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Evolution Events!

For this episode of #EvolutionEvents we discuss why one species has set itself as a record holder amongst all other cetaceans, via its unusual pectoral flippers! 

The humpback whale has the longest pectoral flippers of any whale, dolphin or porpoise (left image). They can measure about ⅓ of the entire whale’s body length, thus, based on an average humpback whale, they can be close to 15 feet long! The humpback whale’s Latin name Megaptera novaeangilae does actually translate to “big-winged whale”. To discover why humpback whales have evolved such large and agile pectoral flipper, read on below. 

Humpback whales are an acrobatic, agile and graceful marine mammal. They are well-known for their complex low-frequency “songs”, that are performed by males during mating seasons. As well, they are recognized for their massive annual migrations and presence of strong maternal care for their young. It is immediately easy to spot their impressive pectoral fins when they leap out of the water, or “breach”. 

Under the surface, these whales are extremely equipped divers and graceful swimmers. Despite their 30-40 ton size, humpback whales have fairly good manoeuvrability and agility. 

So why did they evolve these massive flippers… are they not a hindrance? Or are they actually a benefit to the humpback whale, and why so? 

Humpback whales, and other cetaceans, are tetrapods. This means that they evolved from a common vertebrate ancestor that had two pairs of limbs- we are also tetrapods. That means that ancestors of modern whales eventually evolved their front legs to create the presence of flippers, seen in whales, dolphins and porpoises, while still today retaining a hidden bone structure that proves as much. 

All cetaceans have pectoral flippers, but why did humpback whales evolve their unique flippers that are so different from other cetaceans, even other baleen whales? Well, scientists have gone on to explain that these flippers are probably used for a number of purposes; navigating shallower areas, producing rapid acceleration, increasing manoeuvrability and stability, managing body temperature, communication, fighting rivals and predators, and collecting and herding prey. 

Let’s discuss why each of these factors might be important for a marine mammal, and especially a humpback whale. 

Humpback whales utilise two different locations and seasonality in order to either 1) feed or 2) mate or give birth to their calves. In the late spring, summer and even fall months, humpback whales dwell in cooler northern regions that are high in food, such as krill and herring. During winter months, these whales do not feed and will instead breed or raise their young in warm, tropical regions. For example, for the North Pacific population migrating to Washington, B.C. and Alaska, these tropical regions pertain to Mexico and Hawaii. Warm tropical sites provide especially warm, safe shallow bays that are perfect for socializing and raising new calves.

It is believed humpback whales use these pectoral flippers in shallow spots, for navigation- spots that may prove challenging or dangerous otherwise. Shallow waters can of course occur both in the breeding grounds and in the feeding grounds. Their pectoral flippers also contain “tubercles” at the top edges, which contain one hair each; a sensory system that is believed to help with swimming, feeding and socialization. 

Living in the ocean, with many species completing long dives, cetaceans need a way to stay warm and thus regulate their body temperature. It is suspected that the pectoral flippers of humpback whales may also have a purpose in thermoregulation due to their large surface area. Temperature regulation can be a tricky situation when you live in alternating cool and warm waters!

The ocean is a dynamic and dangerous place. Whales often utilise communications across distances that don’t always include vocalisations. Humpback whales are suspected to use their tails, bodies and flippers for a unique communication purpose; when humpback whales breach or slap their fins on the surface (e.g. pectoral fin slap), this sound resonates and may find nearby whales and distribute information- for example, “I am here” or “a predator is around”. 

Pectoral flippers are also utilized by males during breeding season. In their winter breeding grounds, humpback whale males may use pectoral flippers to fight against other males in competition for females, or even to literally prevent other males from reaching females before themselves, essentially cutting other males off. The “fighting aspect” of these flippers can also be utilized in defence from predators, such as orcas. Often, adult humpback whales cannot be hunted and killed by even large groups of orcas, but it is possible. Most typically, it is humpback whale calves that are sought out by orca pods. Humpback whales may use their large pectoral flippers as weapons, especially as they are usually laden with sharp barnacles- these flippers at force can prove a devastating and even lethal blow to a predator, providing defence for the humpback itself or its calf. 

Humpback whales are acrobatics: performing beautiful, graceful “ballet-like” twirls and rolls under the surface, and even above the surface! How do they do this as one of the largest whales in the world? Well, these pectoral fins help to facilitate these more agile and controlled movements. 

In terms of acceleration, the tubercles themselves allow water to move more efficiently over them, creating a hydrodynamic benefit. Humpback whales travel massive distances for migration purposes (up to thousands of miles in a single year); therefore efficiency can certainly help them when swimming. They also need to perform high-intensity feeding behaviours in which they lunge towards concentrations of prey (e.g. schooling fish), fighting significant drag with their mouths open to capture this prey. These pectorals, used optimally and strategically, can help the whale to generate lift when lunging, stabilise the body, increase thrust and counteract the torque force when engulfing prey and water. So, to perform their complex feeding behaviours and survive (e.g. lunge-feeding, bubble-net feeding), humpback whales need their pectoral flippers! 

The above benefits are significant, but we do have to mention one limitation- sometimes these flippers can be limiting to hydrodynamics if not used properly and exactly- for example, at the wrong angles and un-symmetrically. Therefore, is there a better reason that evolution might have favoured these unusual flippers? 

Scientists have suspected that the pectoral flippers of these whales perform a variety of benefits when hunting; we mentioned feeding earlier, but we only discussed how the pectoral flippers assist the whale to feed indirectly. However, do these pectoral flippers actually have a significant role in garnering and gathering prey, and thus had a strong selection process (i.e. evolved for a significant purpose)? Experts think so!

The pectoral flippers of humpback whales may be used for three techniques when corralling and herding prey during feeding (or more if they have not been studied yet). These include preventing prey physically from escaping a bait ball/ school, affecting water movements to help with concentrating prey (includes affecting the water that prey is held in) and even using the bright white coloration on their flippers to “direct” prey based on light reflections (See specific study here). 

With such a powerful role as a feeding tool (in more ways than one, both directly and indirectly), it is clear that humpback whales and their complex lifestyles require complex pectoral flippers that fulfil the many tasks their lives demand. 

A close modern relative (i.e. possibly originating from a common ancestor) of the humpback whale is the fin whale (right image)- do fin whales have these massive flippers too? They do not! Fin whales have much shorter and taper-shaped pectoral flippers. They are certainly not the same length proportion to the fin whale’s body size as the humpback whales’ is (remember, 1/3rd the length). However, fin whales’ pectoral flippers do still have a hydrodynamic purpose to reduce drag. From this, I believe we can see that different pressures may produce pectoral fin adaptations that are similar, but also very different based on a species’ specific requirements. 

How animals evolved and continue to evolve to become better equipped not only for their environment, but for their specific lifestyle and behaviour, is a topic that is both mesmerizing and incredibly complex. We can clearly see that we have still much to discover about the world of whales! 

Article By: Alexa D., Five Star Whale Watching Naturalist.

Resources:

Merriam Webster, Conscious Breath Adventures, WDC, New Scientist, Pectoral foraging: an innovative tactic for humpback whale foraging, Whales online, Hydrodynamic performance of the flippers of large-bodied cetaceans in relation to locomotor ecology, Whole-genome sequencing of the blue whale and other rorquals finds signatures for introgressive gene flow.