Adaptations for Survival

In fall, a snowshoe hare’s brown fur turns white over several weeks. On snow it is hard to see, but what if the snow melts early and the hare is still white?

Snowshoe hares live in snowy forests in Canada and the northern United States, including Montana. Scientists there put radio collars on hares and checked on them each week. They asked a plain question. Does a coat that stands out cost a hare its life?

Explore: change when the snow melts

In this model, a hillside goes from fall to spring. Each dot is a hare. You choose the week the snow melts and when the hares change color. A warmer climate switch makes the snowy season shorter. Hares that stand out are highlighted, and a bar shows how many hares are lost to predators each week.

Dot color = coat color. Orange ring = coat and ground differ by 60 points or more. ✕ = lost this week. Dashed circle = lost earlier. Chart: bars = this run, black ticks = expected.

When does the snow melt?

Early Normal Late

When the hares change color: usual timing

3 weeks earlier3 weeks later

Warmer climate: a shorter snow season

Week to show: week 1 of 22

Run season Resample Reset

Season’s end: press Run season, or drag the week slider to the last week, to see how many hares are alive.

Made up for this sketch: 100 hares; 22 weeks that are not real dates; snow arrives in week 6 and melts in week 12, 15 or 18; a warmer climate makes snow arrive one week later and melt one week sooner; a coat change takes 4 weeks; hares differ by up to 2 weeks either way; chances between the lesson’s three values are joined by straight lines; a hare that predators spot is lost; the color-change slider moves every hare’s fall and spring change together, by up to 3 weeks; a hare is ringed when its coat differs from the ground by 60 points or more (a threshold chosen for the sketch).

In this sketch, the more a coat differs from the ground, the lower the chance of surviving a week. The lesson’s Montana estimates are 96 percent (matched), 92 percent (60 points different) and 89 percent (100 points different). The model shows only color, covers one season, and shows no inheritance or change over generations. They are model estimates from two Montana sites, not counts, and they do not show that coat color caused each death.

Try this.

1. Set the snow to melt early, and keep the hares’ color change normal. Predict which hares will be lost most. Then run it and check.
2. Shift the hares’ color change earlier. Predict whether more hares will survive the spring. Then test it.
3. Turn on the warmer climate switch. Write down how many hares you think will survive. Then run it and compare.

The model shows only color. Real predators also use smell, sound and movement, and real hares differ in many ways.

Explain: what an adaptation is

Look at a snowshoe hare’s back feet. They are large, and fur covers the bottoms. Big feet help the hare move on top of soft snow. The fur helps keep its feet warm.

An adaptation is an inherited trait that helps a living thing survive and reproduce in its ecosystem. Inherited means it passes from parents to their young. Scientists sort adaptations into two groups.

• A physical adaptation is a feature of the body: a body covering, a color, or a structure.
• A behavioral adaptation is an action or a pattern of actions.

Adaptation	Type	How it helps
Hare’s large, furry back feet	Physical (structure and covering)	Moves on snow; keeps feet warm
Hare’s white winter coat	Physical (coloration)	Harder for predators to see on snow
Eastern box turtle’s shell	Physical (covering)	Shields its soft body from predators
Great blue heron’s long legs	Physical (structure)	Wades into water to catch fish
Monarchs flying south in fall	Behavioral (migration)	Reach a warmer place for winter
Groundhog’s long winter sleep	Behavioral (hibernation)	Needs much less food while food is scarce

The hare’s white coat is camouflage, coloring that helps an animal blend in with its background. Blending in lowers the chance that a predator notices the hare. It does not make the hare invisible.

Bird beaks work a bit like tools. A cardinal’s short, thick beak cracks seeds. A hummingbird’s long, thin beak reaches nectar deep in a flower. A heron’s long, sharp beak grabs fish. The comparison breaks here: you can put a tool down, but a bird cannot swap its beak.

Explain: behaviors, trade-offs and who changes

Migration is a regular journey between places used in different seasons. Many monarch butterflies pass through Virginia in fall on their way to Mexico. The monarchs that make this trip have never made it before. Hibernation is a deep, inactive state some animals enter for winter. A hibernating groundhog’s heart beats much more slowly, and its body cools. So hibernation changes the whole body, not only what the animal does. Many behaviors like these are instinctive: animals do them without being taught. How animals respond to seasons is the topic of the next lesson.

A trade-off means a trait helps in one way but costs in another. A white coat hides a hare on snow. On bare brown ground, the same coat stands out. A box turtle’s heavy shell protects it, but it is also heavy to carry. No living thing is perfectly adapted.

Hares in one forest are not all alike. In Montana, some hares turned white earlier than others. In some places with little snow, snowshoe hares stay brown all winter. These differences between individuals of a species are called variation.

A single hare cannot decide to change color. Its coat changes on a yearly schedule, day length seems to help set it, and snow can nudge the timing a little. Montana studies found no sign that white hares moved to snowier spots. Change in an inherited trait happens to a population, all the members of one species in one area. Hares whose inherited timing fits the snow may survive and have more young. Over many generations, that timing can become more common. This is natural selection, which you will study in a later lesson. If the climate changes faster than that, a population can struggle.

Elaborate: survival of matched and mismatched hares

Scientists followed 186 snowshoe hares with radio collars at two sites in Montana. Each week they found each hare by its radio signal. They scored how white its coat was and how much snow covered the ground. Then a math model estimated the chance of surviving one week for hares at each level of mismatch.

Coat compared with the ground	Difference in whiteness (percent)	Estimated chance of surviving one week (percent)
Matched (white on snow, or brown on bare ground)	0	96
Partly mismatched	60	92
Fully mismatched (for example, all white on bare ground)	100	89

Source: Zimova, Mills and Nowak, Ecology Letters (2016), Discussion and Figure 3. Hares were near Seeley Lake (2009 to 2012) and Gardiner (2010 to 2012), Montana. These are rounded estimates from a model, not counts of hares.

A second team tested color directly, with clay mice instead of live ones.

Model color	Ground	Attacks recorded (number)
Dark	Light beach sand	5
Light	Light beach sand	2
Light	Dark inland soil	16
Dark	Dark inland soil	5

Source: Vignieri, Larson and Hoekstra, Evolution (2010), Table 1. Models sat out in the field at Topsail Hill Preserve State Park (beach) and at an inland site, in Florida.

1. By how many percentage points does weekly survival drop from matched to fully mismatched hares?
2. Imagine 100 matched hares and 100 fully mismatched hares. About how many in each group would you expect to be alive after one week?
3. On each kind of ground, which model color was attacked more? What fraction of all 28 attacks hit models that did not match?

Design problem

You cannot test live hares at school. Plan a safe classroom test of whether matching the background protects prey, using paper “prey” and classmates as “predators.”

One way to do it

• Question: Are paper squares that match the background picked up less often than squares that do not?
• Independent variable: whether a square matches the background (white squares and brown squares on a white sheet, then on a brown sheet).
• Dependent variable: the number of each color picked up.
• Constants: 20 squares of each color, each 1 cm by 1 cm, a sheet about 1 m by 1 m, and 15 seconds per turn.
• Control: the same mix of squares on both backgrounds, so color matching is the only change.
• Repeated trials: five classmates each take three turns on each background. Find the mean for each color.
• Safety: use only paper, keep the floor clear so no one trips, and pick up every square afterward.

If matching squares are picked up less on both backgrounds, the result supports camouflage. Your test is a model: real predators also use smell and movement.

Evaluate: check your thinking

When the snow melts early, a white hare stands out on brown ground. In Montana, mismatched hares had a lower estimated chance of surviving each week. Snow can nudge a single hare’s timing only a little; only a population can shift its timing much, over many generations, if hares vary in an inherited way.

Science practices you used: making predictions and testing them with a model, reading data tables, calculating a difference in percentage points, planning a fair test with constants and repeated trials, and naming the limits of a model and of the data.