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Why have whales and dolphins not evolved to have gills?

Why have whales and dolphins not evolved to have gills?


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It seems at first glance that it would be an evolutionary disadvantage for a sea creature to have to come up to the surface on a regular basis in order to breathe, so why are there animals (e.g. whales and dolphins) which still need to do so?

If there is some advantage to breathing from the air instead of using gills, why haven't animals such as sharks (or indeed fish in general) evolved to do this instead?


I think you're looking at this the wrong way.

Animals evolve to fill niches. Many, many land animals returned to the water as a place to live, including:

whales, dolphins, porpoises, seals, sea lions, walruses, manatees, dugongs, otters, hippos, pygmy hippos, crocodiles, sea snakes, sea turtles, river turtles, river snakes, etc. etc. etc.

Their success in this environment points to no need whatsoever to "evolve" gills to be better suited. Without the interference of man, there would be yet many more of them.


Natural selection can only choose from the diversity available. Sometimes there are physiological or other constraints that prevent a solution we think should be optimal to arise. For instance, insects are the most diversified group of animals, but there are no marine insects. Why? It seems their respiratory tract (a tracheal system) can't deal with salt water. Or we can think of flying as something very advantageous, but flight has only evolved four times (insects, pteurosaurs, birds and bats; or five, if we include our flying machines). So "you can't always have what you want".

The precursor of gills can yet be seen in the early stages of mammal embryological development (they lead to the channel that allows you to unclog your ears by swallowing). But to turn them back into fully functioning gills would take so many mutations and the re-adaptation of so many working systems, that it's not a much viable option for large animals such as cetaceans. Remember that the gills first formed in the very first, very small vertebrates, and slowly acquired it's present shape we see in bigger fishes.


Welcome to evolution.

Just because the idea is great, the path obvious (in our minds), does not mean it will happen.

Evolution works by random mutations followed by selection to weed out nearly all phenotypes that are less successful than the original.

So you need both the mutations and right selection environment.

So while gills on a sea going mammals is a great idea. The path towards that is not an easy outcome without some form of intelligent design. You see those gills arches fishes have to support their gills… well the tetrapod lineage has since re-purpose those bones to make several structures such as the thyroid gland, part of the jaw, the larynx, and the bones in mammalian ear. https://en.wikipedia.org/wiki/Branchial_arch

So a whale that simply reconverts to gills, would be without a larynx (mute), or inner ear (bad hearing) and parts of the jaw (Can't eat). So you can see alot of other work will need to be done to make such an animal work. And if this process is left to nature, each intermediate must also work successfully in some environment.

Such are the constrains of evolution.

However if intelligent design were available. No problem. It can be done. We can ignore temporary lowered fitness, and work with a desired outcome in mind.


How Can Whales And Dolphins Hold Their Breath For So Long Underwater?

Here&rsquos a fun fact: whales and dolphins are not really fish.

This might be one of those truths of the world that make you doubt your existence, but it&rsquos absolutely correct. Whales and dolphins are not fish they are mammals, just like us humans!

This has a few implications that make them stand out from most members of their aquatic neighborhood. First, whales give birth to their young ones (as opposed to laying eggs) and feed them milk. They&rsquore warm-blooded too, and tend to have fur on their bodies (although it&rsquos very sparse). However, the most interesting distinction between whales and other fish is that the former don&rsquot have gills. This means that they cannot breathe the oxygen dissolved in water and therefore have to come to the surface every time they want to breathe air.

The question is &ndash how do whales (and other mammals living underwater) manage to hold their breath for so long before taking another breath?

Short answer: Whales&rsquo lungs make the most of each breath, and store excess oxygen in a special protein found in muscles called myoglobin. Also, they conserve energy underwater by purposely slowing their heart rate, constructing select arteries and limiting blood supply to only a few organs.


6 Answers 6

Many marine creatures temporarily "beach" themselves for a variety of reasons, including food. If climate or geological events left ocean food supplies decimated, critters that could hunt on shore would have the distinct advantage of not starving. Once the "low hanging fruit" close to the water was consumed, differentiation in gene lines would let some individuals spend more and more time out of the water. Mutated traits for heavier limbs, lower body weight and rebalanced metabolism would be selected for. Given time they will shed features that were advantageous in water but a literal drag on land. If the ocean's life and nutrient cycles aren't restored, aquatic life would become a distant genetic memory.

Nature would re-use biological mechanisms. Echolocation hardware can be repurposed for hearing sound through air to escape predators, communicate danger and find mates. Each of these is an evolutionary advantage. Half-working ears that accidentally trigger flight response means you pass your mutated ability on to a new generation while your neighbor gets eaten. At some point the aversion to smoke against what used to be a gill will keep a bloodline from walking into a forest fire. All these little mutations, responses and abilities will eventually result in a variety of land-viable species as they create new niches in the rebalancing ecosystem.

Hemoglobin may not be that big an issue. A newborn human's normal levels vary from 10 to 18 g/dl and a dolphin's levels may reflect its environment more than its genes. We have multiple overlapping systems to preserve homeostasis. Dolphins deal with a greater range of atmospheric pressure and their hemoglobin balancing systems might be fine tuned for this.

Baleen whales do face a new gross physical challenge, but even the krill eaters eat fish by accident. Maybe whalebone would be useful in sifting nutrients from beach sand. If not, and if nature doesn't find another use for those organic mechanisms, they're going to be under heavy evolutionary pressure and be one of the categories that get wiped out.

Now I have images of poisonous tree-climbing squid swooping in unsuspecting prey like a flying squirrel. Nature is metal.

You need to eliminate all vertebrate life on land so they have no competition. This is going to take a long time. Whales and dolphins are extremely specialized, so you need to remove as many obstacles as possible. Any terrestrial vertebrate will be able to fill open niches much faster than whales and dolphins. You need nothing in the way so the the highly disadvantageous specialized adaptations don't matter.

You need to get them into swamps to have them start redeveloping feet and hind limbs. It will be dolphins, whales just won't happen due to their size, you need small animals to make such a transition. So if you absolutely need whales you need to get rid of dolphins so whales will expand into their niche with smaller size.

Hemoglobin is an insignificant problem easily changed, and echolocation works on land (see bats). Baleen again requires the elimination of toothed whales and dolphins to encourage them to re-evolve teeth or an analog.

If you just need one or two species of toothed whale (or dolphins) to do it, it is not to bad. You use killer whale beach hunting behavior to encourage more terrestrial adaptations, but that won't work for baleen whales.

I'm not sure what the etiquette on this site is regarding reposting answers to past questions, but I remember answering a similar question on dolphins readapting for life on land, that might be useful here. Whether the reason I remember this answer is due to my fondness for the goofy MSPaint image I made to accompany it is neither here nor there.

While it is unlikely that cetaceans would regain their back legs, it is possible for tails to be re-purposed for terrestrial locomotion. Sea lions have done just that, and they are quite agile on land for a primarily aquatic species.

It seems unlikely that baleen whales would return to land, as the main motivation for returning to land would probably be to find food and baleen is useless out of the water. Baleen whales could probably return to having teeth, though, given the right circumstances.

The process of returning to land could begin with a carnivorous whale or dolphin species that "beaches" itself sometimes in order to acquire food. Orcas are known to do this to hunt seals, for instance. Individuals that can propel themselves on land somewhat with their flippers have an advantage over those that cannot. Eventually, they might evolve an amphibious shape similar to a seal, which can then branch out into the more terrestrial sea lion shape. If a fully terrestrial niche opened up, they might leave the water completely. This could lead to a whole new branch of various kinds of mammal that uses their tails for back legs.

One of the key factors here is to realize the shift from land animal to whale was over 55 million years of evolutionary pressures. it's not a short shift by any means that will include many staging points along the way. It's also worth noting that there were several failed steps along the way that eventually went extinct as they were not able to adapt.

First and foremost, the pressure to get out of the sea must exist. and to do this I would go one of two routes.

-1 massive predator. with some warmer waters, it is possible to reintroduce the Megalodon, an absolutely massive shark. This shark relies on deep water to hide itself, locating a prey (whale) near the surface, and swimming quickly for a surprise attack from the depths. This makes deep waters exceedingly dangerous to the whale and dolphin populations and forces them to shallow water habitats. This would likely bring about the extinction of larger whale species as they die off to this new competitor. Dolphins and smaller whale species are only capable of surviving because they can function in shallow water.

-2 toxin or 'dead' zones become prevalent. Algae blooms and other natural phenomenon can become rampant. These blooms consume the oxygen from the water making for large dead zones. Since ocean mammals still breathe air, they are not directly impacted, but their food source sure is. This forces them to where the prey is. shallow water. Once again, larger whale species won't be able to transition to shallow or they would be unable to find the scale of food they require in shallow habitats. Small whale species become dominant.

In either case above, you now have the whale population either extinct or in shallow waters. In deep ocean, speed has the advantage. in shallow waters, agility dominates. Over the next million or so years, the whale population slowly adapts to become more suited to quick movements instead of fast speeds.

Out of this comes the ability to 'push off', using the tail to fling itself off of the ground or nearby rocks to reach top speed at moments notice. At this stage (potentially 3-4 million years in the future) the whale now has the shape and predatory tactics closer to that of a crocodile (kutchicetus or potentially ambulocetus) where it lies in wait, in shallow water, until a prey species comes by for them to quickly strike at. This creature would have descended from the dolphin populations, but no longer resemble much of a dolphin. Legs for pushing off could begin development at this stage (potentially 5 million years in the future, maybe longer). Sonar and the sort will have gradually faded away, allowing more of the brain to be used for sight and smell.

And at this phase you need something to push them fully on land. a shifting landscape (like the Americas seperate allowing the ocean current to flow between north and south america) changes currents bringing cold water into the formerly warm shallows these creatures lived in. forcing them onto land to sun themselves for warmth. Or a change in the prey available forcing the former-dolphin-like creatures further onto land for prey (kinda crocodile like). Over the next few million years, these traits slowly become like the Pakicetus until it's once again a land creature.


Why haven't whales evolved gills?

If evolution is parsimonious, why do we have vestigial structures like a tailbone or an appendix? Why is about half of the human genome composed of repetitive sequences that are derived from viruses or mobile genetic elements? I get your general point and agree that an organism with both lungs and gills would likely vestigialize one of the organs if it used only one (it is much easier for random mutation to degrade unecessary function), but I'd disagree with the general statement that evolution is parsimonious.

Evolution cannot be generalised to be either parsimonious or 'extremely conservative'. Without selection pressure, change is inevitable.
And it is often postulated how evolution may be able to occur in bursts.

The best way to describe evolution is to describe the entire process, not to stick on it a few words.

It may actually be the case that one of the best pathways to evolve whales is to do it through land animals. Some adaptations whales have could not happen with only the selection pressures occur in the sea. Being warm-blooded and breathing air may be one among many.

I don't think this is necessarily true, though. Evolution gets caught in local minimums, it may never find the global minimums. It doesn't really matter whether one takes more than the other unless they do so in a way that influences selection in the present moment. For instance, imagine that removing our tongues increased fitness and reduce energy expenditure in ways we can't calculate as a social creature, but if there's no gradual path of increased fitness there, evolution will not likely find that solution through random mutation and selection. Evolution can be wasteful as long as its waste isn't affecting fitness relative to its competitors and even then, it may only lead to a population reduction, not an elimination of the entire population containing such a mutation.

That would be an envrionmental driver, no? Indeed, the history of evolutionary development (which started with their teeth while they were still amphibious) does indicate that they were capitalizing on the easy pickings in the ocean, but this is a matter of ecology (resource availability) which is a subset of environment, no?

Evolving gills would not be a backward step. For a whale to evolve gills would be an absolutely huge step forward. Evolution doesn't take giant steps. It can't. Most evolutionary steps are little tiny baby steps. Every once in a while, evolution takes a bigger stride. But unlike Superman, it never, ever leaps across the Grand Canyon in a single bound.

Whales cannot evolve gills.

There is a lot of unreferenced assertions in this thread, most erroneous, and unfortunately due to time constraints I am going to add my own unreferenced (and therefore likely erroneous) assertions. :-/

As already commented on, evolution always move forward in time. As a process how can it not?

If something like gills evolved again, they could be convergent, as gill arches are now used to form the throat, larynx et cetera. Mind that some land living vertebrates do retain some of their gill openings in development by accident, it is a known phenomena in humans, so such a pathway could use some or all of the earlier trait basis. [The "Why Evolution Is True" blog related such a case a few months ago.]

It is a teleological idea that evolution has a goal, often relying on pre-evolutionary ideas of a "ladder of descent". Sticking pity labels on process characteristics is always problematic, and sticking erroneous labels on it is confusing.

Similarly already commented on, selective pressures on a population may preserve traits or evolve new ones, but the population need not be at a local (or global) optimum but simply surviving. The global optimum (most biomass) for evolution acting on individual populations is the prokaryote unicellular 'body' form. (Ecologically we can expect a spread, and it is beneficial since the world with plants is the most globally productive - seen as net primary productivity - yet.)

And again as already commented on, evolving several complementary ways of air uptake may be advantageous and is how tetrapods switched to lungs. Animals may use skin (amphibians), air sacks (various tetrapods) or intestinal tissue (swim bladder) as supplement. The mechanism may work best under water (gills), with access to water (skin uptake) or be independent of water life (intestinal uptake).

Arguably, no longer. A few months ago the find that there is at least one warmblooded species of fish, the opha, was released, an accidental finding. "Heated blood makes opah a high performance predator that swims faster, sees better" [ https://swfsc.noaa.gov/news.aspx?ParentMenuId=39&id=20466 ]

You can argue that it would be harder, perhaps impossible for whales to evolve similar counterstreaming heat exchangers as the opah use, since the opah body temperature is a lot lower than in mammals. (But it is a recurring development, IIRC the gnu antilope has evolved that to shield its brain from its unusually high body temperature of some 44 deg C or so. And we have some similar heat exchange mechanisms for the brain at a lesser efficiency level.) But it could be possible.

Mostly, evolving gill analogs would be under several constraints (contributes relatively little oxygen must be shielded against heat losses).

Whales were large land animals that only recently (on typical evolutionary time scales) became whales - Thus all the intermediate stages* of fossil skeletons have been found. - Possibly the most complete set of "intermediate stages" that exist. They did slightly evolve their air breathing structures. Their "nose" was near the front of the land animal they evolved from, but migrated towards their developing tail so that it was in the air when most of the animal was under water (and water was supporting its weight).

* Vestiges of the once massive hind legs still remain in most modern whales - just tiny useless internal bone structures that are "floating inside" their red meat flesh - not any more even attached to the back bone. In some of the later stages of the whale evolution fossils, these diminished in size former hind leg bones are still attached to the back bone but are 100% internal, so with little if any utility. Here is that almost to whale stage skeleton with the residual hip bones and greatly reduced hind leg entirely internal, but unlike the whale, these bone structures are attached to the back bone still.

There are some excellent answers here, but many that imply a direction to evolution. Evolution requires variation and selection. The variation is pointless variation (from genetic mistakes). The selection among variants is what leads to differentiation.

Generally, there is step-wise progression. If an entire chromosome is deleted or duplicated, there can be a multi-step change, but generally, single-steps accumulate variation, and sexual reproduction mixes variants.

Gills may be incompatible with lungs via a single step process. The process of acquiring dry-air tolerance by gills may be simpler than the process of acquiring lung tolerance for water (generally called drowning). Perhaps there has been a variant whale born that COULD have inhaled water, but that whale never forgot to hold its breath. And maybe that whale was particularly infertile.

The warm-blooded explanation makes a lot of sense. Whales require thermo-regulation. A whale that took in sea-water would drop in temperature. Again, evolution is step-wise. So if you need to develop a mechanism for oxygenation of blood, and a thermo-regulatory change simultaneously, the odds are long.

There also needs to be selective pressure. If a whale is born with a first step in a hypothetical path to gills . say some blood-skin patch that oxygenates poorly . then that genetic variant has to be passed on, and survive. As random genetic drift, there is a good chance that articular variant is lost. If it is preserved, then a second step has to happen to add to that first-step.

Google tells me that whales have existed for 35 million years, and that there are 84 current living species, along with another 400 that are extinct. If any of the 400 were on a path to gills, that soft tissue change is not available. Perhaps they all were and that particular path is always a dead-end.

A gill is a highly evolved structure, with a suite of genes involved. And there has to be a stepwise path to that structure that is not somehow also a selective problem.

Many mammals live in or around water (beavers, otters, seals, etc). A few live completely in the water (dolphins, whales). It is relatively easy to see how a sea-otter population could evolve to a "seal" population (better food in the water, greater predation on land, etc.). And to see how a seal population could evolve to a pure marine mammal (problem of child-biirth, but evading land predation, perhaps better food). The variation of adding fat layers, and mutations for water maneuvering can be simpler single-step mutations.

So in addition to the problems people pointed out here, where entire gills might be selectively disadvantageous (from cooling, from loss of bouyancy (drowning again!)), I would also point out that gills are variation-ally complex, and intermediate steps may be disadvantageous.


Why havent dolphins evolved to breathe under water?

I understand dolphins are mammals, but why havent dolphins evolved to breathe underwater? What do you think the benefits are for them breathing out of a blow hole?

Edit: thankyou to everyone who took the time to explain this to me in awesome detail. I appreciate it very much. And thankyou everyone for not being condescending at all. You are all very helpful and I definitely learned a lot.

You are given the same organs as your ancestors are. The same tail, the same lungs and the same fingers. Gradually, by natural selection, any variation that gives you an advantage helps you survive and pass on those traits. These is what leads to the development of new organs. That being said, you are restricted by the limited potential of the organs you possess. Animals that thrive in aquatic environments tend to have gills, but if an animal has lungs that achieve the same performance for at least, the whole period of hunting, it's just as good. Evolutionary pressure stops where it is just barely adequate for an animal to survive.

Besides that, Lungs are far more efficient than gills, as water has a far lower oxygen concentration than air. Fish can live with this due to their lower metabolic demand. A dolphin with its Mammalian metabolic demand cannot.

Additionally, dolphins and all aquatic tetrapods excluding amphibians have lost the genetic instruction for gills, most being repurposed into internal ears, trachea and facial structures. Gills themselves started out as food nets that were repurposed for respiration as chordates evolved more complex and larger forms.

Edit: I might add that you cant regain organs your ancestors have lost. Many lineages went extinct simply because their potential for speciation has been exhausted and were out competed by generalists body forms. Birds cant become T-Rexes because they have lost their teeth and fingers. Whales can no longer return to land except to die, but fish, at least the ones with a swim bladder connected to its gut, can and have on multiple occasions adapted to air respiration, and if they still have hind fins, adapt to a tetrapodal body arrangement.


Why have whales and dolphins not evolved to have gills? - Biology

Whales, dolphins, porpoises, seals, walruses, and many other marine animals are mammals, not fish. The marine mammals exist because about 50 to 60 millions of years ago, some mammals wandered off of the land and into the ocean, and there they evolved into different types of marine mammals. For whales and dolphins, their front legs turned into flippers. Their back legs became really tiny, so tiny that you can't even see them when you look at these animals, but they have hind legs still inside their bodies -- if you see a skeleton of a whale you can see it has tiny leg bones near its tail. Here's an interesting discussion of the hind legs in whales and dolphins:

Like mammals, whales and dolphins breathe air through a pair of lungs, they are warm-blooded, their young drink milk, and they have hair (although very little). The whale is actually the closest living relative of the hippopotamus. Here's some good information on the difference between whales and dolphins and fish:

Whales are mammals because they give birth to live young, they have fur (although it is very sparse on their body), they have lungs and breath air and they provide milk for their young.

You have a lot of interesting questions about marine mammals. Dolphins and other whales are all mammals called cetaceans. Mammals, and only mammals, have fur or hair and they feed their babies milk. Whales all feed their babies milk. You may be thinking that whales dont have any fur. You are mostly right. They have fur when they are fetuses, but lose it before they are born. A few species have whiskers as adults and whiskers are a type of hair.

There are certain characteristics that all mammals have in common. Mammals all are warm-blooded animals, they breathe air, have hair, and moms feed their babies milk from mammary glands. Whales actually do all of these things! Whales are warm blooded, which means they keep a high body temperature that does not change in the cold water. Fish are cold-blooded, so their body temperature changes depending on the temperature of their environment. Whales actually breathe air with lungs using their blowholes to breath out! They come to the surface of the water so they can breathe just like you and me. Fish get oxygen directly from the water through their gills. Whales even have a little bit of hair on their smooth skin, usually on the top of their head. Fish have scales. Whales even give birth to live baby whales that will get milk from their mom for food. Fish lay eggs, which must still grow into a baby fish. So whales are indeed mammals and not fish!

Whales are mammals because they share the characteristics of other mammals (like us!). These characteristics include being able to regulate their own body temperature (unlike cold-blooded animals, like lizards), growing hair and producing milk to feed their young (who are born live, not in eggs). Because of all of these traits, even though whales live in the ocean and are generally much bigger than us, just like people, they are mammals.


Facts about dolphins

Dolphins are incredible. They are socially skilled, intelligent, agile, joyful, and playful creatures that share many emotional similarities with humans. There is an impressive range of different species of dolphin and they all have their own unique identities and characteristics!

Top ten facts about dolphins

  • There are currently 42 species of dolphins and seven species of porpoises.
  • Dolphins are marine mammals. They must surface to breathe air and give birth to live young.
  • A dolphin pregnancy last between nine and 16 months. The mother feeds her offspring on milk. The sons and daughters of resident orcas stay with their maternal family for life.
  • Dolphins eat fish, squid and crustaceans. They do not chew their food but may break it into smaller pieces before swallowing.
  • All dolphins have conical-shaped teeth. A Risso's dolphin has 14 while a spinner dolphin can have 240.
  • The orca (killer whale) is the largest dolphin. Hector's dolphin and Franciscana are two of the smallest.
  • The four river dolphin species inhabit the large waterways of Asia and South America.
  • Dolphins have an array of vocalisations such as clicks, whistles and squeals which they use for their well-developed communication and echolocation skills.
  • Lifespan varies from around 20 years in the smaller dolphin species to 80 years or more for larger dolphins such as orcas. is the most endangered dolphin. There are less than 50 individuals remaining. Fewer than 10 vaquitas (a species of porpoise) survive.

How many species of dolphins are there?

Currently there are 49 dolphin and porpoise species which are grouped into six families: the oceanic dolphin family is by far the largest with 38 members the porpoise family has seven members and there are four river dolphin families, each containing just one species. However, classification is not an exact science and as more information and discoveries come to light, deliberations will continue and some dolphin species are likely to be further split into more than one species and/or subspecies.

Names and nick names can certainly be confusing there are eight dolphin names that feature the word ‘whale’, including pilot whales, killer whales, false killer whales and melon-headed whales. Two species have whale and dolphin in their name the northern right whale dolphin and the southern right whale dolphin – no wonder both the public and scientists get confused!

Where do dolphins live?

Dolphins live in the world’s seas and oceans and in some rivers too. Some dolphin species prefer to live in coastal areas, others like shallow water but prefer to live away from the coast close to patches of shallower water which are located further out to sea.

Orcas are the only dolphins which live in the Arctic and Antarctic. Their large size means that they have more protection against the harsh cold of the freezing seas. Most dolphins prefer tropical and temperate waters as they are warm-blooded mammals and so it is easier for them to regulate their body temperature in these environments.

Can dolphins live in fresh water?

River dolphins such as the Amazon River dolphin (boto) and South Asian river dolphins live their lives only in fresh water rivers and lakes, a long way from the ocean they are sometimes known as the ‘true river dolphins’. There is another group of so-called river dolphins these are fresh water populations of marine dolphin species some of whom permanently live in fresh water rivers, these include the tucuxi (or sotalia), the Guiana dolphin, Irrawaddy dolphin and the finless porpoise.

What are the differences between dolphins and porpoises?

Porpoises are smaller than dolphins they are less than 2.5m (8ft) long. They are also characteristically chunkier than dolphins and have a small head, little or no beak, and a small triangular dorsal fin (except the finless porpoises!). Porpoises and dolphins also have a different teeth shape. Porpoise teeth are spade-shaped whilst dolphins are conical.


Whales evolved large brains in the same way that we did

The largest brains ever to have evolved belong to whales. Now we have discovered that the marine mammals gained their big brain size in the same way we did – through massive expansion of two particular brain regions, fuelled perhaps through changes in diet.

Amandine Muller at the University of Cambridge and Stephen Montgomery at the University of Bristol, UK, looked at brain size data from 18 species of whale and dolphin, as well as from 124 different land animals including 43 species of primate. With few exceptions, the whales, dolphins and primates all seem to have gained large brains through dramatic growth of the same two brain regions: the cerebellum and neocortex. Both regions are important for cognitive functions such as attention, and for controlling the movement of the body.

It makes sense that the cerebellum and neocortex evolve in unison, says Montgomery, because they are physically connected by many brain pathways. “It’s possible one can only change so much without being constrained by the performance of its partner, and needing the other structure to ‘catch up’,” he says.

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But what drove these two brain regions to expand so dramatically in whales and dolphins? Muller and Montgomery first explored whether the trigger was a change in social behaviour. In common with some primates – including our species – whales and dolphins can form complex social groups. However, the two researchers found no strong correlation between the whale and dolphin species with the most advanced social behaviour and those with a particularly large cerebellum and neocortex.

But they did discover that the whale and dolphin species with a larger cerebellum and neocortex typically enjoy an unusually broad diet, in terms of the variety of foodstuffs they consume. This might suggest that broadening the diet encouraged the evolution of larger brains.

It is unclear why diet and brain size are linked. Montgomery speculates that a broad diet is more likely to provide the energetic resources needed to fuel brain expansion. Alternatively, it might be that marine mammals with a broader diet need to learn and use a wider range of foraging behaviours to exploit different food resources. This could require a larger brain.

Unravelling how and why brain evolution and diet are linked in whales and dolphins could be important because it might help us understand why primates evolved large brains too.

Many researchers argue that primates gained large brains as their social worlds became more complex. But in the last few years, some have given up on this social brain hypothesis because of evidence that primate brain expansion is actually better explained by changes to diet. The data from whales and dolphins provides new evidence to bolster this idea.

Journal reference: Journal of Evolutionary Biology , DOI: 10.1111/jeb.13539


How do cetaceans feed?

Baleen whales and toothed whales feed very differently. While tooth whales are grabbers and suckers, baleen whales are either skimmers, suckers or gulpers!

Toothed whales (including dolphins and porpoises) all have teeth but the number, size and position, and even purpose of their teeth, varies from species to species. Some, like Orcas, use their teeth for grabbing while the long tusk of a male narwhal acts as a sensory organ and may help them “taste” the surrounding waters. One thing they have in common is that they do not use their teeth to chew their food! Toothed whales do not have molars for chewing their food, they swallow it whole or in large chunks. Some toothed whales use their tongues as pistons to suck in the food, using their teeth more socially than for feeding. One thing all toothed whales have in common is their sophisticated sonar systems called echolocation. This helps them find and target their prey which typically consists of fish, squid and octopus. However, some Orcas also are known to eat sharks and even other marine mammals.

Instead of teeth, baleen whales have hundreds of overlapping baleen plates grow downwards from the gums of the whale’s upper jaw. The number, size, and color of the baleen plates are unique for each whale species. The hairy fringe on the inside of the baleen plates earned these whales their scientific name “mysticete”, or moustached whales. Baleen is made out of keratin, the same protein that makes up our fingernails and hair. Being strong and flexible, baleen makes the perfect filter, or colander, enabling whales to strain out sea water and keep the prey.

Baleen whales are typically categorized as skimmers, gulpers, or suckers. Skimmers, like North Atlantic right whales, swim through patches of plankton with their mouths open as the water washes through the baleen, the tiny plankton are caught by the baleen and remain trapped in the their mouths. Gulpers are rorqual whales. Rorqual whales like blue and humpback whales have a series of pleats running from their lower jaw to the belly button which expand when they are filled with water. They close down with their upper jaw and force the water through the baleen, capturing fish or krill. Gray whales are suckers, they literally suck amphipods from the bottom of the ocean taking in mouthfuls of mud and food before forcing the water, sand and mud through their baleen as they capture their prey.


Why haven't fish evolved to fill the niche whales do? Why haven't marine mammals evolved to be the size of small fish?

My main question is why there isn't a massive fish the same size or as large as whales. They had much more time to evolve this adaptation and grow basically so large nothing could eat them, so why didn't they? Yes, there is the whale shark, but to my knowledge they are fairly rare, the only fish species of that size I can think of, and while they are filter feeders, my impression is they don't consume schools of fish like humpbacks do, for example.

If I had to guess why Iɽ say it's because maybe mammals are warm-blooded and therefore can expend more energy, or it requires more intelligence to do whatever allowed whales to thrive and evolve into many different species. That could be completely wrong, I'm not an expert.

Come to think of it, even marine reptiles evolved to be far larger than most fish. I guess there are and have been huge sharks, but they're mostly non-filter feeding predators.

And thinking about that brings up the reverse question : why didn't any mammals evolve to be basically fully marine creatures, yet smaller sized? For example, a sort of mini-dugong that feeds on moss and seaweed or very tiny, fast dolphin (like the size of a typical small fish. like a sea rat) that feeds on fish in relatively shallow waters and reefs.


How do whales and dolphins breathe?

Whales and dolphins are mammals and breathe air into their lungs, just like we do. They cannot breathe underwater like fish can as they do not have gills. They breathe through nostrils, called a blowhole, located right on top of their heads.

This allows them to take breaths by exposing just the top of their heads to the air while they are swimming or resting under the water. After each breath, the blowhole is sealed tightly by strong muscles that surround it, so that water cannot get into the whale or dolphin’s lungs.

When a dolphin surfaces for air, he breathes out (exhales) first and then breathes in (inhales) fresh air it only takes a fraction of a second for the dolphin to do this. If you are close by, it is easy to hear a dolphin’s ‘blow’ at the surface in fact you will often hear a dolphin before you see him! The blow is the sound you hear, and the spray of water you see, when the dolphin forcefully breathes out and clears away any water resting on top of his blowhole. The water spray is not coming from the dolphin’s lungs it is just water sitting on top of its head around the blowhole being blown away before he inhales.

Dolphins do not breathe through their mouths in the same way as people can, they only breathe through their blowholes. In this way, breathing and eating are kept entirely separate in dolphins so that they can capture prey in their mouths and swallow it without the risk water getting into their lungs.

Dolphins are able to hold their breath for several minutes but typically they breathe about 4 or 5 times every minute.

Until recently it was thought that dolphins could not breathe through their mouths in the same way as people can, only through their blowholes. However, in 2016 scientists discovered a New Zealand dolphin with a damaged blowhole who had learnt to breathe through his or her mouth.

Dolphins are able to hold their breath for several minutes but typically they breathe about 4 or 5 times every minute.

Deep-diving whales such as sperm whales or Cuvier's beaked whales may go well over an hour between breathes. The record is held by a Cuvier's beaked whale that dived for 137 minutes (well over two hours!).

They have high amounts of haemoglobin and myoglobin to store oxygen in their blood and muscles, can reduce their heart rate and even shut down some organs when they dive to help them survive in the deep.



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