In a fantasy world I am constructing, a planet goes through regular, but extreme temperature cycling. Roughly every 500 years, the planet begins to heat or cool, dependent on its current temperature. Transition periods between high and low temperatures are relatively short (~20 years). If plotted, this would resemble a square wave.

The amplitude of this wave is significant, with peak temperatures similar to those of the Eocene thermal maximum (27° C), and trough temperatures similar to the last glacial maximum (9° C). This would mean that the planet also goes through intense hydrological cycling as ice sheets freeze and thaw, coastlines drift, and weather patterns change. Thus warm periods also correspond to wet periods, and cold periods correspond to dry periods.

Given this harsh oscillation, most species on Earth would not do well. The equator would remain relatively stable from what I can tell, resulting in similar conditions to what we would see on Earth, but living beyond even 30° N/S would likely require extreme migratory patterns or specific adaptations to accommodate the climate shifts. Thus I am wondering what adaptations and behaviors would allow organisms to survive away from the equator?

To answer this question, consider both plants and animals. It may be assumed that all organisms have developed some sort of sense to allow them to predict when these climate shifts will occur a few years in advance to accommodate any behavioral shifts or morphological changes that need to occur. Additionally, even though this is a fantasy world, assume that the only explicitly magic effects visible is the ability to slowly redistribute body mass, and the natural generation of heat within specific kinds of rocks.

Assume that the first ability takes longer given greater physiological changes, for instance a complete change of body plan might take a week, whereas superficial changes like changes in coloration patterns might only take a few minutes. Greater changes additionally require significant brain power to accomplish. This brain power cannot be sacrificed during the transition. Assume that the ability of stones to generate heat is limited to some volcanic rocks always feeling slightly warm, and a small subset of these stones being hot enough that they would be uncomfortable to touch, but would not results in damaging burns on contact.

My own research into the subject suggests that species that can fly and have significant thermal insulation (birds) would do well in this environment given their high mobility and ability to survive the cold. Small mammals like foxes, rodents, and cats would also do well in this environment due to their resistance to temperature shifts, as well as their low energy demand. Grazing animals like deer and bison may also be able to tolerate the shift given the ability of grasses to survive cold and hot conditions, as well as their own natural tolerance for ranges of environments.

Plants would likely undergo extreme blooming periods during warming cycles to reseed rehydrated and thawing regions, but it seems that many temperate would be able to persist with small variations in sap chemistry. An interesting point of note is that deciduous trees may not lose leaves in the cold phases as the solar flux remains constant.

  • $\begingroup$ If it's regular it cannot be extreme, by definition. $\endgroup$
    – L.Dutch
    Oct 14 '19 at 15:25
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    $\begingroup$ Do you think you could provide values for the temperatures you're talking of? And I assume you mean 20 years in between each extreme when you describe the "plotted" temperature, it was a little unclear to me $\endgroup$
    – Whitehot
    Oct 14 '19 at 15:31
  • $\begingroup$ If we handwave away the improbability of the environment switching between hot and cold in such a sharp fashion (say, an artificial solar system controlled by advanced beings) - there are definitely organisms on Earth which have developed reproductive cycles which would potentially fit in and survive (if the temperature swings are not too big). $\endgroup$ Oct 14 '19 at 15:34
  • $\begingroup$ @Whitehot I've added the temepratures and adjusted the explanation to make it a bit more clear, you had it right. $\endgroup$
    – Zer0ah
    Oct 14 '19 at 15:45
  • $\begingroup$ @L.Dutch The Eocene thermal maximum had modern equatorial conditions at the poles, whereas the last glacial maximum had ice sheets down to 50deg N, so there's quite the shift relative to Earth conditions $\endgroup$
    – Zer0ah
    Oct 14 '19 at 15:47

Mass migration

The simplest way for organisms to adapt to such changes is not to change so rapidly (a couple hundred years isn't much) but rather to migrate, as whole ecosystems, along with the changes in habitats. The periodic changes would mean that the following adaptations would be useful:

  1. The ability to migrate (obviously) - while easy for many animals over the course of 20 years, it means that various plants would be adapted to ensure that their seeds would be carried away and distributed by the animals that need these plants to live.

  2. The desire to migrate. It means that they would be "instinctively trigger-happy" in interpreting changes in temperature as a signal requiring long-distance travel instead of local adaptation to survive the changes where they are; behaviors that we see in migratory birds on Earth would be widespread in most types of animals (and in a way, plants) on that world. Also, while animals could be territorial, long-lived animals would not expect to live their whole life in one territory but they would consider just moving acceptable (which is not the case for all animals on Earth).

  3. The ability to flourish after migration - in such a world, the ability to rapidly repopulate after migrating to a new habitat (and losing most of population in the process) would be even more valuable than in our world, so this is something that most species would be optimized for. Larger litters, faster breeding cycles, possibly the ability to self-fertilize or preserve fertilized eggs so that after a migration a single individual could have lots of offspring even if they're the only one of their species that manages to successfully migrate.

  • $\begingroup$ I was going to suggest an answer like this. Essentially the whole ecosystem spends a whole lot of time in a "recent crisis and resulting colonization" type of status. 20 years isn't enough time to change much, every 500 years each species is moving into a newly unpopulated area. en.wikipedia.org/wiki/Ecological_succession $\endgroup$
    – Zwuwdz
    Oct 14 '19 at 19:41

I think that, away from the equator, it would be very difficult for most living organisms to live through the cold part of your climate cycle with just some extra fur or such. Most organisms that would want to keep active through that period would need to migrate towards the equator.

Tardigrades can survive pretty extreme conditions, so I would imagine that this type of micro-organism would be pretty widespread throughout this world.

However, it could create some interesting hyper-hibernation behaviours.

A lot of insects would revolve around colonies dying during the "winter", then regrowing in "summer", perhaps by having eggs that can survive freezing and thawing.

Same for plants, they would mainly die and spread seeds as winter approaches, or revert to just being a tuber-centric root system, then regrow once conditions are favourable.

I'm fairly certain that your large animals would need to migrate. I can't really imagine any bear-like animal that could hibernate for 500 years and continue its life unfazed afterwards. Anything smaller would probably lack the fat storing capabilities to do so. I'm unsure about rodents or similar critters, but once again I doubt it would be possible.

Large grazing animals, like mammoths, might stand a chance against the cold, but with no plants around to eat, they'd die out quite quickly from starvation.

Anything cold blooded is out of the question, unless you want to go for a Loch Ness Monster type scenario. The problem with that is that thawing doesn't usually occur instantaneously or uniformly. If your brain thaws before your heart, you're gonna be in deep trouble.

Any organisms regrowing after this period probably wouldn't invest as much into size, as the "race" to claim ressources after the Great Thaw would be too important. Apart from that, you could probably imagine a fair number of ecosystems that could exist stably during your summers.


For animals, migration appears to be the best option. You could have heat loving animals that retreat towards the equatorial regions during the'cold' period, and expand into the higher latitudes during the 'hot' period. A population of cold loving animals would similarly migrate between polar and 'temperate' latitudes.

Regarding alternatives: I don't think that hibernation for 500 years is a realistic option, and I don't think that an animal that could adapt to large extremes in climate could compete with mobile species 'designed' for specific climates.

For plants: a more practical option might be seeds that can remain dormant in the soil for extended periods (seeds can remain viable for thousands of years). One set of plants (multiple species) could have seeds that only germinate after exposure to frosts - they would grow in cold conditions. The other set would germinate after a period of warm seasons. A period of 500 years would be more than long enough for mature forests to grow over the decay of the previous vegetation type.

Other options: Rather than long-lasting seeds, you could rely on plants with sticky seeds and/or light flying seeds - the former would catch a lift with migrating animals (see above) so the effective latitude of each species would drift over time, the latter would need enough seeds that, given suitable wind currents, could re-establish new populations as the climate changed.

Aquatic species: It is worth considering that a lot of aquatic species sometimes spend considerable amounts of time in widely ranging climates (whales for example, which can migrate from equatorial to polar seas). The sea acts as a natural thermal reservoir so even with significant climactic changes, such animals might be able to adapt their behavior to suit the large long-term variations you propose.

Outside the box: As an alternative, you could have bimorphic species. As an analogy, imagine a tadpole/frog system, where the animal matures to adult-hood as a heat-loving-'tadpole' when it grows in warm conditions, but metamorphose or mature as a cold-loving-'frog' if brought up in cold conditions.




Simple environment examples of temperature

One can provide a simple explanation of refugia involving core temperatures and exposure to sunlight. In the northern hemisphere, north-facing sites on hills or mountains, and places at higher elevations count as cold sites. The reverse are sun- or heat-exposed, lower-elevation, south-facing sites: hot sites. (The opposite directions apply in the southern hemisphere.) Each site becomes a refugium, one as a "cold-surviving refugium" and the other as a "hot-surviving refugium". Canyons with deep hidden areas (the opposite of hillsides, mountains, mesas, etc. or other exposed areas) lead to these separate types of refugia

Your world contains hot and cold refugia. During cold times, as warm-adapted creatures flee south or die out, cold adapted species spread down from the poles and out from their mountain "cold surviving refugia" and retake the newly encoldened lands. Likewise during the hot times - cold species retreat to the cold refugia and hot species migrate up from the south and outwards from their "hot-surviving refugia". It takes a few decades for the change of shifts, which then persists for the next few centuries until the change takes place again.


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