Male Animals Are Generic Female Animals Are Special
As with humans, male is seen as "generic" and female is seen as special with animals. This leads to Tertiary Sexual Characteristics put more on female animals than on male ones.
Animal Are Male Assumption
Animal Gender Assumption is guessing an animal's gender without actually knowing what the animal's gender is. Male Animals Are Generic Female Animals Are Special leads to the tendency for people to refer to any animal that doesn't look obviously feminine by a masculine pronoun like "he" and assume it to be male.
Examples come in three stock situations:
Instances of characters seeing an animal and automatically referring to it as a male. For example, Bob sees a dog from a distance and calls "Here, boy!"
Instances in which a character sees an animal that is obviously female, like a cow with udders or a bloodsucking mosquito and referring to it as male anyway. This leads to Animal Gender Bender.
Complicating this trope, we have Animals Lack Attributes in drawn/animated works. In the case of mammalian animals, the characters might be able to clearly see that an animal is male, even if the audience can't.
There is one main exception; any animal that has a baby in tow is (usually) unanimously referred to as she. This could it confusing for the male penguins and sea horses who work as primary caregivers. Also, this is often inverted with cats as they are often associated with femininity. This is what leads to Female Feline, Male Mutt whenever a cat is shown or partnered with a dog.
Results of Male Animals Are Generic Female Animals Are Special
Male Animals Are Generic Female Animals Are Special contributes to gender disparity among animal characters in which most animal characters are male. In fact, Florida State University's findings show that this gender disparity among animal characters is worse than that among human characters. Its authors found that the number of male and female characters in children's books is only equal among human characters. Among animal characters, even cats, males are represented more than twice as much as females. This tendency of readers and viewers to interpret any not obviously feminine looking animal character as male exaggerates this pattern of female animal underrepresentation.
The Sex, Cats, and Stereotypes post on the Redeemed Reader blog not only talks about this trope and its effects with animal characters, but how women are seen as the steady ones and stabilizers in society. To many authors and educators, the way to gender parity is having girls behave more like boys, whether the characters are humans, Demi Humans, animals (both real and fictional), aliens, monsters, mythical and fantastical creatures, or deities.
For example, the blogpost lists The Cat in the Hat, who is actually supposed to be androgynous, but readers and viewers see him/her as male. First, it takes a little more effort to imagine a female Cat In the Hat wreaking havoc in someone’s living room than a male one because girls being destructive seems more wrong than boys being destructive. We subconsciously rely on girls to be the steady ones and stabilizers and if they reject that role, we think that society will be toast. Second, he/she doesn't look overtly feminine or have overtly feminine facial features. In fact, he/she has big jowls, which are a typical tomcat facial feature.
The Peanut Butter Fingers blogpost, Assuming Animal Genders talks about how dogs (and other animals, but focuses on dogs) are assumed to be male or female without knowing for certain.
Hens (as in female chickens) are often simply referred to by the gender neutral species term "chicken," while roosters (male chickens) are more likely to be called the masculine term, "rooster," than just a chicken (save for Chicken Boo from Animaniacs).
Female ducks are simply referred to as, well, "ducks," but male ducks are called "drakes." Female ducks are also called "hens," but very rarely so.
"Goose" is female or gender-neutral, "gander" is male only.
The German word for cat, "die katze," is used both generically and to refer to female cats in particular, but "der kater," the term used to male cats, is never used generically.
The video game, Red Dead Redemption calls male deer by the exclusively male term, "buck," but female deer (or does) are simply referred to by the gender neutral, "deer."
In Secondary Sexual Characteristics
Lions of both sexes can be maneless, but only males have manes.
This means that a picture of a lion will almost always be a male, as a lioness could be easily mistaken for another big cat (by someone who is not an expert).
In Universe Fictional Animal Are Male Assumption Examples:
Inverted in Ladyhawke, in which Phillipe assumes the horse is female until Navarre points out he's overlooked something.
Kevin the bird from Up was mistaken for male at first, but it turned out that she had chicks in tow.
In Priest-Kings of Gor Tarl meets the Priest-Kings, the Physical Gods of the planet, which are basically 20 foot tall locusts. Misk the Priest-King, his friend, shows him the last male Priest-King, which Misk had hidden away from those which don't want another generation of Priest-Kings to replace the current generation. Until he sees that male, Tarl assumed that Misk and most of the other Priest-Kings he met were male. Since this is the only male, he now assumes that they're female. But no, they're not female either, they're neuter. The Mother of the Nest is the only female, and the male is the only male. There is one female egg hidden away; finding it beomes Tarl's Quest in the following book.
Stand Up Comedy
A routine by Dan Rowan & Dick Martin (later of Rowan and Martin's Laugh-In) involves a discussion about bees, during which Rowan (the straight man) says of the drone "he is neuter. He is neither male nor female." In real life, it's the workers bees, who are all female, who are sterile and neuter. The drones in a beehive are reproductive males.
The Simpsons: In the educational/propaganda film "Meat and You: Partners in Freedom" Troy McClure refers to a cow as a he. "If a cow ever got the chance, he'd eat you and everyone you care about!"
Real Life Animal Are Male Assumption Examples:
Averted, Inverted, or played with in Poland where animal gender is assumed based on gender of that animal name. For example Mickey Mouse (in Polish: Myszka Miki) is often mistaken to female and mosquitoes (in Polish: komar) are always asumed to be male even if female are one who suck blood.
In a Real Life example, medieval beekeepers assumed that the Hive Queen bees were male, simply because they were bigger. Old records of beekeeping practices call them "kings".