Is the following sentence missing an apostrophe? If so, where?: The gazelles biggest threat is from a lions hunger.
Yes. There should be an apostrophe in this sentence.
There should be apostrophes to indicate the Saxon Genetive form of the noun 'gazelle' and the noun 'lion'. They could be placed in 2 possible ways.
One of the possibilities is: The gazelle
In this sentence both nouns are singular and are treated as a general name.
The other possibility is: The gazelle
Here both nouns are plural and can be treated as the description of a specified group of animals.