Many animals have a mating season or an estrous cycle (referred as "going into heat"). Depending on the animal, this can be very messy, smelly, or (especially when it comes to males in rut) dangerous.
In fiction, mating seasons and estrous cycles are presented as a hindrance to characters. How this is presented depends on whether the character is the one in season or not.
Usually, this trope occurs for dog or cat owners during heat cycles. It's usually depicted as their female pet having gone into heat. The owner doesn't know how to deal with the mess, smell, and/or noise of an animal in heat. If they have a male pet, they'll have to quarantine them. Even if they don't have a male pet, a common gag is male animals (usually stray or free-roaming cats) hanging around their house. Sometimes this occurs with owners of tomcats, where the joke is instead that their cat is always roaming around and looking for mates.
This trope can also appear for animal or animal-like characters, ranging from Partially Civilized Animals, humanoids with animal characteristics, or aliens/monsters. In this case, heat is depicted as a more annoying version of menstruation. The trouble comes from the character's need to breed, as well as oftentimes heat itself being depicted as uncomfortably hot to experience. Mating season is prime humor potential because even normally mellow or sophisticated characters can't avoid their base instincts.
Another common joke is people in the wilderness during mating season. This usually leads to an animal attack due to the high amount of animals in the area.
- The Pokémon episode "Forest Grumps" revolves around the gang being attacked by angry Ursaring during mating season.
- Chapter 81 of Miss Kobayashi's Dragon Maid deals with Ilulu going into heat and attempting to force herself onto Kobayashi. Tohru's attempts to tire her out with a sparring match fail, as Ilulu simply goes onto her instead.
- Being a Cat Girl Nekomata, Koneko of High School D×D can go into this. However being also fairly physically small at her age means that actually going through with it and getting pregnant would be very dangerous and thus she avoids actually going through with the instincts of heat.
- In From Behind Bars, the zoo keepers introduce a female in heat to Mufasa and Scar in hopes that they'll have cubs. This annoys Mufasa because he has difficulties staying faithful to his mate Sarabi, who is still back home in Africa.
- Heat is a Naruto fanfic where Naruto (a boy) has been having heat cycles since he was six due to the Nine-Tales Fox (who is also male) being trapped inside him. He finds them annoying but usually secludes himself until they're finished. Now that he's sixteen, however, he's having troubled dealing with his attraction to Hinata during heat.
- The plot of Linked in Life and Love kicks off when Blake goes into heat and she runs out of the medication normally distributed to Faunus to suppress it while stuck in her room due to a lockdown.
- Triptych Continuum: In Cutie Mark Crusaders Alpha Pack Leader, Apple Bloom winds up with just about every male dog (of a certain age) in Ponyville following her — because unknown to her, Winona is in heat and AB's picked up the scent. They're waiting for her to lead them back to the border collie. She decides she may be on the verge of manifesting a mark for canine training. Dogsledding ensues. Using a "borrowed" sleigh. On grass. In the summer.
- The Teen Titans fic Prey Mate starts when the half-demon Raven hits her second heat. She looks for someone to help with her heat and ends up attracted to Jinx (due to Jinx's magical abilities).
- Played for Drama in Chemistry. If Cadance doesn't take her medication, she enters a heat-like state which is very uncomfortable and hard to temper.
- Fantastic Beasts and Where to Find Them features a female Erumphent (a rhinoceros-like creature) who escapes from Newt Scamander's Bigger on the Inside magical briefcase. As it's her mating season, he finds her at a zoo, attempting to mate with a much smaller hippo and having already destroyed much of the area with her large size. Newt even attempts to lure her back into the case using the musk of a male of her species and imitating the male's mating dance.
- A Dog's Purpose:
- In his first life, Toby lives in a compound with several other dogs. One day he notices that he's discovered a new "game" to play with his best friend, a female named Coco. She doesn't like it so much. Toby's annoyed that all the other males also want to play his new "game". As it turns out, Coco was in heat and all the males wanted to mount her. Coco, Toby, and a few other dogs are promptly sterilized.
- Three reincarnations in, the protagonist reincarnates into a bitch for the first time. Having always been neutered, Ellie doesn't understand her first heat cycle. She just finds it uncomfortable and smelly. Ellie is promptly spayed after her cycle ends.
- Stray: In his youth, Pufftail was a Casanova who would chase after any female cat in heat that he could scent. His yowling and flirting often annoyed both owners and other humans.
- It's briefly mentioned in Tailchaser's Song that Roofshadow avoids males during heat in order to stop herself from having a litter she doesn't want yet.
- In Mark Of The Cat, the women of the Outer Regions may go into heat. note Those that do emit pheromones that attract the attention of every man in the vicinity. They compete to impress her, and she chooses her favorite to mate with. Rather like cats, in fact.
- Hares in Frost Dancers call mating season "frost dancing". It's a turbulent season where jacks fight over jills. It's the only time where hares have a hierarchy.
- Star Trek:
- Vulcans experience pon farr every seven years, a hormonal imbalance that will kill them unless they mate or participate in a Duel to the Death.
- Star Trek: The Next Generation episode "Manhunt". Lwaxana Troi is going through the Betazoid equivalent of menopause (called "The Phase"), which causes a tremendous increase in her sex drive. She tries to mate with Captain Picard and get married to Commander Riker.
- When she visits Deep Space Nine, she turns her attentions to Odo, of all people.
- In Dark Angel Max's feline DNA causes her to go into heat a few times a year. In the second season, when she's carrying a virus tailored to kill her boyfriend Logan, this causes... issues.
- In Monster Hunter, the Black Diablos are female Diablos in heat. Their shells turn black during the mating season and they are much more aggressive and hostile toward intruders than usual, meaning that both the hunters and the creatures around face huge trouble when a Black Diablos is nearby.
- In Don't Starve, normally peaceful beefalo become extremely aggressive during mating season, during which baby beefalo spawn offscreen.
- In the furry comic S.S.D.D Annie, as an anthropomorphic red fox, goes into heat once a year. It starts out as her acting even hornier than normal, but then turns into a massive case of PMS.
- Schlock Mercenary: Musth has been bred out of Uplifted elephants, though occasionally used as an insult. In the non-uplifted ones it's usually suppressed by nanobots, but those can be hacked.
- Sorcery 101 mentions that werewolves experience this during January, and unlike with real wolves, it affects both sexes. A Breather Episode chapter explores this for comedy.
- On the South Park episode "Cat Orgy", Cartman's cat is in heat and attracts every male cat in town to his home, where they have the titular orgy.
- One episode of Futurama had Zoidberg behaving erratically because it was mating season for his species.
- In an episode of Space Ghost Coast to Coast, Zorak is in a frenzy because it's mating season (especially because it's his first time). The issue is complicated by the fact that females of his species eat the male after mating.
- Star vs. the Forces of Evil: Mewman females such as Star go through a process known as "Mewberty." It starts with them shedding little hearts whenever they're around attractive boys, and ends with them transforming into a six-armed butterfly monster that captures and cocoons every male it comes across. Despite how horrifying the whole event is for everyone involved, apparently the best way to deal with it is to just let it run its course. After Star's Mewberty, she has tiny wings that she can barely use to flutter, and her mother later demonstrates the ability to go into six-armed butterfly mode at will.
- Male elephants go into a period called musth when they're ready to mate. The condition can be identified by what look like tear streaks on the head and dribbling urine. They're extremely dangerous and unpredictable during this time due to the pain caused by the swollen glands on their head combined with a massive boost in their testosterone levels.
- Male deer, elk, moose, and caribou all enter rut during mating season. Their antlers become extremely itchy, causing them to scratch the velvet off them and reveal the sharp bone underneath. They also become extremely aggressive and begin gathering into groups to fight, which can sometimes be to the death if the loser doesn't flee fast enough. They're also known to occasionally attack and sometimes even kill humans in this state.