The Evolution of Baleen Whales
Baleen whales (Mysticetes) are some of the largest animals on Earth. They are major predators and large-scale nutrient distributors in the ocean ecosystem (Marx and Fordyce, 2015). Along with their gigantism, their evolutionary success and ecological diversity are accredited to their method of filter-feeding (Peredo et al., 2017). The baleen structure originated during a transitional phase from the use of raptorial feeding to baleen-assisted filtering, which is executed by suctioning prey, a process that circumvents the problem of function interference between teeth and baleen (Marx et al., 2016). The lack of functional teeth in all Mysticetes alters their feeding transport mechanisms by forcing them to use suction to ingest their prey directly into their mouth and into the oropharynx. Mysticetes exemplify the evolution of baleen in whales and help us gain a better understanding of these elaborate keratinous plates that assist them in feeding and growing. It is suggested that the origin of baleen occurred due to various factors such as mandibular variation and decoupling tooth loss. The stepwise evolutionary transition from teeth to baleen can be traced through fossil and molecular records. (Deméré et al., 2008).
Mysticete evolution is most likely to have been affected by the palaeoenvironmental change, however, the lack of accurate and early fossil records makes it difficult to support that statement. Scientists have attempted to address this issue by investigating the mysticete macroevolutionary dynamics. They have provided clarification on the recognition or homology of characters by scoring ninety taxa across 257 specimens and their 272 morphological characters. The results of these findings now provide a basis on which features may have contributed to the patterns of diversity and the origin of baleen. Four different hypotheses are suggested for the origin of baleen. The first, the dental filtration hypothesis, suggests that filter-feeding began when the use of dental cusps was adopted, similar to that of the crabeater seals. The second, medial baleen hypothesis, suggests that baleen evolved near the median plane of a functional dental row. The third, posterior baleen hypothesis, suggests that baleen is aligned in the rostrum along with the dentition. The fourth hypothesis, the suction feeding hypothesis, suggests suction feeding is a condition involving the transition from raptorial feeding to suction feeding and eventually bulk filter-feeding (Peredo et al., 2017).
The fourth hypothesis is the most plausible explanation because evidence shows that mandibular and dental variation is responsible for the evolution of baleen, specifically: the reduction in the size of jaws and the absence of teeth (Werth, 2006). In terms of mandibular variation, the term amblygnathy was coined to express the condition of a shortened and rounded jaw. Therefore, many amblygnathous odontocetes were studied to create a sound conclusion on the relationship between jaw size and loss of teeth, and accordingly baleen evolution. A basic measurement known as the mandibular blunt index (MBI) was calculated by using the ratio of jaw width to length and was compared against the number of teeth in the specimen. As the bluntness of the jaw increases, so does the MBI and inversely, a decrease in tooth count is observed (Fig. 2; Werth et al., 2006). These findings are specific to male odontocetes and further investigation into female amblygnathous odontocetes must be done to generalize this trend to all odontocetes.
Furthermore, elongated, narrow jaws with an abundance of teeth were replaced by diverse forms of short, blunt jaws with dental reduction or a considerable loss of teeth. In fact, the paleontological record shows that a common ancestor of odontocetes, known as the dorudontine basilosaurid archaeocete, possessed a long head filled with numerous teeth suggesting that blunt heads and jaws with baleen were not inherited but arose over time (Fig. 3; Werth et al., 2006). These alterations have significant implications on odontocete feeding and in turn affect the pre-existing structures. Once several deep and horizontal striations began forming, officially known as baleen, the structure of teeth weakened and consequently affected their functionality. The remaining teeth from fossils were heavily eroded and portrayed a significant loss of enamel along with low remnants of dentine (Marx et al., 2016). Due to this, the purpose of their teeth no longer included capturing or chewing prey but was cultivated as strong selective pressures as secondary sexual characteristics, used for intraspecific competition, or fighting other whales, as well as for display (Werth, 2006).
Research on baleen in whales has allowed for greater insight into evolution, a change of characteristics of a species over several generations that mainly relies on the process of natural selection. Many generations of different families or species of baleen whales have helped us formulate a conclusion on the development of baleen and investigated the factors that contributed to its evolvement. Inadequate phylogenies pushed scientists to examine hundreds of fossils and create an illustration of a Mysticetes skeleton which helped create a foundation for further research to build upon. These investigations led to the conclusion that mandibular and dental variation are the main causes of raptorial feeding being replaced by baleen-assisted filtering, and that new foraging methods are being used by these large aquatic mammals to maintain their position in the hierarchy of the ocean ecosystem.
