Evolution of Paternal Behavior in Voles
Males of some arvicoline species exhibit care behaviors un-
reported in other species of Microtus. These behaviors are
routinely observed in laboratory environments and are con-
sistent with field data of males sharing a nest with females
and young. Do these behaviors represent a derived condi-
tion within Microtus,and if so have they evolved more than
once? Although relationships among arvicoline species re-
main unclear (Hinton 1926; Anderson 1985; Musser and
Carleton 1993; Martin et al. 2000; Conroy and Cook 2000),
we have comparable behavioral data for four species of Mi-
crotusand two outgroups: the sagebrush vole (Lemmiscus
curtatus) and red-backed vole (McGuire and Novak 1984,
1986; Hofmann et al. 1989; McGuire 1997). These data
permit us to make a preliminary phylogenetic analysis based
on behavioral data (fig. 20.2).
Our analysis groups prairie and pine voles, a clade that
is not reported in either morphological or molecular phy-
logenetic studies of arvicolines. Of more interest is our re-
covery of the subgenus Mynomes,which includes Microtus
pennsylvanicusand M. montanus,a group typically recov-
ered in morphological and molecular studies (e.g., Musser
and Carleton 1993; Conroy and Cook 2000). An interest-
ing pattern in our analysis is the loss of paternal behavior
in subgenus Mynomes(Character 7). Interpreting this as
a loss (rather than three separate gains of paternal behav-
ior) also is supported by the observation that montane and
meadow vole males facultatively spend time in the natal
nest, depending on the testing environment (fig. 20.1).Parental Care 241Figure 20.2 Phylogeny of parental behaviors in species of arvicolines for which comparable behavioral data are available (sources are McGuire
and Novak 1984, 1986; Hofmann et al. 1989; McGuire 1997). The tree is based on an exhaustive search (PAUP*4.0b10, Swofford 2003; all charac-
ters treated as unordered; red-backed vole specified as outgroup). This single most parsimonious tree has a length of 9 and an unscaled consistency
index (CI) of 0.89. Character definitions: 1. Tenacious Nipple Attachment. Pups cling so tightly to nipples that they remain attached even when
mother moves. 2. Number of Mammae 8. Number varies from four (pine vole) to six (prairie vole) to eight (other species). Character 3. Pups Eat
Solid Food Early. The day that pups first eat solid food varies from 13 or 14 days postpartum (meadow, montane, and sagebrush voles) to 15 or
17 days (red-backed, prairie, and pine voles). Character 4. Pups Weaned Early. Weaning varies from 13 or 14 days postpartum (meadow and mon-
tane voles) to 19, 20, 21, or 23 days (red-backed, prairie, pine, and sagebrush voles). Character 5. Mother Spends Little Time In Nest. Montane and
meadow vole females spend 45% and 50% of their time in the natal nest during first ten days postpartum. Values for pine, sagebrush, prairie, and
red-backed vole females are 65%, 75%, 81%, and 81%. Character 6. Mother Shares Nest with Weanlings. Sagebrush, prairie, and pine vole fe-
males continue to nest with young after weaning; red-backed, montane, and meadow vole females nest separately from weaned young. Character- Father Spends Time in Natal Nest. Red-backed, pine, prairie, and sagebrush vole males spend time in the natal nest (27%, 32%, 63%, and 69%
respectively); montane and meadow vole males spend less than 1% of their time in the natal nest. Character 8. Male and Female Coordinate Care.
Of six species studied, only prairie voles coordinate their visits to the natal nest so that pups are rarely left alone.