Group df by animal and return one row per animal with two plain (non-MultiIndex) columns: total_visits, the sum of that animal's visits, and avg_age, the mean of that animal's age.

Sample data: df:

  animal  age  visits priority
a    cat  2.5       1      yes
b    cat  3.0       3      yes
c  snake  0.5       2       no
d    dog  NaN       3      yes
e    dog  5.0       2       no
f    cat  2.0       3       no
... (10 rows total)