Since kangaroo joeys are so small at birth (about the size of a honeybee), they often remain undetected by zookeepers for some time. That being the case, the Basel Zoo explains that it marks the "birthday" of a joey as the date its head first pops out of its mother's pouch.
During the western gray kangaroo breeding season, males compete for the affection of females in "boxing" matches. More often than not, only the most dominant male breeds with the females in his group. The Basel Zoo's recent joey additions are all half-siblings, since they have different mothers but the same father: the zoo's only male western gray kangaroo, Eddy.
The Basel Zoo is also home to another, more famous animal baby: Farasi, the baby hippopotamus who recently became the source of controversy when reports surfaced that the zoo might resort to killing him if it couldn't find him a home at another zoo before he reached maturity. (The zoo later announced that, although there are "rare cases in which we have to kill an animal," this was unlikely to be an option in Farasi's case.)
More photos after the jump!