How Garbage Trucks Reproduce
As you can see the smaller male truck has backed up to the rear of the larger female and is making its deposit. If he has been successful and has made a large enough deposit she will compact the raw material and after a period of gestation, a new truck will be born. If his deposit is not large enough it may be mixed with the deposits of other trucks, but in this way his contributions, diluted as they are, will extend unto further generations.
Interesting about this species is that the mothers rarely care for their young, and even a newly born is set to forage for garbage on the city streets on its own. Typically very shy, the new garbage trucks scurry about consuming refuse put out by the empathetic citizens of Florence, hoping one day to grow full size, and to make babies of their own.
At least, this is how I explained what was going on to the kids.