Can Beast Boy mate with any animal? He can fully transform into any animal, with the exception of having with green skin, scales, feathers, hair, etc. Is it possible for him to mate with an animal of the opposite sex?
Technically, the mechanism for Beast Boy's transformative DNA has not ever been truly explained or explored. Is it magic? Is it science? Is it nanotech? We are never truly aware of the capacity of his DNA. Whether he would be able to create viable offspring due to the unusual nature of his DNA has never been determined.
Beast Boy has the ability to morph and transform into any animal that he has seen himself or has seen in an illustration (as is the case when he shapeshifts into an animal such as an extinct dinosaur).
These transformations take only a second and Beast Boy has demonstrated that he is capable of rapidly changing his form with little or no effort expended. As a corollary to this power, he can transform back into his default form if an external force like magic transforms him into an animal.
We know his power can be affected by magic, it can be augmented by technology, and he can add alien DNA to his transformative matrix, but the underlying function has never been explicitly explained. We know he was a human who acquired this ability either through natural mutation or through induced mutation (early origins).
- His power allows him to genetically replicate any animal in question. He is able to replicate this animal without any knowledge of this creature's DNA which implies he has the capacity to spontaneously replicate the DNA by touch, by sight, and his body automatically makes his body into a replica of the innate DNA held within his body.
While an animal, he gains all of the physical abilities, attributes, and characteristics of said animal, such as greatly enhanced superhuman strength (a gorilla), superhuman speed (a cheetah), and enhanced durability (a turtle), and abilities such as flight (various birds), and aquatic breathing (various fish).
He can even gain the specific poisons and toxins produced by specific snakes. He also gains the ability to survive or otherwise live in the same hostile environments some animals can, as seen in one comic where he morphs into a parasitic worm to become immune to the stomach acid and the immune system of Brother Blood when he dived into the villain's intestinal tract to retrieve Raven's chakra.
While in animal form, Beast Boy retains his human intellect, memories, and the ability to speak.
- His ability however, allows him to replicate an animal but overlaying his intellect in place of the instinctive nature of the animal in question. The question should be WOULD he want to be able to mate with animals he is imitating. He would have none of the instinctive clues necessary to attract the attention of the members of the species, should he NEED to mate with those animals. Especially if they are not Earthborn animals. He would have absolutely no frame of instinctual reference.
- Beast Boy's transformations have not been limited to Earth life forms. When on a rescue mission in the Vegan system to free Starfire, he easily transformed into a Gordanian, one of the green-skinned alien warriors native to the system, in order to sneak Robin and Cyborg onto a warship.
In the "Who is Wonder Girl?" story arc, he was able to infiltrate another alien world after being shown a green-skinned creature native to the planet; he had some difficulty with this change, claiming that Earth-animal forms he knew instinctively, but was successful in the transformation.
When he pursued Madame Rouge in revenge for her role in killing his foster mother, Rita Farr, he began transforming into creatures that do not even exist, products of his subconscious rage against the villainess.
- And more importantly, would animals WANT to mate with him, given his scent profile would still have trace elements of humanity around him, on him from his interactions with other humans. Given the ferocity of some animal mating rituals, he might find himself in danger due to his lack of animal instincts necessary for this particular species.
As of late, he has demonstrated the ability to transform into animals many times larger than their actual forms. Among others, he's been seen transforming into a human-sized tarantula and a giant snake.
Whether this is a more advanced progression of his abilities or a new ability altogether is unknown. Like his Kingdom Come incarnation, he also recently gained the ability to transform into mythological creatures, namely a phoenix and a chimera.
Recently he has also shown the ability to partially transform into an animal, such as acquiring octopus tentacles on the lower half of his body, while his upper-torso remains unchanged.
These recent transformation may indicate his power may have more than a genetic aspect to them and perhaps in the Rebirth of the DC Universe, we may get a more complete understanding of how his power works.
In Summary: I would suspect he might be physically able to mate, but given the genetic variability of his DNA and mutability of his flesh, he would probably not produce viable offspring.
When Beast Boy changes form, all of his cells presumably change to that form. The first question is whether his cells retain their form when separated from the rest of him. If so, are his genetics when transformed compatible with the animal he is imitating, and viable?
Assuming the answers to the questions above are "yes", for him to conceive with an animal, he would have to remain in that form, at least until his original contribution is fully assimilated into the offspring. It is then a question of whether his greeness, retention of his intellect and identity and speech in animal form, and ability to shape shift are heritable. I don't believe that any of this has been established in-universe.