The Animals with Largest & Smallest Brain Size (by Class)

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It’s just fun to know what’s the largest reptile or fish in the world. And it’s even more entertaining to learn about the animal that has the largest brain size.

We wanted here to provide complete, definite information regarding the largest and smallest brain size (and brain-body mass ratio) in each animal class. To do that, we have researched relevant datasets in many scientific journals online, as referenced at the end of article.

Before moving further, please keep in mind this article is exclusively about vertebrates which includes mammals, birds, fish, reptiles and amphibians.

We haven’t looked into any invertebrates because there are far too many species. According to National Geographic, over one million species have been described most of which are insects. In other words, there would never be a big enough data sample to be representative.

With that said, following are the research results.

Animals with Largest Brain Size

Here is the list of animals with largest brain weights by class:

Animal ClassLargest Brain Size
AnimalBrain Size (g)
MammalsSperm whale8200
BirdsEmperor penguin48
FishGiant manta ray122
ReptilesAmerican crocodile16

As you probably know, the largest brain among all animals on earth belongs to the sperm whale (Physeter macrocephalus) weighing over 8kg.

emperor penguin with its little baby

Unlike what many people mistakenly believe, the ostrich does not have the biggest brain among birds. That title goes to a giant species of penguin called the emperor penguin (Aptenodytes forsteri). The brain size of this animal is 48g versus the ostrich’s brain of 42g.

Until now, most people probably know that the largest brain in the reptile world belongs to crocodiles but we didn’t know to which species exactly. Our research revealed that the American crocodile (Crocodylus acutus) has the largest brain size among reptiles. This crocodile species has a brain that’s about 2g heavier than the next reptile.

The giant manta ray (Mobula birostris) has the largest brain size across all fish. It’s no wonder that manta rays are the most intelligent fish out there.

It’s worth noting that with the exception of emperor penguin, all animals on this list are one of the largest of their classes.

Animals with Smallest Brain Size

This is the name of mammal and bird with smallest brain weights:

Animal ClassSmallest Brain Size
AnimalBrain Size (mg)
MammalsEtruscan shrew62
BirdsBee hummingbird<130

As widely accepted, the smallest brain among mammals belongs to the tiny Etruscan shrew (Suncus etruscus) where it’s only 62mg. This animal is actually the smallest mammal in the world.

Based on the available data, the bird species with smallest brain size are all hummingbirds, which is expected.

The hummingbird species with the absolute smallest brain appears to be vervain hummingbird where the weight is 130mg. But since this is the second smallest bird in the world, we would assume that the smallest bird ever, bee hummingbird (Mellisuga helenae), has the smallest brain size overall and it will probably be less than 130mg.

Animals with Smallest Brain-Body Mass Ratio

These are the animals with smallest brain-body ratio by class:

Animal ClassSmallest Brain-Body Mass Ratio
AnimalBrain Ratio
MammalsBlue whale0.0032% (1:31282)
BirdsOstrich0.034% (1:2983)
FishWhale shark0.0026% (1:39182)
ReptilesAmerican alligator0.0065% (1:15446)

The huge blue whale (Balaenoptera musculus) ironically has the smallest brain-body mass ratio among all mammals. When it comes to birds, the ostrich (Struthio camelus) holds the title of having the lowest brain-to-body weight ratio. The reptile with smallest brain-body ratio is interestingly the American alligator (Alligator mississippiensis).

Many people may not know that the whale shark (Rhincodon typus) has the lowest brain-to-body mass ratio among fish and even all vertebrates.

whale shark swimming in the sea

It’s wrongly accepted that bony eared assfish has the smallest brain-body weight ratio among vertebrates, which is certainly not true. The whale shark has the smallest brain-to-body ratio at 0.0026% (1:39182) versus 0.027% (1:3704) for bony eared assfish. Even the blue whale and American alligator have a far lower brain ratio than this bony fish species.

We also want to note that all animals on this list are the largest of their kind excepting the American alligator which is still one of the largest. This follows the trend that the bigger the animal gets, the smaller the brain-to-body mass ratio is (based on Wikipedia).

Research Method

Here is how we performed this comprehensive research:

  1. We extracted the brain weights and body weights of over 7,700 different animal species (vertebrates) from datasets of 22 scientific journals.
  2. We used that data to calculate the brain-body mass ratio of each animal species record.
  3. For each animal class, we sorted the data in order to find the animal species with the largest/smallest brain size and largest/smallest brain-body mass ratio.

To be more certain about the results, we considered only animal species that have at least two consistent data records (if not more).

For each animal species added to our lists here, we looked at all its data records available then included the highest or lowest value depending on the list (i.e. highest number for largest brain size/ratio and vice versa).

Please note the number of 7,700 mentioned above doesn’t account for the number of data records per animal species, so the total amount of data available is much higher.

As you may have noticed, we didn’t include a result for some of the animal classes. Because there wasn’t enough data records or the numbers were too close to make a definitive decision. This applies to the largest brain-body mass ratio for all animal classes, the smallest brain size for birds/fish, in addition to the amphibians in all lists.


There you have it, the vertebrate animals with largest and smallest brain size by class and also those with smallest brain-body weight ratio by class.

Here is a summary of the most interesting findings:

  • The bird with largest brain size is the emperor penguin (not the ostrich).
  • The crocodile (and reptile) with largest brain size is the American crocodile.
  • The animal with smallest brain-body mass ratio among vertebrates is the whale shark (not bony eared assfish).
  • The reptile with smallest brain-body mass ratio is the American alligator.

We hope that you have learned something new and interesting about animal’s brain size here.

Don’t forget to share the article with others so they can learn this new information.

In case you want to learn more about brain sizes, you can check out our article about brain weights of animals.