et al. 1994; Huchon and Douzery 2001; Honeycutt et al.
2003). The octodontoids are thought to have originated 13
to 18 million years ago (Houchon and Douzery 2001),
perhaps in response to major vegetation changes associated
with the uplifting of the Andes and concomitant changes in
rainfall regimes (Honeycutt et al. 2003).
Within the Octodontoidea, the exact relationship be-
tween octodontids and ctenomyids is disputed. Some au-
thors consider the tuco-tucos to be a subfamily (Ctenomy-
inae) within the Octodontidae (Reig et al. 1990) whereas
others recognize them as a distinct family, the Ctenomyi-
dae (Woods 1993). Recent molecular analyses (Mascheretti
et al. 2000; Gallardo and Kirsch 2001; Honeycutt et al.
2003) support the latter interpretation, with divergence be-
tween the octodontids and ctenomyids occurring ca. 10 mil-
lion years before present, during the late Miocene (Lessa
and Cook 1998). For the purposes of this review, we con-
cur with Wilson and Reeder (1993) in treating the Octo-
dontidae and Ctenomyidae as distinct families and appar-
ent sister taxa to one another.
Although closely allied phylogenetically, octodontids and
ctenomyids exhibit strikingly different patterns of evolu-
tionary diversification. The Ctenomyidae consist of a single
genus (Ctenomys) containing fifty to sixty named forms
(Reig et al. 1990; Woods 1993; Mascheretti et al. 2000).
In contrast, only ten species of octodontids are recognized
(Woods 1993), but these taxa are divided among six genera
(Aconaemys, Octodon, Octodontomys, Octomys, Spalaco-
pus, Tympanoctomys;Woods 1993, although see Hutterer
1994). Thus, while ctenomyids show extensive species-level
diversification but relatively little other intrafamilial phylo-
genetic structure, octodontids are characterized by multiple,
deeper (generic) phylogenetic divisions but little species-
level diversification within each genus. The reasons for these
differences in diversity are unclear. Although early cteno-
myids are thought to have undergone a burst of specia-
tion (Lessa and Cook 1998), rates of diversification within
this lineage are not significantly greater than those for oc-
todontids (Cook and Lessa 1998), suggesting that the dif-
ferent patterns of phyletic diversity in these families are not
due solely to more rapid evolutionary divergence among
ctenomyids.
Ecological diversity
In addition to their distinct patterns of phyletic diversity,
the Octodontidae and Ctenomyidae display markedly dif-
ferent patterns of ecological specialization. Both families
are endemic to subAmazonian South America. The geo-
graphic distribution of ctenomyids, however, is more ex-
tensive (fig. 34.1), suggesting that members of this family
may occupy a greater diversity of habitats. Although the ar-
eas in which ctenomyids are found range from arid, coastal
sand dunes to wet, montane meadows (Redford and Eisen-
berg 1992), the actual variety of environmental conditions
experienced may be less than is implied by this description;
all ctenomyids are subterranean (Reig et al. 1990) and,
hence, the environments in which they live are generally as-
sumed to be similar due to the buffering of surface condi-
tions by subterranean burrows (Reichman and Smith 1987;
Buffenstein 1996; but see Lacey et al. 2000). While some
aspects of ctenomyid ecology clearly differ among species
(e.g., type of soils inhabited, nature and distribution of
plants consumed; Reig et al. 1990; Busch et al. 2000), how
these differences have interacted with the shared challenges
of subterranean life to shape ctenomyid evolution remains
largely unknown.
In contrast, although geographically more restricted,
the octodontids are ecologically more diverse, and include
surface-dwelling and semisubterranean as well as truly sub-
terranean forms. Collectively, these animals occur in a wide
array of habitats, including open grasslands, dense forests,
and extremely arid salt flats (Mares and Ojeda 1981; Red-
ford and Eisenberg 1992). Although most octodontids use
subterranean burrows as nest sites, members of many spe-
cies are thought to spend a substantial proportion of their
time above ground (e.g., Vásquez 1997). As a result, these404 Chapter Thirty-Four
Figure 34.1 Geographic distributions of the Octodontidaeand Ctenomyidae.
Range data were compiled from geographic data provided in Woods (1984) and
Redford and Eisenberg (1992).