and occur in blind mole-rats (Nevo, chap. 25), North Amer-
ican ground squirrels (Hare and Murie, chap. 29), desert
rodents (Randall, chap. 31), some groups of South Ameri-
can rodents (Lacey and Ebensperger, chap. 34), and African
mole-rats (Faulkes and Bennett, chap. 36).
Experimentation and Hypothesis Testing
Science does not advance by gathering data that support hy-
potheses, but by conducting “strong inference” tests (Platt
1964) of the alternatives. Such tests involve developing com-
peting, critical predictions from each hypothesis, and then
conducting experiments or observations that attempt to fal-
sify these predictions. The hypothesis that is left standing at
the end of this process becomes the front-runner as the most
likely explanation for the phenomenon at issue.
To meet our primary goals, we asked authors to empha-
size their experimental and strong inferential approaches
whenever possible. For example, the role of hormones and
neural control of behavior (Curtis and colleagues, chap. 16)
have been elucidated experimentally. The expression of hor-
mones such as vasopressin and oxytocin are positively cor-
related with monogamy, which in turn affects mating be-
havior, paternal care, and juvenile development. Rigorous
testing of alternative hypotheses is used by Carroll and
Potts (chap. 5) to examine the genetic consequences of mate
choice and sexual selection in house mice via the major his-
tocompatibility complex (MHC), by Drickamer (chap. 9)
to determine the role of chemical signals in accelerating or
suppressing reproduction, and by Sikes (chap. 11) to dem-
onstrate how field and laboratory experimentation was used
to test hypotheses for facultative sex ratio adjustment. Sikes
concludes that the theory for adaptive sex ratio adjustment
in rodents has thus far exceeded the data, and that no mech-
anism for varying the sex ratio adaptively has been discov-
ered. Experimentation is also used to determine what fac-
tors contribute to stress and how stress affects fitness and
demography (Boonstra et al., chap. 12) and how rodents
learn what foods to eat and avoid (Galef, chap. 18). Mc-
Guire and Bemis (chap. 20) use cross-fostering experiments
to discern the role of paternal care in polygynous and
monogamous species of voles. An experimental approach
also is used to address mechanisms of reproductive sup-
pression and self regulation (Krebs et al. chap. 15), scent
marking (Roberts, chap. 22), antipredator processes (Ow-
ings and Coss, chap. 26; Ylönen and Brown, chap. 28), the
genetic basis for aggression in mole-rats (Nevo, chap. 25),
alarm calls (Blumstein, chap. 27; Hoogland, chap. 37), and
behavior conducive to commensal living in rats and mice
(Berdoy and Drickamer, chap. 32).
Proximate Mechanisms and Ultimate CausationWhy do animals do what they do? There are multiple, com-
plementary answers to such a question, which lie at differ-
ent levels of analysis (Tinbergen 1963; Sherman 1988). One
type of answer addresses the mechanistic (hormonal, neu-
ronal) causes of a behavioral phenomenon, while another
addresses the ontogeny of the behavior in each individual’s
lifetime, still a third examines the effects of the behavior on
the individual’s fitness, and the fourth type of answer ad-
dresses the evolutionary history of the behavior. The first
two levels are known as proximateand the latter two as
ultimateexplanations. Complete understanding of any be-
havioral phenomenon requires answers at all four levels
and their integration.
Synthesis of mechanism and function is well illustrated in
chapters 3 (Waterman) and 4 (Solomon and Keane), which
describe how the behavior of one sex becomes a stimulus
for the opposite sex to utilize a particular reproductive tac-
tic, such as wandering versus mate guarding, or territori-
ality versus searching. Alternative reproductive tactics are
typically functions of both social and ecological variables, as
is also illustrated in chapters 6 (neotomine and peromyscine
rodents, Kalcounis-Rüppell and Ribble), 7 (tree squirrels,
Koprowski), 30 (marmots, Armitage), and 34 (South Amer-
ican hystrigonath rodents, Lacey and Ebensberger). In chap-
ter 8, Dobson and Oli explain how reproductive effort is
tied to altricial and precocial development of offspring. The
presence of an opposite-sex parent is the stimulus for sex-
biased natal dispersal in mice, which ultimately prevents in-
breeding and intrasexual competition with male relatives
(Nunes, chap. 13). Male affiliative behavior leading to
monogamy can be explained proximately by neuronal and
hormonal mechanisms and ultimately by the value of pa-
ternal care in contributing to survival of young (Curtis et al.
chap. 16). Although density of voles and mice, or more likely
number of adult females, seems to be a proximate mecha-
nism to suppress reproduction in young females, the ulti-
mate benefit apparently is to conserve reproductive effort
by aborting embryos that would likely be killed by infanti-
cide at birth (Krebs et al. chap. 15). Conversely, Ebensper-
ger and Blumstein (chap. 23) discuss how the act of ejacu-
lation inhibits infanticide by males, which in turn might be
a selective factor for multi-male mating by females in some
species, to confuse paternity and deter infanticide. Finally,
Mateo (chap. 17) uses experimental and observational data
from ground squirrels to demonstrate how learning, matu-
ration, and ecological selective pressures intertwine to shape
the ontogeny of adaptive behavior. Although we gave each
author guidelines and a general subject area to cover, we
found that many authors were able to go beyond a summa-Rodent Societies as Model Systems 5