Both articles are acceptable, but it depends on the statements being made. In the context of a paragraph about tigers in general, "the tiger" refers to the specific species in comparison to other animal species, and "a tiger" refers to any example of a single tiger.
If your statements apply to the species as a whole, then use "the tiger." For example, "the tiger is the largest cat species." This statement is not necessarily true for all individual tigers, because tiger cubs are smaller than adult cheetahs, so "a tiger is the largest cat" does not work.
If your statements apply to all individual tigers, then you can use "a tiger" or "the tiger". For example, "a tiger is a large striped cat" or "the tiger is a large striped cat": both statements are true.
You could also say "tigers" to refer to all tigers in general, as in "tigers are very large striped cats."