﷽̶̨̛̫̰̲̲̣̞̱̻͕̮̬̮͍͔̹̍͆̏̏͋͂̿͌̓﷽̴̨̡̨̡̛̛̛̛͙̹̬͈̭̣̥̙̮̺͈̗͓̘̰̣͕̱̹̼̳̜͈̥̭̲̤̭͉̭̟̳̳̗̳̈́͊̓̍̐́̂́͋̌̓̄̂̄̉̽͛̄͗̐͗́̂̅̊̍͛̓̕̕̚͝﷽̸̖̫͍̀͑͒̒̒͋̎̊̏͊̈̆̉͂̕

Heat

When a female animal is in the fertile and receptive phase of her estrous cycle, she is said to be ‘in heat’.

This is often accompanied by physical changes such as engorgement of the vulva and reproductive tissues as well as behavior indicators such as flirtatious behavior, increased urine-marking, and even masturbation or mounting and humping behavior in some females!

When the female is in heat, regardless of her species, she will oftentimes desire sexual release. This is the best time to introduce them to more involved intimate contact, such as eager cunnilingus or slow and gentle digital stimulation. Once the zoo has assessed her arousal-level and degree of interest, he may decide (depending upon her size) to attempt penetration at this time.

Having her first experience in the frenzy and desire of heat will leave a female many positive memories, and she may sometimes choose to engage in sex only during her fertile phases or possibly, if she likes it quite a lot, she may enjoy this activity year-round.

References

http://beastiality.club/beastiality-club-extreme-animal-sex-content/the-encyclopedia-of-zoophilia/