 60 species of fish in 25 families (five cephalopod families and one crustacean family, penaeid shrimps) have been documented as prey of Guiana Dolphins. Fish prey is generally schooling species but include both demersal and pelagic species, suggesting the Guiana Dolphin is an opportunistic feeder. Smaller, younger classes of fish, usually less than 20 cm, are preferred. In south-eastern Brazil, the Guiana Dolphin prefers trichiurids (e.g. largehead hairtail, Trichiurus lepturus), sciaenids (e.g. croakers, Cynoscion spp. , and whitemouth croaker, Micropogonias Sfurnieri ), and cephalopods (e.g. Loligo spp. and Atlantic brief squid, Lolliguncula brevis). In north-eastern Brazil, shorthead drum ( Larimus breviceps ), white grunt ( Haemulon plumaierii), lane snapper (Lutjanus synagris), largehead hairtail, and white mullet ( Mugil curema) are preferred. Off Santa Catarina, Brazil, the Guiana Dolphin feeds primarily on anchovies ( Engraulidae ). Geographical variation in prey composition suggests that there may be some degree of local specialization. This may have a role in the patchy distribution of the Guiana Dolphin, and may contribute to restriction of gene flow among populations. Given that various species of croaker are preferred prey, the Guiana Dolphin likely uses passive listening while hunting. Individuals may forage alone or cooperatively in groups. For example, Guiana Dolphins in the Cananeia Estuary beach-hunt; groups of dolphins herd fish onto sloping beaches and strand themselves temporarily in the process. This technique is similar to strategies that have developed in populations of bottlenose dolphins and Killer Whales ( Orcinus orca ), but it is used mostly by females. A recent study found evidence of passive electroreception abilities in specialized crypts on rostra of Guiana Dolphins that were originally associated with mammalian whiskers. This trait has only been documented among mammals in the order Monotremata and invites the possibility that other cetaceans may have this ability also.

Breeding. There is no confirmed seasonality in breeding of the Guiana Dolphin. Males mature at c.7 years of age and females at 5-8 years. A high ratio of testes mass to body mass (¢.3%) suggests that the Guiana Dolphin has a promiscuous mating system, dependent on sperm competition. Birth interval is thought to be c¢.2 years, although longer intervals have been recorded in some populations. Gestation lasts 11-12 months. Females more than 25 years old appear to have senescent ovaries, suggesting menopause. Maximum known age is 29 years for males and 30 years for females.

Activity patterns. Feeding and traveling are the most commonly observed behaviors of Guiana Dolphins, and individuals may “porpoise” while swimming at faster speeds. In some regions, the Guiana Dolphin enters and leaves bays with a regular diurnal pattern, entering bays early in the morning and leaving in the afternoon. Long dives last 20-120 seconds and are spaced apart by a series of shorter dives of only 5-10 seconds. Social vocalizations are 0-21-24 kHz, and whistle patterns seem to reflect geographical variation. The Guiana Dolphin is known to be aerially active, sometimes spy hopping or leaping acrobatically. They are not known to bow-ride but may surf in boat wakes.

Movements, Home range and Social organization. More than 40 individuals can be in a group of Guiana Dolphins, but groups of 2-10 individuals are most commonly observed in south-eastern Brazil. Groups of up to 60 individuals have been observed in Brazil’s Cananeia Estuary, and aggregations of several hundred have been reported occasionally in a few areas. Nursery groups (which include young offspring) tend to be larger. Studies of association patterns of Guiana Dolphins in Cananeia Estuary suggest groups are fission—fusion societies where most associations among individuals are weak and shortterm. Socializing activities may include herding of females by males—a behavior also observed in some populations of bottlenose dolphins. In fact, socio-sexual behaviors are observed between Guiana Dolphins and bottlenose dolphins in mixed-species groups off the coast of Costa Rica. Bottlenose dolphins appear to form long-term male alliances that behave coercively toward female Guiana Dolphins, and some hybridization may occur. The Guiana Dolphin generally displays high site fidelity; individuals have been consistently identified in the same areas for up to ten years at a time, and young offspring do not appear to disperse much from their region of birth. Patchy distribution of Guiana Dolphins likely derives from small home ranges. For example, at Baia Norte, Brazil, home ranges were estimated to be only 10-15 km?. In the Cananeia Estuary, however, home ranges were 1-:6-22-9 km?, suggesting large variation in home range size.

Status and Conservation. CITES Appendix I. Classified as Data Deficient on The IUCN Red List. No abundance estimates or data on population trends and demographic parameters are available for the Guiana Dolphin. Local estimates of abundance and density are often inconsistent and vary in methodology. MtDNA analyses suggest that there are highly differentiated populations with restricted gene flow among them. Distinctions were clear enough for management units (MUs) to be delineated: there are six MUs along the Brazilian coast, one MU comprising Central America to Venezuela, and one MU comprising Guyana to French Guiana. Proximity of Guiana Dolphins to human activity means they are especially vulnerable to threats from fisheries interactions, boat traffic, contaminants, and residential and industrial waste throughout their distribution. Because populations of Guiana Dolphins are highly localized, with restricted gene flow, locally directed conservation efforts with a priority on reducing gillnet entanglement have been suggested. The Guiana Dolphin is occasionally caught on the coast of Brazil as direct catch for use as bait in shark and shrimp fisheries and possibly for human consumption. Incidental catch is a much more concerning threat to Guiana Dolphins because they seem especially susceptible to entanglement in monofilament gillnets, seine nets, and shrimp traps. An estimated 90 Guiana Dolphins are killed per year in set gillnets along the Brazilian coast. There is some evidence that “pingers” may help deter Guiana Dolphins from approaching gillnets. Molecular sampling from local markets in the Brazilian Amazon has shown that parts of Guiana Dolphin, such as eyes, are illegally sold as amulets or love charms. These are likely taken from incidentally caught individuals or individuals killed for bait, but there is no robust data on whether any individuals are killed expressly for these products. River-damming projects planned in the Amazon River Basin will reduce freshwater outflow into coastal estuaries, decreasing productivity and prey availability. Aquaculture along the coast also degrades habitat, particularly highly productive mangroves, estuaries, and salt marshes. Effluents from large harbors and agricultural areas, including heavy metals such as mercury and selenium and insecticidal substances that have been banned elsewhere in the world, are also a problem. Guiana Dolphins sampled from Cananeia Estuary, an area impacted by PCBs, did not have tissue concentrations of PCBs as high as those in small cetaceans from other developed areas, but samples from Guanabara Bay (a world hotspot for persistent organic pollutants) revealed tissue concentrations of DDT, PCBs, and PBDEs comparable to small coastal cetaceans in North America. Some Guiana Dolphins have been taken for the captive industry in the past, but live capture has been illegal since 2005.

Bibliography. Caballero et al. (2007, 2010), Czech-Damal et al. (2012), Daura-Jorge et al. (2011), Di Beneditto & Ramos (2004), Dorneles, Lailson-Brito, Eppe et al. (2008), Dorneles, Lailson-Brito, Fernandez et al. (2008), Edwards & Schnell (2001), Flach et al. (2008), Flores & Bazzalo (2004), Flores & da Silva (2009), Gravena et al. (2008), Hollatz etal. (2011), Jefferson et al. (2008), Monteiro-Neto, Alves et al. (2000), Monteiro-Neto, Avila et al. (2004), Monteiro-Neto, Itavo & Moraes (2003), Oshima et al. (2010), Pansard et al. (2011), Rosas & Monteiro-Filho (2002), Rosas et al. (2010), Santos (2010), Santos & Rosso (2008), Santos et al. (2001), Secchi (2010d), Sholl et al. (2008), da Silva & Best (1994), Thompson (2010), Yogui et al. (2003).

Figures

On following pages: 11. Long-beaked Common Dolphin (Delphinus capensis); 12. Fraser's Dolphin (Lagenodelphis hosel); 13. Tucuxi (Sotalia fluviatilis); 14. Guiana Dolphin (Sotalia guianensis); 15. Indo-Pacific Humpback Dolphin (Sousa chinensis); 16. Atlantic Humpback Dolphin (Sousa teuszii).

Distribution. Coasts of E Central America and NE South America from c.16° N in Honduras to ¢.27° S in S Brazil. Range into the Orinoco River system in Venezuela is mapped for this species, but it is uncertain if this population belongs to the Guiana Dolphin or the Tucuxi (S. fluviatilis ).