Learn Spanish (Latin America): La cría volverá a la selva cuando pueda sobrevivir sola. - The baby animal will return to the jungle when it can survive alone.

Because the sentence talks about the future. When we use time words like cuando (when) for an event that hasn't happened yet, Spanish requires the present subjunctive, which is pueda here. If this were a routine ('it always returns when it can'), we would use the indicative puede.

Can I use la cría to talk about a human baby?

Generally, no. La cría refers to a baby animal, a hatchling, or a litter of offspring. If you are talking about a human baby, you should use words like el bebé or el nene / la nena.

Why is it sola with an 'a' at the end? What if the baby animal is biologically male?

The adjective sola has to match the grammatical gender of the noun it refers to. Because la cría is always a feminine noun, any adjective describing it must also be feminine (sola), no matter the biological sex of the actual animal.

Why is there an a after volverá?

The verb volver means to return. When you return to a destination, Spanish uses the preposition a (to) to connect the verb of motion with that place, which in this case is la selva (the jungle).