The majestic orca (image from The New Scientist)
Orcas may be some of the most iconic and beloved animals on the planet, and they are certainly the champions of our podcast, as they are the winners of our March Mammal Madness! There is so much amazing research about orcas, we couldn’t cram it into one episode, so we have two to cover this incredible species. This writeup will focus on their evolution, physiology and ecology, and the next writeup will focus on fascinating facts about their intelligence, conservation and future.
Orcas today and yesterday: their evolution
Although they are often called Killer Whales, thanks to the historical view of them as vicous predators, Orcas are actually part of the dolphin family. Orcas are the only species in their genus, Orcinus, and belong to the suborder Odontoceti, also called toothed whales. This makes them very different from the Baleen Whales we have covered, like the Humpback Whale, that have the huge plates of fibrous baleen in place of teeth, and filter feed, straining krill out of water. Toothed whales hunt and kill prey. Most evolutionary hypotheses point to a common carnivorous land animal ancestor, called Mesonychids, whose fossils have been found. There were roughly the size of a wolf, and in the process of searching for more food, they began to forage deeper and deeper into the water. Eventually, in an evolutionary process taking millions of years, they lost their fur, developed fins for more efficient swimming and became fully aquatic. The earliest ancestors of the Orca appeared around 11 million years ago, with the orca lineage branching off a relatively short time later. Orca look morphologically very similar to modern day pilot whales and pygmy killer whales, but analysis of a common gene in the 90s revealed that their closest relative is actually the snubfin dolphin. More studies done in recent years placed the orcas as a sister clade to the oceanic dolphin family.
The early ancestor of the modern day toothed whales, a mesonychid, which resembled a striped wolf (image from facweb.furman.edu)
Orcas exist in every ocean, but there are three to five populations that are considered different enough to be classed as separate races or subspecies, or even species. These groups are called ecotypes, and they differ in size, appearance, diet, hunting styles, behaviours, social groups, and even have different dialects, just like different parts of the world have different languages and accents. Some groups live on fish, whilst others survive on mammals, and others on birds and reptiles. Their chosen homes can differ, with some living in Arctic waters and others in the Antarctic. Some are called transients, moving across oceans, whilst others tend to stay in one place.
In the Northern Hemisphere, there are 5 major groups:
1.) Residents: fish specialists, these orcas tend to have small home ranges that keep them in close proximity to large fish populations. Each community are genetically distinct and even communicate differently, with unique traits like the beach rubbing behaviour of the Northern Residents.
2.) Bigg’s Orcas: also called Transients, this population eats mammals from harbour seals to whale calves. They live in small groups and travel large distances. Unfortunately, as they are eating prey higher up the food chain, these orcas accumulate pollutants transferred up the food chain up to their mammalian prey. These pollutants end up stored in the blubber of killer whales, which can affect their reproduction, suppress their immune system and lead to deformities and cancers.
3.) Offshore: this is probably the least understood ecotype, as they live far off the coast, making them more challenging to study as they are so rarely encountered. Unlike other ecotypes, their diet and social structure is unknown. Their teeth have been found to be worn down, suggesting they are dining on something with rough skin, most likely sharks!
4.) North Atlantic Type 1: although research is still ongoing into this group, they are known to be generalist eaters and have been observed using the carousel feeding technique, herding herring into dense balls and slapping them with their tails to stun the fish. They are seen around the coasts of Norway, Iceland and Scotland, feeding on herring and Mackerel.
5.) North Atlantic Type 2: this group predominantly feeds on whales and dolphins and are rarely seen. They have very distinctive back sloping eye patches and much larger teeth than Type 1 whales. They have only been sighted a handful of times, and are likely fish specialists, although no one knows for sure.
There are also 5 major groups (or ecotypes) in the Southern Hemisphere.
1.) Type A: some of the largest orcas, reaching up to 31 feet, this population ranges throughout the Southern Ocean and primarily hunt minke whales, following them on their migrations.
2.) Type B (large): this population is famous for their distinctive wave washing hunting, in which they use their tails and bodies to flick seals off pack ice into the water. This gives them the alternative name of Pack Ice Orcas. They are often brown or yellowish in colour due to algae growing on their skin.
3.) Type B (small): often called the Gerlache Ocras, as they are most often found in the Gerlache Strait in the Antarctic peninsula. They also have a brownish-yellow colouration, but are smaller than other orcas. Their diet is unknown, butas they have often been seen around penguin colonies, it is fairly likely they make up most of their diet.
4.) Type C: the smallest of all the orcas, with the same yellowish colouration as the other Antarctic orcas, they are found in the Ross sea. They have a very distinctive dramatically sloped eye patch, and are often seen feeding on fish.
5.) Type D: this population has smaller eye patches, shorter dorsal fins and rounder heads.
In a 2016 study, researchers sequenced the entire genomes of 50 Killer Whales from across these population groups. They compared and contrasted the differences between each individual to identify the most recent common ancestor that could have been as recent as 250,000 years ago. The variety of orca behaviours we see can be linked to genetic variations. For instance, variants in genes that control metabolism have led some orcas to favour eating dolphins and seals, and they can turn excess amino acids into methionine, an amino acid that mammals cannot generate on their own. Fish eating orcas do not have this mutation; they can reliably feed on abundant fish, whereas mammal eating orcas have a more precarious diet of less abundant dolphins. This requires them to have a safety mechanism to store excess amino acids for days when they don’t get chance to catch a meal. This and other gene variations has been likened to the vast diversity we see in the human race. There are few other mammals that spread across such a large geographic range and display such variation in their behaviour and genomes.
The different ecotypes of the Orca, as we currently understand. This poster showcases the different marking patterns and colourations that enable these ecotypes to be identified (image from www.simrstofino.org)
Anatomy of an Apex predator
Often called the ‘Wolves of the Sea’, orcas are highly efficient apex predators and have no natural predators. They have a variety of adaptations that make them perfect for their role as the wolves of the sea. Like Great White Sharks, orcas are counter shaded, with black patches on top and mostly white on the bottom. This means that when seen from above, the black blends into the dark ocean, and when seen from underneath, the white allows them to blend in with the sunlight on the water, making them almost impossible for ocean going animals to see them until it is too late. An orcas’ core body temperature sits between 36.4 and 38 degrees Celsius, but it’s a challenge to keep this temperature
in the chilly waters orcas call home, as water conducts heat about 25 times faster than air of the same temperature. To tackle this, orcas have a thick layer of blubber that insulates them from heat loss. Like other marine mammals we have seen, they have a counter current heat exchange system, with arteries and veins positioned close to each other. This allows warmed blood travelling through arteries to heat the cooler blood in veins, rather than the heat being lost to the environment. When orcas dive to forage, circulation at the skin is decreased and blood is shunted towards the core. If the orcas get too warm, through exercise or time spent in warmer waters, circulation near the surface of the flippers, flukes and dorsal fins increases, allowing excess heat to be shed into the surrounding waters.
A simple example of how the countercurrent heat exchange works. Orcas utilize this physiology to manage their temperature in the cold waters they live in (image from www.biology-pages.info)
Although orcas don’t dive anywhere near the depths of other whales, foraging whales can reach recorded depths of at least 100m, with some studies estimating maximum depths of 452m. Some ecotypes, like the transient orcas in the North Pacific, have been recorded diving for as long as 11.2 minutes in search of schools of fish. All marine mammals have a dive response, called the mammalian dive reflex. Their heart rate slows down to as little as 30 beats per minute from 60 beats, and blood is diverted away from low oxygen tolerant tissues in the body extremities towards vital organs. Marine mammals also have specially adapted hemoglobin and myoglobin that can store higher levels of oxygen in tissues. Orcas and other cetaceans, most famously dolphins, engage in something called unihemispheric slow wave sleep (USWS). During this, one half of the brain goes into a sleeping state, whilst the other half stays awake, allowing the orca to maintain their sight and hearing, enabling them to resurface for respiration. Interestingly, studies in captive orcas revealed that for the first few months of life, an orca calf and its mother barely rest at all. Over time, the mother and calf will gradually increase the amount of time spent sleeping to normal adult levels. This pattern has also been seen in dolphins and their calves, and researchers speculate that this is an adaptation to avoiding predators and maintaining body temperatures while calves build up a vital layer of blubber.
Working for your dinner: the amazing hunting techniques of the orca
As mentioned above, different populations of orcas prefer to chow down on different food sources, either fish or mammals and birds, and they have very different techniques depending on their prey choice.
The famous spy hopping behaviour of the Orca, They lift their heads out the water to check for prey on ice floes (image from whalemuseum.org)
As mentioned above, orcas in the Antarctic exhibit the famous wave washing technique, as made famous by nature documentaries. Initially, orcas will ‘spy-hop’; a well-documented behaviour where whales pop out of the water vertically, like a human treading water, to look onto the ice and locate their next seal dinner. Next, the pod will start chipping away at the chunk of ice on which the unfortunate seal sits. Once the ice has shrunk down, the pod will swim incredibly fast towards the icecap, ducking underneath it at the last minute to create a wave that will fling the seal off into the water. This is not the only clever way orcas can catch seals. In Argentina, orcas are known to surf waves right onto the rocky shores where elephant seals and sea lions care for their young and can snatch them right off the beach. This is particularly impressive, as in all the recorded instances of this behaviour not one orca has ever been recorded beaching itself. Often, calves beach alongside the adults, and there is speculation that this how they teach the young this complex behaviour.
The daramtic sight of an orca beaching itself in the hunt for a seal on the coast of Argentina (image from NBC news)
Orcas are also famous for preying on larger species, from dolphins all the way up to huge marine mammals like the blue whale. In 2019, there were three separate blue whale kills by orcas off the coast of Australia. Many large whales are sighted with tooth rake marks on their sides from orca attacks.
Incredible drone footage of Orcas hunting sharks, confusing them with nudges and then using their flippers to slip the shark and put them into tonic immobility (image from CNN)
It takes an impressive animal to take on the great white shark, but drone footage has shown pods of whales chasing down and attacking sharks. Prior to this amazing footage, there was long held speculation that orcas attacked sharks when great whites began washing up on beaches with massive chunks bitten out of them. There are even a pair of now infamous orcas called Port and Starboard, that are suspected to be behind the discovery of eight great white carcasses off the coast of South Africa. After these orca attacks, great sharks are even known to vacate the waters for some time afterwards, even as long as a year. It is even a concern now that these orcas may be unbalancing the ecosystem in the area by driving away apex predators and allowing their prey populations to grow unchecked. No one knows exactly why orcas are attacking great whites as they don’t always eat them, but some suspect it could be due to competition for the same resources. But the toothy business end of a great white can do some serious damage even to hardy orcas, so how exactly do they kill a great white without injury? The orca will push a shark to the surface of the water using its broad tail, then slam it down on the shark’s head. Suggesting some understanding of shark biology, the orca will then flip the shark onto its back, putting it into ‘tonic immobility’, paralyzing the shark and making it and easy kill.
Smaller fish like herring are an even easier snack for orcas. Pods off the coast of Norway use bubbles to force schools of fish towards the surface of the water into a ‘bait ball’, then use their powerful tails to smash down into ball, stunning the fish and gobbling them up.
Awesome videos!
References
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MASSIVE THANK YOU TO RACHAEL DA SILVA FROM THE UK FOR THIS WRITE UP! PLEASE FOLLOW HER ON INSTAGRAM AND HER WILDLIFE ARTWORK AT TILLY_MINT08
