Why do women have periods when most animals don't?

EDIT: Oh, I think I get it now. "Period" in this context does not refer to the general fertility cycle. It is specifically a euphemism for menstruation. That is why you'll hear people saying things like "I'm on my period", "my period started late/early", etc. These phrases would not make sense if it referred to the fertility cycle in general, since from puberty to menopause, most healthy women are in some phase of a fertility cycle.

"Of course, I wasn't the only one: most women menstruate. But most other female animals don't bleed outwardly like us. Even among those that give birth to live young as we do, only a handful of species menstruate."

So the term menstruate is used to mean to "bleed outwardly like us".

"If the woman doesn't get pregnant, progesterone levels begin to fall. The thick endometrial tissue with its blood vessels then begins to slough off, and passes out through the vagina. This bleeding is menstruation."

So they are expanding on the term menstruate. Now it's both "bleed outwardly like us" and "The thick endometrial tissue with its blood vessels then begins to slough off, and passes out through the vagina"

"On average, women lose 30 to 90 ml of fluid over 3-7 days of menstruation. We know, because scientists have given women pre-weighed pads and tampons, and weighed them again after use."

Clarifying the term menstruate and showing how to measure it. I'm going to paraphrase the definition of the term. So at this point, when they talk about menstruation, they are talking about a blood like fluid, that can be measured, and that happens when the the tissue and blood vessels pass out of the vagina.

The article has a break to talk about the history, theories, and research that has happened over the years. After that they get to the main point. "To figure out the truth, we need to compare animals that do and don't menstruate."

"Great apes do it too. Menstrual bleeding is easily detectable in chimpanzees and gibbons. However, gorillas and orang-utans bleed less copiously, so menstruation is only visible on closer inspection. Other primates, such as tarsiers, may also menstruate, but there is little hard evidence."

They re-iterate key parts of the term. When this article talks about menstruation, they are specifically talking about detectable bleeding.

Now we can get to the heart of my objection. They spent most of the article defining their usage of the term menstruation, and how it relates to the research.

The article claims "It seems the list of animals that menstruate is quite short: humans, apes, monkeys, bats and elephant shrews. What do these seemingly disparate animals have in common?", remember the definition they use for menstruate, blood and tissue passing though the vagina.

If you've ever owned an unfix female dog during heat, you'll be quite aware that they do, in fact bleed out their vagina for a period of time during their reproductive cycle. This fits with their definition.

The only mention of dogs is: "In dogs and cats, the foetuses dig in a little more. But in humans and other primates, a foetus will dig through all the womb lining to directly bathe in its mother's blood." This does not specify why dogs do not fit into their term. In fact this seems to support that dogs have a very similar cycle to humans.

So my question remains, Why are dogs excluded? They never acknowledge the contradiction. So either the article incorrectly sets up the terms, or they don't want to talk about dogs because it doesn't fit the narrative.

If you're utterly convinced that estrous and menstruation are the same, on the basis of some blood-like discharge occurring in both, then you might want to explore a career in academia. I'm sure there's a journal somewhere that would take your well-reasoned and compelling thesis on the subject. (You might need to pay them for the privilege, but as assiduously as you're defending your position here, that doesn't seem too unreasonable.)