Which pair correctly describes obligate vs facultative symbiosis in animals?

Which pair correctly describes obligate vs facultative symbiosis in animals?

• A. Obligate: termites rely on gut microbes to digest cellulose, with trophallaxis transferring microbes; Facultative: ants tending aphids for honeydew, which alters foraging and movement patterns.
• B. Obligate: termites rely on gut microbes to digest cellulose, with social transfer of microbes between individuals; Facultative: ants tending aphids for honeydew, which alters foraging and movement patterns.
• C. Obligate: birds relying on seed dispersal; Facultative: fish cleaning symbioses.
• D. Obligate: dogs and fleas; Facultative: bees and flowers.

Answer: (D)

In symbiotic relationships, obligate means one (or both) partners cannot survive or reproduce without the other, while facultative means the association is beneficial but not essential—each can live independently if needed. Dogs and fleas fit the idea of obligate in the sense that fleas are obligate parasites on mammals; they depend on a host to obtain blood and to complete their development. The host (the dog) isn’t dependent on the fleas for survival, but the fleas cannot persist without a host. Bees and flowers, by contrast, exemplify a facultative mutualism: bees rely on floral nectar and pollen, but they can and do forage on many plants, and flowers can be Pollinated by other animals or by non-biological means. The interaction boosts both parties but is not strictly required for either to survive. Other options either describe relationships that aren’t clearly obligate (like many seed-dispersal or cleaning interactions) or pair an obligate host–parasite example with a non-obligate partner, making them less precise for the labels.

In symbiotic relationships, obligate means one (or both) partners cannot survive or reproduce without the other, while facultative means the association is beneficial but not essential—each can live independently if needed.

Dogs and fleas fit the idea of obligate in the sense that fleas are obligate parasites on mammals; they depend on a host to obtain blood and to complete their development. The host (the dog) isn’t dependent on the fleas for survival, but the fleas cannot persist without a host. Bees and flowers, by contrast, exemplify a facultative mutualism: bees rely on floral nectar and pollen, but they can and do forage on many plants, and flowers can be Pollinated by other animals or by non-biological means. The interaction boosts both parties but is not strictly required for either to survive.

Other options either describe relationships that aren’t clearly obligate (like many seed-dispersal or cleaning interactions) or pair an obligate host–parasite example with a non-obligate partner, making them less precise for the labels.