It is mostly about the degree of likelyhood implied regarding the content of the subordinate clause.
「としたら」 would generally express a lower degree of likelihood in the eyes of the speaker than 「たら」 or 「なら」 would. The difference, however, is often fairly subtle in actual usage for many speakers.
You could always lower the degree of likelyhood in question by adding 「もし」 to any one of the aforementioned expressions.
Thus, your first pair of sentences have nearly the same meaning. As I have explained, the speaker of the first sentence would be feeling that what he has heard is less likely to be true than the speaker of the second sentence would be feeling.
The same can be said about your second pair of sentences. You would say the first sentence in a place where one would be less likely to encounter a lion than you would say the second.