Gratuities are entirely optional and always greatly appreciated.
Traditionally, tips reflect a guest's satisfaction with the capabilities of their guides or chefs during the trip. Gratuities may be offered by individual guests, or organized as a "group tip" at the end of the tour.
Gratuities are not required. There is always another way for guests to express their gratitude; guides and chefs certainly appreciate hearing a heartfelt thank you from satisfied guests.