This isn't a question of grammar, but of meaning. The choice is the speaker's.
Ich mag die Blumen, weil sie gelb sind.
I like flowers when they are yellow and these flowers are yellow; it follows that I like them.
Ich mag die Blumen, obwohl sie gelb sind.
I like flowers, but not when they're yellow; these flowers are yellow, yet I like them nonetheless.
An easy way to think about this is that obwohl indicates the violation of some kind of expectation (for instance, expecting not to like yellow flowers).
Given your example, I assume obwohl was intended. If a creature has a very long neck one might expect it to be able to drink from a pond without lowering itself to the ground. However, in the case of the giraffe, that expectation is violated: they do need to get lower to the ground by spreading their legs in order to drink.