In Life of Pi, what makes the animal story more likely to be the truth about how it really played out on the lifeboat?I personally belive that the...

This is an interesting story, because the animal one, when compared to the human one, becomes, probably the less likely scenario, and rather something that Pi concocted in his mind to survive the harsh realities he had to endure.  The "dry yeastless factuality" of the human story is also incredibly brutal and devastating; refashioning it into animals makes that reality something Pi can cope with without cracking or suffering major repercussions.

To argue FOR the animal story, take into account the thing that makes it so unbelievable:  because it is such a fantastic story, and so out there, it is probably the true one.  How could Pi fashion such a story, with such incredible detail, out of thin air?  Because it is so bizarre (consider the acidic island with lemur-like creatures, for example), it has to be true.  It's like the saying, "You can't make that stuff up."  Also, Pi was very well equipped to deal with the animal scenario--his background in zoology made surviving with a tiger on a lifeboat a very plausible scenario.  He had learned quite a bit at the zoo about alpha males, territories, training and tigers, and he put all of that to use.  If anyone was going to survive, it was going to be him.  He describes such factual clarity and detail on his journey, that is easy to believe that it actually happened.  There would have been stranded animals, after all.  Pi even has explanations for why Richard Parker left, and how he could still be around even though there wasn't evidence of him.

As far as which story you believe goes, Yann Martel himself leaves it up to the reader.  In interviews, he has said that which ending you pick

"will often reflect how they look upon their lives, that people who are more able to make leaps of faith will believe the first story."

So, Martel contributes an ability to have faith and belief to the ability to believe the animal story.  You can go back and forth, debating the possibilities of each ending, and both have sound arguments.  What it really boils down to in the end is which one you want to believe, and whether your belief in the power of the story is enough to sustain you through the animal story.  I hope that those thoughts helped; good luck!