This is a DFhack lua script to fix sterility in animals that won't breed. It currently works with DFHack 40.12 r1.
The script functions as follows:
Required arg:
fert: sets the flag to make creature(s) are fertile
ster: sets the flag to make creature(s) are sterile

Optional args:
With no optional argument, the script will only process the currently selected creature
all: process all creatures currently on the map
animals: processes everything but dwarves on the map
only:<creature>: processes all of the creatures matching the specified creature on the map

Examples:
Makes the current selected unit fertile
Makes all creatures on the map sterile
Makes all the non-dwarven creatures on the map fertile.
Sterilizes all cats on the map

NOTES:
-Pregnant animals will still carry to term.
-This only affects currently spawned units. Any units that spawn after the script is used will need to have the script run on them to be effected.
-Currently non-dwarven civilized creatures are still effected when "animals" is picked. Working on a more robust solution to this so that only non-learners are processed.
-There is currently no "help" arg to print out the usage. Probably make that part of the next release.