I believe that there is a contract formed the same way it is with other summoning animals. The same ritual is performed in order to summon it after all. Of course the contract is done unwillingly, but with the MS's powers, it is quite easy to form it on the spot. Rather, MS's properties seem to allow to form a summoning contract without need of the blood contract. Even Sasuke formed a contract with a hawk, by seemingly only using his MS (hax ). Perhaps it is then done on spiritual level, by sharing a bit of their spiritual energies, rather than through blood, which seems to have rather physical aspects. This might suggest that it was the Elder Brother's or his descendants that first formed summoning contracts with animals. This isn't surprising at all, because it's another watered down power of the Rinnegan's Animal Path, that Sharingan/MS inherited.