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)
Manger (2006) implied that the origin of large brains in odontocetes is linked to the cooling of oceanic temperatures, in contrast, most studies imply that cetacean evolution was not caused directly by climate change, instead it is believed that it affected ocean circulation and food chains (Perrin, F. 2008). Current studies suggest that the large size of dolphin brains was primarily a response to social forces, with the need of effective functioning within a complex society where communication and collaboration as well as competition among group members is really important as individuals can benefit from recognition of others and knowledge of their relationships, and from flexibility in adapting or implementing new behaviours as the social or ecological context changes (Marino, L. 2007). Results in a study made by Marino. L (2004) show an increase in encephalization at the origin of Odontoceti that may be related to the development echolocation, the ability to process high-frequency acoustic information
The killer whale brain is really elaborated in the insular cortex, surrounding operculum, limbic lobe and a small hippocampus. This finding is intriguing taking in consideration the fact that killer whales exhibit highly sophisticated ranging and distribution patterns that depend heavily on spatial memory skills (Marino, L. et. al., 2004).
There is also a probability that the operculum in cetaceand could serve a similar function as the speech-related opercular cortex in humans, which makes sense as there are adaptive features of the killer whale brain associated with the evolution of complex communicative abilities with a highly complex social structure (Marino, L, et. al., 2004).
4. EFFECT ON COGNITIVE ABILITIES AND LEARNING
Killer whales display a range of complex behaviours that indicate social intelligence, the problem is that these are difficult to study in the open ocean where protective laws may apply or in captivity, where access is constrained for commercial and safety reasons. (Anderson, R. 2016). However, a few studies reveal their high cognitive skills such as cooperative hunting, imitation, echolocation, dialects and social structure.
Killer whales are social animals that can usually be observed travelling in groups of a few to 20 or more individuals and show social and cooperative hunting skills when they forage and hunt (William, F. 2008). A few examples of foraging techniques that no other marine mammals use are wave wash hunting behaviour and carousel feeding. There is a study that observed how a group of 10 killer whales located a crabeater seal on an ice floe, the whales then swam away from it and then turned and went rapidly towards the floe in echelon formation. They deliberately created a wave that broke up the floe and washed the seal into the ocean (Pitman, R. and Durban, J. 2012). There is also evidence of carousel feeding, where killer whales were seen using percussive actions such as tail lobbing, releasing blasts of bubbles, flashing the white ventral side of their bodies and getting close forming tight ball close to the surface. They then confused their prey by striking the edged of the ball with their tail flukes and ate the debilitated fish (William, F. 2008).
Moreover, light detection is believed to be essential for predator avoidance behaviours, mate selection and foraging, therefore its evolution has been directly linked to survivorship and reproductive fitness (Avango, D. et.al., 2013).
In a study made by Abramson, J. (2012), three killer whales living in an aquarium were studied. Over the study, using a previously learned “do that” command, the researchers asked one of the whales to imitate an action that another was performing. Each whale imitated 15 behaviours that they already knew, such as slapping the water with their fins, and four that they had never seen or attempted before, including barrel rolls. The whales quickly successfully imitated the behaviour, and even the new behaviours not previously learned were performed after less than 16 tries. Furthermore, in a different study, a group of them were observed doing what appeared to be tests of trust, pranks, emotional self-control, limited use of tactical deception, and empathetic behaviours (Anderson, R. 2016).
4.3. Echolocation and dialects
An example of cognitive social skills is echolocation and the different dialects within the various ecotypes and pods of killer whales. Each pod shares an acoustic dialect, for example, residents often utilize echolocation and communicate within and between hunting groups, with the seasonal presence of their prey strongly influencing the distribution of resident groups of their range (William, F. 2008).
It is believed that these calls may be composed of subunits, which implies that they have high cognitive skills as vocal production learning is the process by which vocal signals are modified by experience with the signals of other individuals. For species capable of vocal learning, neural mechanisms for learning subunits and sequences of those subunits may enable production of novel calls. There are three functionally and structurally distinct types of vocalizations, such as echolocation clicks, whistles and pulsed call (William, F. 2008).
A pod can have seven to seventeen discrete calls and some killer whales vocalize significantly less than residents as surprise is an important element of foraging success, making both vocalizing and echolocating limiting (Burham, R. et. al., 2016).
The pronounced difference in the extent to which fish-eating and mammal-eating killer whales vocalize is consistent with a difference in the ecological cost for vocal communication arising from eavesdropping by potential prey (Hawkins and Johnstone, 1978). The cost of vocal communication for residents is limited to the energetic cost of generating the calls. In contrast, marine mammals taken by transient killer whales can detect killer whale calls over long distances and respond to the calls of transients with antipredator behaviour. Killer whale pulsed calls are thought to be such communicative signals and it has been suggested that they function to coordinate direction of movement and behaviour state among members of a group. these functions are essential to maintain group cohesion, killer whales have no natural predators, and decreased risk of predation is not therefore a possible reason for food-related calling (William, F. 2008).
4.4. Social structure:
Killer whale populations are structured into several social tiers, which possess distinctive cultural attributes in vocal, social, feeding, and play behaviour. Cultural learning of behaviours may proceed through motor imitation or perhaps even through direct teaching. Vocal imitation also occurs, such as the development of dialects among killer whale family groups. The close synchrony seen among wild dolphins is a form of imitative behaviour that may serve in part to express their affiliation and culture (Marino, L. 2006). The transmission of learned behaviour, is one of the attributes of cetaceans that most sets them apart from the majority of other nonhuman species (Marino, L. 2007).
Social groups in a common area encounter others frequently, providing opportunities for cooperative foraging, mating, and shared territorial defence. All of these factors promote social cohesion, shared cultural traditions and the development of community. As previously mentioned, members of a community actively avoid close physical contact with members of other communities. The reason for this apparent xenophobia is unknown, but one could speculate that inter-community contact has few benefits and many risks, such as competition for food, disease transmission, and aggression arising from the lack of shared social traditions. In any case, by presenting a behavioural barrier to social contact, and hence mating, social exclusion restricts gene flow and allows populations to diverge genetically (Hoelzel, A. 2001)
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