One of the many Stenella species, these animals were previously known as Stenella plagiodon (Cope, 1866). Now known as Atlantic spotted dolphin, Stenella frontalis (G. Cuvier, 1829), this species is also known as Stenella froenata (F. Cuvier, 1829). Other common names used in the past include Gulf Stream spotted dolphin, spotted porpoise, Cuvier's porpoise, Cuvier's dolphin, long-snouted dolphin, and bridled dolphin.
Recent genetic work suggests that the genus Stenella is paraphyletic (LeDuc et al. 1999). This species might need to be moved to another genus.
Order: Cetartiodactyla
Cetacea (unranked)
Odontoceti (unranked)
Family: Delphinidae
Genus: Stenella
Size, shape and distinctive characteristics
Atlantic spotted dolphin are a small delphinid. Mature adults reach around 2.2 m and calves are born between 70-90 cm. Weight of adults reaches 120-140 kg. The species is not considered sexually dimorphic, but mature males have an extremely pronounced white-tipped rostrum. Body is usually spotted but differs by region and the degree of spotting decreases with increased depth. Some Atlantic spotted dolphin off the Azores have virtually no spots on their ventral area.
Atlantic spotted dolphin followed the general spot pattern for S. attenuata, first described by Perrin (1987) and modified for this species since 1985 by Herzing (1997). As neonate and calves (two-tones) no spots are present and by age 2-3, small black spots have developed on the ventral side. As they approach their juvenile (speckled) ages (4-8) they develop small white spots on the dorsal area and more black spots on their ventral side. After age 9 the young adults (mottled) have a clear increase of white spots on the dorsal side. After 15 years the full adults (fused) will continue to add spots which can merge over their entire body. There can be slight variation by individual with age classes, with some dolphins having fairly white ventral areas for a typical dolphin of comparable age.
Young Stenella frontalis can be mistaken for bottlenose dolphin due to their lack of spots. Both male and female adults develop white-tipped rostrums with the males being the more extreme.
This species is heavier bodied than pantropical spotted dolphins, Stenella attenuata. S. frontalis has distinct coloration patterns with a weakly defined cape, a flipper stripe terminating between the end of the gape and eye, and a uniformly colored peduncle. The spinal blaze is typically present and strong, sweeping back and up the body. Pantropical spotted dolphin have a strongly defined cape that passes high over the eye, distinct from S. front
Stenella frontalis is endemic to the warm temperate waters of the Atlantic ocean and Gulf of Mexico. The species has a distribution up and down the east coast of the U.S. from 45 degrees north (New Jersey) 35 degrees south (Venezuela and Brazil). In the Eastern Atlantic Stenella frontalis is found off North Africa from Mauritania south to Gabon. This species is especially prevalent through the islands of the Bahamas, and in the Gulf of Mexico.
S. frontalis can be resident year round, such as in the Bahamas, and migratory, such as along the eastern seaboard and other locations. Griffin and Griffin (2004) have documented temporal variation of this species on the West Florida Continental Shelf.
Ecology and Behaviour
Atlantic spotted dolphins often prefer the continental shelf areas of the U.S. and shallow sandbanks of the Bahamas, staying typically within the 100 fathom curve. Stenella frontalis is a gregarious species often observed with mixed-age groups numbering in the hundreds, not thousands. When in smaller subgroups, this species can be age and sex segregated to a small degree.
This species is highly social and has been studied extensively in the Bahamas, including three generations of dolphin and their detailed underwater behavior and sound (Herzing 1996, 2000). Tightly bonded mother and calf pairs are typical to the age of 3. Juvenile subgroup are often separate part of the day and observed practicing foraging and social skills. Adult males form coalitions and engage in herding and courtship coordination. Females have a larger network of associates, but weaker bonds than males.
Social behaviors include play, discipline, aggression, rest, forage, and travel. Underwater behavior are correlated with distinct sounds such as contact/reunions with signature whistles, burst-pulsed sounds with aggression, and echolocation clicks with foraging. Atlantic spotted dolphin also show synchronized surface and underwater behaviors, usually during male coordination during courtship or fighting.
Extensive acoustic work has been done with S. frontalis in the Bahamas including high frequency measures of echolocation clicks (Au and Herzing, 2003) and broadband recordings of whistles and burst-pulsed sounds (Lammers et al., 2003). Whistles comparisons have also been made in the Gulf of Mexico (Baron et al., 2008) and are underway in southeastern Brazil (Azevedo and colleagues).
This species also has regular interspecific interactions with bottlenose dolphin both two locations in the Bahamas on Little Bahama Bank (Herzing 1996;Herzing and Johnson 1997), and on Great Bahama Bank (Herzing et al., 2003; Melillo et al., 2009) and includes the identification of a suspected hybrid (Herzing et al, 2003) indicating that not only recreational sexual interaction occurs between these sympatric species, but occasional reproduction. Like many other cetaceans it is possible that hybridization occurs on a regular basis with such closely intertwined species.
Life History
Life history of this species has only recently been documented in one part of the world (Herzing 1997). This species has distinct age classes and color phases. Calves stay with their mothers through about the age of three. When the mother becomes pregnant, older calves join juvenile subgroups usually consisting of familiar nursery group members. After the age of 8 males will form coalitions and females, who become sexually mature around this time, will often become pregnant for the first time. The age of male sexually maturing for this species is unknown although in the Bahamas preliminary paternity determination through genetics (Green and colleagues) suggests that only fused males, greater than 15 years of age, sire offspring.
Females have calves an average of every three years. If a calf is lost, females quickly go into estrus and become pregnant again, shortening their calving cycle to every 1-2 years. Mortality rates for calves is around 25 % the first year. In many cases first time mothers do very well tending their calves, likely due to much socialization and learning babysitting skills throughout their juvenile period (Herzing, 2005). Calving season is considered spring and fall in the Bahamas.
In the Bahamas estimated maximum age for S. frontalis is over 50 years of age. Females stop reproducing around forty but it is unknown whether they display senescence like pilot whales and other species.
Welsh and Herzing (2008) have also described preferential associations between kin for this species, indicating that long-term associations can be driven by family relationships. This species has also been observed to engage in active teaching of calves which probably increases their survival skills at an early age (Bender et al, 2008).
Natural predators include tiger sharks, bull sharks, and orcas.
Diet
Prey species in the shallow waters include both bottom fish, flying fish and squid. In the Bahamas, where underwater observations have been possible (Herzing, 2004) this species feeds on a variety of fish from diverse families including conger eels (Congridae), snake eels (Ophichthidae), razorfish/wrasses (Labridae), flounder (Bothidae), snakefish (Synodontidae), needlefish (Belonidae), ballyhoo (Hemiramphidae), flying fish (Exocoetidae), and various species of squid (Doryteuthis). In the Gulf of Mexico this species has been observed in coordinated nocturnal feeding (Fertl and Wursig, 1995).
Global Abundance
Unknown for the most part. Estimates vary from 3,213 for the northern Gulf of Mexico to over 52,000 for the east coast of the U.S. although survey methods are considered challenging. Most other areas have not been surveyed.
IUCN status
Data Deficient. Year Assessed: 2008. The species is widespread but abundance has not been estimated for the mid and eastern Atlantic. Bycatches in West Africa are of unknown scale and potentially large. This species may also be subject to targeted hunts in some Caribbean islands.
Some possible direct kills in the Caribbean and off West Africa. Incidental catches in fisheries off Brazil, the United States and Mauritania.
Abundance and bycatch in fisheries off West Africa are in need of investigation.
Denise
Herzing, Wild Dolphin Project and Florida Atlantic University wdpdenise@earthlink.net
Au, W.W.L. and Herzing, D.L. (2003). Echolocation signals of wild Atlantic spotted dolphin (Stenella frontalis). J. Acoustical Society of America 113(1): 598-604.
Azevedo, A.F., Flach, L, Bisi, T.L., Andrade, L.G., Dorneles P.R. and J. Lailson-Brito (submitted for review). Whistles emitted by Atlantic spotted dolphins (Stenella frontalis) in southeastern Brazil.
Baron, S.C, Martinez, A, Garrison, L.P. and E.O. Keith (2008). Differences in acoustic signals from Delphinids in the western North Atlantic and northern Gulf of Mexico. Marine Mammal Science, 24 (1), 42-56.
Bender, C.E., Herzing, D.L. and Bjorklund, D.F. (2008).Evidence of Teaching In Atlantic Spotted Dolphins (Stenella frontalis) by Mother Dolphins Foraging in the Presence of their Calves. Animal Cognition, 12 (1), 43-53.
Fertl, D. and B. Wursig.(1995). Coordinated feeding by Atlantic spotted dolphins (Stenella frontalis) in the Gulf of Mexico. Aquatic Mammals 21 (1), 3-5.
Green, M.L., Herzing, D.L. and J. D. Baldwin (submitted for review). Noninvasive genetic analysis of paternity and reproductive success in Atlantic spotted dolphins, Stenella frontalis.
Griffin, R.B. and N.J. Griffin (2004). Temporal Variation in Atlantic Spotted Dolphin (Stenella frontalis) and Bottlenose Dolphin (Tursiops truncatus) Densities on the West Florida Continental Shelf. Aquatic Mammals 30 (3), 380-390.
Hammond, P.S. et al. (2008). Stenella frontalis. In; IUCN 2009. IUCN Red List of Threathened Species. Version 2009.1
Herzing, D.L. (1996). Vocalizations and associated underwater behavior of free-ranging Atlantic spotted dolphins, Stenella frontalis, and bottlenose dolphins, Tursiops truncatus. Aquatic Mammals 22 (2), 61-79.
Herzing, D.L. (1997). The natural history of free-ranging Atlantic spotted dolphins (Stenella frontalis): Age classes, color phases, and female reproduction. Marine Mammal Science 13(4), 576-595.
Herzing D.L., and Johnson, C.M. (1997) Interspecific interactions between Atlantic spotted dolphins (Stenella frontalis) and bottlenose dolphins (Tursiops truncatus) in the Bahamas, 1985-1995. Aquatic Mammals, 23 (2) 85-99.
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Herzing, D.L. (2004). Social and Non-Social uses of Echolocation in Free-Ranging Stenella frontalis and Tursiops truncatus. In: Advances in the Study of Echolocation in Bats and Dolphins. Springer-Verlag Press, 404-410.
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LeDuc, R.G., W.F. Perrin, and A.E. Dizon. (1999). Phylogenetic relationships between the delphinid cetaceans base on full cytochrome b sequences. Marine Mammal Science, 15: 619-648.
Melillo, K.E., Dudzinski, K.M, and L.A. Cornick. Interactions between Atlantic Spotted 9Stenella frontalis) and Bottlenose (Tursiops truncatus) Dolphins off Bimini, The Bahamas, 2003-2007. Aquatic Mammals. 35 (2), pp. 281-291.
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Welsh, L. S. and Herzing, D. L. (2008). Preferential Association Among Kin Exhibited in a Population of Atlantic Spotted Dolphins (Stenella frontalis) in the Bahamas. International Journal of Comparative Psychology, 21,1-11.
Stenella frontalis
