American Robin has to detect which fruits are safe to eat, or he
may become intoxicated.
by Ann Cook
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.
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!
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.
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.
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.
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.
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.)
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)?
Science Education Standards
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.
evolution accounts for the diversity of species through gradual processes
over many generations.