Comment by jcfrei

I believe (maybe someone has a study on this) one of the main drivers of changing social habits among humans is the ratio of dopamine / cortisol. When we're younger dopamine releases happen more frequently, we feel energized, ready to take risks and are optimistic. As we grow older the ratio of cortisol releases increases, making us anxious, risk averse and depressed in our outlook. Might be similar for these deer.