Humans cannot digest cellulose, but rabbits can digest cellulose because

Mammals cannot digest cellulose because they lack the enzyme cellulase essential for breaking the beta-acetyl linkages. Animals rely on symbiotic microbes for the digestion of cellulose. Rabbit is a hindgut fermenter and has symbiotic bacteria that digest cellulose in the caecum. Ruminant mammals are foregut fermenters in which symbiotic bacteria that digest cellulose occur in the compound stomach. Termites have symbiotic protozoans (e.g., Trichonympha) in their gut which digest cellulose.