Chapter 1: An Evolutionary Approach to Animal Behavior
1.1 Use the data in Figure 1.4 and information in Box 1.1 of the textbook to outline
the history of the polygynous mating system of the montane vole. Why does the
meadow vole also exhibit polygyny? How similar genetically are the montane and
meadow voles? The red-backed vole and the montane vole?
One hypothesis for the evolution of vole mating systems is that the ancestor of the group
of species shown in Figure 1.4 was polygynous but that this trait was then lost in a
subsequent (monogamous) ancestor that gave rise to the cluster of species from the
sagebrush vole to the meadow vole. But the monogamous lineage eventually produced a
polygynous species that gave rise to both the montane and meadow voles. The mating
system of this recent ancestral species has been retained in the montane and meadow
voles, which are highly similar genetically, thanks to their recent common ancestor.
1.2 The great behavioral biologist Niko Tinbergen said that there were four main
questions for behavioral researchers [1196]. I will paraphrase these questions as
follows:
1. How does the behavior promote an animal’s ability to survive and reproduce?
2. How does an animal use its sensory and motor abilities to activate and modify
its behavior patterns?
3. How does an animal’s behavior change during its growth, especially in
response to the experiences that it has while maturing?
4. How does an animal’s behavior compare with that of other closely related
species, and what does this tell us about the origins of its behavior and the
changes that have occurred during the history of the species?
Place these questions within the four levels of analysis framework and then assign
each to the proximate or ultimate category. If you heard that because evolutionary
questions are “ultimate” ones, they are therefore more important than questions
about proximate causes, you would respectfully disagree. Why?
Question 1 deals with issues of adaptive value (ultimate); Question 2 focuses on the
physiological level of analysis (proximate); Question 3 has to do with developmental
causes of behavior (proximate), while Question 4 deals with the historical origins and
modification of behavior (ultimate).
Although ultimate sounds more important than proximate, both levels of analysis are
essential for a complete understanding of behavior; ultimate analyses complement
proximate ones, and vice versa; a total understanding of the ultimate causes of behavior
would still leave a huge explanatory gap that could only be filled by information on the
immediate causes of behavior.
1.3 When a female baboon copulates, she vocalizes loudly, but her cries are longer
and louder if her partner happens to be a high-ranking “alpha” male [1085]. A
primate researcher has suggested that females cry out more vigorously when
copulating with top males because this warns low-ranking baboons to stay clear.
(Subordinate males sometimes harass mating pairs to such an extent that the
copulation ends prematurely, but if that happens, they may be attacked by the
dominant male whose mating has been so rudely interrupted.) The same researcher
also says, however, that the more vigorous cries may simply reflect the fact that
females are more strongly stimulated by the larger, more energetic, alpha males.
Are the two explanations really in competition with each other? Explain why they
could both be right and how they could be shown to be complementary.
What we have here is a dispute based on thinking that proximate and ultimate
explanations are in competition when in fact they are complementary. The proximate
cause of the female’s loud calls could be related to the nature of stimulation provided by
a dominant male; the adaptive consequence of giving those calls in response to that
stimulation could be a reduction in interference from subordinate harassers.
1.4 Imagine that it would be possible to inject prairie voles with a chemical that tied
up the V1-a receptors in the ventral pallium. If that experiment were done, what
outcome would result in the rejection of the vasopressin receptor hypothesis for
monogamy in the prairie vole? Which is better, a tested hypothesis that is rejected,
or a tested hypothesis that is accepted?
The vasopressin receptor hypothesis yields the prediction that if these receptors were
blocked, then the intense social bonding that normally occurs between males and their
sexual partners would not occur. If it did occur, then the hypothesis would have to be
rejected.
Although human psychology being what it is, we tend to think that accepted hypotheses
are somehow superior to ideas that have to be thrown out, nevertheless, science advances
whenever a hypothesis is adequately tested no matter whether the outcome is the
rejection or acceptance of the explanation.
1.5 Take the material in the paragraph on page 5 of the textbook in which work by
Larry Young’s team on the V1aR gene hypothesis for prairie vole monogamy is
described. Rewrite this material so that you present a single sentence each for the
question about a cause that motivated the research, the hypothesis about what the
cause was, a prediction that was derived from the hypothesis, the test evidence that
was gathered to check the prediction, and the conclusion that the researchers
accepted on the basis of their research.
Causal question: What causes prairie voles to be monogamous?
Hypothesis: The information in a gene, the V1aR gene to be specific, contributes to the
development of monogamous behavior in the prairie vole.
Prediction: If one were to do an experiment in which cells in the appropriate portion of
the brain of the prairie vole were to receive extra copies of the gene in question, these
genetically altered individuals should behave in an even more monogamous fashion than
unaltered prairie voles.
Test evidence: The experiment, when done, revealed that male prairie voles with extra
copies of the V1aR gene did, in fact, bond more tightly with a female than genetically
unchanged voles.
Scientific conclusion: The V1aR gene does contribute to the development of
monogamous behavior in the prairie vole.
1.6 Here’s an imaginary quote from a scientific paper: “We hypothesized that house
mice that received the prairie vole V1aR gene should exhibit a greater tendency
toward monogamy than others that were not genetically modified in this fashion.”
What’s the problem here?
The problem is that the statement is a prediction, not a hypothesis. The claim about what
should happen as a result of a planned experiment with house mice is a claim about the
expected results of the experiment, not a tentative explanation of why monogamy occurs
in prairie voles.
1.7 Imagine that a mutation occurs this year in the gene that codes for the V1-a
receptor protein in prairie voles. The altered protein increases the tendency of a
male to form social bonds with a mate, in effect making him somewhat more
monogamous than the typical male prairie vole. What is required if enhanced
monogamy is to become more common in this species over time? If monogamous
males have 3.7 pups on average during their lifetime while enhanced monogamous
males have 4.1 on average, will the allele associated with enhanced monogamy
necessarily make up an increasing proportion of the population? (“No” is the
correct answer. Why? Hint: How does natural selection “measure” individual
reproductive success?) Now imagine two types of prairie voles, one type that usually
lives for 1.5 years and another for 0.8 years. Would it ever be possible for the one
that died sooner to replace the longer-lived type over time?
Any change in monogamous behavior will spread only if individuals carrying the altered
gene that influences the development of their behavior have more surviving offspring
than others with a different hereditary basis for their behavior. The number of babies
individuals have is not necessarily an indicator of the number of surviving offspring they
will have, which is why males with 4.1 babies on average will not necessarily leave more
copies of their alleles than males with 3.7 babies on average.
1.8 If you wanted to invent the term “genetic success” for use in evolutionary studies
based on natural selection theory, what definition would you come up with?
An individual’s genetic success could be quantified as the number of copies of that
individual’s genes passed on to the next generation as a result of its attributes, including
its behavior.
1.9 Lemmings are small rodents that live in the Arctic tundra. Their populations
fluctuate wildly. At high population densities, large numbers leave their homes and
travel long distances, during which time many die, some by drowning as they
attempt to cross rivers and lakes. One popular explanation for their behavior is that
the travelers are actually committing suicide to relieve overpopulation. By heading
off to die, the suicidal lemmings leave shelter and food for those who have stayed
behind. These surviving individuals will perpetuate their species, saving it from
extinction. What theory was used to produce this hypothesis? How would George C.
Williams use Gary Larson’s cartoon (Figure 1.13 of the textbook) to evaluate the
hypothesis critically?
The idea that lemmings commit suicide to help their species as a whole is based on group
selection theory. The problem is that suicidal individuals are said to die to help others,
which should have the result of eliminating any distinctive alleles possessed by the
suicidal types. In short order, only individuals that did not commit suicide at high
population densities would be left and their alleles would predominate in the population.
Natural selection typically trumps group selection in causing evolutionary change to
occur.
1.10 Male Hanuman langurs apparently use a rule of thumb when deciding which
infants to kill: they attack youngsters of females that they did not copulate with
prior to the birth of those offspring. In light of this finding, some observers have an
explanation for why it is that after a takeover pregnant females may mate with
newcomers. These researchers have suggested that the females’ behavior may create
confusion about the paternity of their babies, the better to limit male infanticidal
behavior. If females also engage in paternity confusion when their band contains
several adult males, what predictions can you make about (1) the duration of female
receptivity each reproductive cycle, (2) the relationship between the period when
females are in heat and the timing of ovulation, and (3) the occurrence of
copulations by subordinate males as well as the dominant male langur, who
attempts to monopolize sexual access to the female?
(1) If the paternity confusion hypothesis is correct, female receptivity should last longer
than the period in which egg fertilization can occur. (2) Ovulation and estrus need not be
closely correlated. (3) Subordinate males should succeed in copulating with females,
although not necessarily when ovulation is actually taking place.
1.11 In an assassin bug, a predatory insect, males stand guard over clutches of eggs
that females (usually their mates) have laid on leaves [1186]. The males protect the
clutch against parasitic wasps that attempt to lay their eggs in assassin bug eggs.
From time to time, a male stabs one of the eggs in his clutch with his beak, injecting
it with digestive enzymes before drawing out the fluid. Needless to say, this act
destroys the egg and so constitutes a kind of infanticide. If infanticide of this sort
has evolved through sexual competition for quicker reproduction, as in Hanuman
langurs and lions, how should males treat clutches laid by their copulatory partners
versus those that the males “adopted” when they forced another male to leave an
egg mass he had fertilized, something that happens occasionally? Once you have a
prediction in hand, consider these two findings: (1) the parentage of the eggs has no
effect on the number eaten, which increases steadily as the period of egg guarding
lengthens, and (2) males do not lose weight while egg guarding, even though they
cannot feed elsewhere. Are these findings relevant for the quicker reproduction
hypothesis? Do they support another hypothesis for the tendency of males to eat
some of the eggs under their care?
The prediction is that males should tend to destroy the eggs fertilized by other individuals
rather than consuming the eggs they have fertilized themselves.
However, the failure of males to discriminate against eggs fertilized by other males
suggests that sexual competition is not responsible for the fact that males destroy some
eggs under their care. Instead, it appears that males eat some eggs in order to maintain
body condition during the long period when they are forced to remain close to the brood
they are guarding.