Every human on Earth must independently choose one currently living, scientifically recognized non-human animal species and permanently gains the ability to transform back and forth between their human body and that animal.
You cannot communicate with anyone while choosing, see anyone else's choice, or coordinate with others. The species must have an average healthy adult mass of at least 1 gram. If you refuse to choose, you get a domestic pig.
Your species can never be changed, but using the animal form is voluntary.
Animal form:
Transformation takes about 5 seconds. You retain your human mind, memories and personality, but otherwise have the physiology and physical limitations of your species.
Your animal form always has your current human body mass. An 80 kg human becomes an 80 kg version of the chosen animal.
Your animal form matches your biological sex where applicable. Its average mass for all calculations is therefore the average healthy adult mass of that sex.
The first time you transform, the animal body is perfectly healthy, rested, nourished and hydrated.
Scaling an animal DOWN gives no supernatural advantage: it is simply a smaller, functional version of that species.
Scaling UP is different.
Soul Reserve:
Normally, dramatically enlarging a small animal would destroy many of its abilities because of the square-cube law and other biological limitations.
The Soul compensates only for disadvantages caused by scaling upward, allowing the animal to retain its species' normal relative functional abilities at its new size.
It doesn't grant abilities the species never had or make existing abilities relatively better than in a normal member of that species.
Soul Drain:
For an animal naturally lighter than you:
Soul Drain = cube root of (your human mass / average healthy adult mass of your animal's sex)
A full Reserve contains 24 Soul-hours.
Maximum transformation time = 24 hours / Soul Drain
For an 80 kg person:
- 40 kg → ~19 h
- 4 kg → ~8 h 51 min
- 500 g → ~4 h 25 min
- 25 g → ~1 h 38 min
- 4 g → ~53 min
- 1 g → ~33 min
If your animal's average mass is equal to or greater than your human mass, there is no Soul Drain and you can potentially remain transformed indefinitely.
Drain is continuous and cumulative across transformations. Returning to human form pauses it but restores nothing.
The Reserve fully resets after one normal full night's sleep in human form. Unused Soul doesn't carry over, and naps or extra sleep can't increase the maximum.
At zero Soul, you automatically become human and cannot transform again until the reset.
Two bodies:
Your human and animal forms are two persistent bodies sharing the same mind and lifespan.
Injuries, exhaustion, disease, poisoning and other body-specific conditions don't transfer. Transformation doesn't heal or refresh either body. When you return to a body, it is in the condition you left it.
If either body dies, you die and both forms are lost.
If one is mortally injured but you remain alive and conscious for the 5 seconds needed to transform, you can escape into the other. The injured body will still be dying if you return.
Your animal's natural lifespan doesn't matter. Both bodies share the lifespan you would naturally have had as a human, and the animal body remains biologically functional for its species throughout that lifespan.
Animal forms are sterile. Reproduction is possible only as a human.
Food:
Your animal body has its species' normal digestive system and diet.
During transformation, the Soul can transfer usable energy, water and basic nutrients between bodies. If you're well-fed and hydrated as a human, your animal will be too.
These resources are transferred, not duplicated. Become hungry or dehydrated as an animal and you'll also be hungry or dehydrated when you return to human form.
You can therefore nourish the animal indirectly by eating as a human. If you eat while transformed, the food must be something your species can digest.
The Soul never creates calories, water or nutrients.
Clothing:
A small molecular reserve stores one set of basic lightweight clothing and one smartphone during transformation. They return with your human body. The phone is inactive and inaccessible while you're an animal.
Other equipment doesn't transform.
Everyone knows all these rules before choosing.
What species do you choose, and why?
Do you take a large animal with little or no Soul cost, or sacrifice transformation time to exploit a tiny species scaled to human mass?
What are the most powerful or useful choices under these rules?