Google now uses stemming technology. Thus, when appropriate, it will search not only for your search terms, but also for words that are similar to some or all of those terms. If you search for "pet lemur dietary needs", Google will also search for "pet lemur diet needs", and other related variations of your terms. Any variants of your terms that were searched for will be highlighted in the snippet of text accompanying each result.
This is taken from the Google Search Tips. Until recently it said quite the reverse. That's a pretty fundamental change to bring in without a fanfare!