To further study the evolution of paternal care, it is first
important to increase the number of taxa for which com-
parable behavioral data are available. Such data could read-
ily be collected for additional species of Microtusas well as
other genera (e.g.,Synaptomys, Dicrostonyx, Arvicola, On-
datra). Second, we need molecular and morphological char-
acter data for all species studied behaviorally. Finally, with
increasing knowledge of the comparative neurobiological
bases of vole social behavior (e.g., Wang and Insel 1996),
such data should be incorporated into phylogenetic data
matrices and analyses. Such a three-pronged approach has
the potential to make arvicolines a central example for un-
derstanding the evolution of parental behaviors in rodents
(Curtis et al., chap. 16, this volume).
Summary
Rodents display direct parental behaviors (nursing, groom-
ing, retrieving, and huddling) and indirect parental be-
haviors (food caching, nest building, and defending young
against conspecifics and predators). Except for nursing, fa-
thers can perform all of these, and in some species fathers
show levels of care comparable to mothers. Species with
precocial young typically exhibit lower levels of nest build-
ing and pup retrieval than species with altricial young. Lit-
ter size affects aspects of maternal behavior. Mothers of
large litters spend less time in the natal nest and wean their
offspring later than do mothers of small litters. Also, ma-
ternal aggression toward male conspecifics increases with
litter size. Differential prenatal and postnatal investment in
male and female offspring is documented for some species,
but the topic requires further study. Mothers that are preg-
nant while caring for a litter show lower levels of maternal
care than mothers that are not pregnant, but such differ-
ences appear only late in lactation. In several species, pater-
nal presence is associated with decreased maternal care; this
may result from a decreased maternal workload (if males
care for young) or disruption (if males do not contribute
to care). Mothers regulate paternal interactions with young
by excluding fathers from the natal nest; such exclusion
typically occurs on the day of parturition, but may extend
throughout the preweaning period. Male rodents decrease
care when paternity is uncertain and when their mating op-
portunities increase, although data are limited. Experience,
gained either by alloparental care or by caring for young
in successive litters, has little effect on parental behavior.
In contrast, characteristics of the physical environment can
dramatically influence the level of care. Paternal presence
increases survival and growth of offspring in challenging but
not in standard or seminatural laboratory environments;
field data are limited and conflicting. A preliminary phylo-
genetic analysis of parental behaviors in six species of voles
suggests that paternal behavior may have been present in the
ancestor of Microtusand subsequently lost in the subgenus
Mynomes(M. pennsylvanicusM. montanus). Future
phylogenetic, field, and seminatural studies of parental be-
havior are needed, especially for species of Hystricognathi.242 Chapter Twenty