Having pets is usually considered an entirely human endeavor. However, this notion isn’t strictly true. While humans have been domesticating dogs, cats, and livestock for many centuries, our animal cousins have mostly preferred the company of their own species. There are many thousands of examples of relationships between animals, and in some cases, one species would not be able to survive without the other. Let's take a tour into the strange world of mutualistic and symbiotic relationships.

What Is Biological Mutualism?

Mutualism is a type of symbiotic relationship where both organisms benefit. Mutualism isn't a conscious, altruistic process, but rather both species act to fulfill their own selfish needs, and both benefit while no harm is caused.

One of the most interesting examples of a mutualistic relationship is between a species of burrowing Tarantula (Xenesthis immanis) and the Dotted humming frog (Chiasmocleis ventrimaculata), in Peru. This giant spider species is capable of swiftly killing and eating the frog, but it wouldn't. Instead, the spider would let the frog live right next to it. But why? Scientists determined that the frog benefits because it eats the leftover meal scraps from the spider. It also gains protection from predators that don't want to risk becoming the spider's next meal. However, the spider also benefits. The frog specializes in eating ants, and ants are one of the primary predators of spider eggs.

But how does the spider tell the difference between this species of frog and the ones it would normally eat? Scientists believe it's down to the chemicals on the frog's skin. Young burrowing tarantulas have been observed picking up the frogs and examining them with their mouths before putting them down.^1

Is Mutualism the Same As Symbiosis?

No. Some symbiotic relationships can be mutualistic, but not all. So, what's the difference? A symbiotic relationship is a relationship between two organisms, and this relationship can take many forms. For example, it could be beneficial (where both organisms benefit), commensal (one organism benefits), or parasitic (one organism benefits, but the other is harmed). We’ve already looked at mutualistic relationships, but let’s now look at a few examples of parasitic and commensal relationships.^2


Parasitic Wasps

Many species of parasitic wasps exist, but one of the most extreme is a species from the genus Glyptapanteles. This species injects its eggs into caterpillars. To grow, the larvae feed on the caterpillar's body fluids, essentially eating it from the inside out. Eventually, the wasps hatch, breaking through the caterpillar's skin. However, they're not done yet. These infant wasps still need to finish maturing, and to do this; they need protection. Alarmingly, the caterpillar provides this protection, guarding the infant wasps until it starves to death. To summarize, this wasp species has evolved to control caterpillar's minds and turn them into slaves.^3

Ticks and Leeches

Other common examples of parasitic relationships include ticks and leeches. While these creatures don't kill the host directly, they carry many pathogens that can severely sick the host. Some of these pathogens can be fatal. Ticks and leeches both feed off the blood of larger animals to survive.^4


In commensal relationships, only one organism benefits, but the other organism is left unharmed.

  • Cattle egrets - A bird species that eat insects and worms brought to the surface by grazing cattle. The cow's hooves disturb the ground, bringing insects to the surface, where the birds can feed. The cows are unaffected.
  • Nurse plants - These are trees and larger plants that offer protection to seedlings and other smaller plants. Due to their size, they ward off grazing herbivores and provide shelter against extreme weather.^5