Animal communication in cetaceans like Dolphins

The Cetaceans are regarded as the most distinctive, evolved and exclusive sequence of mammals. The sequence covers aquatic animals such as the dolphins and whales which are regarded as very intelligent mammals with advanced expressive abilities. Species of whales include the blue whale, tusked narwhals and singing humpback whales. The cetaceans adapts well to aquatic life as most of them are marine based and notably peculiar. Reportedly, dolphins could be found in these regions; the Amazon, Yangtze, Parana including other regions. There some notable features of the Cetaceans, mostly attributed by flattened neck vertebrae, dorsal fins, a tail, and horizontally set out flukes (Lande 1976).
The evidence of the process of evolution of cetaceans is derived from various studies in different fields including molecular biology and paleontology among others; which conclude that whales among other cetaceans evolved from terrestrial mammals. Cetaceans evolved into the two modern classifications of Odontocetes (those that have teeth) and Mysticeti (those with baleen). A significant number of Odontocetes have the capability to navigate through the use of echolocation (Thewissen et al. 2006). They produce sound waves by using an intricate system of nasal passages and sacs which use echoes for navigation.

Known Ancestry of Cetaceans

The earliest known Cetaceans were primitive creatures with teeth; however, they did not possess Mysticetes and Odontocetes cranial specializations. The Cetaceans formed the paraphyletic sub-order Archaeoceti (Lande 1976). Organisms of the archaeocete order developed a wide range of morphologies that led to their classification into various families such as the Pakicetidae, Protocetidae, and Basilosauridae (Geisler and Sanders 2003).
The first documented whales are known as the Pakicetus and were considered as classic land animals. These Pakicetus had structural characteristics that included large teeth and elongated skulls. Their exterior appearance is described as having no semblance to modern whales. However, their skulls had a strong resemblance to those of modern whales especially around the ears that are encircled by thick bones that resemble a wall. In most cases, minor features that may ordinarily overlook provide the needed evidence to establish a link between animals that are highly developed and specialized such as whales with lesser feature-laden relatives (Miller 2007).

Discussion of Sub Group Limb Usage

A significant number of whales are categorized as Odontocetes, a name derived from the fact that they have fully developed teeth in their adult stages. The presence of teeth distinguishes these cetaceans from extant Mysticetes. Odontocetes have a major distinction in the telescopic capability that is located in their maxilla area of the skull and may extend outwards (Murray and Zalmir 2001).
In Odontocetes that are alive, the supraorbital process acts as a base for the facial muscle and inserts around the nasal passages including the blowhole. Essentially, the complex facial muscles and nasal apparatus are used for the generation of high-frequency sounds that are used as echolocation mechanisms in Odontocetes (Thewissen and Williams 2002). Among the unique attributes of Odontocetes are their features that make echolocation possible. The features include the melon which is an area that comprises of adipose tissue above the skull and may have varying quantities of tissue that connects others. The second feature is the cranial/facial asymmetry which is described as a condition where bones and soft tissue on the right side of the facial structure are more developed and larger in contrast to similar structures on the left part. Facial and cranial symmetry are features that are present in all odontocetes families; however, fossil evidence suggests that there is less prevalence of cranial symmetry in the more basal members and is fully unavailable in some extinct taxa (Thewissen and Williams 2002).
Stem Odontocete: Stem Odontocetes include the Simocetus Rayi that had a downwardly arched upper jaw characterized by a toothless tip which may have been used in grubbing through the sediments. Another early stem Odontocete is the Agorophius though it is not closely related to the Simocetus. Other Odontocetes that are widely known include the Xenorophius and Archaeodelphis from the later Oligocene age. Though the two taxa have unique facial structures, the Archaeodelphis do not have well developed nasofacial muscles which make echolocation challenging (Price et al. 2005).
Crown Odontocete: There are various families of crown odontocetes and include Ziphiidae, Physeteridae, kogidae, Platanistidae, pontoporiidae, iniidae, lipotidae, delhinidae, phocoenidae, and Monodontidae. Within these families of crown Odontocetes, there are at least seventy three species (Price et al. 2005).

Mysticetes

The feeding dynamic of whales is a major consideration in their classification. The whalebone or baleen whales are named on the basis of their feeding mechanism. Evidently, plates of baleen can be found on the upper-side or roof of a whale’s mouth and functions as a mechanism for straining planktonic foods. Significant evolutionary trends among the Mysticetes include developing larger body size and gigantic heads, loss of teeth, shortening of the neck and the inter-temporal region (Lande 1976).

Stem Cetacean

The stem Cetacean included Pakicetidae, Ambulocetidae, and Remingtonocetidae that were semiaquatic, as such; they were able to move in land and water. Their different forms and number of species suggest a significant diversity especially during the transition from land to sea. The Pakicetids include Pakicetus, Itchyolestes, and Nalacetus which have demonstrated a similarity to extant whales with their wolf-life proportions, long noses, and tails. The discovery of Pakicetid skeletons suggests that they possessed well-developed forelimbs and hind limbs. Pakicetids were largely freshwater animals and the appearance of wear on their teeth suggests that they ate fish. Ambulocetids are described as having long, snouted skull that is similar to Pakicetids apart from the fact that their eyes were facing upwards instead of being on the sides of the head. They had a larger body mass than the Pakicetid (Thewissen et al. 2006).
Crown Cetaceans differ from stem whales in having a telescoped skull, in which the nostrils have moved to the top of the head and from the blowhole. Air and water vapor released from the blowholes are referred to as the blow and forms shapes that are species-specific and useful in field identification.

Vestigial Pelvic Girdle

The vestigial pelvic girdle is part of the vestigial structure of Cetaceans considering that they do not have hind limbs. Their bodies are hydrodynamic and fusiform, as such, they have backbones that are flexible and can resist hydrostatic pressure as they make deep dives (Thewissen et al. 2006). The vestigial structures in cetaceans are characterized as clear evidence to support the evolutionary theory especially since they are homologous structures as well. Though the pelvic girdle was at fully functional at some point, it degenerated like the rest of the vestigial structure in Cetaceans since they were no longer important or needed for survival in a rapidly changing environment. Since these were structures that were no longer required, they diminished gradually; however, in other evolutionary lineages derived from the same ancestor, the vestigial structure may be modified or retained (Murray and Zalmir 2001).

Conclusion

Various studies have succeeded in obtaining reliably molecular and morphologic data that supports the relationships among the cetaceans. A progressive and rapidly growing record continues to provide evidence that supports considerable morphologic diversity in Cetaceans and specifically, in whales, most of which had well developed hind limbs. There are divergent estimates suggesting that baleen and toothed whales had the same Archaeocete ancestor. Also, there is evidence suggesting that some of the older Mysticetes had both baleen and teeth; however, later Mysticetes evolved losing their teeth but retaining baleen whereas the Odontocetes retained the teeth.

References

Geisler, J H., and Sanders, E.A. 2003. Morphological evidence for the phylogeny of cetacean. Journal of mammalian Evolution 10.1/2: 23-89.
Lande, R. 1976. Evolutionary Mechanisms of Limb Loss in Tetrapods. Evolution 32: 73-92.
Miller, D. L. 2007. Reproductive biology and phylogeny of cetacean: Whales, Dolphins and Porpoises.” Taylor & Francis Group.
Murray, F.and Zalmir S. 2001. Biology, Medicine, and Surgery of South American wild animals. Iowa State University Press.
Price, S. A., Olaf R. P., Bininda-Emonds, and John L. G. 2005. A complete phylogeny of the whales, Dolphins and even-toed hoofed mammals (Cetartiodactyla). Biological Review 80: 445-473.
Thewissen, J. G. M., and E. M Williams. 2002. The early radiations of cetacean (Mammalia): Evolutionary pattern and developmental correlations. Annual Review of Ecological Systems 33:73-90.
Thewissen, J. G. M., Cohn, M. J.,Stevens, L. S., Bajpai S., Heyning, J. and Horton jr. W. E. 2006. Developmental basis for hind-limb loss in dolphins and origin of the cetacean bodyplan. PNAS 103.22 2: 8414-8418.