Cetacea (whales, dolphins and porpoises)

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Introduction

The cetaceans (whales and dolphins), are the largest and most diverse order of marine mammals and previously were divided into two suborders, the Odontoceti (toothed whales) and the Mysteceti (baleen or rorqual whales), which are separated primarily on the basis of feeding strategies and accompanying morphological transformations. As the Mysteceti are believed to have evolved from an Odontoceti ancestor, the Odontoceti is a paraphyletic group. Hence, in the classification presented here, we have dispensed with this division of the Cetacea. Nevertheless the two groups are discussed below in the context of their differing morphology and biology.

A characteristic of most modern toothed whales is the development of a high frequency echolocating system which primitive toothed whales (such as the archeocete or ‘archaic whale’), probably did not possess. Baleen whales do emit sounds, but they have a cochlea (ear bone), developed for the perception of low frequency sounds, making it appear doubtful that echolocation, which typically uses high frequency sounds, is utilized in the feeding strategy of filter feeding whales.

Baleen whales have evolved a filter-feeding system consisting of fibrous baleen plates in the place of teeth. They are surface feeders, feeding mainly on zoo-plankton, which are harvested mostly in the upper 100metres. The toothed whale strategy is to focus on more discrete forms of prey, such as squid and fish, hence their development of an echolocating system that assists them to locate and hunt such prey in poor visibility or at great depths. A good example of this feeding strategy is the sperm whale which has been know to sound (dive), to over 1000 and possibly 2250 meters in it’s hunt for squid. To achieve this, they have also needed to develop morphological characteristics quite different from the surface feeding baleen whales, such as relative buoyancy control, that facilitate energy conservation at great depths.

Palaeontological evidence suggests that both the echolocating toothed whales and the filter feeding baleen whales diverged in the early Oligocene (38-31.5 Mya) from a common archeocete (primitive toothed whale) ancestor, which possessed neither echolocating nor filter feeding abilities. More specifically, Fordyce feels that the three major groups of modern cetaceans which are the toothed whales, the beaked whales and the baleen whales had a monophyletic point of origin (that is, of singular ancestral origin), rooted in a group of archaic toothed whales called the Dorudontinae. As mentioned above, early speciation of intermediates from this group about 38 to 31.5 million years ago led to the evolution of modern whales as we know them today.

The presence of embryological dentition appearing before the development of baleen plates and the discovery of fossilised intermediates such (such as Aetiocetus) between the Archaeoceti and the Mysteceti, further provide evidence that the Mysteceti evolved from a primitive toothed whale.

The extant toothed whales are more diversified than the baleen whales, with the dolphins (Delphinidae) being the most speciose and, on the basis of paleontological evidence the family of toothed whales which radiated most recently.

The earliest ‘archaic whale’ yet discovered is Pakicetus, a fossil about 50 million years old from Pakistan. Biochemical and genetic studies suggest that these early archeocetes themselves evolved from ungulates (hoofed mammals). These early hoofed mammals gradually made their way back to a marine habitat from a terrestrial one. According to Fordyce the most likely ancestral group of early toothed whales is the Mesonychidae, a family of primitive terrestrial hoofed mammals that lived in North America, Europe and Asia, that adapted to fish eating in the shallows of an ancient ocean basin known as the Tethys which stretched from the present Mediterranean to beyond India. The closest living ungulate to modern cetaceans is the hippopotamus.

Molecular study

Deoxyribonucleic acid (DNA) is the molecule that houses the genetic code. Methods have evolved that enable the researcher to both sample and analyse the genetic structure, most generally by selecting and reading gene sequences. The identified genetic sequences are then converted into numerical data which is statistically analysed to infer phylogenies (that is, to reconstruct the statistically most probable evolutionary history of a selected species or group of species). Palaeontologists on the other hand, do a comparative analyses of morphological similarities and differences of fossilised skeletal material characteristics in their efforts to trace evolutionary histories. Today, modern systematists use both the morphological and the molecular approach to best infer phylogenies. An example of such a study is included below. This study also well illustrates that evolutionary histories are inferred probabilities (when considering an incomplete fossil record as well as limited sampling of the DNA), and that changes, in the light of new evidence, will be made.

Highly conserved mtDNA sequences of whales and dolphins - Phlyogenetic Implications:

Sequence divergence values, estimated from the pairwise proportion of shared restriction sites of mitochondrial DNA (mtDNA), in the cetacean taxa, are low, reflecting an unexpectantly high conservation of mtDNA sequences.
This implies that either the rate of nucleotide substitution of cetacean mtDNA is slower than that of terrestrial mammals, or that the fossil record of cetaceans needs to be re-interpreted.

For example, fossil evidence suggests that the baleen and toothed whales evolved from an archeocete common ancestor about 35 million years ago, which means that the substitution rate for cetacean mtDNA is about 0.22% per million years, as compared to the terrestrial mammalian mtDNA rate of 1% per million years – a fourfold difference in mutation rate between marine and terrestrial mammals. Conversely, if the calibration of the mammalian mtDNA molecular clock originally established by Brown et al. is valid for cetaceans, then the low sequence divergence values indicates a recent and rapid radiation of the major cetacean groups: the toothed and baleen whales would then have shared a common ancestor as recently as 7-8 million years ago (compared to the fossil based estimates of 35 million years ago). Whether the earlier forms of toothed whales and baleen whales are directly ancestral to the extant forms, or whether they are possibly an example of parallel evolution with the extinct forms having morphological features similar to those of the living suborders, remains an unresolved issue. However, when considering these results, it should be noted that it has been demonstrated (by comparison with complete genomic sequences), that the sampling method of mtDNA used in this study was skewed towards the conservative. None the less this bias alone is not nearly great enough to explain the difference in mutation rates between marine and terrestrial mammals meaning that the above argument does hold true.

It is important to note that the relative rate of evolution implied by this molecular study (Ohland et al 1994), of the groups of cetaceans sampled is concordant with that of fossil-based studies. The initial splitting of the baleen and toothed whale lineages is followed by the divergence of the lineages leading to the sperm and beaked whale families, with the dolphins being the most recent family to evolve.

Families found off the coast of southern Africa

Physeteridae (Sperm whale)

One species: Physeter cotodon (Sperm Whale).

Kogiidae

 

Ziphiidae (beaked whales)

Delphinidae (dolphins, pilot whales, killer whales and false killer whales)

Balaenidae (right whales)

One species iindigenous to southern Africa: Eubalaena australis (Southern right  whale). This large baleen whale is by far the most common of the whale species found off the Cape coast. Their social group is normally a family unit that consists of less than 6 individuals. They can be seen most prolifically between July and September along the Cape coast, particularly in False Bay, the Hermarnus area and off the coast opposite de Hoop Nature Reserve.

Neobalaenidae (pygmy right whales)

One species indigenous to southern Africa: Caperea marginata (Pygmy right whale). This is the smallest of the baleen whales, having a length of about 5 metres and a weight of around 4 500kg. It has a small triangular dorsal fin, set back towards the tail, and has throat grooves. Colouration is a deep blue grey that lightens towards towards the belly side. Little is know of these scarcely seen animals.

Balaenidopteridae (rorquals)

Other families, not encountered off southern Africa: Eschrictiidae (Grey Whale Eschrichtius robustus) - Pacific), Monodontidae (white whales - Arctic Ocean), Platanistidae (river dolphins - South America, S and E Asia).

 

 

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