Killer whales are fast-swimming, long-lived, intelligent, social animals and the largest apex predators in the ocean.
2. EVOLUTIONARY HISTORY
From the fossil record, we know that early cetaceans evolved from terrestrial quadrupeds (Figure 1) to obligate swimmers (William, F. 2008) 50 million years ago (Hoelzel, A. 2002).
The early cetaceans adapted to their aquatic environment through millions of years of evolution. It is believed that the ancestors of whales and dolphins began their migration to the sea by foraging food and hunting in the shallows near shore, using legs to walk and swim (William, F. 2008). As time went on, the whales specialized in feeding on different prey, branching off into two different groups, and one group (Figure 2), called Odontoceti, kept its teeth (Bird, J. 2007).
Figure 2. Evolutionary tree of Killer whales (Dahan, J. 2016)
Figure 1. Traditionally accepted chart of Whale Lineage, leading to the Odontoceti, the suborder that killer whales belong to. (Dahan. J. 2016)
There are various ecotypes recognised, differing in dorsal shape, pigmentation, dialect, DNA, physiology and behaviour (Hadoram, S. 2006). All different types possess very pronounced morphological differences, with three different ecotypes appearing in the Antarctic form and two in the North East Pacific. The Antarctic forms are colloquially known as ‘resident’, which are coastal fish eaters, ‘transient’, mammal eaters, and ‘offshore’, which dietary habits are unknown. (William, R. 2002). There are genetic differences among all these forms, with particularly marked differences between resident and transient forms (Hoelzel, R. 1998).
These dietary specializations likely evolved slowly refining their foraging strategies that were learned by individuals and passed across generations. Foraging specializations may have played a role in the historical separation of ancestral resident and transient groups, leading to the social and the eventual reproductive isolation of the two populations. (William, F. 2008).
The taxonomy of this genus is in need of review, and it is possible that killer whales will be split into a number of different species or subspecies over the next few years (Reeves et al. 2004), but for now, all forms and populations are considered a single, rather variable species (Shirhai, H. and Jarret, B. 2006).
3. MORPHOLOGICAL AND NEURAL ADAPTATIONS
In terms of morphology, killer whales slowly lost their hind limbs over thousands of years, while their tale was tucked within the body into a fluke becoming vestigial structures (Hoelzel. A, 2002). The fingers or toes on their front limbs became webbed, eventually metamorphosing into the flippers we see today and the nose moved from the front of the head to the top, becoming a blowhole (William, F. 2008). One of the most remarkable evolutionary developments of cetaceans was the ‘telescoping’ movement and elongation of the premaxillary and maxillary bones in the skull as the nasal openings migrated to the more effective position on top of their head. (William, F. 2008).
Rapid encephalization within the orca line of evolution developed during the Oligocene period and orca and dolphin lineages differentiated around 8 million years ago (Anderson, R. 2016).
Current cetacean brains are among the largest of all animals in the animal kingdom, in mass comparison to body size, possessing a unique underlying neocortical organizational scheme that allows them to display cognitive and behavioural complexity that could match with our closest phylogenetic relatives, the great apes (Marino, L. 2004).
Understanding killer whale neuroanatomy is vital because, like other dolphins, they show evidence of many complex social, communicative, and cognitive capacities different to other species (Marino, L. et. al. 2004). Studies of their brain reveals a structural complexity that could support complex information processing, allowing for intelligent, rational behaviour (Marino, L. 2007). This shows that based on the size of their brain and encephalization quotient, orcas are among the most intelligent animals in the world (Anderson, R. 2016).
There are some obstacles understanding the brain evolution of these animals as the brain does not fossilize, only the outer shape can be studied in natural endocasts (William, F, 2008). Furthermore, the little information and studies on killer whale brains could also be due to the difficulties associated with preparing and examining such a large brain. (Marino, L, et. al. 2004)
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