This American Robin has to detect which fruits are safe to eat, or he may become intoxicated.
Copyright by Ann Cook

Toxic Lunches

What would happen if you left fruit sitting out on the kitchen counter for a few weeks? Mold would probably grow on it after several days, and the sight of that would probably keep you from eating it.

But what if mold didn't grow? How would we know if something had happened to the fruit to make it dangerous for us to eat? We would use our senses. The fruit might look, smell, feel, or taste different. Have you ever wondered if animals in nature face similar dangers? The answer is yes!

Berries Gone Bad
Fruit grows on trees during spring, summer, and fall. When winter sets in, the fruit stops growing, and sap and other fluids from the tree stop flowing into the fruit. Sometimes molds form, but not once the temperatures freeze. As fruits get old and stop getting fresh supplies of sap, they often shrivel and get wrinkled, but if they taste and smell okay, birds still eat them. Some fruits even get tastier as winter progresses! Usually this is good, because robins and several other species need fruits to survive in winter. But once in a while fruits become toxic. Robins that eat toxic fruits can get sick and sometimes even die.

Toxic Fruit
What causes fruits to become toxic? One set of culprits are microscopic bacteria and yeast spores that are found in the air all around us. They feed on the fruit sugars and other carbohydrates. As they grow, they produce organic compounds such as alcohol in a process called fermentation. Some kinds of fruits (e.g., tatarian honeysuckle, nighshades, Chinaberries, and chokecherries), produce other kinds of toxins (e.g., saponin, solanine, and cyanide) that that can paralyze nerve centers, cause problems ranging from vomiting to convulsions, and even kill animals.

When humans or animals eat or drink take in certain toxic chemicals (like alcohol), they become intoxicated. Sometimes we laugh at intoxicated people or animals because their movements are uncoordinated and awkward. But these responses happen because parts of the nervous system are being paralyzed. When birds are intoxicated, they have trouble perching, hopping or walking, and controlling their flight. They are also slow to notice and react to predators and other dangers, and often crash into branches and each other.

How Birds Protect Themselves
Ornithologists used to believe that birds had a very poor sense of smell, but researchers have discovered that some songbirds are able to detect odors fairly well. This ability can help birds detect fermented berries. Of course, odors are much easier to detect when berries are warm than when they are frozen. Think of how a frozen turkey smells compared to a cooked one!

Birds also protect themselves from the effects of bad fruits because they tend to wander, especially during the seasons when they feast on fruits. This means they aren't in one area eating fruits from the same trees for too long. Normally a few fermented morsels won't cause too much trouble. It's when food is scarce or when birds stay in the same area for several days that they are most likely to eat too many bad berries. This is especially true when fruits are frozen and odors are hard to detect.

(Note: This behavioral adaptation probably developed over thousands or even millions of years. Those birds that had a tendency to wander were more likely to live and pass their genes — and that behavior — on to future generations.)

Try This! Journaling Questions
  • If fruits can become toxic, why do you think robins and other birds haven't given up on this food source?
  • What do you think some advantages might be of a having fruit diet in winter?
  • What things do you and your family do to make sure the food you eat is safe (for instance, wash fruits and vegetables)?

National Science Education Standards

  • Organisms have basic needs.
  • All animals depend on plants. Some animals eat plants for food.
  • All organisms must be able to obtain and use resources, grow, reproduce, and maintain stable internal conditions in a constantly changing external environment.
  • An organism's behavior evolves through adaptation to its environment.
  • Biological evolution accounts for the diversity of species through gradual processes over many generations.