subset a data frame by randomly selecting values based on 2 columns

I have a data frame that I want to subset by randomly selecting 25 values of ID based on spp=cat and 25 values of ID based on spp=dog.

Here is my example data:

ID  spp category    prop
1   cat small_mam   0.99
2   cat small_mam   0.8
2   cat birds       0.15
3   dog large_mam   1
4   dog med_mam     0.4
4   dog emu         0.6
10  dog med_mam     0.8
10  dog birds       0.2
12  dog reptiles    1
13  dog large_mam   1
14  dog large_mam   1
15  dog large_mam   1
27  cat birds       0.2
28  cat small_mam   1
29  cat small_mam   0.75
29  cat birds       0.25
30  cat small_mam   0.7
30  cat birds       0.2

ID values for spp are unique meaning that cat and dog never have the same ID value. ID ranges from 1 to 696 but is not necessarily unique, this is because ID can be composed of up to 7 categories so randomly sub-setting 25 rows for each species does not work.

The context behind this question is that I will be drawing 1000 random samples of 25 cat and 25 dog scats (UID = the scat ID number) for a bootstrap calculation of dietary overlap using the piankabio function in package(pgirmess).

Thanks in advance for any help.

I am using R version 3.1.3