harry potter - Why did Sirius succumb to the Dementors without resistence?

In the Prisoner of Azkaban, Sirius in his Animagus form, after driving away Lupin in his werewolf form, goes after Pettigrew who escaped in his Animagus form in the meantime. But Harry finds him shortly thereafter turned back into human form and crouching with his hands over his head, all apparently because of the Dementors swarming around.

Why did Sirius not fight the Dementors? Did he not have a Patronus? Sirius is shown in the series to be, along with James Potter, an extremely talented wizard. If James and Lupin had their own corporeal Patronuses (or Patroni?) why did Sirius not master the charm?

Just before his transformation, Sirius had been using Snape's wand. It has been answered previously in this site that Animagi could retain their (clothes and hence) wands in animal form. It also seems to be a consensus that Pettigrew retained Voldemort's wand in Animagus form. Thus, it was definitely possible for Sirius to have retained Snape's wand when he transformed.

Moreover, he was forced out of his animal form, a not so easy feat, which in case of Pettigrew a little earlier, had required the combined magical might of Sirius and Lupin. It cannot be that Dementors always have this prowess, since Sirius is the man who escaped Azkaban using his Animagus form.

Of course, there were a lot of Dementors there, but there were Harry and Hermione in the scene too, and even the latter who was untrained in the matter didn't pass away as easily or as soon. And Sirius is the man who is known to have defied the Dementors right inside Azkaban.

Answer

I don’t know a canon answer, but I can come up with some plausible explanations:

1. 
   

   Casting a Patronus was probably impossible for Sirius.

   
   

   Remember that he’s spent the last twelve-or-so years in Azkaban, without a wand, and without casting spells with wands. The Patronus (even a wispy one) is a fairly advanced piece of magic, and Sirius would probably need a full corporeal Patronus to drive away that many Dementors.

   
   
   

   It’s unlikely that he could have conjured that quality of Patronus after such a long dry spell.

   
   

   (This is, of course, assuming that Sirius even knows how to produce a Patronus. As far as I know, there’s no canon evidence that Sirius could perform the Patronus charm.)

   
   


2. 
   

   He’s at a particularly low ebb.

   
   

   For the first time in thirteen years, he’s had the prospect of redemption and freedom. Now it’s been snatched away, because Pettigrew escaped (again). That would be a fairly miserable state to be in, making you more susceptible to the effect of the Dementors, and less able to conjure a Patronus (see point 1).

   
   


3. 
   

   He’s never had to resist a full-blown attack while in animal form.

   
   

   The Dementors in Azkaban are just guards, not executioners. It’s unlikely they would have attempted to perform the Dementor’s Kiss on an inmate, unless they were a threat to the security of the prison. Regular Dementor attacks would probably be frowned upon by the Ministry (who consider themselves the only people with the authority to sanction such attacks).

   
   
   

   Perhaps the bond between soul and body is weaker when in your animal form? There are wizards and witches who forget themselves if they spend too long in their animal form. Maybe it’s easier to defend yourself as a human? (Forgetting that he’s probably unable to cast a Patronus.)