We are running a hypothesis test to see if blood pressure of African zebras differs significantly from Arabian horses. We assume blood pressure in both populations is normally distributed and both...

We are running a hypothesis test to see if blood pressure of African zebras differs significantly from Arabian horses.  We assume blood pressure in both populations is normally distributed and both populations have the same population variance in blood pressure which is unknown to us, so we use the sample variances to estimate the standard error for our calculations.  We have a sample of size 9 for the African zebras and a sample of size 16 for the Arabian horses.  How many degrees of freedom does our test statistic have?