Biological significance of solstice?

It is Winter Solstice in the Northern Hemisphere, Summer Solstice in the South. The solstices have had cultural significance in many parts of the world for thousands of years. This makes me wonder, is there any organism whose phenology responds to solstices in particular? I know there are organisms that respond to season more generally, to full moon or new moon, to light and dark periods, etc. But is there anything that breeds yearly exactly on the solstice, or starts a migration like clockwork on the solstice, or anything similar? It seems like there should be but I can’t think of a good example.

Thank you.

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There are many organisms that have biological responses that are triggered by the amount of light (or UV radiation) received per day (and the timing of said light). In a lot of the scientific literature the solstice is used as a marker to measure how long after/before said date these changes manifest, but it doesn’t appear that the solstice itself has any real biological significance as a specific moment in time. It’s a useful tool the indicate that roughly X units of time after said date/light regime Y species has Z change (eg. starting to grow thicker seasonal hair, leafing out, starting migration, etc).

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I agree, that is often used as the marker, and there are many things that seems to happen about x days before or after a solstice. But isn’t it a little bit surprising that we don’t know of any examples of phenological events exactly at the solstice?

With summer one day is getting shorter after it, as it’s a stimuli to start migration, you can say that it plays a role in all long migrations. Events never happen just on one day and as effect is a hormonal change, it can’t happen with one day, for brain to register the difference, some days should pass. With winter one, tits start singing in early January, you could speculate they’re triggered by longer days.

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You can pick any day and say that there are lots of things that happen X days after said day.

The solstices are useful in that they are regular, deterministic markers that are stable over time (although when they occur des drift over time, but the significance of what they indicate remains the same) due to the geometry of the solar system and the tilt of the earth, but they themselves aren’t the stimulus that triggers a change. The ratio of light to dark is, and that will reach a specific threshold X days before/after the min/max (which is what the solstices mark).

Temperature is another common indicator for species to fire up metabolic processes and that can be roughly linked to the solstice/day marker as well, but it’s not fully depended on the same sort of orbital mechanics and axial tilt aspects.

It’s not at all surprising that we don’t see biological events that take place exactly at the solstice. Biological events take time, and there are usually multiple different influencing factors. The effects of the light dark cycle that the solstice marks varies according the latitude, and thermal cycles lag behind them. In addition, due to the long-term variations in our orbit, the wobble of the Earth, and how our planet processes, the solstices (and equinoxes) drift over time (look at Milankovitch Cycles to see how this plays out). The relationships between the solstices and equinoxes remains the same, as does what they indicate, but they drift in terms of their timing.

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It’s an interesting question but I suspect the solstice has little biological significance. Where I live (Winnipeg, 49.5 North) there is no major change in daylength for at least a month around the solstices. So for me, and I suspect for most of the living things not hibernating in some way, the shortest day has little meaning. Just another long night. It won’t be for four to six weeks that the daylength change will be evident. Perhaps further south, or in coastal areas where the ground is not frozen solid some organisms do have a response!

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i agree (i think you mean solstice?)… during the summer and winter solstice the amount of light changes very little whereas around the equinoxes the change is rapid, but i am not sure the equinox plays any particular role compared with other days around that time either. They are important to humans for plenty of good reasons but there’s also a reason people had to set up ceremonial devices to detect the solstice, it isn’t obvious if you don’t have a device or calendar to tell you when it is.

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Ooops. Yes I did mean solstice! Thanks for that catch. I’ll edit it now.

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i personally always mix those up too. Have to remind myself - equinox means equal day and night.

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Interesting fact that I drop into any conversation at the slightest opportunity: the winter solstice isn’t the day with the latest sunrise nor the earliest sunset. The earliest sunset is about 12 December while sunrise carries on getting later until the end of December. That is in Britain. I can’t explain it so I don’t know if it could differ in other parts of northern hemisphere.

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That makes a lot of sense, thank you all. A full moon is a discrete event that one can detect just by looking at the sky. A solstice requires more data to determine exactly. Any biological event that requires exact timing for coordination (like corals releasing their gametes) will tend to follow a cue that can be determined with very low error by every individual.

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One date is astronomical, the other is geographical (or something like that)

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It is to do with the Equation of Time, whatever that is. The similar phenomenon at the summer solstice is ‘explained’ in a paper by D.W. Hughes in The Horological Journal, December 1993, pages 189-192. I have read it a few times but it is way beyond my grasp.

I have also seen it explained as follows: Imagine you are sitting in a chair facing a vertical line on the wall. The line represents midday. You have your arms held out in front of you an equal distance either side of the midday line, and you slowly move your hands away from each other and back towards each other. One hand represents sunrise, the other sunset. When they are farthest apart, that is mid-summer. When they are closest together, they represent mid-winter. Now imagine it is a rotating chair and someone behind you is jiggling the chair from side to side slightly. While I can visualise all that, it doesn’t really help me understand the solstice business any more than Hughes’ paper.

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The way I’ve always vaguely understood this is that the Earth is an irregular lumpy sphere-like shape wobbling around its axis, which itself wobbles as it moves around its mostly elliptical orbit. Combine with the fact that most people aren’t in the middle of their time zone, and I’d need to actually look at the math to understand the details.

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Not being in the middle of your time zone can explain why you have more morning than afternoon on a particular day. But it completely flummoxes me that the time of sunset can be moving in the opposite direction from the time of sunrise for about three weeks.

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As I understand it, the solstice is the day on which day length is the shortest. The earth wobbles in relation to the sun. In winter, no matter which hemisphere, the earth is tilted away from the sun, so as earth rotates daily, each hemisphere receives the shortest exposure to the sun. The actual amount depends on latitude. Land of the midnight summer and all that. Since the wobble is along a vertical axis, each hemisphere is opposite - North is winter, South is summer. Since the wobble is predictably periodically, as the northern/southern hemispheres pass through this time, it starts to reverse so the days get longer/shorter depending on the hemisphere.
Oddly, London, and much of GB and Eurasia is north of Winnipeg. Our cold winters are a result of both daylength and our physical position on the NA continent. In comparison, the Falkland Islands are about the same latitude south as London.

All I can say is I’m glad the winter solstice is now behind us and look forward to a little more daylight each day, even if it takes a while to make a difference. And I don’t even live in a high latitude.

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Winnipeg versus London is also partly Gulf Stream effect.

Inverewe garden in Scotland is ‘closer to the Arctic Circle than St Petersburg and most of Labrador’ 57.8 degrees North.

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True. I had neglected to mention that. It still amazes me how much of Eurasia is “North of 60”. For a lot of Canada that is where the Arctic begins. We really have no sizable towns north of about 55.

I wonder if this is why New Year’s Day is ten days after Solstice instead of on Solstice – people needed to make sure that the daylight really was coming back.

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