Delphinids, Overview (marine mammals)

The family Delphinidae is one of three extant (widi Phocoenidae and Monodontidae) families in the cetacean superfamily Delphinoidea (which also includes three extinct families, Kentriodontidae, Odobenocetopsidae, and Albireonidae).

The popularity of oceanic dolphins (family Delphinidae) is arguably among the highest for wild animals, bodi for die general public and for scientists conducting research. However, despite this, they are very poorly understood compared to most terrestrial mammals of similar size. For most species of delphinids, basic aspects of their evolution, physiology, ecology, behavior, and population structure are virtually unknown. For die biologist, this presents real challenges and opportunities for dolphin research, not just to understand individual species, but also toward a better understanding of how they fit into marine ecosystems.

I. Taxonomic Overview

Delphinids likely arose in the mid- to late Miocene (11-12 mya) from kentriodontid-like ancestors and quickly radiated into many different morphological and ecological types. This early radiation produced precursors of many modern forms; many of the early delphinid fossils can be assigned to extant genera, particularly Tursiops. Today the Delphinidae is the most speciose family of marine mammals, with 34-36 recognized extant species arranged into 17-19 genera. At present, there is much uncertainty about the evolutionary relationships among the species of delphinids. Of the many recent classifications that have been proposed, two are depicted here. One represents a more traditional view of dolphin taxonomy (Table la) and the other a recent provisional classification based on molecular phylogenetic analyses (Table lb). It should be mentioned diat the latter classification is considered tentative and is based on analysis of a single gene. A more highly resolved representation of systematic relationships among delphinid species is not currently available. Also, there will no doubt be additional revisions proposed in the future, especially involving the apparently paraphyletic genera Stenella and Tursiops. In part, this changing nature of delphinid taxonomy is due to the new molecular and analytical tools currently available to researchers, but it also reflects the uncertainties about evolutionary relationships that have long been recognized by morphological systematists but have yet to be addressed.

II. Morphology

Dolphins have the typical morphological characteristics of toothed whales, such as spindle-shaped bodies, single external blowholes, telescoping of the skull such that the maxillary bones overlap the frontals in the supraorbital region, left-skewed eranial asymmetry, polydonty, and homodonty (in most). In some species, there has evolved a secondary reduction in the number of teeth, often seen as an adaptation for feeding on squid. The evolution of the delphinoid lineage saw the development of elaborate systems of pterygoid sinuses and better-isolated earbones, probably increasing their ability to echolocate and perhaps giving them an advantage over some of the other odontocete groups of the Miocene. The ancestors of the delphinids, the kentri-odontids, were small dolphins with short to medium length rostra and, unlike most modern delphinoids, had symmetrical cranial vertices. The development in the Delphinidae of asymmetry in the cranial vertex and in the premaxillary bones suggests a further refinement of their echolocation capabilities and may partly explain their evolutionary success. The most noticeable difference between delphinids and their closest relatives, the pho-coenids (true porpoises), is that the latter have spade-shaped teeth, whereas delphinids have conical or peg-like teeth, as do most other odontocetes (toothed whales). They also differ from phocoenids in the shape of the facial region of the skull, including having a more distinct vertex. Within the Delphinidae, the most obvious variation among species relates to the feeding apparatus: the development of the rostrum, jaws, and teeth. There is a broad spectrum of rostrum lengths and widths, tooth counts, and toodi sizes, reflecting the range of ecological niches occupied by the different species. For example, total tooth counts range from less than 10 in Risso’s dolphin (Grampus griseus) (which has no teeth in die upper jaw) to 250 in the spinner dolphin (S. longirostris).

TABLE 1

Two Recent Classifications of the Family Delphinidae

a. A classification of the family Delphinidae  reflecting a traditional view of species interrelationships

b. Revised classification of the family Delphinidae based on molecular systematic analysis.

Delphinids also show wide variation in their external morphology. Only a few species (e.g., killer whale, Orcinus orca; pilot whales, Globicephala spp.) are dramatically sexually dimorphic, although many others may have more subtle dimorphism in body size and shape, coloration, and dorsal fin shape. In;size they are small to medium cetaceans, with adults ranging’from less than 1.5 m (S. longirostris roseiventris from the Gulf of Thailand, some species of Cephalorhynchus) to over 9 m (killer whale). Beak length varies widely, from very long on some (e.g., Indian Ocean common dolphin, Delphinus tropicalis) to very short on others (e.g., white-beaked dolphin, Lagenorhynchns albirostris). The external beak is completely absent in a number of delphinid genera, particularly in Orcinus. Globicephala, Feresa, Pseudorca, and Peponocephala. Those species that lack a beak often have heads that are rounded or even bulbous in profile. In most delphinid species, the dorsal fin is pointed and falcate, although it is triangular in some subspecies of S. longirostris and in male O. orca, round in Hector’s dolphin (C. hectori), and even forward canted in males of the eastern spinner dolphin (S. longirostris orientalis). The dorsal fin is completely missing in the right whale dolphins (Lissodelphis spp.), and some hump-backed dolphins (Sousa spp.) have a pronounced hump at the base of the dorsal fin. The color patterns of delphinids are similarly varied, from bold black and white patterns (e.g., O. orca and some of the Cephalorhijnchus spp.) to complex patterns of black, white, and gray (e.g., Delphinus spp. and Stenella spp.) to rather simple patterns of black (e.g., Globicephala spp.) or gray (e.g., Sousa spp.). The complex color patterns exhibited by some delphinids are composites of various elements, including stripes, capes, overlays, spots, and blazes. Regardless of their overall color and pattern, all dolphins tend to have a countershaded aspect, where the ventral surface is lighter than the dorsum. This countershading reduces an animal’s visibility in the marine environment, where the ambient light comes from above. Besides being a mechanism for species recognition, the more complex patterns may also play a role in camouflage against waves at the surface or against the dappling of light penetrating the water.

III. Distribution and Habitat

As a group, the family Delphinidae reaches its highest diversity in tropical and warm temperate latitudes. There are numerous species with pantropical distributions and others that occur in tropical waters but are limited to one or two ocean basins. For example, the Atlantic spotted dolphin (Stenella frontalis) and the Clymene dolphin (S. clymene) are limited to the lower latitudes of the Atlantic Ocean, whereas the Indo-Pacific bottlenose dolphin (Tursiops aduncus) and the Irrawaddy dolphin (Orcaella brevirostris) only occur in the Indian and west Pacific Oceans. In colder areas one can find species in the genera Globicephala, Cephalorhynchus, Lissodelphis, and Lagenorhynchns (including Sagmatias and Leucopleurus of Table lb). Interestingly, new genetic evidence suggests that the majority of these cold-temperate species appear to be closely related (subfamily Lissodelphininae in the classification of Table lb). Only one recognized species, the long-finned pilot whale (G. melas), has an antitropical distribution, although some an-titropical species pairs [e.g., Lissodelphis spp., and the dusky and Pacific white-sided dolphins Lagenorhynchus (or Sagmatias) obscurus/obliquidens] have been hypothesized as being single species with antitropical populations. Only one species, O. orca, ranges into high latitudes near the polar ice. Indeed, occurring also into mid- and low latitudes, this species is probably the most cosmopolitan of all the cetaceans.

Within this broad range of geographic distributions, delphinids occupy an equally diverse array of habitats. Many species occur far offshore in deep water, where the specifics of their ecological requirements are poorly known. In fact, in tropical seas, only the cetacean fauna of the eastern tropical Pacific has been studied extensively and systematically, and some differences in the species composition have been observed in different water masses. Here, areas with a stable mixed layer and a shallow ther-mocline are frequented by Stenella attenuata, S. longirostris, and the rough-toothed dolphin (Steno bredanensis), whereas areas with more variable conditions and some amount of upwelling contain species such as the short-finned pilot whale (G. macrorhynchus), short-beaked common dolpliin (Delphinus delphis), striped dolphin (Stenella coendeoalba), and melon-headed whale (Peponocephala electra). In any ocean, some of the offshore species may also range closer to the coast (e.g., D. delphis) or even have populations or sister species that are restricted to the coastal waters or the nearshore habitat [e.g., the long-beaked common dolphin (D. capensis), coastal populations/species of Tursiops spp]. In a few cases, coastal populations may ascend a short distance up rivers, but only two species [the tucuxi (Sotalia fluviatilis) of eastern Soudi America and Orcaella brevirostris of the Indo-West Pacific] regularly occur far upstream. Both of these riverine species also have marine coastal populations.

IV. Social Organization and Behavior

All dolphin species are social to some degree. However, characteristic group sizes for the different species range from small pods of just a few individuals to large schools numbering in the thousands. Due to the difficulty of observing dolphins in the wild, especially those occupying offshore habitats, very little is known about the behavior and social organization of most species. The populations that have been studied over longer time scales (e.g., Orcinus orca in the northeast Pacific and T. truncatus in the western Adantic) are those that form relatively stable and small social groups within a short distance of the coast and whose movements do not regularly take them out of their study areas. Using photos to identify individuals via fin markings and color patterns, and supplementing observations with genetic data, associations of individuals and their genealogical relatedness have been recorded and monitored over generations. In these populations, some long-term associations and patterns have been noted. For example, it appears that bonds between individuals in a pod and/or between individuals and a particular area (philopatry) tend to be stronger for females than males; the society can even be considered matrilineal for O. orca. A similar pattern was inferred from molecular data on G. rnelas in the North Atlantic. However, one must be cautious in extrapolating these social patterns to other delphinid species. For example, some species that occur on the high seas are found in schools that number in the thousands, and these associations appear much more fluid in their composition. In fact, social patterns observed for inshore groups of species like T. truncatus and O. orca may not even reflect the organization of offshore populations of the same species. What little is known about the social organization of offshore dolphins comes from direct observations of school sizes and life histoiy data collected from mass strandings and fishery kills. In the few pelagic species studied (primarily Stenella attenuata and S. longirostris), there is evidence for promiscuity, strong mother-calf bonds, and some segregation by age and sex both within and between schools.

In addition to their intraspecific social organization, most dolphin species are seen at least occasionally in the company of other species. One famous association is between S. attenuata and S. longirostris in the eastern tropical Pacific, an aggregation that also includes yellowfin tuna (Thunnus albacares) and numerous species of seabirds. Other associations are frequently observed, such as P. electra associated with Fraser’s dolphin (Lagenodelphis hosei) or Tursiops truncatus with G. macro-rhtjnchus. A few species, such as the northern right whale dolphin (Lissodelphis borealis), have been obsen’ed with a wide variety of marine mammals, including mysticetes and pinnipeds.

Most of the information on the behavior and cognition of individual dolphins comes from studies of captive animals, primarily of T. truncatus. Apart from humans, dolphins have the highest ratio of brain size to body mass of any animal. Their intelligence and behavioral versatility are legendary and are still being explored. Using controlled experiments, dolphins have been shown to be capable of understanding complex commands, including the incorporation of abstract concepts and variations of syntax, and of devising novel behaviors of their own volition. Although behavioral activities in the wild are more difficult to interpret, there are well-documented observations of cooperative hunting (e.g., O. orca) and play behavior (e.g., T. truncatus). In fact, T. truncatus has been observed surfing in many coastal areas, and many dolphin species are avid riders of the bow wakes of ships and large whales. Some species regularly perform aerial maneuvers such as high leaps and flips and, in the case of S. longirostris, spins. At the present time, the function of many of these behaviors can only be guessed at. Dolphins also have an array of vocalizations, such as clicks, whistles, and squeals, that are used in part for their well-developed echolocation and in part for communication. However, only those species kept in captivity have been studied extensively. For the rest, due to the logistic difficulties of collecting data on fast-swimming dolphins in the pelagic realm, the vocalizations of only a few species have even been recorded, and those vocalizations are difficult to understand in terms of functionality.

V. Feeding

Ecologically, dolphins have also radiated dramatically. There are species that forage on fish, squids, and/or other invertebrates. A few species (e.g. O. orca, false killer whale, Pseudorca crassidens) even take mammalian prey, including large whales and pinnipeds. While some have fairly specific diets, a few species have rather broad tastes. In those species that eat a wide variety of prey items, one type of food (e.g., fish vs squid) may predominate. For some species (e.g., S. coendeoalba), the preferred food type varies among populations, whereas in others (e.g., S. attenuata), it may even vary among individuals within a population, depending on their sex and life history stage. Perhaps the most dramatic segregation of foraging strategies is seen in the populations of O. orca in the eastern North Pacific, where two distinct groups exist sympatrically: one specializing on mammals and the other on fish. Individual species of delphinids usually forage in a particular part of the water column, specializing in epipelagic prey (e.g., S. attenuata), mesopelagic prey (e.g., Lissodelphis spp.), or even benthic prey (e.g., some species of Cephalorhynchus). The few measurements of swimming speeds (often anecdotal) suggest that some may exceed 20 knots in short bursts, although prolonged cruising speeds are generally on the order of 5-9 knots (9-17 km/hr). Direct data on diving depths for wild delphinids are practically nonexistent. However, S. longirostris, a relatively small species, is thought to dive to at least 200-300 m, based on an analysis of prey items. Greater depths are no doubt possible for larger species (e.g., Globicephala spp.) that feed mainly on larger squid. In fact, a trained pilot whale (Globicephala sp.) is thought to have reached 610 m in an experimental situation. At the other extreme, certain populations of two dolphin species have been known to intentionally beach themselves in pursuit of prey. In one case, groups of T. truncatus drive and pursue schooling fish onto the beach, and in the other, O. orca beach themselves to take pinnipeds hauled out near the waters edge. The behavioral adaptability of dolphins may be best illustrated by those species who incorporate human activities into their foraging, such as the Sousa chinensis that feed on trawl discards or the O. orca that raid long lines for their catch. Along the Gulf of Mexico coast of the United States, there are even some T. tnincatus that feed within shrimp trawl nets, many of whom are “caught,” released, and netted again.

VI. Life History

As with many aspects of dolphin biology, there is much that is unknown about the reproductive biology of most species, although many of the parameters seem to be correlated with body size. Estimated ages at sexual maturity range from about 6 (D. delphis) to 16 (O. orca) years. Like most mammals, when the age at sexual maturity differs between the sexes, it is usually the females that reach maturity at a younger age. Like other cetaceans, dolphins bear single young. Gestation periods are rarely well documented and are thought to range from about 9 to 16 months, although most have a period of less than a year (16 months was an estimate for G. melas). Most species appear to show at least some seasonality to their breeding, although this varies in degree. Estimates of calving intervals similarly vary, even among populations within species or among studies, ranging from just over a year for many species to approximately 8 years in one study of O. orca from the eastern North Atlantic. Reliable estimates of longevity are quite rare, but range from around 20 years for smaller species up to about 60 years for females of the larger species. In the large species, which tend to be more sexually dimorphic, females may live 15-20 years longer than the males. The causes of natural mortality, when they can be ascertained, are usually attributed to parasites and pathogens or to predation by killer whales or sharks.

VII. Abundance and Conservation

Some delphinid species are no doubt the most numerous of cetaceans, occurring in schools of thousands in large portions of the world ocean. Reliable global estimates for most species do not exist, but in the eastern tropical Pacific alone, some species (Stenella attenuata, D. delphis) number in the low millions. At the other end of the spectrum, species with restricted ranges (e.g., C. hectori) may have total population numbers of only a few thousand. Despite (and in some cases because of) their general abundance, dolphins face numerous anthropogenic threats. They are still hunted in some parts of the world by harpoon, drive fisheries, or nets. Two well-known examples are the drive fisheries for S. coendeoalba in Japan and for G. melas in the Faroe Islands. There are many other dolphin fisheries still in operation, mostly in developing countries, often despite the protection of assorted laws and treaties. Usually the meat from intentional takes is for human consumption, although in some areas it is used as bait in crab pots or shark long lines. In addition to the mortalities from directed fisheries, many dolphins are also taken incidentally in the course of other fishing operations. The dolphin mortality from the eastern tropical Pacific tuna purse seine fishery has been well studied and gready reduced in recent years. However, many dolphins of a variety of species are still caught in coastal and offshore gill nets all over the world. It is difficult to accurately assess the severity of these threats to dolphin populations given that the extent of the mortality can only be roughly guessed at in most cases and the sizes of the affected populations are largely unknown. Nevertheless, mortality from fisheries bycatch may present major threats to some dolphin populations.

Compounding these direct kills are indirect effects from human activities. In some areas, large-scale fishing operations may adversely affect dolphin populations, either by direct competition for prey or by alteration of a region s ecology. Pollution also undoubtedly takes its toll on dolphin health, particularly in coastal areas, either by direct poisoning effects or by making the animals more susceptible to pathogens and parasites. Subtle effects on fitness such as decreases in reproductive capacity or shortened life spans are almost impossible to detect, but large-scale mortalities are difficult to ignore and are not at all unusual. There have been mass strandings in recent years along the Mediterranean coast, in the southeastern United States, and in the Gulf of California in Mexico. These large-scale die-offs often involve multiple species (although one species usually predominates) and may occur over periods of several months. It is difficult to determine if such mass mortalities are increasing in frequency in recent years because historical events may not have been documented adequately. It is also difficult to assess their impact on dolphin populations. Nevertheless, they are a cause for concern if for no other reason than as indicators of the declining health of marine ecosystems.