Biogeography of Bottlenosed Dolphins or Tursiops Truncatus - Research Paper Example

 TABLE OF CONTENTS INTRODUCTION 3 BIOGEOGRAPHY 5 EVOLUTIONARY HISTORY 7 CONSERVATION/ENVIRONMENTAL ISSUES 8 REFERENCES 11 Abstract Bottlenose dolphin or Tursiops truncatus is a marine mammal inhabiting both coastal and inshore areas of the oceans worldwide except the Polar Regions. The origins of the organism can be traced back to early Oligocene with fossil records available. It has evolved to adapt to its aquatic habitat after evolution trail going through an intervening terrestrial form. During the course, it has developed such unique features as vocal learning through echolocation. The large brain size and the vocal learning on one hand are the cause of the organism being a major attraction and a source of amusement, and on the other hand render it vulnerable to noise and chemical pollution resulting from anthropogenic activity. The conservation strategies directed at habitat preservation of the organism are recommended. BIOGEOGRAPHY OF BOTTLENOSE DOLPHINS INTRODUCTION Bottlenose dolphins belong to the order Cetacea, the term derived from Greek word ketos meaning whale. The order includes large aquatic mammals with forelimbs modified into flippers, a horizontally flattened tail, lacking hind limbs and with one or two nostrils. The order includes the suborders Odontoceti (toothed whales) and Mysticeti (baleen whales). Most Dolphins are included in the family Delphinidae, belonging to suborder Odontoceti, including the bottlenose Dolphin or genus Tursiops. The genera Tursiops (derived from Latin word Tursio meaning Dolphin or Dolphin like), has been further classified to include two species on the basis of biochemical and genetic studies: common bottlenose dolphin or T. truncatus and the Indopacific bottlenose dolphin or T. aduncus (Rice, 18). Besides these clearly identified species, several potential species have been described such as an offshore and a coastal form in North Atlantic distinctly identified on the basis of distinct morphology and ecological markers (Mead & Potter, 165). Thus the accurate classification of this genus remains uncertain and the genus has been proposed to be polyphyletic (LeDuc et al., 619). Biogeography relates to the study of patterns of the geographical distribution of organism along with factors determining the specific distribution patterns. The biogeography of an organism provides significant information regarding its evolutionary history and adaptation. Thus biogeography can be considered to include the spatial as well as temporal distribution of an organism. The common bottlenose dolphin T.trucatus enjoys worldwide distribution including tropical and temperate, inshore, coastal, shelf and oceanic areas (Reynolds et al., 12), except the polar region, northward of 45◦, though they do occurs far as 62◦N7◦W in northern Europe. Found rarely in Baltic Sea and vagrant in Newfoundland and Norway, the species is present is as yet uncertain in other areas. The author aims to present in the current paper an account of the biogeography of the common bottlenose dolphin or T. truncatus; along with the evolutionary history of the organism on the basis of fossil records and the threat to conservation programs for the organism specially those relating to its biogeography. BIOGEOGRAPHY Figure 1 The major distribution areas of common bottlenose dolphin (figure 1) include the western margins of North Atlantic where the coastal forms occur within a distance of 5km from the land and in bays and estuaries (Mead and Potter, 165). The offshore forms occur within 200-2000m range along the shelf break in regions north and north east to Georges bank (Kenney, 369); and in the Scotian shelf; with the distribution of the two forms overlapping in some zones. The coastal forms are prevalent throughout the year continuously along the North Carolina to Texas (Mead and Potter, 165). In southeastern US they also occur as communities exhibiting specific association patterns and individual home ranges. In the eastern Pacific region, T. truncatus occurs in the neritic zone up to the Southern California Bight in north. The dolphins have been reported to extend their range further north of Point of Conception during the El Nino event in 1982-82 and have been reported from San Francisco (Wells et al., 421). In the eastern tropical Pacific, the species is present in pelagic waters with the communities associated with the islands and archipelago such as Hawaiian, Marquesas and the Tuamotus. T. truncatus inhabits brackish waters of semi-enclosed estuaries to pelagic zones. The coastal forms with smaller ranges have significant plasticity in their movement patterns with Californian dolphins travelling up to 286 kms in 14 days (Defran et al., 366). Along the Atlantic coast, these forms migrate from North Carolina in winters to New York in summers (McLellan et al., 297). Dolphins in Florida bay on the other hand have a restricted home range of 125km2. Though the reasons for such wide differences are not fully understood, but it has been speculated that seasonal variations affecting prey abundance and distribution along with water temperature could have lead to these differences. Individual variations in movement within an area with males dispersing far from there natal homes more than the females, probably in search of receptive female partners, are also observed (Connor et al., 91). However it is only the offshore forms that such movements are observed, the coastal forms remaining closer to their homes. Dispersal has also been observed to areas with better prey availability, such as in organisms in southeastern US. These organisms disperse to areas near seagrass beds which have better assembly of potential prey. This theory was tested in eastern Gulf of Mexico, where though the prey density was lesser in seagrasses but the prey size was greater (Allen et al., 253). Thus seagrass beds provide a suitable niche with ‘nursery’ habitat to the younger ones of bottlenose dolphins, thus are critical to the health of the organism. EVOLUTIONARY HISTORY The suborder Odontoceti has been reported to appear in early Oligocene, radiating dramatically in to the Miocene. The fossil records of early cetaceans indicate the transition from an intermediate land ancestor to a presently extinct aquatic form termed archaeocetes. Data gathered from brain size studies or encephalization records from the fossils suggest significant changes in brain size during Oligocene, characterized by the transition from archaeocetes to early odontocetes. The diversity in the morphological and behavioral characters of the genus Tursiops is limited. The pattern of diversity within a taxonomic rank is indicative of the evolutionary trends and the factors responsible for generating the specific trends. Unlike terrestrial environments, marine environments have fewer restrictions causing differentiation and hence speciation. Marine mammals too have high mobility and long range dispersion leading to limited genetic variations across the population, the extent of which varies due to local resources, social structure and in certain cases due to historical environmental changes (Hoelzel, 1177). Tursiops has been identified as a polytypic genus with numerous morphotypes differing in color pattern, body dimensions and cranial structure with several of these characters overlapping in most types. On the basis of biochemical and genetic studies only two species have clearly identified, however whether more exist is still uncertain. Wide differences have been observed in coastal and pelagic populations of T. truncatus, with the two differing in morphology, feeding ecology, parasite load, hemoglobin profile, microsatellite DNA, etc with distinctions evident in each of these characters. Though the level of genetic differences in these two forms is lesser than that between the two species of the genera; yet the data still remains insufficient and inconclusive. The high level of differentiation indicated by the mitochondrial DNA and microsatellite DNA marker studies however, is indicative of high probability of speciation in this species. Many of the populations of T. truncatus exhibit reduced levels of variation and a pattern of a variation indicative of population expansion. It has been speculated that peripheral population of T. truncatus could have been formed as a consequence of dispersion of larger and more diverse pelagic population at different times during the course of evolution (Natoli et al., 363). One of the major adaptations of bottlenose dolphins forming an evolutionary milestone is vocal learning or ability to respond with a specific verbal output as a consequence of an auditory input. Three theories have been proposed to explain the evolution this character: echolocation, sexual selection and need for maintaining a stable signature among diving mammals. Echolocation has been found to be the probable force leading to development of vocal learning in this species (Moore and Pawloski, 305). CONSERVATION/ENVIRONMENTAL ISSUES Proximity of the bottlenose dolphin habitats to human habitations often exposes the species to anthropogenic disturbances and habitat degradations. The major threat to the populations of bottlenose dolphins include industrial and agricultural pollutant contamination, disturbances from marine industrial activities, by-catch mortality such as those from accidental entanglement in fishing gear or prey mortality, disturbances from dolphin watcher’s boat traffic, physical and acoustic disturbances from ships and other marine traffic (Englund et al., 7). One of the major threats to the bottlenose dolphin population is noise pollution threatening its physiological balance, feeding and breeding behavior, hearing and migration. The importance of sound to bottlenose dolphins is comprehended by the fact that they rely on sound for many of the processes important for their survival. The underwater sound levels are predominantly of low frequency (>1KHz). The response of the organism to acoustic signals is dependent on varied factors such as age, sex, and state of activity along with extrinsic factors such as location, time of day, season etc. Thus even though it is difficult to estimate the nature of impact of alterations of acoustic signals on the organism, it is definitely established that the organism is adversely affected by noise as a consequence of several studies conducted in past. Startled responses to sounds of speedboats have been experimentally demonstrated in bottlenose dolphins, along with rarer surface appearances in response to dolphin watching boats, being indicative of anthropogenic disturbances (Perry, 1). Industrial activities, seismic explorations, sonar, acoustic thermography, acoustic deterrents, and aircrafts have been further implicated in causing disturbance to the cetacean populations in general and bottlenose dolphin populations in particular. In fact several cases documenting the abandonment of areas with one or more of above mentioned activities leading to displacement of bottlenose dolphin population are known. The immediate implications of displacement is not known, but displacement from areas important for breeding such as nearshore areas and calving grounds may have an adverse impact on the survival and growth of the population in general. Another negative implication of stress resulting as a consequence of above mentioned activities is disturbance of energy budget of the organism thereby affecting its survival and reproductive abilities; and results in lower disease resistance. Hearing impairment as a consequence of noise leads to hearing impairment leading to lowering of acoustic cues and masking. Besides the short term impacts of the anthropogenic activities, there are also long term impacts which have not yet been estimated due to the large number of variables involved and several other procedural difficulties. Several agencies and regulations are in force to ensure safety and survivability of bottlenose dolphins. They are protected by the Marine mammal protection act of 1972 (MMPA) which regulates activities that affect dolphins in US. It has also been provided special protected status under Annex II of the European Union’s habitats directive. Conservation measures involving restriction of human activity in habitats of bottlenose dolphins; reducing boat traffic; means to reduce by-catch tragedies, disturbances by shipping, tourism etc need to be brought in to force to prevent habitat degradation and disturbance to survival and displacement of bottlenose dolphins. Further caution should be exercised in breeding and nursery areas of the species and during the breeding periods. Besides Bottlenose dolphin being a highly mobile marine mammal, coordinated efforts at global level need to be made to ensure the survival of the species. REFERENCES Read More