What evolutionary function is served by squinting one eye at lemons?

We react to sour\bitter things like this because that is how our body would tell us that something was poisonus/not good to eat.

It could be an unconscious signal to others around them that what they are eating isn’t safe for them. Such strong tastes could be seen as dangerous like @XOIIO says.

But what reason for the eye squinting? Unless the citrus juice is squirting into their eyes. And the babies seem to be using both eyes.

Adorable video, btw, and good question.

I don’t think it’s necessarily “instinctive.” I think it’s a physical reaction to “tartness,” a very specific flavor and more intense when directly applied to the tongue in the form of a lemon. The taste buds are pretty much hard wired into the nervous system and there are, I believe, certain involuntary reactions that occur to certain stimuli that the nervous system encounters, taste included.

Surely there are better ways to signal “poison” to other members of the tribe than squinting. And you’ll notice that the most common reaction is the squinting of one eye. I wonder if which eye we squint is dependent on handedness. In any case, it’s clear that there is an involuntary, physiological reaction involved. My question is why this would evolve, since the squinting of the eyes doesn’t seem on the surface to serve any useful function.

I think it’s a sympathetic muscular contraction resulting from the grimace being made by the muscles around the mouth.

@zenvelo said it best. It isn’t really a behavior as much as it’s a reflex.

Here is a good explanation.

Excerpted:
“Firstly, the nerves that control your facial muscles must pass through the large salivary glands that tuck between your jaw hinges and your ears…Fourthly, sour food provokes your salivary glands to swell with saliva. They actually stretch and anything threaded through them goes along for the ride. If your facial nerve is already nearly pinched this will clench it; the nerve is tweaked and you suffer muscle spasms.”

It seems like a reflex. They all look like they’re trying to pull away from it. They don’t just squeeze both eyes shut (in most cases I see both eyes closing), they also jerk backwards. The same thing would happen with a splash of cold water and a faceful of dust or sand—and probably also with a blast of loud noise right in the face. Anything that causes such a strong aversive response is something to stay away from, whether it’s a lemon, a barking dog, or something with a terrible smell—right?

A lot of behaviors, traits, and physical characteristics persist that aren’t necessarily adaptive. It’s just that they’re not maladaptive, so they don’t have to be eliminated. Isn’t that so?

I can’t accept the idea that a reaction like this is more about protecting others than it is about one’s own comfort and self-preservation.

Maybe it is a reflex response to prevent the acid from the fruit, getting into the eye, since most acidic fruit is often juicy and can have a huge squirt factor when bitten into.

We really only have five senses: sight, touch, hearing, taste and smell and our minds and nervous systems only have input from those five senses with which to navigate and survive in an extremely complex environment, an extremely complex world. We can’t stop to think about and process every bit of all of it so our nervous systems sometimes simply react and then our minds sort it out later, at their leisure. Kind of. We can’t grab a burning log from a fire and wait until our minds sort out whether it hurts and is dangerous or not. Eating is essential to our survival and our mouths are essentially the main point of entry into our body for things that could harm us. Our taste buds have to be on the alert and ever vigilant. That isn’t a very scientific way to describe it but I think that’s basically the way some of it works, it kind of has to work that way.

I wouldn’t survive without sour patch kids!.........Yummy little bastards.

I think it’s a reflex action as others have suggested. If I think about my own reaction to something very sour it starts in the mouth. The salivary glands react; the back of my throat tightens; the feeling spreads to my ears and brow area and up into my sinuses; my eyes squinch a bit. I love sour food. Some of the babies did and some didn’t enjoy the taste.
When my brother was about 1 year and half he discovered that adults laughed when he made faces eating a lemon. He loved that and would ham it up to get attention. This is the observation of his older sister who was jealous. . . .(me.)

I find that when I squint, using both eyes and not thinking about it, my mouth changes shape. There does seem to be a powerful muscular connection.

In fact, it takes a conscious effort to both squint and not involve my lips and teeth.

A large section of the human brain is devoted to interpreting the expressions of other people’s faces, especially when it comes to detecting primary emotions such as fear, anger and disgust.