Do lemmings really leap into the sea en masse?
— The famous cliff-fall footage was staged for the camera
Lemmings are small rodents that live on grasslands near the Arctic. The story that "when their numbers grow too large, they march in lines and throw themselves into the sea" is famous the world over. But lemmings never choose death on their own. What really happens is a far stranger cycle: every few years their numbers explode, scatter, and then crash.
People sometimes describe a crowd all drifting the same way as "marching like lemmings" — following the one in front without thinking, all tumbling off a cliff together. Many of us picture exactly that image.
Trace this image back and you land on a nature documentary released in the US in 1958. A scene of lemmings plunging one after another into the water from a clifftop left a lasting impression.
But that scene, it turns out, was not something wild lemmings did on their own. So what was the "real event" behind the rumor?
The rumor is a twist of two separate facts
Lemmings surge in number once every few years. In a boom year, the young set off separately in search of new ground. Along the way some swim across rivers or lakes and run out of strength. Death isn't the goal — it's a failed leg of the journey.
Reports say that in the famous film scene, the crew drove lemmings they had brought in toward the cliff edge. The staged footage spread as a "nature record" and cemented the rumor for good.
So it's true that "there are years with mass movement and mass death." What got added afterward was the idea that it was "on purpose." Let's look at this step by step.
Why do they multiply so fast?
Lemmings reach breeding age remarkably quickly. Within a few weeks of birth they can bear young of their own, with several pups per litter. What's more, they dig tunnels under the snow and keep breeding there through winter if conditions allow. Under the snow it's far warmer than outside, and predators have a harder time finding them.
Pups become parents almost at once, and their pups do too. Once this "rabbits-multiplying" chain gets going, numbers grow not by addition but by multiplication. Look at Figure 1. Even if the number only doubles each generation, after 8 generations it has grown 256-fold.
In reality, many pups are eaten by predators or die of cold. Even so, when just slightly more survive than die, numbers can surge the way Figure 1 shows. In a lean year there may be barely a handful across a wide grassland; in a boom year, they're said to be scurrying past your feet everywhere you look.
What happens in a boom year?
When numbers get too high, food — grass and moss — runs short, and fights between rivals increase. The young in particular then set off, scattered, in search of new ground. Each one is moving on its own, but when they funnel along the same valleys or shorelines, it can look like a single marching column.
Lemmings can swim, and they will try to cross a river or lake if one lies in their path. If the water is too wide or too rough, some run out of strength before reaching the far side. It's no surprise that people who saw many bodies washed up on shore concluded they had "entered the water on purpose."
And after every boom comes a crash. As Figure 2 shows, numbers plunge right after a peak year, followed by one or two lean years. In many regions this cycle repeats roughly every 3 to 4 years.
The crash isn't down to a single cause. Predators such as stoats and weasels multiply by following the lemmings up. Because predator numbers stay high for a while even after lemmings decline, lemmings struggle to recover quickly. The time it takes for grazed-down grass and moss to grow back is also thought to play a part.
The sudden mass appearance of lemmings, and their equally sudden disappearance, has startled people for centuries. Some 16th-century European books reportedly claimed they fell from the sky during storms. That's how inexplicable the sudden booms and crashes once seemed.
Snowy owls and Arctic foxes in the Arctic rely on lemmings as a main food source. In lemming-rich years they reportedly raise large broods; in lean years they may barely breed at all. The rise and fall of a small rodent ripples through the lives of Arctic wildlife as a whole.
Summary
The story of lemmings leaping into the sea en masse is a myth. What really happened was a periodic boom every few years, followed by dispersal and then a sharp crash. Staged footage was layered on top of that, and together they built the legend of creatures "choosing death on their own."
Lemmings aren't marching toward death — they're searching for somewhere to live.
What looked like tragedy was the fate of a creature that multiplies by multiplication.
For why it stays warm under the snow, see "Why does it stay around 0°C under the snow even when it's -20°C outside?"; for another example of how good or bad food years change animal behavior, see "Why do bears come down into towns in autumn?"
- Fold a sheet of paper in half. The stack is now 2 sheets thick — "doubling once per generation."
- Keep folding in half and count how many folds you manage before you can't anymore. Most people hit a limit at 7 or 8 folds.
- At 8 folds, the stack is 256 sheets thick. For ordinary paper that's about 2.5 cm — enough to feel, by hand, how fast repeated doubling grows in a short time.
Once the crease gets too stiff, don't force it — stop there. Compare your fold count with family members: notice how just one extra fold doubles the sheet count.
Want to go deeper? — Terms, formulas, and textbook connectionsWe mark which level each part belongs to, from middle-school science up to university-level courses
- MSCovered in middle-school science
- HSCovered in high-school biology
- HS+Advanced high-school content, or a textbook sidebar topic
- Univ.Not covered in high school — university-level ecology/mathematical biology
- ResearchNot yet settled even at university level — an active research question
MSTerminology: this phenomenon has names
- Food chain: the "eats and is eaten by" relationship. Lemmings eat grass and moss, and are eaten by stoats and owls.
- Population cycle: when a species' numbers rise and fall on a roughly fixed schedule. Lemmings are famous for 3–4-year cycles.
- Dispersal: moving away from where you were born to a new area. The mass movement of lemmings in boom years is an example of dispersal.
MSHSChecking the math: how many-fold growth happens in a year
Starting from a lean-year density, we estimate with simple multiplication how crowded things get after a year. The figures are illustrative assumptions meant to show the mechanism, not precise measurements.
| In symbols | N = N₀ × Rⁿ |
| In words | Number after n generations = starting number × (per-generation multiplier) multiplied together n times |
| Where it comes from | Since newborns become parents too, the increase is proportional to the current number. This is the exponential-growth formula, where the same multiplier compounds each generation. |
| Symbol | Meaning and units |
| N | Number after n generations (individuals per hectare) |
| N₀ | Starting number (individuals per hectare) |
| R | Per-generation multiplier (unitless) |
| n | Number of generations |
| Lean-year density N₀ (estimate) | about 1 individual per hectare |
| Generation length (birth to first litter) | assumed ≈ 6.5 weeks |
| Per-generation multiplier R (survivors after predation) | assumed 2× |
| Area of 1 hectare | 10,000 square meters |
| Generations per year, n | 52 ÷ 6.5 = 8 generations |
| Multiplier over 2 generations | 2 × 2 = 4× |
| Multiplier over 4 generations | 4 × 4 = 16× |
| Multiplier over 8 generations (R⁸) | 16 × 16 = 256× |
| Density after 1 year, N | 1 × 256 = 256 individuals (per hectare) |
| Area per individual | 10,000 ÷ 256 ≈ 39 square meters |
The square root of 39 square meters is about 6.2. So a year earlier there was only 1 lemming per 100-meter square; a year later, roughly 1 per 6-meter square. If the multiplier were just 2.5× instead, after one year the total would be about 1500×. Small differences change the outcome enormously, which is why year-to-year snow and food conditions cause such large swings in growth.
HSHS+Growth has a ceiling — and a repeating cycle
HSHigh-school biology teaches that as a population grows, food and space run short and growth slows down. Graphed, this traces an S-shaped curve, called the population growth curve (logistic curve). The calculation above captures the early part of that curve, before crowding sets in.
HS+Lemmings, though, don't settle onto the S-curve — they overshoot the ceiling by a wide margin before crashing. One reason is "time lag": predator numbers rise, and later fall, with a delay behind the lemmings. The slightly offset, chasing rise-and-fall between predator and prey appears in textbook sidebars as the predator–prey cycle.
Univ.Predator–prey equations, and density dependence
The basic equations describing how predator and prey numbers chase each other are the Lotka–Volterra equations. These yield solutions where the two populations rise and fall periodically, offset from one another. To explain the actual lemming cycle, models add density dependence (growth slowing as crowding increases) and time lags on top of this. In Greenland collared lemmings, predation by a single stoat species alone has reportedly reproduced the 4-year cycle quite well.
📖 For the derivation and further reading: Lotka–Volterra equations (Wikipedia, Japanese) / Exponential growth (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- Do predators or food drive the cycle? Studies disagree by region — some point to predators as the main cause, others to grass and moss supply. The question remains unsettled.
- Could warming erase the cycle? In southern Norway, clear cycles have been largely absent since the 1990s. Reports link this to rising winter temperatures making it harder to maintain livable space under the snow.
- What triggers dispersal? Is it crowding itself, rising conflict, or dwindling food? What decides which individuals set off, and when, is still not well understood.
In other words, even this article describes things "as currently understood." The rise and fall of this small rodent remains one of ecology's great puzzles, studied for nearly a century.
Textbook connections, by level
| Level | Subject/unit | Where in this article |
|---|---|---|
| MS | Science, "Relationships among living things (food chains)" | How predator increases reduce lemming numbers |
| HS | Biology, "Populations and communities" | The growth curve, the rabbit-math calculation |
| HS+ | Biology (advanced), "Predator–prey cycles" | Delayed predator rise and fall, Figure 2 |
| Univ. | Ecology / mathematical biology | Lotka–Volterra equations, density dependence |
| Research | Population ecology / climate-change ecology | Causes of the cycle, changes under warming |
| — | Everyday connections | Rethinking the "everyone heads for the cliff" metaphor; not taking footage at face value |
- Wikipedia, "Lemming" (レミング, Japanese)
- Kausrud, K. L. et al. (2008) Linking climate change to lemming cycles. Nature 456
- Gilg, O., Hanski, I., Sittler, B. (2003) Cyclic dynamics in a simple vertebrate predator-prey community. Science 302
- Elton, C. (1942) Voles, Mice and Lemmings: Problems in Population Dynamics. Oxford University Press
- Wikipedia, "Lotka–Volterra equations" (ロトカ・ヴォルテラの方程式, Japanese)
※This article is a general-audience science explainer. The figures given are illustrative estimates meant to aid understanding, not precise data. The multiplier and generation length can vary widely by species, region, and year.