Are humans special in that they collapse wave functions?

that all particles exist as clouds

This is a miss-representation of the mathematics that describes quantum mechanically elementary particles and their composites.

Look at the orbitals of the hydrogen atom:

If one is not careful to understand the postulates and mathematics of quantum mechanics one ends up with particles as clouds, because of the above pictures.

What the image conveys is that each point is a probable point for findng an electron if one studies an excited hydrogen atom. Quantum mechanics predicts probabilities to find a particle at (x,y,z,t), not trajectories or orbits.

when humans are not looking but exist as particles when humans look at them

Any interaction with a particle "finds it", according to the probabilities that its wavefunction $Ψ$ dictates. The probability is calculated by $Ψ^*Ψ$, and particles interact with other particles according to the theory. So humans are not necessary as observers to get interactions and "collapse" the wavefunction into another wavefunction (that is what collapse means, different conditions impose different wavefunctions).