THE LIVING WORLD
Unit Eight. The Living Environment
37.5. Instinct and Learning Interact
Some animals have innate predispositions toward forming certain associations. Certain pairs of stimuli can be linked by operant conditioning, others not. For example, pigeons can learn to associate food with colors but not with sounds; on the other hand, they can associate danger with sounds, but not with colors. This sort of learning preparedness demonstrates that what an animal can learn is biologically influenced; that is, learning is possible only within the boundaries set by instinct.
The innate programs that make up an animal’s instincts have evolved because each of them reinforces an adaptive response. The seed a pigeon eats may have a distinctive color that the pigeon can see, but it makes no sound the pigeon can hear. The approach of a predator a pigeon fears may generate noise, but involves no distinctive color.
Behavior Often Reflects Ecological Factors
Knowledge of an animal’s ecology is key to understanding its behavior, as the genetic component of behaviors has evolved to match animals to their habitats. For example, some species of birds, like Clark’s nutcracker, feed on seeds. These birds bury seeds in the ground when seeds are abundant so they will have food during the winter. Thousands of seeds may be buried by a bird and then later recovered, sometimes as much as nine months later. One would expect these birds to have an extraordinary spatial memory, and this is indeed what researchers have found. One Clark’s nutcracker can remember the locations of up to 2,000 seeds, using features of the landscape and other surrounding objects as spatial references to memorize their locations. When examined, Clark’s nutcrackers turn out to have an unusually large hippocampus, the center for memory storage in the brain.
The Interaction Between Instinct and Learning
The way in which white-crowned sparrows first acquire their courtship songs provides an excellent example of the interaction between instinct and learning in the development of behavior. Courtship songs are sung by mature birds and are species specific. By rearing male birds in soundproof incubators provided with speakers and microphones, animal behaviorist Peter Marler could control what a bird heard as it matured, and then record the song it produced as an adult, a recording called a sonogram. When compared to a normal sonogram, in figure 37.5a, he found that white-crowned sparrows that heard no song at all during development had a poorly developed song as adults, shown in the sonogram in figure 37.5b. The same thing happened if they heard only the song of a different species, the song sparrow. But birds that heard the song of their own species sang a fully developed white-crowned sparrow song as adults. This was true even if the young birds also heard the song sparrow song along with their own.
Figure 37.5. Song development in birds involves both instinct and learning.
The sonograms of songs produced by male white-crowned sparrows that had been exposed to their own species' song during development (a) are different from those of male sparrows that heard no song during rearing (b). This difference indicates that the genetic program itself is insufficient to produce a normal song.
Marler’s results suggest that these birds have a genetic template, or instinctive program, that guides them to learn the appropriate song. During a critical period in development, the template will accept the correct song as a model. Thus, song acquisition depends on learning, but only the song of the correct species can be learned.
Although the song template is genetically determined, Marler found that learning also plays a prominent role in song development. If a young white-crowned sparrow becomes deaf after it hears its species’ song during the critical period, it will sing a poorly developed song as an adult. The bird must “practice” listening to himself sing, matching what he hears to the model his template has accepted.
The males of some bird species have no opportunity to hear the song of their own species. In such cases, it appears that the males instinctively “know” their own species song. For example, cuckoos are brood parasites; females lay their eggs in the nest of another species of bird, and the young that hatch are reared by the foster parents. When the cuckoos become adults, they sing the song of their own species rather than that of their foster parents. Because male brood parasites would hear the song of their host species during development, it is adaptive for them to ignore such “incorrect” stimuli. They hear no adult males of their own species singing, so no correct song models are available. In this species, natural selection has provided the male with a genetically programmed song totally guided by instinct.
Key Learning Outcome 37.5. Behavior is both instinct (influenced by genes) and learned through experience. Genes are thought to limit the extent to which behavior can be modified and the types of associations that can be made.