What is the weakest animal in the world?

Do you mean in absolute terms or in relative terms? In other words, do you want to know the weakest animal pound for pound?

Let me put it yet another way. I can lift more than an ant even on my worst and laziest day. But pound for pound, I cannot come close to lifting as much as an ant does. An ant can lift many many times its own body weight, but I can hardly lift a 12-oz beer to my lips some days.

Okay, I never have any trouble lifting a beer to my lips, but you get the point.

Come to think of it, pound for pound, human beings aren't that strong. We rely mostly on our superior brain power to survive.

Which, for some of us, is not very good news.

Because all animals use similar molecules in their muscles, the muscles of all animals is (very roughly) the same strength in terms of force per cross sectional area.

However, an animal twice as long typically has muscles roughly four times the cross sectional area (resulting in 4 times the force), but a body weight roughly eight times as much.

Because of that power law scaling, the bigger an animal is, the more absolute force its muscles can produce, but the less of that force can be applied to the outside world -- more of that force is required just to move the muscle itself around.

And so the animals that can lift the most weight relative to the total weight of the animal are generally the smallest animals -- ants, fleas, etc.

The single animals that can move the biggest weights (absolute) strength are generally the largest single animals -- elephants, Clydesdale horses, etc.

The weakest animal would be bacteria because they have no form of defense and can be killed easily.

However, most people put "bacteria" in its own category, neither animal nor plant.

There are a few species in the family "Animalia" that have no muscles: the sponges and Placozoa.

Some people say that Humans are the weakest animals in the world but think about ants, cats, hamsters, stuff like that . So really there's no right answer.