When your total earnings reach 25,000g Demetrius will come by and tell you he has been doing research. He'll then give you the options if you want bats to leave fruits in your cave or to have mushrooms randomly grow in your cave. This choice is permanent and cannot be changed in game.
Fruit Bats
Fruitbats leave various fruits in the Cave on your farm. This is a great option if you are large into making Artisan Goods as it gives you a slow but steady supply of fruit even out of season. This is also helps speed up finishing the Artisan Bundle in the first year or two before you have enough money to buy the all the Fruit Trees necessary for the bundle. Though sadly, there seem to be no other use for the fruits outside of making Artisan Goods, Shipping them, finishing the Artisan Bundle, or eating them for extra energy as they don't replenish very much health.
Mushrooms
Six plots will be added to the cave where certain types of mushrooms will randomly grow. Five different types will appear, including all four that are required to craft the Life Elixir. Even though Common Mushrooms, Morels, and Chanterelles, are the ones that most commonly appear, this is still the best way to craft the elixir as often as possible. This makes the Mushroom option a great one for those who love to go into The Mines, especially since most mushrooms can be hard to find otherwise. This is also a good choice for players that have grown, or plan to grow, fruit trees and do not need the extra fruit provided by the fruit bats.