Table of Contents
Table of Contents

Table of Contents… 1-2
Introduction… 3-4 Part I: Evolutionary History… 5 * Pakicetus… 5-7 * Ambulocetuss… 7-8 * Rodhocetus… 8-9 * Basilosaurus and Dorundontidae… 9-10 * Kentriodontidae… 10-11
Part II: Evolutionary Biology, Ecology, and Behavior… 11 * What is a Dolphin?... 11 * General Anatomy… 11-12 * Integumentary, Sensory and Urinary systems… 12-18 * Musculoskeletal system and Locomotion… 18-20 * Respiration, diving, and breath-hold physiology… 20-22 * Sound Production, communication, echolocation… 22-23 * Diet, foraging strategies… 24-27 * Mating, Breeding, and social organization behaviors… 27-29 * Reproductive structures, patterns and strategies… 29-31
Part III: Exploitation and Conservation… 31 * Commercial exploitation… 31-32 * Progress… 32-33 Conclusion… 34 References… 35-37 Figures… 38-39

Dolphins are marine mammals that are closely related to whales and porpoises. They vary in size from 1.2 meters (Maui's dolphin), up to 9.5 meters (the orca or killer whale); part of the largest family in the Cetacean order, the Delphinidae , dolphins have evolved relatively recently, about ten million years ago, during the Miocene. Fig. 1. Geological range of extinct and extant cetaceans.

There has been a trend within mammalian evolution towards colonization of land, however in some instances this trend has been reversed; there are three large orders of mammals that have separately re-adapted to marine life, the Cetacea (whales, dolphins and porpoises), the Pinnipedia (seals, sea lions and walruses), and the Sirenia (dugongs, sea cows and manatees). This “re-adaptation” is most extreme in the cetaceans which have evolved a totally fish-like form, masking their true mammalian ancestry (Coffey, 1977). The evolution of cetaceans is so well documented, and is considered one of the best examples of macroevolution, as documented by fossils (Thewissen &Williams, 2002), that is has become known as the Rosetta stone to evolution itself. The most primitive ancestor of cetaceans in fact looked nothing like modern day species; rather, they resembled small wolves. There are six families of prehistoric cetaceans; the pakicetids, ambulocetids, remingtonocetids, protocetids, dorudontids, and basilosaurids. Cumulatively, they illustrate the transition from terrestrial quadruped to a fully aquatic marine mammal. The oldest of the cetacean ancestors are the pakicetids, which lived in the floodplains of present-day Pakistan region, approximately fifty million years ago. Pakicetids then evolved into the semiaquatic ambulocetids, which inhabited bays of the Tethys Ocean in Northern Pakistan. Remingtonocetids were also found in marine deposits of Pakistan; however, they appear to have been more aquatic than ambulocetids. Protocetids were increasingly more aquatic than remingtonocetids, although they had retained their hind limbs. By the late Eocene period, dorudontids and basilosaurids had had successfully assimilated into marine life, losing nearly all vestiges of a terrestrial life (Thewissen & Williams, 2002). The following is a more in depth description of each cetacean ancestor.
Part I: Evolutionary History

Fig. 2. Illustrates the evolution of dolphins in reference to geological ages.

Pakicetus
Previous lack of fossil evidence had sprung much debate about the exact origin of dolphins; however, recent excavations in Pakistan have clarified a once mysterious evolution. Approximately fifty-three million years ago Pakicetus roamed the earth. This hoofed mammal more closely resembled modern wolves than dolphins. Fossils of Pakicetus were found during a 1979 North Pakistan excavation. At first glance, Pakicetus seems like an uncharacteristic ancestor to the dolphins, however upon examination of the skull, scientist Philip Gingerich was finally able to identify which order Pakicetus belong to. Here, he found a tiny ‘S’ shaped bone, known as the sigmoid process, that is only found in modern day cetaceans.
Fig. 3. The sigmoid process is an inner ear bone unique to modern cetaceans. This is the very bone that helped indentify pakicetus as belonging in the heritage of cetaceans.
From this, Gingerich confirmed Pakicetus to belong to the cetacean order. In order to understand why a land animal evolved into the modern dolphin, we must understand the conditions in which Pakicetus lived. During the Eocene period, land masses drifted further apart, this created new ocean currents; these currents directed warm water and warm air from the equator to the polar regions of the earth. In effect, the warming of the poles meant an overall warming of the planet. Increasing temperatures were limiting terrestrial food sources for Pakicetus. In order to survive, Pakicetus needed to quickly find an alternative food source. Warmer planet temperatures boosted photosynthesis, which in turn produced an abundance of marine life, it is for this reason Pakicetus took its first precarious steps back into aquatic life. Pakicetids have never been found associated with marine fauna or deposits, they were clearly terrestrial animals. The morphology of their limbs, comprised of long thin legs and short hands suggests they were poor swimmers (Thewissen & Williams, 2002). Approximately fifty million years ago, Pakicetus disappeared from fossil records. This poses the question: what happened after Pakicetus?
Ambulocetus
In 1994, Hans Thewissen, a former student of Gingerich returned to the same region of Pakistan at which remains of Pakicetus had been discovered. Here he discovers the bones of the next ancestor in the lineage of dolphins. This creature had legs like a land animal, but feet like an aquatic wader. Like Gingerich, Thewissen was unable to identify the order of the animal until he examined the skull. Here he found the same unique inner ear bone structure found in both modern cetaceans and Pakicetus, confirming his fossil discovery to be an ancestor of modern cetaceans. Thewissen named the new species Ambulocetus. This newly discovered ancestor is the reason why Pakicetus disappeared from fossil record; Pakicetus had simply evolved into the new species Ambulocetus. The poor swimming skills of Pakicetus obliged three adaptations to improve locomotion in water; the tail developed muscles that flattened like that of an oar, the hind-limbs had widened and shortened to act as flippers in propulsion, and the overall body plan had become more streamlined, making for a more efficient swimmer.
Fig. 4 The skeletal structure of Ambulocetus.
In order to permanently inhabit marine waters, this mammal would have to evolve the ability to drink and inhabit waters of high salinity. Isotopic analysis done on teeth samples indicate Ambulocetus drank fresh water; this suggests that Ambulocetus still lived primarily on land.
Rodhocetus
Although Ambulocetus had evolved adaptations to enable aquatic living, it only partially inhabited water. Forty-nine million years ago, Ambulocetus vanished from fossil record. Rodhocetus was the first fully aquatic ancestor to dolphins. It had a shorter, more powerful neck that enable efficient diving. The hind legs had lengthened and widened, becoming more like flippers than terrestrial limbs. The tail developed musculature associated with propulsion, however the most notable adaptation of Rodhocetus lies in the function of the kidney. Evolution modifies the existing organ to process high salinity associated with marine life. Another important adaptation of Rodhocetus is that of the inner ear, particularly, the organ of balance; it consists of three canals filled with fluid. The fluid moves when the head moves, firing nerves cells that send signals to the brain. The brain then interprets these signals and in turn causes balance adjustments. Abrupt and erratic movement sends scrambled messages to the brain, this in turn causes dizziness. In cetaceans these three canals have been reduced in size over time thus inhibiting movement of the inner ear fluid. This adaptation allows dolphins to turn and spin in water without risking disorientation. In Rodhocetus, this adaptation increased agility to avoid predation.
Basilosaurus and Dorundontidae
The next ancestor in dolphin lineage is Basilosaurus; they are late Eocene cetaceans most commonly found in association with dorudontids. Although skull morphology and habitat are similar, basilosaurids and dorundontids are very different in body form. Dorundontids are proportionally similar to extant dolphins; basilosaurids have an extreme elongation of the lumbar vertebrae, resulting in long snake-like bodies.
Fig. 5. Skeletal structure of a basilosaurid: (a) full skeleton of a basilosaurid with hyper extended vertebrae, (b) proposed pelvic girdle of basilosaurids.
Dorundontidae were dolphin-like in their body form. They were also the dominant cetaceans of the late Eocene period. They had a powerful vertebral column and short, flipper shaped forelimbs. Their external hindl-imbs were small and did not function in locomotion. Their pelvis was not attached to the skeleton that indicates the presence of a fluke; this further indicates that members of this family swam similarly to that of extant dolphins (Thewissen & Williams, 2002). Fossils are generally found in marine deposits, which indicates this family had adapted to being fully aquatic.
Kentriodontidae
Very little is known about these cetaceans of the Late Oligocene to the Middle Miocene, beyond the fact that species were some of the first radiations of true dolphins. These were medium-sized odontocetes with largely symmetrical skulls, and thought likely to include ancestors of some modern species. The Kentriodontids may have been related to the family Iniidae (freshwater dolphins), although characteristics shared with this family, including long mandibular symphyses, rugose enamel on teeth, large cranial crests and unfused cervical vertebrae are primitive (Gaskin, 1985).
Part II: Evolutionary Biology, Ecology, and Behavior
What is a Dolphin?
Dolphins are a part of the order Cetacea; they are part of the sub-order Odontoceti; as the name suggest, members of this suborder have teeth rather than the baleen that is present in whale species. Dolphins, whales and porpoises comprise the order Cetacea; they are characterized by their need to breathe air, presence of true bones, vertical movement of the spine and internal fertilization.
General Anatomy
When their ancestors ventured from land to water they were obliged to modify their anatomy in order to adapt to marine life. Despite their superficial resemblance to fish, dolphins are of course mammals.Their bodies resembles that of a typical fish, apart from their tail fluke which is on a horizontal plane; their bodies are fusiform to move through water more efficiently. They have no external ears and their mammary glands, genitals and anal openings are recessed into body slits to reduce drag. The forelimb structures have been modified into a flipper for stabilization. Most species possess a dorsal fin, which is strengthened by strong fibers of connective tissue. All members of the order Cetacea have lost their hind limbs, however there are remnants of a pelvic girdle found internally. From the tail, two flattened structures called ‘flukes’ have been developed; they don’t contain any bone or cartilaginous substances but are strengthened, like the dorsal fin, by fibrous connective tissue. Aside from structural changes, the anatomical structure has remained mammalian; dolphins breathe air and produce live young. Their physiology, including respiration, heart and blood, reproduction, urinary, and digestive systems have also remained mammalian (Coffey, 1977).
Integumentary, Sensory and Urinary systems
Before investigating the behavioral and ecological adaptations of dolphins to their environments, it is necessary to discuss how the internal workings of these animals and what makes it possible for them to live in such diverse ways. The following summarizes the functional anatomy and physiology of the sensory and urinary systems of dolphins. Review of the integumentary system is inclusive of descriptions of components of the skin, hair, glands etc. Discussion of sensory system includes development of a visual system, olfaction, taste, and sound production. Lastly the urinary system and maintenance of water balance are reviewed.
As present in all mammals, dolphin skin consists of three layers; and outer layer (the epidermis), a middle layer (the dermis), and a deep layer, which forms the blubber (the hypodermis. The skin of dolphins is distinguished by the absence of glands and hair. The structure of the epidermis in marine mammals is multilayered stratified epithelium that consists of layers typical to that of terrestrial mammals: stratum basale, stratum spinosum, and stratum corneum. The outermost layer of the epidermis, the stratum corneum, consists of a layer of flattened, keratinized cells (Berta & Sumich, 1999). Dolphin skin is smooth with a rubbery feel. Cutaneous ridges have been described on the surface of the skin of some dolphins, however the function of these structures is still unknown. It has been suggested that these ridges may play a role in tactile sensing or in hydrodynamic characteristics (Shoemaker & Ridgway, 1991). Dolphin epidermis is rather thin, however it is 10-20 times thicker than in terrestrial animals. This characteristic makes them susceptible to scarring. Scarring helps scientists differentiate and identify individuals within a dolphin population.

Fig. 6. Dolphin skin, illustrating the three layers of that comprise epidermis, the dermis and hypodermis.
The dermis is composed of dense connective tissue. It is vascularized and contains fat cells proximal to the hypodermis layer. The most notable feature of dolphin dermis is the absence of hair follicles, sebaceous glands and sweat glands. One can speculate that as dolphins evolved from terrestrial mammals to marine mammals, presence of fur and ability to sweat became unnecessary traits, therefore eventual loss of follicles and respective glands logically followed.
The hypodermis, or blubber, is loose connective tissue composed of fat cells interlayered with collagen bundles; it is loosely connected to the underlying muscles. In dolphins, this blubber layer evolved in lieu of hair as means of insulation. Thickness of the blubber varies among species, and is also dependent on age, sex and seasonal variations (Berta & Sumich, 1999).
The outer epithelial layer of the skin may be pigmented. Coloration patterns are determined by regional differences in the concentration of melanocytes in the epidermis. There are three basic color patterns in dolphins: uniform pattern, spotted or striped pattern with sharp colored areas on the head, side, belly, and flukes, and counter shading pattern, in which the dolphin is dark above and light below. Counter shading and striped patterns function mainly in concealment. Other functions of coloration patterns relate to protection from enemies, search for food and intraspecific communication.
Fig. 7 illustrates the three coloration patterns found in cetaceans: (a) uniform patter, (b) striped or spotted pattern, (c) countershading.

It is estimated from the limited data available on dolphins of known age, that dolphins reach full brain development at approximately nine to ten years of age. This is about half the growth period required for human brains. Dolphins, similar to humans, have large cerebral hemispheres; the surface of the brain is convoluted, and in some species is even more convoluted than the human brain. Dolphins are also competent task performers. The length of the dolphin spinal cord varies amongst taxa. It is cylindrical throughout the length of the animal with a prominent cervical enlargement associated with development of the flippers. There is a less prominent lumbar, due to the recession of hind limbs.
Sight: From behavioral observation, it is evident that dolphins utilize their sight, however the importance of this sense is not completely known; in animals that are primarily hunters it would be expected that the vision is stereoscopic: able it perceive two separate images from each eye. Their eyes are located on the sides of the head but are directed forward. Rather than being spherical as in most mammals, dolphin eyes are anteriorly flattened; they possess a curtain-like extension of the upper iris that comes down over the pupil when exposed to bright light, leaving only a tiny openings in the lower portions of the eye. These smaller formed pupils may help to avoid problems focusing in intense light and allows dolphins to have sharp vision both above and below the water during strong daylight. Although dolphins do not have tear glands, the harderian glands as well as glands within the conjunctiva of the eyelids bathe the cornea in a viscous solution, acting in protecting the eye from the effects of seawater (Dawson et al., 1987).
Hearing: There is little doubt that this is the most important sense of dolphins. It is used in communication and echolocation (see Sounds production, communication, echolocation, and prey capture).
Taste: Research on captive dolphins have shown there to be definite food preference. Even if the fish had been disguised, dolphins showed a clear preference. This suggests that their preference was based on taste and/or texture (Coffey, 1977). Anatomical studies have showed that dolphins possess taste buds on the base of the tongue. Additionally, research on captive bottlenose dolphins revealed they are able to distinguish different chemicals.
Touch: As discussed above, dolphin skin is tough and seemingly quite ill-suited for sensitive reception, however, behavioral patterns suggest otherwise. Many dolphins spend considerable time in physical contact with other conspecifics; this is especially evident in mating behavior where biting, mouthing and rubbing are exhibited. These sensory behaviors would suggest touch to be important
Dolphins possess a reniculate kidney; each kidney is made up of small lobes, known as reniculi. Each reniculus is comprised, similarly to the organ itself, it’s own cortex, medulla, and calyx. The ducts from reniculi join to form the ureter. Specialized adaptations in the kidney such as large reservoirs of glycogen and unique medullary blood vessels may facilitate diving (Berta &Sumich, 1999). Although dolphins are hypoosmotic (their body fluids have a lower salt content then the surrounding environment), water conservation is not a characteristic of dolphin physiology.
Analyzing isotopic concentrations in cetacean tooth phosphates have shed light onto when cetaceans adapted to excess salt load associated with inhabiting a marine environment. The transition from terrestrial mammal to marine mammal reflects adaptations in kidney function. By the middle of the Eocene period, osmoregulatory organs of cetaceans had adapted to the increased salt loads.
Muscoloskeletal system and Locomotion
In comparison to a typical mammalian skull, the dolphin skull size and shape has altered; for example the external narial openings have migrated to a more dorsal position on the head. This allows dolphins to take breathes from the waters surface with much more ease. The vertebral column does not contain a sacral region, typical of terrestrial mammals, because as dolphins lost their hind limbs they also lost a fully formed pelvic girdle. The posterior vertebrae extend as far as the notch of the tail flukes. The thoracic and lumbar vertebrae are restrained by extremely strong collagenous subdermal connective tissue sheath. This sheath gives rigidity to the thorax (Berta & Sumich, 1999).
Dolphin flippers vary in size however all dolphin flippers have a cross-sectional design, typical of airplane wings; this suggests flipper functions in ascending and descending in dolphin locomotion. Modern cetaceans are propelled by dorso-ventral movements of the tail fluke; fluke-base locomotion was present in dorudontids and basilosaurids which suggest that this type of locomotion dates back to the late Eocene period (Thewissen & Williams, 2002).
Fig.8 illustrates the caudal tail movement in propulsion and locomotion.

The flukes of dolphins have three basic components: a cutaneous layer, a blubber layer, a ligamentous layer that extends from the caudal keels and sides of the tail, and lastly a core of dense fibrous tissue within the ligamentous envelope which forms the bulk of the fluke. There is a prominent dorsal fin located along the spines of dolphins. It is supported by tough fibrous tissue, also found in the fluke. It has been suggested that the dorsal fin assists in maintenance of balance.
Respiration, diving, and breath-hold physiology
The respiratory tract of dolphins begins at the blowhole and ends in the lungs. Opening and closing of the blowhole is controlled by the contraction of musculature surrounding the blowhole. The lungs of dolphins are distinct from other marine mammals in that they lack lobes. There is some difference in size between the left and right lung; the right usually longer and heavier, the left smaller to accommodate the asymmetric position of the heart in the thoracic cavity. Dolphins exhale and inhale singly but very rapidly upon surfacing, preceding a dive. During inhalation elastic tissue in the lungs and diaphragm is stretched by muscular activity in the diaphragm. The elastic tissue recoils during exhalation, rapidly and almost completely emptying oxygen content of the lungs. In some species, alveoli are highly vascularized to enable rapid reuptake of oxygen (Berta & Sumich, 1999). A prominent feature of marine mammals is their frequent prolonged breath-hold diving. The details of diving behavior are difficult to interpret however possible motives for this adaptation include foraging for food, increase swim efficiency or to avoid predation. Over twenty centuries ago, Aristotle recognized that dolphins were air-breathing mammals, however it was not until the twentieth century that exploration on the physiology of this trait was conducted. Marine mammals must cease breathing during dives. This leads to several conflicting physiological conditions; firstly, the moment inhalation occurs without exhalation, oxygen stores begin to deplete. This is especially true during increased intensity of activity. Secondly, without ventilation, carbon monoxide content increases in blood and muscle tissue, thus making cellular fluid and blood serum more acidic. A prolonged period of increase in CO2 is called hypoxia. Another factor to take into account is water pressure. When a dolphin dives below the sea surface, water pressure increases one atm (atmosphere: 760mmHg) for each ten meters descended. Increased water pressure can pose serious threat to dolphins; it can compress air filled spaces throughout their anatomy, causing distortion and even collapse of those spaces. Processing of gases at increased pressure can also cause serious problems in diving dolphins; oxygen at high concentrations can be toxic and nitrogen can have a sleep-inducing effect. Dolphins have adapted various oxygen stores to supplement depletion when diving. Large oxygen stores are maintained with hemoglobin or with myoglobin. Red blood cells of diving and non-diving mammals are relatively the same size; however diving mammals have more red bloods cells, and higher overall blood volume. The total blood volume relative to mass is three times greater in diving mammals than in that of non-diving mammals. Packed red blood cell volume, known as the hematocrit, ranges from 40-50% (Berta & Sumich, 1999).
Sounds production, communication, and echolocation
Unlike sharks and rays that have a well-developed sense of smell, dolphins generally do not use olfaction in gaining information about their environment. Their sight is moderately developed and their sense of touch is restricted to close physical contact, thus their sense of sound has become their primary sense. Dolphins use a series of clicks and whistles of varying frequencies to communicate to one another. These sound waves are also used in echolocation. Essentially, echolocation is a specialized type of acoustic communication in which an animal sends information to itself. Much like sonar, dolphins emit three basic groups of sounds, low frequency clicks, high frequency whistles and high frequency clicks. The high frequency clicks are used to locate food and to gain information about the general topography of an area. As each click strikes a target, a portion of the sound is reflected back to the source of the click, the dolphin. The time required to reflect these clicks determine the distance of the target.
Fig.9. The pattern of click production and returning echo. This illustrates a dolphin utilizing echolocation to find a specified target, generally prey.
In addition to clicks used in echolocation, dolphins also produce whistles. It is hypothesized that these whistles are used in social settings to communicate with conspecifics; because each dolphin has a unique whistle, this suggests that whistles are also used to broadcast an individuals’ identity among social groups. The origins of these sounds remains debatable; one theory suggests that all sounds associated with echolocation arise in the nasal diverticula surrounding the blowhole; the air stream is thought to pass over the nasal passage and the vibrations in these result in clicks and whistles.

Diet and foraging behaviors
The taxonomic division of Cetacea into Mysticeti and Odontoceti (of which dolphins belong to) reflect differences in diet and feeding methods. Species of Mysticeti (whales) are known as ‘strainers’ whom utilize baleen to strain plankton and small marine invertebrates from seawater. Species of Odontoceti are known as ‘graspers’; as the term suggests, members of this division utilize their teeth to grasp onto prey. Teeth of Odontocetes are simple, peg-shaped, with single, open roots. Odontoceti can be further divided into three groups pertaining to food choice: fish-eaters called ichthyophagi, squid-eaters called teuthophagi, and flesh-eaters called sarcophagi. Dolphins are generally strictly ichthyophagous, however if the opportunity arises, some members may eat suitable sized squid; dolphins do not chew their food, instead their slender rostrum, containing many sharply pointed teeth, have been adapted solely to grasp small fish.

Fig. 10 Size range among food preferences of cetaceans: 1. Calanoid copepods; 2: Thysanoessa; 3. Euphasia and Meganyctiphanes; 4. Myctophid lantern fish; 5. Capelin; 6. Mackerel; 7. Onycoteuthid squid; 8. Bathypelagic angler fish; 9. Architeuthid squid. Dolphins prefer species 4, 5 and 6.
Most dolphins are opportunistic feeders and their food preferences are dictated by environmental circumstances. Such environmental circumstances include sounds emitted by prey. With this, dolphins utilize echolocation in locating schools of fish to feed upon. Species such as the bottlenose dolphins Tursiops truncates, have been shown to carry out complicated tasks in complete darkness, indicating emission of broad-band ‘clicks’ to scan their environment acoustically. They have been appeared to be able to control the frequency at which they emit these sounds. Frequencies ranging from 2.0-220 kHZ have been recognized as discrimination clicks; whereas the lower frequency clicks, ranging from 0.25-1.0 kHZ are called orientation clicks (Gaskin, 1985). As the name suggests, the lower frequencies emitted indicates that information regarding the general environment gathered.
All cetaceans are characterized by a complex stomach with multiple divisions. There are four compartments: a forestomach, a fundic chamber with folded mucosa and gastric glands, a connecting stomach between the fundic chamber and pyloric stomach, and lastly a pyloric stomach. The forestomach of dolphins is reported to hold several liters of water and is separated from the esophagus by a muscular sphincter. A possible function of the forestomach may be to hold swallowed water while dolphins secure their prey (Berta & Sumich, 1999).

Fig. 11. The structural division of the stomach in a fin whale, bottlenose dolphin and beaked whale.
Mating, breeding, and social organization behaviors
Males can mate with many females during a breeding season, however females, no matter how many times they mate, can only achieve one pregnancy. Sexual selection for males is usually fundamentally different from those for females because reproductive success is defined differently between the two genders. Male mating strategy of polygyny may pay off well with numerous offspring sired. In females, mating strategies result in low risk of reproductive exclusion, but the payoffs are also low, generally one offspring per breeding season. Dolphins have promiscuous mating systems, in which several adult males are associated with a group of females.
The term breeding is inclusive of two components: mating (copulation) and parturition (calving). Mature males are less dependent on social contact and usually spend their time swimming alone. However, during the breeding season, males spend more time in close proximity to females. The females in turn become submissive to the males, only leaving him to feed for short periods of time (Coffey, 1977). Dolphins, like humans, have been observed to have sex not just to reproduce, sex also is performed as a means to form bonds within social groups.
All dolphins mate and give birth in water. Only the general outlines of reproduction in dolphins are understood from research done in captivity, where underwater courting, mating, and calving behaviors can be observed. Courtship displays vary from species to species. These displays can include swimming skills, leaps and fluke/flipper signals. Touch is important in pre-copulation behaviors; tactile behaviors exhibited are nibbling, brushing against one another and biting.
Mothers care and nurture their young. During the months proceeding birth, the mother –young relationship is intense; she is very attentive to her young and keeps her young close. As with most maternal care, intensity of the relationship declines as time progresses; the young spends increasingly more time away from its mother and, within a year, becomes independent.
Dolphins exhibit all types of behavior in which one member gives care to a conspecific. These behaviors can be split up into behaviors between adults and their young, and behaviors between two adults. With regards to care-giving behaviors between two adults, Dolphins have been observed supporting wounded members of their group. They have also been observed attacking hunting boats.
Dolphin species tend to group together in structured social groups, characterized by long-term association among individuals. The sizes of these groups vary among species and location. Members generally maintain close and synchronize their movements. Most commonly, they exhibit a staggered arrangement called echelon formation. This type of formation is also commonly found in birds.
Reproductive structures, patterns and strategies
Dolphins exhibit internal fertilization, characteristic of all mammals. Both genders have genital slits on the underside of their body. The male is able to retract his penis into the genital slit, whereas the female is able to close off her vaginal opening by the contraction of the genital slit. Both adaptations evolved to decrease drag by external appendages, simultaneously increasing hydrodynamics. In males, testes enlarge during the mating season; more sperm increases the success of spawning. They are located in the abdominal cavity, slightly to the side of the kidneys.

Fig. 11. The reproductive organ of the male dolphin. The penis is able to retract and expand out of the genital slit opening.
In females, ovaries lie in the abdominal cavity, within deep ovarian pouches. Ovaries of dolphins are generally spherical and smooth.
Fig. 12. The reproductive organ of the female dolphin.
Based on anatomical constraints, they appear to copulate belly to belly, generally lasting no longer than a few seconds. In this time, the genital slits are opened and sperm is passed from the penis into the vagina. Gestation periods vary from species to species however it typically lasts 12 months.
The mammae of cetaceans consist of long, flat glands located along the belly. The mammae ducts open to a single nipple on each side, which are retained within mammary slits.
Part III: Exploitation and Conservation
In this final section I will survey both the commercial exploitation and conservation of dolphins; the overexploitation of dolphins has resulted in significant decreases in population size, the extinction and endangered status of some species.
Commercial Exploitation
Dolphins have few natural enemies; humans pose the biggest threat to the dolphin population. We pollute their habitat, dumping trash, pesticides and other harmful chemicals. Collision with boats or its’ propellers cause serious injuries and death in some instances. The most notable cause of unintentional death of dolphins is that caused by the fishing industry, here dolphins are unintentionally caught in tuna fishing lines and dragged onto fishing boats. Loud noises produced by naval sonar, offshore drilling and construction can also be harmful to dolphins. These noises can induce stress and damage their hearing.
The behavioral associations between dolphins and yellowfin tuna is not well understood, however tuna purse seine fisheries use this association to locate schools of tuna. When fisherman sight schools of dolphins and tuna, they set nets surrounding both animals. The nets are then tightened and closed, trapping both tuna and dolphins.
Progress
The United States has passed numerous agreements regarding conservation and/or protection of marine mammals. The Marine Mammal Protection Act (MMPA) was established in 1972. MMPA regulates the taking of marine mammals in U.S. waters and on importing marine mammal products into the U.S. The MMPA along with the U.S. Endangered Species Act of 1973 were the first legislative acts to recognize the values of non-consumptive uses of protected species. The protected species were listed as either endangered: in danger of becoming extinct, or threatened: likely to become endangered (Berta & Sumich, 1999).

Fig.13. A partial list of agreements and treaties passed for the conservation of marine mammals.
In 1973, the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) created a mechanism to regulate the international trade of endangered plant and animal species. The mechanism is divided into two appedices: Appendix I: include all species deemed to be threatened with extinction which are or may be affected by international trade. Appendix II: includes species that are not currently threatened with extinction, but are at risk unless international trade is restricted or regulated (Berta & Sumich, 1999).

Conclusion
Dolphins have evolved from terrestrial mammal, to a skilled fully aquatic mammal. Their body form has adapted to enable them to not only be fully aquatic, but enables them to be one of the most efficient swimmers in the ocean. There has been speculation on the intelligence of dolphins in comparison to humans; if they had hands could they replace us as the beings of highest intelligence? What the evolutionary future holds is questionable, however, it is indisputable the great adaptations evolved in dolphins. Venturing from land back to the sea, dolphins remain one of the key proofs of evolution.

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Figures
Figure 1: Coffey, D. J. 1977. Dolphins, whales, and porpoises: an encyclopedia of sea mammals. New York: Macmillan.
Figure 2: Berta, A., and J. L. Sumich. 1999. Marine mammals: evolutionary biology. San Diego, CA: Academic Press.
Figure 3: Berta, A., and J. L. Sumich. 1999. Marine mammals: evolutionary biology. San Diego, CA: Academic Press.
Figure 4: Berta, A., and J. L. Sumich. 1999. Marine mammals: evolutionary biology. San Diego, CA: Academic Press.
Figure 5: Berta, A., and J. L. Sumich. 1999. Marine mammals: evolutionary biology. San Diego, CA: Academic Press.
Figure 6: Berta, A., and J. L. Sumich. 1999. Marine mammals: evolutionary biology. San Diego, CA: Academic Press.
Figure 7: Berta, A., and J. L. Sumich. 1999. Marine mammals: evolutionary biology. San Diego, CA: Academic Press.
Figure 8: Coffey, D. J. 1977. Dolphins, whales, and porpoises: an encyclopedia of sea mammals. New York: Macmillan.
Figure 9: Berta, A., and J. L. Sumich. 1999. Marine mammals: evolutionary biology. San Diego, CA: Academic Press.
Figure 10: Gaskin, D.E. 1985. The Ecology of whales and dolphins. London: Heinemann Educational Books, Inc.
Figure 11: Coffey, D. J. 1977. Dolphins, whales, and porpoises: an encyclopedia of sea mammals. New York: Macmillan.
Figure 12: Coffey, D. J. 1977. Dolphins, whales, and porpoises: an encyclopedia of sea mammals. New York: Macmillan.
Figure 13: Berta, A., and J. L. Sumich. 1999. Marine mammals: evolutionary biology. San Diego, CA: Academic Press.