...and that's even before the first migrant wave! Minding the Shaft of Enlightenment glitch, I now have 40% of my fortress population legendary. Sweet. Here's what happened:
Before I started the fort, I made an adventurer to create an emergency heap of some items, mostly for early moods: giraffe horns, dragon bones, ☼adamantine long swords☼ and so on. Among them was some fat from a roc I killed. I got greedy and decided to multiply it, using the old liquid cloning trick - in this case, I melted the fat by igniting a nearby tree. (!!Science!!, yeah.) A minecart borrowed from preparation fortress held a total of 833 roc grease, enough for some small soap tower I guess. I decided against making more.
So now I just got through the process of processing my starting supplies and re-claiming the pile left by the adventurer (for some reason the candy swords got this way are unequippable by military until you run them through some job like encrusting or melting - it doesn't have to be completed though) and what's left was that pile of fat. About time to do something with it lest it rots. My cook painfully slowly carried the glob to the kitchen, then I prepared to wait forever until he is done with it. Surprisingly it took no time at all, and I got an announcement stating that Unib Alathumstiz has become a Cook. But wasn't he already? Actually, he was both Proficient in Cook and Tanner, so I presumed he got 6th level at last? Then I notice he's flashing. Then I open DT and see he's Legendary +5. Then I flip out.
Someday, in the center of newly founded dwarven outpost there will be a temple made exclusively of roc soap. In the center of it there will be kitchen of Unib Boltsizzle, the Cook, the Enlightened by the spirit of great bird. And no one, not a lowly hauler in this settlement will have less than ecstatic thoughts form a meal they last ate. But it's not for now. Today we must carve bedrooms and produce something for our first caraven to trade. ...Maybe it will be prepared meals.
tl;dr: Rendering huge stacks of fat quickly gives equally huge amounts of experience. Breeding giant sperm whales suddenly has one more benefit.