Do chanting mantras, devoting merit etc. to animals actually help & how do we know that?

I always hear of Buddhists encouraging people to chant mantras, dedicate merits, pray etc. for animals & pets so that they may be exposed to the dharma and take a better rebirth. Also, chanting for dead animals; the insect you unintentionally killed, the carcass on the dinner table, pets who have passed on etc. How do we know that? Do animals have the capacity to comprehend dharma?