Complex sentence: Two stem mothers sharing a leaf spend a significant amount of their time engaging in territorial behavior instead of feeding or gall forming, and so stem mothers sharing a leaf produce less offspring than single stem mothers on leaves of the same size.
Split into simpler sentences: Two stem mothers sharing a leaf spend a significant amount of their time engaging in territorial behavior instead of feeding or gall forming. This means that stem mothers sharing a leaf produce less offspring than single stem mothers on leaves of the same size.