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Ant reproduction is strange

5 min readlife, science

First, credit where it’s due: go watch Kurzgesagt on ants. Start with this one, then this, this, and this. This channel’s animations are full of information, humor, and knowledge. Every time I watch their videos, even when it’s something I already know well, I come away with new takeaways. They are thought-provoking and clearly made with good intent.

After these ant videos, I lost a few hours to a Wikipedia deep dive. This post is what fell out.

Here they are, roughly in the order I found them.

1. Male ants have no father, but have grandfathers

A male ant hatches from an egg that was never fertilized. One parent, the queen, full stop. He carries a single set of chromosomes instead of two, half the usual deck.

It gets stranger. He can never have a son of his own. But he does have a grandfather, because his mother had a father. So somewhere in his family tree there are grandfathers with no fathers in between:

A male ant's family tree A male ant descends from the queen and her father (his grandfather), but has no father of his own, and can produce daughters but never sons. ♂ Grandfather the queen's own father ♀ Queen, his mother she does have a father ♂ A male ant one parent, half a genome ♀ His daughters grown from his sperm ✕ no father his egg was never fertilized ✕ no sons he cannot produce a male
Solid lines exist; dashed ones can't. Read top to bottom as descent.

2. A queen mates once and lives off it for decades

A queen mates a single time, on one flight, when she is young. She never does it again. She stores what she collected inside her body and rations it out for the rest of her life, and some queens live thirty years.

The strange part is the storage. Sperm is fragile, it dies within days almost anywhere else. Inside her it stays alive and ready for twenty or thirty years, a living reserve she keeps in the dark and spends one cell at a time.

One afternoon of her youth quietly funds three decades of a city.

3. She chooses each baby’s sex with a muscle

When the queen lays an egg, she decides whether it becomes a daughter or a son. Not with hormones, not with a roll of the dice. She does it by flexing a muscle, opening or closing a valve to let the stored sperm reach the egg or not. Open, it gets fertilized, daughter. Shut, son.

She is making the single most consequential decision about a life by tensing a tiny muscle, thousands upon thousands of times, and she is never wrong about which one she meant. No press release. No committee that studied the matter. It makes a certain government’s mountain of rationale for its choices look like what it is: decoration.

How the queen sets each egg's sex Stored sperm sits behind a muscle-controlled valve. With the muscle relaxed the valve opens and the egg is fertilized into a daughter with two chromosome sets. With the muscle flexed the valve stays shut and the unfertilized egg becomes a son with one set. Stored sperm held since her one flight valve muscle relaxed, open muscle flexed, shut ♀ Daughter egg meets sperm two chromosome sets ♂ Son egg sealed off one chromosome set
One muscle, two outcomes. Open the valve and the egg is fertilized; shut it and the egg develops untouched.

4. Every worker you have ever seen is female

Every ant marching across your kitchen counter is a she. The whole visible society, the foragers, the builders, the ones carrying a crumb four times their size, all sisters. The colony you picture when you think “ants” is a world run entirely by daughters.

5. A queen is made, not born

Here is the part that undoes any tidy story about destiny. A larva that grows into a queen and a larva that grows into a plain worker can be the same underneath. The difference is how the sisters raise it, how it’s fed and tended while it develops. Feed it one way, you get a worker. Feed it another, you get a queen who may outlive everyone tending her.

Nobody is born royal. The colony simply decides, and then makes it true.

6. Some queens skip the mixing entirely

Just when the rules start to feel settled, a handful of species tear them up. In the little fire ant, a new queen is not a blend of her parents at all. She is a clone of her mother, a straight copy. The males are clones of their father. Two lineages, one female and one male, each photocopying itself down the generations and never truly merging.

The only ants that are a real mix of both parents are the workers, the sterile ones who will never pass anything on. The queen keeps her line pure, the king keeps his, and the blended daughters do the labor.

Two lineages that copy themselves and rarely merge In the little fire ant the queens are clones of their mothers and the males are clones of their fathers, so each sex forms its own unbroken copy line down the sides. The two lines combine only to produce workers in the middle, who are a genuine mix of both parents and are sterile, so that branch does not continue. ♀ Queen founds the colony ♂ Male her one mate ♀ Queen, a clone identical to her mother ♂ Male, a clone identical to his father ♀ Workers a true mix of both clones herself, on and on clones himself, on and on ✕ the line ends here
Two copy lines that stay separate. They combine only to make workers, and that branch ends there. Read top to bottom as descent.

The strangeness has a reason

Pull on all of this and one thread runs through it. Because a male carries only a single set of chromosomes, every sperm he makes is identical, there is no second copy to shuffle. A queen who mated once, with one male, therefore produces daughters who share not half their genes, the way you and a sibling do, but roughly three quarters.

How much a worker shares with each relative A sister's genome is half from her father and half from her mother. If the queen mated once, every daughter has the same father, so his half is shared in full and the mother's half is shared by about a half, giving three quarters. A half sister with a different father shares only part of the mother's side, about one quarter. A queen shares one half with a child of her own. So a full sister is the only relationship closer than her own offspring. her father's half her mother's half A full sister same father = ¾ A half sister different father = ¼ if she bred instead of helping Her own child = ½
A full sister is the only relative closer than her own child, and only when the queen mated once.

A worker is more closely related to her sisters than she would be to her own daughters. The surest way to spread her genes is not to breed at all, it is to raise more sisters. The sterile workforce, the daughters who give up children to tend a queen, stops looking like sacrifice and starts looking like arithmetic.

There is a catch, and the honest version keeps it. The three quarters only holds if the queen mated once, so every daughter shares one father. Let her mate with two males and the brood splits into full sisters, still three quarters, and half sisters who share only a quarter, and the colony average slides to one half, no better than a child of her own. Most ant queens do mate a single time, which is likely why the arithmetic ever favored sisterhood at all. But some mate with several males, and an army ant queen may mate with a dozen, and in those colonies the tidy advantage frays.

The rest follows from there. It is the shape a family takes when the numbers reward sisters over daughters.

So that is where my time went. A society of sisters, ruled by a queen who was fed into existence, sons who never had fathers, all of it running on one long-ago flight and the flex of a muscle.

Facts like these make me wonder, for a moment, whether I picked the wrong career and still have time to switch. I don’t, and I wouldn’t want to. I love building with code. I’m just glad the ants are out there being this strange, and that I get to sit here amazed.