Unit Eight. The Living Environment
37.3. Genetic Effects on Behavior
Although most animal behaviors are not “hard-wired” instincts, such as those studied by the early ethologists, animal behaviorists who followed these pioneer researchers have clearly demonstrated that many animal behaviors are strongly influenced by genes passed from parent to offspring. In other words, “nature” plays a key role in determining patterns of behavior.
If genes determine behavior, then it should be possible to study their inheritance, much as Mendel studied the inheritance of flower color in garden peas. This sort of investigation is called behavioral genetics.
Studies of Genetic Hybrids
Behavioral genetic studies have revealed many examples of behaviors that seem to be inherited in a Mendelian manner. William Dilger of Cornell University examined two species of lovebirds that differ in the way they carry twigs, paper, and other materials used to build a nest. One species, Fisher’s lovebird, holds nest materials in its beak, while another, the peachfaced lovebird, carries material tucked beneath its flank (tail) feathers. When Dilger crossed the two species to produce hybrids, he found that the hybrids carry nest material in a way that is intermediate between that of the parents: They repeatedly shift material between the bill and the flank feathers. Other studies conducted on courtship songs in crickets and tree frogs also demonstrate the intermediate nature of hybrid behavior; hybrids, possessing alleles from both parental species, produce songs that are a combination of the songs of their parents.
Studies of Twins
The influence of genes on behavior can also be seen in humans by comparing the behavior of identical twins. Identical twins are, as their name implies, genetically identical. Because most sets of identical twins are raised together, any similarities in their behavior might result either from identical genes, or from shared experiences as they grow up. However, in some instances twins have been separated at birth and raised apart in different families. A recent study of 50 such sets of twins revealed many similarities in personality, temperament, and even leisure-time activities, even though the twins were often raised in very different circumstances. These results show that genes play a key role in determining human behavior, although the relative importance of genes versus environment is still hotly debated.
A Detailed Look at How One Gene Affects a Behavior
One well-studied gene mutation in mice provides a clear look at how a particular gene influences a behavior. In 1996 behavioral geneticists discovered a new gene, fosB, that seems to determine whether or not female mice will nurture their young. Females with both fosB alleles knocked out (experimentally removed) will initially investigate their newborn babies, but then ignore them, in stark contrast to the caring and protective maternal care provided by normal females (figure 37.3).
Figure 37.3. A gene alters maternal care.
In mice, normal mothers (a) take very good care of their offspring, retrieving them if they move away and crouching over them. Mothers with the mutant fosB allele (b) perform neither of these behaviors, leaving their pups exposed.
This inattentiveness appears to result from a chain reaction. When mothers of new babies initially inspect them, information from their auditory, olfactory, and tactile senses is transmitted to the hypothalamus. There, fosB alleles are activated, producing a particular protein, which in turn activates both enzymes and other genes that affect the neural circuitry within the hypothalamus. These modifications within the brain cause the female to react maternally toward her offspring. In a general way, the information gained from inspecting the newborn babies can be viewed as acting like a sign stimulus, the fosB gene as an innate releasing mechanism, and the maternal behavior as the resulting action pattern.
In mothers lacking the fosB allele, this innate behavioral pattern is stopped midway. No protein is activated, the brain’s neural circuitry is not rewired, and maternal behavior does not result.
Key Learning Outcome 37.3. The conclusion that genes play a key role in many behaviors is supported by a broad range of studies in many animals, including humans.