What Are the Advantages of Bilateral Symmetry in Animals?

Butterfly Credit: Garoch/Pixabay

While scientists are still trying to determine the different ways that bilateral symmetry in animals is advantageous, the most obvious benefit is that it enhances an animal’s mobility. It allows fluid movement in a particular direction, which helps when an animal is searching for food or trying to escape danger.

All vertebrates have bilateral symmetry as well as some invertebrates. Bilateral symmetry refers to both sides of the body being a mirror image of each other when viewed across a central axis. On either side of the central axis, the arrangement of an animal’s body parts is the same. This only applies to the body shape, not the internal organs.

What Is a Sagittal Plane?

To explain bilateral symmetry, an imaginary line can be drawn along the length of an animal. Starting at the tip of its nose and ending at the tip of the tail, this line splits the body into two halves. The left side and right side mirror each other, and the line that divides these halves is called the sagittal plane. In most animals, the sagittal plane runs horizontally. However, in humans, it runs vertically because of our upright orientation.

Despite this left and right symmetry, the two halves aren’t necessarily identical. For example, a whale might have a slightly bigger flipper on one side than on the other.

What Are the Main Advantages of Bilateral Symmetry?

Animals with bilateral symmetry have greater motion capabilities in a particular direction. Their bodies are also streamlined, which is very beneficial for some animals, such as marine animals, as it allows for motion at a faster speed. 

The sensory organs are all clustered in the head, which is called cephalization. As forward motion is used to travel, this means an animal’s head meets and reacts to stimuli before the rest of the body. As a result, the hearing and eyesight of animals with bilateral symmetry are more enhanced than in animals displaying radial symmetry.

What Other Characteristics Are There?

Animals with bilateral symmetry all have a head (anterior) and a tail (posterior) area, a left and right side, and a top (dorsal) and bottom (ventral). The majority have a central nervous system with a complex brain and facial features that include a mouth and two eyes. A benefit of having a separate tail and head area is that food is consumed at one end of the body and waste is excreted at the other end.

Bilateral symmetry helps animals move easily in a forward direction and helps animals keep their balance. For example, a lioness with four normal legs can run and hunt efficiently whereas one that has been injured and has a damaged paw or limb is at a disadvantage when trying to do either activity. 

Examples of Animals With Bilateral Symmetry

Bilateral symmetry, which is also called plane symmetry, exists in 99 percent of animals. Most phyla are included in this, such as Arthropoda, Annelida, Chordata, Nematoda, Platyzoa, and the majority of Mollusca. 

Therefore, as well as humans and marine mammals, such as whales, examples of animals with bilateral symmetry include cats, dogs, mice, bears, elephants, crabs, wolves, frogs, birds, giraffes, and badgers.

Butterflies have exceptional bilateral symmetry. As well as having a symmetrical body shape, the patterns on their wings are almost identical.   

What Is Radial Symmetry?

The other main type of symmetry in animals is called radial symmetry. This means an animal has numerous axes passing through the center of the body with equal halves displayed along any of the axes. They can move in any direction as opposed to the forward movement favored by animals with bilateral symmetry. A starfish has radial symmetry, as does a jellyfish.

These animals move slowly and have sensory organs dotted around rather than fixed in a central nervous system. They have a top and a bottom as well as an oral side (including a mouth) and an aboral side (without a mouth).

Animals with radial symmetry can regenerate parts of their bodies. For example, if a sea star loses an arm, it can grow a new one.

Exceptions to Bilateral and Radial Symmetry

Some animals have biradial symmetry or pentaradial symmetry. Biradial combines both bilateral and radial symmetry. The body has four sections across a central axis, and each of these is the same as the one opposite it but different from the one next to it. Pentaradial has five sections, which are all equal, such as in sea urchins and sand dollars. Echinoderms are special because they start life with bilateral symmetry then switch to pentaradial symmetry as adults. Some animals don’t fall into any of these categories as they are asymmetrical, such as sponges.