Key concepts

Have you ever woken up on a cold, frosty morning and just wanted to snuggle down deeper under the covers? Other mammals, such as puppies and piglets, do not like being cold either, but they do not have hands or blankets to wrap themselves up. So when these animals get chilled they change their behavior and do things like huddle—curl up close to other animals. In this science activity you'll see just how much huddling can help reduce heat loss.

Want makes a puppy want to sleep in an adorable heap? Snuggling, also known as huddling, does more than create cute puppy piles. Puppies, like other animals, huddle to keep warm. Heat is continuously flowing into and out of everything around you. Just as a ball rolls downhill, heat rolls away, too, flowing from an object that's hotter to one that's colder. If, for instance, you put a cup of hot chocolate in a room-temperature room, the heat will flow from the hot chocolate to the room, cup and table. These surroundings will actually warm up (ever so slightly) from the hot chocolate's heat. When the temperature of the hot chocolate cools enough to reach the surrounding temperature, the heat flow stops.

Heat flows from a hotter object to a colder object in several different ways. For example, when puppies are huddling together to keep warm, they are reducing how much of their bodies (specifically surface area) are exposed to the colder open air, and so are reducing heat loss in two ways: via radiation and convection. A puppy also gains heat another way—conduction—if the puppy it's snuggled up with happens to be warmer.

• Four glass jars or bottles that are the same size and shape, with lids that can be discarded (The lids will be punctured in this activity. Use jars whose lids do not stick out farther than the glass sides of the jars. If using canning jars, you will need rings for the lids.)
• Thermometer with a stem that has an upper range of at least 200 degrees Fahrenheit, such as a candy thermometer
• Large soup pot
• Nail
• Hammer
• Measuring cups
• Water
• Oven mitts
• Towel
• Timer or stopwatch
• Piece of paper and pen or pencil

• Using a hammer and a nail, carefully make a hole in the center of each jar's lid just big enough for the thermometer's stem to fit through. If needed, slightly adjust the shape of each hole (using the nail) so that the thermometer's stem sticks straight—vertically—down into the jar. Why do you think you'd want to measure the core temperature of the jars?
• Fill the jars about three fourths full with equal amounts of water (of the same temperature for each jar). Each jar will be a model of a puppy.
• Fill the large soup pot with enough water so that when three jars are in the pot, the water level within the jars equals the water level outside of the jars (in the pot).
• Be careful when working with the hot water. Adult supervision is required when using the stove.

• Set a timer for 10 minutes or have a stopwatch ready.
• Put the large soup pot on the stove and put one jar inside. Put the thermometer inside the jar (going through the lid's hole) so that the thermometer is held upright inside the water in the jar. Turn on the stove.
• When the temperature of the water inside of the jar reaches 190 degrees F, turn off the stove, immediately remove the jar from the water bath using oven mitts and dry the outside of the jar quickly on a towel.
• Immediately start the timer or stopwatch. Write down the water temperature inside the jar when you do this. Then continue taking water temperature measurements every two minutes for at least 10 minutes. How does the temperature change over time?
• Repeat these steps, but this time put three jars inside of the pot (instead of just one). Put the thermometer into one of the three jars. When the jar reaches 190 degrees F, remove all of the jars, quickly dry them and arrange them so that two sides of each jar are touching two other jars, forming a triangle. Write down the initial temperature in the jar, and then again take temperature measurements every two minutes for at least 10 minutes. How does the temperature of the jar huddled with two other jars compare with that of the one jar that stood alone over time? At what time was the difference in their temperatures the greatest?
Overall, what do your results tell you about how and why puppies and other warm-blooded animals huddle to keep warm?
Extra: Try repeating this activity but compare three jars touching with two jars touching or with three jars separated by an inch. How does the heat loss compare between these different configurations as well as with the solitary jar?
Extra: Think of how you could add "fur" or "fat" to your puppy models in this activity and then try it out. How do these additions affect heat loss?
Extra: You could investigate increased heat loss through convection by blowing a fan on the jars. How does this affect how much heat is lost over time?

Observations and results
Did the three jars huddled together lose less heat over time compared with the one jar alone? Was their difference in temperature the greatest at the end of the 10-minute period?

Over time, you should have seen that the water in the jars—alone and in a huddled group of three—lost heat, but the single jar lost heat faster than the three jars huddled together. The temperature of the water in the jars should have started out at about 190 degrees F, because this was the temperature immediately when they were removed from the pot. As just one example, the temperature of the water in the solitary jar may have then dropped to about 172 degrees F after 10 minutes whereas the temperature of the water in a jar huddled in a group of three may have only dropped to about 176 degrees F. (The exact size and shape of the jars affects how quickly they lose heat, as a larger amount of hot water will take a longer amount of time to cool down compared with a smaller amount of the same-temperature water; and the shape of the jars affects their total surface area.) When the jars are close together, just like puppies snuggling together, they help keep each other warm, primarily through conduction.

More to explore
How Do Animals Spend the Winter?, from Science Made Simple
How Penguins Survive Cold Conditions, from Paul Ward
Energy, Heat Flow and Life , from David Watson
Huddle and Cuddle: How Puppies Keep Warm, from Science Buddies

This activity brought to you in partnership with Science Buddies