Stenella longirostris (Gray, 1828)

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 : 511-512

21.

Spinner Dolphin

Stenella longirostris

French: Dauphin a long bec / German: Spinnerdelfin / Spanish: Delfin girador

Other common names: Long-beaked Dolphin, Long-snouted Dolphin, Pantropical Spinner Dolphin, Spinner; Gray's Spinner Dolphin (longirostris); Central American Spinner Dolphin (centroamericana); Eastern Spinner Dolphin (orientalis); Dwarf Spinner Dolphin (roseiventris)

Taxonomy. Delphinus longirostris Gray, 1828 ,

no type locality given. Later designated by J. E. Gray in 1850 to “Inhab. Southern Ocean.”

Taxonomy of Stenella is currently in dispute. The genus is likely polyphyletic and will probably be revised in the near future.

According to molecular studies, S. frontalis is most closely related to S. coeruleoalba and S. clymene , followed by Delphinus and Tursiops aduncus . These species appear to be more closely related to each other than any of them are to 7. truncatus , S. attenuata , or S. longurostris. Similarities in appearance between S. attenuata and S. frontalis are thus likely due to homoplasy or pleisiomorphy. If these relationships are confirmed, taxonomic assignment of these species will need to be revised. Four subspecies recognized.

Subspecies and Distribution.

S.l.longirostrisGray,1828—oceanicislandsintropicalwatersoftheAtlantic,Indian,andWPacificoceanstoc.145°W.

S.l.centroamericanaPerrin,1990—coastalwatersofthetropicalEPacificOceanfromtheGulfofTehuantepec,MexicotoCostaRica.

S.l.orientalisPerrin,1990—pelagicwatersofthetropicalEPacificOceannofartherWthanc.145°W,primarilybetween24°NoffBajaCalifornia,Mexico,and10°SoffCPeru.

S. l. roserventris Dumont d’Urville in Schreber, Goldfuss & Wagner, 1846 — shallow, nearshore waters of SE Asia, Sunda Is, Wallacea, New Guinea, and Australia.

Descriptive notes. Total length 129-280 cm (males) and 138-211 cm (females); weight up to 80 kg (males) and up to 63 kg (females). Neonates are 50-80 cm long and weigh 5-10 kg. Generally, “Gray’s Spinner Dolphin” ( S. I. longirostris ) and the “Central American Spinner Dolphin” (S. L centroamericana) are heavier and longer than the “Eastern Spinner Dolphin” (S. L orientalis). The “Dwarf Spinner Dolphin” (S. I. rosetventris) is smallest subspecies, as the name implies. The Spinner Dolphin has slender build; very long, thin beak; and slender curved flippers. Depending on subspecies and sex, moderately sized dorsal fins vary from falcate to triangular, with males tending to have more triangular dorsal fins. In the Eastern Spinner Dolphin, dorsal fin may be recurved, giving a look of backward placement—especially prominent in older males. Older males also have post-anal protrusion or keel, which is particularly sexually dimorphic in the Eastern Spinner Dolphin and the Central American Spinner Dolphin, but less so in Gray’s Spinner Dolphin. Adult male Eastern Spinner Dolphins and Central American Spinner Dolphins may also have upturned fluke tips. Beak tips and lips are dark gray, and bodyis three-toned, comprising a dark-gray dorsal cape, paler gray flanks, and white (pink-tinged) belly, throat, and lower jaw. The Eastern Spinner Dolphin and the Central American Spinner Dolphin are not three-toned but instead are monotone dark gray, with white patches around urogenital area and in “armpit” areas. There is also usually dark strip extending from eye posteroventrally to flipper joint. Another form, called the white-belly Spinner Dolphin, seems to be a hybrid between Gray’s Spinner Dolphin and the Eastern Spinner Dolphin. It is characterized by more robust build and two-tone color pattern, but sexual dimorphism is not as exaggerated as it is in the Eastern Spinner Dolphin. There may be other, more subtle morphological differences among populations. Neonates have more muted color patterns that are otherwise similar to adult patterns. There are 40-60 pairs of small, conical teeth in each jaw, except the Dwarf Spinner Dolphin has only 40-52 teeth in each jaw.

Habitat. Inshore waters ofislands and banks in tropical areas. In Hawaii, Spinner Dolphins are known to use shallow bays for resting during the day. In the eastern tropical Pacific Ocean, they prefer warm pools on high seas. These are warm water masses characterized by a shallow mixed layer overlying a thermocline, high sea-surface temperatures that vary little throughout the year, and high primary productivity. The Dwarf Spinner Dolphin prefers shallow coral reefs, and the white-belly form prefers habitat similar to the Pantropical Spotted Dolphin (S. attenuata ).

Food and Feeding. The Spinner Dolphin is primarily a nocturnal forager and generally feeds on small fish (less than 20 cm) and cephalopods. The Dwarf Spinner Dolphin feeds during the day on reef species such as benthic fish and invertebrates. Off Hawaii, the Spinner Dolphin forages cooperatively by herding schooling fish into large donutshaped balls close to the surface; then individuals take turns swimming through the donut and picking out prey. Foraging dives may descend to 200-300 m, and sometimes even up to 600 m. Natural predators include the Killer Whale ( Orcinus orca ) and possibly the False Killer Whale ( Pseudorca crassidens ), the Pygmy Killer Whale ( Peponocephala electra ), and the Short-finned Pilot Whale ( Globicephala macrorhynchus ).

Breeding. Breeding of the Spinner Dolphin may peak from late spring to autumn, depending on population and geographical location. Gestation is ¢.10 months, and offspring are nursed for 1-2 years. Females reach sexual maturity at 4-7 years and males at 7-10 years. Breeding females give birth roughly once every three years. Longevity is thought to be ¢.26 years. Subspecies with stronger sexual dimorphism likely have a polygynous mating system based on male-male competition. In contrast, the other subspecies and the white-belly form have larger testes and so are likely more polygynandrous and more dependent on sperm competition.

Activity patterns. Pelagic groups of Spinner Dolphins may travel more than 50 km/ day. In Hawaii, they stay in bays during early morning and afternoon to rest in a location safe from predators and then move offshore ¢.400 m for night-time foraging. The Spinner Dolphin is famously known forits highly acrobatic leaps, which often involve spinning on the longitudinal axis up to seven times. Spinning is thought to be a useful technique for generating enough force on water reentry to dislodge attached remoras. It is also possible that the bubble plume created on reentry acts as a distinct acoustic target for communication purposes. The Spinner Dolphin is also known to bow-ride, except in the eastern tropical Pacific Ocean, where it avoids purse-seine vessels.

Movements, Home range and Social organization. In French Polynesia, groups of Spinner Dolphins consist of a few dozen to more than 100 individuals, but groups of more than 1000 individuals have been documented. In the eastern tropical Pacific Ocean, Spinner Dolphins travel in large mixed-species groups with the Pantropical Spotted Dolphin and yellowfin tuna ( Thunnus albacares). Interactions with Pantropical Spotted Dolphins include traveling, aggression, and sexual activity. Social structure of the Spinner Dolphin appears, at least in some cases, to be based on family units with more fluid associations among unrelated individuals. In this fission—fusion structure, large groups will often break apart and reform with different subgroup memberships throughout the day and for different activities. A population of Spinner Dolphins off the south-eastern Hawaiian Archipelago seems to be structured this way. This contrasts with another population in the north-western part of the Archipelago that is characterized by highly stable, long-term associations. These more stable associations have been linked to greater gene flow and may be influenced by habitat and resource variability throughout the Archipelago. This population appears to be isolated from other populations of Spinner Dolphins in the Pacific Ocean and demonstrates that there can be variable genetic and social structures even between closely related populations. In offshore waters of the eastern Pacific Ocean, there is some evidence of segregation by sex and age classes. Movements of Spinner Dolphins may be associated with warm-water currents, such as the Kuroshiro Current offJapan. Site fidelity of Spinner Dolphins is high off Hawaii and in French Polynesia.

Status and Conservation. CITES Appendix II. Classified as Data Deficient on The IUCN Red List. The Eastern Spinner Dolphin is listed as Vulnerable, and the Central American Spinner Dolphin and the Dwarf Spinner Dolphin have not been assessed. There are ¢.801,000 Spinner Dolphins in the eastern tropical Pacific Ocean. The Eastern Spinner Dolphin has been impacted tuna purse-seine fisheries in the eastern tropical Pacific Ocean and currently numbers ¢.613,000 individuals;it is recovering only at a growth rate of 1-1% /year. There are 11,971 Spinner Dolphins in the northern Gulf of Mexico and 3351 individuals in Hawaiian waters. There are currently no abundance estimates for the Dwarf Spinner Dolphin. In the Philippines and Malaysia, there are ¢.35,000 Spinner Dolphins. Many regions in the Pacific, Indian, and Atlantic oceans have not yet been surveyed, but known abundances add up to more than 1,000,000 individuals overall. Gray’s Spinner Dolphin is likely composed of several distinct populations; genetics has revealed limited gene flow among neighboring island communities in the western tropical Pacific Ocean, and even among the Hawaiian Islands gene flow is limited. The Spinner Dolphin is susceptible to tuna purse-seine fisheries in the eastern tropical Pacific Ocean. The Eastern Spinner Dolphin is estimated to have been depleted by c.65% since 1959 as a result of kills in this fishery. Mortality has been greatly reduced since the introduction of mortality limits by the Inter-American Tropical Tuna Commission, but the Eastern Spinner Dolphin appears to be recovering very slowly. Stress from continued fisheries encounters and mortality due to offspring separation or orphaning may be having long-term negative effects on the population. Spinner Dolphins also are taken incidentally in gillnet, trawl, and purse-seine fisheries throughout their distribution. They are the most abundant delphinid in the Indian Ocean, and annual takes in fisheries number in the hundreds per year off India and up to several thousand per year off Sri Lanka. Catch rates are unknown in other areas, including the Atlantic Ocean, especially in fisheries operating off West Africa. Incidental catches off the Philippines and Venezuela are used for shark bait and human consumption. The Spinner Dolphin is intentionally caught for these purposes in the Caribbean and off Sri Lanka, Philippines, Indonesia, and Taiwan. The Spinner Dolphin is also vulnerable to shrimp trawlers in the Gulf of Thailand. Spinner Dolphins rest in shallow bays during the day in some areas off Brazil, Hawaii, Bali, and Indonesia, and they may be increasingly susceptible to harassment from growing dolphin-watching activity.

Bibliography. Andrews et al. (2010), Archer et al. (2001), Benoit-Bird & Au (2003), Cannier & Petiau (2006), Delfour (2007), Dolar et al. (2003), Gerrodette & Forcada (2005), Gray (1850), Hammond et al. (2008n, 20080), Jefferson et al. (2008), Karczmarski et al. (2005), Lammers (2004), LeDuc et al. (1999), Marten & Psakaros (1999), Perrin & Gilpatrick (1994), Perrin & Mesnick (2003), Wang & Yang Shihchu (2007).

Figures

17. Pantropical Spotted Dolphin (Stenella attenuata), 18. Clymene Dolphin (Stenella clymene), 19. Striped Dolphin (Stenella coeruleoalba), 20. Atlantic Spotted Dolphin (Stenella frontalis), 21. Spinner Dolphin (Stenella longirostris), 22. Rough-toothed Dolphin (Steno bredanensus)

Subspecies and Distribution. S.l.longirostrisGray,1828—oceanicislandsintropicalwatersoftheAtlantic,Indian,andWPacificoceanstoc.145°W. S.l.centroamericanaPerrin,1990—coastalwatersofthetropicalEPacificOceanfromtheGulfofTehuantepec,MexicotoCostaRica. S.l.orientalisPerrin,1990—pelagicwatersofthetropicalEPacificOceannofartherWthanc.145°W,primarilybetween24°NoffBajaCalifornia,Mexico,and10°SoffCPeru. S. l. roserventris Dumont d’Urville in Schreber, Goldfuss & Wagner, 1846 — shallow, nearshore waters of SE Asia, Sunda Is, Wallacea, New Guinea, and Australia.