Until recently all bottlenose dolphins were considered members of a single species, the Common bottlenose Dolphin (T. truncatus), which was thought to show considerable variation in morphology and behaviour. In 1998 the Indo-Pacific bottlenose dolphin was recognised as a separate species based on genetic, osteological and other morphological differences, including the presence of ventral speckling (Reeves et al. 2002, Wells & Scott 2002, Kemper 2004).
Recent studies suggest that the Indo-Pacific bottlenose dolphin is more closely related to genera Stenella and Delphinus than it is to the Common bottlenose Dolphin, making the genus Tursiops paraphyletic. More revision to the taxonomy of these species is likely to occur as further genetic research is carried out (LeDuc et al. 1998, Wells & Scott 2002, Natoli et al. 2004).
Size, shape and distinctive characteristics
The Indo-Pacific bottlenose dolphin has a medium-sized, fusiform body with a short rostrum, although it is comparatively longer and more slender than T. truncatus relative to body size. The dorsal fin is triangular or falcate, and dorsal colouration is dark grey, ventral colouration is very light grey in juvenile animals, developing dark speckling with age.
Adults are slightly smaller and less robust than their Common bottlenose relatives, reaching about 2.6 m in total body length with some geographical variation.The pectoral fins are also comparatively larger, possibly to allow greater manouverability in a shallow water habitat (Shirihai & Jarrett 2006).
This species has a wide but discontinuous distribution in the warm temperate to tropical Indian and Pacific, from South Africa in the west, along the rim of the Indian Ocean to southern Japan and southeast Australia in the east (Wells & Scott 2002).
Ecology and Behaviour
This species inhabits inshore coastal waters, usually in groups of 2-15 individuals, although group size can number into the hundreds and may vary due to variation in prey availabilty and predation risk (Mann & Barnett 1999). They form complex fission-fusion societies and associations between individuals are usually weak, and may be dynamic over a few hours or stable for longer periods of time. They are sometimes seen in mixed groups with other dolphin species (Mann et al. 2000).
Populations that have been well studied for long periods of time, such as those in Shark Bay, Western Australia, have shown some of the most complex social behaviours seen in non-human animals.
One example is the formation of alliances between related males in order to gain and keep access to receptive females. Some of these alliances are stable over very long periods of time, while the different alliances sometimes co-operate or compete with each other against others depending on the circumstances (Krützen & Sherwin 2004). Individuals also form "super alliances" of up to 14 individuals (Connor et al. 2006), with complex internal structure, and this, along with the fact that alliance membership is not a pre-requisite for fathering offspring (Krützen & Barre 2004) show that courtship behaviour involves a variety of strategies and complex interactions in this species. In Shark Bay, males are much more aggressive than females (Scott et al. 2005) and infanticide has been reported elsewhere (Patterson et al. 1998; Dunn et al. 2002).
Females range from being almost solitary to highly gregarious (Gibson & Mann 2008), and form loose social networks with other females of various age and kinship. Variation in female social behaviour is probably driven by ecological factors (e.g. foraging tactics, Mann & Sargeant 2003; predation risk, Heithaus & Dill 2002), and may have social benefits (e.g. reduced male harassment, calf care, social learning).
For example, females with newborns occur in larger groups than those with older calves (Mann et al. 2000), and mothers and calves are most sociable during the calves' first year (Gibson & Mann 2008), when mortality rate is highest (Mann et al. 2000).
Bottlenose dolphin groups containing females and calves are often referred to as ‘nursery' groups, implying that these groups cooperate to provide protection and/or social opportunities for calves. Wells (2003) proposed that females in large, stable nursery groups might have higher reproductive success than lone females, a pattern not found in Shark Bay, where habitat use is a better predictor of reproductive success (Mann et al. 2000).
Females typically reach sexual maturity at between 7 and 12 years of age, with peak mating and calving season occuring in spring and summer. Calves are born after a gestation period of about 12 months, and are weaned at between 3 and 8 years of age. Females tend to wean mid-pregnancy, and have an inter-birth interval of about 4 years (Mann et al. 2000).
Calves must learn to navigate their social and physical environments by the end of infancy (typically 3-6 years, from birth through weaning), despite a prolonged juvenile period (>8 years, from weaning to reproductive maturity; Mann et al. 2000). They appear to emulate the adult social structure by joining and leaving their mothers from an early age, typically to forage or socialize (Mann & Watson-Capps 2005). These temporary separations are frequent and often long distance; even newborns, 0-3 months old, have been observed greater than 100 m from their mothers (Mann & Smuts 1999). During separations from their mothers, calves may be alone or with others from within or outside of their mothers' network of close associates (Mann & Smuts 1999).
Sociosexual and aggressive behaviour of male calves resembles adult male alliance patterns in that their interactions are often symmetrical, synchronous and involve multiple males; calves of both sexes also mimic the courtship behaviour of adults (Mann 2006). In Shark Bay, some males that played together extensively as calves were still being sighted together 15 years later (Mann 2006), when alliances are thought to stabilize (Connor et al. 2006).
Bottlenose dolphns feed on a wide variety of schooling, demersal and reef fish as well as cephalopods, particularly squid (Amir et al. 2005), and several populations have developed very specialised feeding strategies, such as the "sponging" dolphins in Shark Bay, Western Australia, which use conical sponges to protect their rostra while pursuing prey on the ocean floor. Females, more than males, show diverse foraging tactics (Mann & Sargeant 2003; Sargeant et al. 2005).
Recent studies suggest that females teach these techniques to their offspring, providing the first example of culturally transmitted tool use in bottlenose dolphins (Kruetzen et al. 2005).
Due to the unresolved taxonomy, very little long term data is available for the species, although it is widespread in Indo-Pacific coastal waters.
The largest and best known populations include those in Shark Bay, Western Australia with at least 2000 individuals (Preen et al. 1997); Queensland, Australia with an estimated 1000 individuals (Chilvers & Corkeron 2003); the Persian Gulf with at least 1200 individuals (Preen 2004); and KwaZulu Natal with over 500 individuals (Cockcroft et al. 1992).
The IUCN lists both Tursiops species as Data Deficient
Natural threats include high levels of predation by sharks, particularly tiger sharks (Galeocerdo cuvier), and great white sharks (Carcharodon carcharias) in Australia and South Africa, where a large percentage of the population bear the scars of such attacks.
Direct harvest for human consumption was outlawed as recently as 1990, although a number are still taken in small cultural harvests globally.
Incidental bycatch continues to be a major concern, particularly in purse seine and gill net fisheries which take many thousands of dolphins each year in Taiwan, Bangladesh, South Africa and Australia, and many other fisheries across the species range. Due to their global popularity as a captive display species live captures have also occurred, particularly in Taiwan, Indonesia, Japan, and the Solomon Islands (Reeves et al. 2003).
Due to their preference for inshore waters in close proximity to human populations, Indo-Pacific bottlenose dolphins are exposed to increasing disturbance from a variety of sources. These include coastal pollution from run-off and sedimentation, reduced inshore prey populations due to overfishing, bio-accumulation of harmful chemicals in the marine food chain, disturbance by recreational and commercial boat traffic and increasing levels of anthropogenic noise.
Dolphins are also the target of a rapidly expanding eco-tourism industry (watching, swimming, provisional feeding of animals etc.), and these activities have been shown to cause changes in their behaviour and distribution (Bejder et al. 2006, Constantine 2004).
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