Morphological description and comparison of sperm from eighteen
species of cricetid rodents
Luis F. Rossi,† Noé u. de La saNcha,†, JuaN P. Luaces, daNieLa Y. estevez, aNd MaRia s. MeRaNi*
Laboratorio de Biología Cromosómica, Facultad de Medicina, Universidad de Buenos Aires, C.P. ABG1121, Ciudad Autónoma
de Buenos Aires, Argentina (LFR, JPL, DYE, MSM)
Consejo Nacional de Investigaciones Cientíﬁcas y Técnicas (CONICET), C1425FQB, Ciudad Autónoma de Buenos Aires,
Argentina (LFR, JPL, MSM)
Department of Biological Sciences, Chicago State University, Chicago, IL 60628, USA (NUDLS)
Integrative Research Center, The Field Museum of Natural History, Chicago, IL 60605, USA (NUDLS)
* Correspondent: email@example.com
† These authors contributed equally to this research.
Analyses of the dimensions and morphology of spermatozoa can be useful in the identiﬁcation of mammalian
species. We compared and contrasted sperm morphology and dimensions in 9 genera and 18 species of the family
Cricetidae. Spermatozoa were obtained from the cauda epididymes of animals previously ﬁxed in 10% formalin,
and stained with Giemsa or silver-nitrate staining methods. At least 50 spermatozoa from different specimens
were examined for each species. Discriminant function analysis was used to distinguish between the spermatozoa
of different species and to identify the best discriminating characteristics. MANOVA revealed that differences
between species were signiﬁcant. Species in the same genus tended to group together. Qualitative characteristics
that discriminate between species are discussed.
El análisis de las dimensiones y la morfología de los espermatozoides puede ser útil en la identiﬁcación de
especies de mamíferos. La morfología y dimensiones de espermatozoides (largo y ancho de la cabeza, longitud
de la pieza intermedia, pieza principal con la pieza ﬁnal y longitud total) de 9 géneros y 18 especies de cricétidos
fueron comparados y contrastados. Los mismos se obtuvieron de la cola del epidídimo de animales previamente
ﬁjados en formalina al 10%, y fueron teñidos con Giemsa o con nitrato de plata. Al menos 50 espermatozoides
(de diferentes especímenes) por especie fueron estudiados. El análisis de la función discriminante se utilizó
para distinguir entre los espermatozoides de las diferentes especies e identiﬁcar las mejores características
discriminantes. Una prueba de MANOVA reveló que las diferencias entre especies son altamente signiﬁcativas.
Las especies del mismo género tienden a agruparse juntas. También se discuten las características cualitativas que
ayudaron a discriminar entre especies.
Key words: Cricetidae, rodents, spermatozoa, sperm morphology, taxonomy
Spermatozoa are likely under intense selective pressure given
their crucial role in reproduction. Many studies have evaluated
the phylogenetic relationships and evolution of vertebrate and
invertebrate spermatozoa (Austin and Bishop 1958; Baccetti
and Afzelius 1976; Jamieson 1987; Roldan et al. 1992; Cética
et al. 1997; Gage 1998; Swallow and Wilkinson 2002; Breed
et al. 2014). Although the sperm cells of all mammalian
species possess the same basic structural components, they can
vary widely in size and morphology (Cummins and Woodall
1985; Pitnick et al. 2009). The evolution of features such as
sperm shape, size, and count is probably the result of natural or
sexual selective forces (Roldan et al. 1992). Mammalian sper-
matozoa are diverse in form and size across species (Roldan
et al. 1992).
Journal of Mammalogy, 99(6):1398–1404, 2018
© 2018 American Society of Mammalogists, www.mammalogy.org
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ROSSI ET AL.—COMPARISON OF CRICETID SPERMATOZOA 1399
Members of the order Rodentia exhibit the greatest inter-
speciﬁc variability of any mammalian taxa and account for the
highest species richness of all mammals (Burgin et al. 2018).
Their sperm are complex and show considerable differences in
terms of head shape (e.g., ovoid to falciform, with 1 or more api-
cal hooks, and with the presence of nuclear caudal extensions)
and measurements (e.g., total length 34.64 and 258.32 µm
in Myocastor coypus and Cricetulus griseus, respectively—
Roldan et al. 1992; Gallardo et al. 2002; Breed et al. 2014).
Understanding the diverse morphology and dimensions
of spermatozoa can provide valuable information for taxo-
nomic studies (Rouse and Robson 1986; Harding et al. 1987;
Roldan et al. 1992). The accurate identiﬁcation of rodent spe-
cies is problematic: dental, cranial, and external morphology
cannot always resolve cryptic yet genetically distinct species
(Roldan et al. 1985). Unequivocal species identiﬁcation is
essential when studying rodent ecology, especially when some
sympatric species host different zoonotic diseases (e.g., Junin
virus, hosted in Calomys musculinus; Machupo virus hosted
in Calomys callosus; or hantavirus hosted in Oligoryzomys),
while similar taxa pose no health risk (Weissenbacher et al.
1990). Accurate identiﬁcation is also essential in biodiversity
and conservation studies, especially at greater biogeographical
scales (de la Sancha 2014; de la Sancha et al. 2014). The utility
of information about sperm morphology is clear when compar-
ing sperm of Calomys hummelincki, which has sperm with a
hooked head and an eccentrically inserted tail, with those of
Calomys laucha, which has sperm with a hookless head and a
centrally inserted tail (Pérez Zapata et al. 1987). Some cryptic
rodent species are known to have very different spermatozoa
(Gordon and Watson 1986; Pérez Zapata et al. 1987; Roldan
et al. 1992).
Sigmodontinae is the most diverse subfamily of the
Cricetidae in South America (Reig 1986). They are found over
most of South American, in most habitats, and extend as far
north as southern North America (D’Elía and Pardiñas 2015).
There is some debate about which species should be included
in the Sigmodontinae. Should the subfamily include only
South American cricetids with a complex penis morphology,
or should it also include North American forms with a single-
pronged baculum, namely the subfamilies Neotominae and
Tylomyinae that include neotomine-peromyscines (D’Elía and
Pardinas 2015). Regardless, all complex-penis cricetids form
a monophyletic group with neotomine-peromyscines as out-
groups (Steppan and Schenk 2017). Currently, complex-penis
sigmodontines are divided into 9 tribes, including Oryzomyini,
Thomasomyini, Wiedomyini, Ichthyomyini, Abrotrichini,
Reithrodontini, Sigmodontini, Phyllotini, and Akodontini
(D’Elía and Pardinas 2015). However, clear afﬁnities for many
of these groups are not yet resolved (D’Elía 2015).
In this study, we surveyed sperm morphology and pro-
vided morphometric comparisons across 9 genera and
18 species of South American sigmodontines, represent-
ing the tribes Akodontini, Oryzomini, and Phyllotini sensu
D’Elía and Pardiñas (2015). While the total number of sig-
modontine species is unclear (D’Elía and Pardiñas 2015), we
include descriptions for 3 major tribes. This adds considerably
to our understanding of sigmodontine spermatozoid morphol-
ogy, and to rodent spermatozoid morphology as a whole. Our
results show that these characteristics are useful in making
taxonomic distinctions and can be used to infer phylogenetic
relationships when combined with biogeographical, morpho-
logical, chromosomal, or genetic data.
Materials and Methods
Animals used in this study (n = 58; see Supplementary Data
SD1 for species [as identiﬁed by their original collectors] and
sample sizes) were trapped following protocols and guidelines
established by the American Society of Mammalogists (Sikes
et al. 2016). The Argentinean specimens (n = 29) belong to the
mammal collection of the Museo Lorenzo Scaglia, Mar del
Plata, and contain karyotypic and cytotaxonomic descriptions
(Vitullo et al. 1982, 1983). The Paraguayan specimens (n = 29)
were collected in 4 forest reserves in eastern Paraguay. Their
capture was approved by the Texas Tech University Animal
Care and Use Committee (reference number 07045-10) for the
provision of voucher specimens for identiﬁcation purposes.
These specimens are now deposited at the Field Museum
of Natural History (FMNH), Chicago, Illinois; the Natural
Science Research Laboratory, Texas Tech University (TTU),
Lubbock, Texas; and at the Colección de Zoología, Facultad
de Ciencias Naturales y Exactas (CZ), Universidad Nacional
de Asunción, San Lorenzo, Paraguay (for details, see de la
All samples were obtained from testes ﬁxed in 10% formalin.
We dissected the cauda epididymis to obtain tissue, and these
were macerated in phosphate-buffered saline (PBS: 137 mM
NaCl, 2.7 mM KCl, 10 mM Na2HPO4, and 2 mM KH2PO4) and
centrifuged at 800 rpm. The supernatant was discarded. The
pellet was then resuspended in PBS and passed through a series
of ﬁlters to remove large epididymal debris. A drop of the ﬁnal
suspension was spread on a microscope slide, air-dried, and
stained either with modiﬁed Giemsa (Watson 1975) or via the
silver-nitrate staining method (Cética et al. 1997). This made
it possible to clearly identify the differentiated structure of the
sperm (Cética et al. 1997; Andraszek and Smalec 2011). The
morphology and linear dimensions of at least 50 spermatozoa
per species were then measured from their digitized images:
total length (from tip of the head to the lowest point of the tail),
length and width of the head (across the longest and widest
parts of the head), length of the midpiece (from the base of the
head to the beginning of the principal piece), and combined
length of the principal and end piece (from the beginning of the
principal piece to the end of the end piece). The morphology
of the sperm heads was characterized as pyriform (pear-shaped
and the base of the head is concave), polygonal (3 or more usu-
ally straight sides and the base of the head is ﬂat), or oval (a
hookless head in the shape of an egg). Insertion of the tail in the
base of the head was characterized in all species (centrally or
eccentrically). Only spermatozoa that were complete, including
the end piece, were used.
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1400 JOURNAL OF MAMMALOGY
To study the location of the nucleus within the sperm head,
spermatozoa in a second drop of prepared sperm suspension
were stained with DAPI (4′,6-diamidino-2-phenylindole,
0.2 μm/ml in McIlvaine’s buffer pH 6.8) for 5 min, mounted
in glycerol, viewed under a Leitz DMRB epiﬂuorescence
microscope (Leica Microsystems, Wetzlar, Germany) (magni-
ﬁcation 1,000×), and photographed using a Leica DFC 300
FX digital camera (Leica Microsystems). Images were pro-
cessed using Leica Application Suite v.3.6.0 software (Leica
Species-speciﬁc descriptive statistics were computed for
each sperm characteristic. Discriminant function analy-
sis (DFA—Strauss 1985; dos Reis et al. 1990) was used to
assess morphological differences and to maximize group
discrimination (Strauss 2010). We plotted DF1 and DF2
scores to show group discrimination. Multivariate normal-
ity was assessed with the Mardia normality test using PAST
software (Hammer et al. 2001). Because the data were not
normally distributed, we used a nonparametric approach
to assess signiﬁcance of differences between spermato-
zoa of rodent species following the protocol of Hernández
et al. (2017) to improve conﬁdence in our results. Thus,
the DFA was followed by a nonparametric MANOVA with
10,000 permutations to determine signiﬁcance with Wilks’
lambda using Matlab functions “Dfa” and “MANOVA”
(Strauss 2015). We analyzed differences between groups for
the entire sample of species, and for pairwise comparisons
between all 18 species.
Finally, we used the recent rodent phylogeny by Steppan and
Schenk (2017) to create a trimmed phylogeny with the taxa
used in our study to test if rodent sperm shape has a phyloge-
netic signal. We used our class mean DF1 and DF2 scores as a
proxy for shape and then used the R package picante function
“phylosignal” to calculate the K statistic for phylogenetic signal
and test signiﬁcance based on the variance of phylogenetically
independent contrasts relative to tip-shufﬂing randomization
(Kembel et al. 2010).
Differences in total length and size of the sperm head, mid-
piece, and principal piece + end piece were seen between most
species (Table 1). Sperm length varied from 59.35 ± 3.33 µm
(SE) in C. laucha to 99.13 ± 3.43 µm in Scapteromys aquati-
cus (Table 1). DFA discriminated between the tribes and gen-
era examined. Species of Calomys were completely separated
from all others except C. musculinus (Fig. 1). Akodon spp.
and S. aquaticus grouped together and were distintic from all
other species (Fig. 1). Other Akodontines examined, including
Necromys lasiurus, Oxymecterus rufus, and Thaptomys nigrita
completely discriminated from the genus Akodon. However,
Necromys lasiurus and O. rufus grouped with Orizomines,
but not Sooretamys angouya (Fig. 1). DF1 accounted for
53.97% of the variation in morphometric measurements for
the spermatozoa, and was associated primarily with total tail
length (Fig. 1; ρ = 0.75), while DF2 accounted for 39.09% of
the variation and was strongly most associated with the mid-
piece (Fig. 1; ρ = 0.94; Supplementary Data SD2). MANOVA
revealed differences between rodent species were signiﬁcant
(Wilks’ λ = 0.001, F85,4.12 = 501.00, P < 0.01). Pairwise com-
parisons conﬁrmed signiﬁcant differences between all pairs of
species (Supplementary Data SD2). Sperm morphology can
be used as a diagnostic character that clearly discriminates
between species. This idea is supported by the fact that DF1
Table 1.—Summary statistics
± SE for the 5 characters used to describe the morphology of spermatozoa for each cricetid species evaluated.
Tribe Species Head Midpiece (µm) Principal piece + end
Total length (µm)
Length (µm) Width (µm)
Calomys callidus 6.1 ± 0.30 2.1 ± 0.16 19.4 ± 0.25 59.7 ± 1.38 85.7 ± 1.74
C. laucha 6.7 ± 0.08 2.4 ± 0.02 17.8 ± 0.08 37.6 ± 1.64 59.3 ± 3.33
C. musculinus 6.5 ± 0.31 2.1 ± 0.17 15.2 ± 0.47 57.1 ± 1.58 72.6 ± 2.06
C. venustus 7.2 ± 0.18 3.0 ± 0.10 21.3 ± 0.26 56.3 ± 1.01 87.2 ± 1.47
Akodon azarae 6.5 ± 0.31 2.6 ± 0.15 15.2 ± 0.44 77.3 ± 1.35 98.2 ± 1.25
A. dolores 6.4 ± 0.23 2.7 ± 0.14 15.1 ± 0.29 72.1 ± 0.93 93.5 ± 1.18
A. albiventer 5.9 ± 0.24 2.7 ± 0.19 15.2 ± 0.24 70.7 ± 2.34 89.6 ± 1.41
A. montensis 6.3 ± 0.44 2.3 ± 0.12 15.5 ± 0.59 71.9 ± 1.09 93.7 ± 2.26
A. paranaensis 7.1 ± 0.27 3.2 ± 0.16 15.7 ± 0.20 63.4 ± 1.43 87.7 ± 1.43
Oxymycterus rufus 6.3 ± 0.18 2.2 ± 0.16 13.5 ± 0.49 53.8 ± 1.73 73.9 ± 1.73
Scapteromys acuaticus 5.9 ± 0.49 2.5 ± 0.39 13.6 ± 1.10 74.9 ± 3.41 99.1 ± 3.43
Thaptomys nigrita 6.1 ± 0.22 2.4 ± 0.13 14.6 ± 0.27 62.5 ± 1.94 77.3 ± 1.32
Necromys lasiurus 5.8 ± 0.27 2.9 ± 0.06 11.3 ± 0.73 56.6 ± 3.46 71.7 ± 2.63
Oligoryzomys ﬂavescens 7.0 ± 0.27 2.1 ± 0.19 12.2 ± 0.31 53.3 ± 1.85 70.1 ± 1.76
O. longicaudatus 6.2 ± 0.34 2.4 ± 0.23 12.5 ± 0.40 54.3 ± 2.00 75.6 ± 1.93
O. nigripes 7.4 ± 0.14 2.1 ± 0.20 12.9 ± 0.32 52.2 ± 1.35 72.2 ± 2.03
Hylaeamys megacephalus 5.9 ± 0.38 2.2 ± 0.33 13.8 ± 1.26 52.6 ± 2.66 72.1 ± 1.60
Sooretamys angouya 6.1 ± 0.30 2.7 ± 0.31 14.81 ± 0.39 62.0 ± 1.98 83.1 ± 1.73
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ROSSI ET AL.—COMPARISON OF CRICETID SPERMATOZOA 1401
has a signiﬁcant phylogenetic signal (K = 0.94301, P = 0.002,
Z-score = −1.529).
Most species (16 of 18) had sperm heads with a single hook
in the apical portion of the head (Figs. 2–4), while 2 species
(T. nigrita and C. laucha) had oval heads. The apical hook
observed in most species consisted mainly of acrosomal mate-
rial, with the nucleus terminating near the base (observable
with DAPI staining).
Tribe Phyllotini.—The spermatozoa of C. musculinus,
C. callidus, and C. venustus had hooked heads. However, in
C. callidus and C. venustus, the sperm head was pyriform and
the tail centrally inserted (Fig. 2), while in C. musculinus the
sperm head was polygonal and the tail inserted eccentrically
(Fig. 2). The sperm heads of C. laucha were hookless and the
tail inserted centrally into the ﬂat base of the head (Fig. 2).
Considerable variation was recorded in sperm cell dimensions
across Calomys (Table 1), with a large difference seen between
sperm cell total length in C. laucha (59.35 ± 3.33 µm; the short-
est sperm cells) and C. venustus (87.25 ± 1.47 µm; the longest).
Sperm length variables on both discriminant axes separated
these species completely (Fig. 1; Table 2).
Tribe Akodontini.—Sperm heads of Akodon spp. had an api-
cal hook (Fig. 3A). However, morphological differences were
seen in terms of sperm cell head shape, differences in the base
of the head, and the point of tail insertion. Akodon montensis
and A. azarae had pyriform sperm heads, and showed central
insertion of the tail at the base (Fig. 3A). In A. paranaensis,
A. dolores, and A. albiventer, the sperm heads were polygo-
nal (Fig. 3A). The location of tail insertion differentiated
A. paranaensis (eccentric) from A. dolores and A. albiventer
(central). Total sperm length across the genus ranged from
87.75 ± 1.43 µm to 98.21 ± 1.25 µm (Fig. 1). No signiﬁcant
differences were seen in length of the midpiece between any
members of Akodon. Although sperms cells of A. azarae and
A. montensis showed no morphological differences, they were
distinguishable by overall sperm cell length. Indeed, A. azarae
had the longest spermatozoa of all Akodon species studied,
and were much longer than the sperm cells of A. montensis
Fig. 2.—Sperm heads in the tribe Phyllotini, genus Calomys: C. cal-
lidus and C. venustus showed sperm heads with a single hook and
pyriform head shape, C. musculinus showed sperm heads with a single
hook and polygonal head shape, and C. laucha (Cla) sperm heads were
oval and hookless. Arrow: sperm cell with a centrally inserted tail.
Arrowhead: sperm cell with an eccentrically inserted tail. Scale bar:
5 μm. Sperm stained with modiﬁed Giemsa.
Fig. 1.—Morphometric analysis. Discriminant function analysis (DFA) scatter- and bi-plots for the examined species: Calomys callidus (Cca),
C. laucha (Cla), C. musculinus (Cmu), C. venustrus (Cve), Akodon albiventer (Aal), A. azarae (Aaz), A. dolores (Ado), A. montensis (Amo),
A. paranaensis (Apa), Oxymycterus rufus (Oru), Necromys lasiurus (Nla), Scapteromys acuaticus (Sac), Thaptomys nigrita (Tni), Oligoryzomys
ﬂavescens (Oﬂ), O. longicaudatus (Olo), O. nigripes (Oni), Hylaeamys megacephalus (Hme), Sooretamys angouya (San).
Table 2.—Summary of discriminant function analysis (DFA) of 5
linear measurements of spermatozoa from 18 cricetid rodent species.
Variable DF1 DF2
Head length −0.142 −0.024
Head width 0.069 −0.130
Midpiece −0.320 −0.940
Principal piece + end piece 0.751 −0.105
Total length 0.624 −0.312
1402 JOURNAL OF MAMMALOGY
(98.21 ± 1.25 and 93.73 ± 2.26 μm, respectively). Akodon
dolores and A. albiventer had similar spermatozoa, and no con-
sistent differentiating characteristics were found. Oxymycterus
rufus, N. lasiurus, and T. nigrita were clearly distinguishable
from all other Akodon species (Fig. 1). Thaptomys nigrita had
an oval sperm head like that of C. laucha, and the insertion
of the tail was central in both (Fig. 3B). However, total sperm
length of T. nigrita was longer (77.26 ± 1.32 μm) than that of
C. laucha (57.35 ± 3.33 μm; Table 1). Scapteromys aquaticus
had a pyriform sperm head (Fig. 3B) and the longest sperm
cells of all species studied (99.13 ± 3.43; Table 1). Oxymecterus
rufus and N. lasiurus were characterized by spermatozoa with
polygonal heads, but they showed a central and an eccentric
insertion of the tail, respectively (Fig. 3B).
Tribe Oryzomini.—The spermatozoa of O. nigripes, O. lon-
gicaudatus, and O. ﬂavescens were similar. All showed polyg-
onal heads and centrally inserted tails (Fig. 4A). The sperm
morphometrics of Oligoryzomys spp. overlapped considerably
(Table 1); DFA did not completely discriminate between them
(Fig. 1). The remaining Oryzomini had sperm cells with hooked
heads. The heads of Hylaeamys megacephalus sperm were pyr-
iform and the tail centrally inserted, while those of S. angouya
showed a polygonal head with a centrally inserted tail (Fig. 4B).
Additionally, the sperm of S. angouya was longer than that of
any other member of the tribe (Table 1). With the exception of
some overlap between Hylaeamys and Oligoryzomys, there was
little overlap among other species of the tribe (Fig. 1).
Of the 18 species studied, 16 showed a characteristic hooked
sperm head and 2 (C. laucha and T. nigrita) had spermato-
zoa with oval heads. It has been postulated that species with a
sperm head showing an apical hook (that largely contains acro-
somal material) generally have longer sperm tails, and that spe-
cies with oval sperm heads have shorter tails (Ding et al. 2010).
While this may be true for C. laucha, it appears not to apply
to T. nigrita or the rest of the species examined. Our results
also show that South American cricetid species are more likely
to have apically hooked sperm heads than oval heads. Roldan
(1992) concluded that oval sperm heads were the ancestral con-
dition for Calomys, but we did not evaluate this hypothesis.
The genus Calomys showed the greatest variation in sperm
dimensions and head morphology. Roldan et al. (1992)
described a hooked heads in C. callidus and C. musculinus,
while the sperm heads of C. laucha are hookless. The sperm of
these 3 species differ in their linear dimensions. Because earlier
studies presented no measurements of the head, midpiece, or
tail (Pérez Zapata et al. 1987; Roldan et al. 1992), it is difﬁ-
cult to provide a more comprehensive comparison with earlier
Differences between the sperm cells of Akodon spp. are
not as evident as in Calomys. The DFA did not separate most
Akodon species, and only the spermatozoa of A. azarae can be
easily differentiated in terms of length from those of the others.
The size of the midpiece was similar (ca. 15 μm) in all Akodon
Fig. 3.—Sperm heads in the tribe Akodontini. A) Sperm heads in Akodon. Species with pyriform head shapes: A. azarae and A. montensis; species
with polygonal sperm heads: A. paranaensis, A. dolores, and A. albiventer. B) Sperm heads of members of other genera in the tribe Akodontini.
Thaptomys nigrita with oval head shapes, Scapteromys acuaticus with pyriform sperm heads, Oxymycterus rufus and Necromys lasiurus with
polygonal sperm heads. Arrow: sperm cell with a centrally inserted tail. Arrowhead: sperm cell with an eccentrically inserted tail. Scale bar: 5 μm.
Sperm stained with modiﬁed Giemsa and silver nitrate.
Fig. 4.—Sperm heads in the tribe Oryzomini. A) Sperm head morphol-
ogy in members of the genus Oligoryzomys. All specimens showed
polygonal sperm heads. B) Sperm heads of other members of the tribe
Oryzomini. Polygonal sperm head of Sooretamys angouya and the
pyriform sperm head of Hylaeamys megacephalus. Arrow: sperm cell
with a centrally inserted tail. Scale bar: 5 μm. Sperm stained with
ROSSI ET AL.—COMPARISON OF CRICETID SPERMATOZOA 1403
sperm cells; it may represent a conserved character within the
genus. However, no midpiece measurements have been reported
for any other members of this genus. The hooked heads of
Akodon spermatozoa are consistent with those described by
Roldan et al. (1992). Thaptomys nigrita is phylogenetically
close to the genus Akodon, and in fact was once referred to as
a subgenus of Akodon (Cabrera 1961; Reig 1987). However,
our work shows that sperm of these taxa can be clearly dis-
tinguished by their different head morphologies (oval with no
apical hook in T. nigrita versus nonovoid and with a hook for
members of Akodon). The DFA separated T. nigrita completely
from Akodon, and the other Akodontines examined (N. lasi-
urus, O. rufus).
The phylogenetic position of species within the genus
Oligoryzomys has long been unclear. Indeed, several authors
have questioned the distinctions between different species of
Oligoryzomys (Carleton and Musser 1989; Andrades Miranda
et al. 2001; Musser and Carleton 2005; Weksler and Bonvicino
2005; Frances and D’Elía 2006). Our study shows that sperm
dimensions in this genus are very similar. Thus, sperm dimen-
sions do not provide an easy tool to separate Oligoryzomys spe-
cies that are difﬁcult to identify.
We show that sperm cell morphology and morphometrics
provide a valuable tool for identifying some cricetid rodents,
which reinforces the idea that sperm morphology and dimen-
sions likely have a strong phylogenetic signal. Future work
should examine spermatozoa of other species in an effort to
assess the mapping of sperm morphology on phylogenetic his-
tory. Sperm morphology also should be considered in concert
with biogeographic, chromosomal, and genetic data so that
comprehensive evolutionary analyses of related species can be
performed. We hope that the ideas developed here will stimu-
late further research in this ﬁeld, and perhaps ultimately sug-
gest new approaches.
This work was supported by the Consejo Nacional de
Investigaciones Cientíﬁcas y Técnicas (CONICET) (grant
number PIP-0168, MSM). Financial support to NUDLS was
partially provided by the Marshall Field Collection Fund of
the Field Museum of Natural History, a Fulbright Fellowship
(Institute of International Education, U.S. Department of State),
a Latin American Student Research Award (American Society
of Mammalogists), the Mary Rice Foundation, the American
Philosophical Society through the Lewis and Clark Exploration
Fund, a Hispanic Scholarship Fund Award, an AT&T McNair
Fellowship (TTU), research assistantships (Department of
Biological Sciences, TTU), a Texas Tech Association of
Biologists Minigrant (TTUAB), the J. Knox Jones, Jr. Memorial
Endowed Scholarship (TTU), and the Michelle C. Knapp
Memorial Scholarship (TTU). Curatorial assistance with speci-
mens was provided by B. Patterson, B. Stanley, and J. Phelps
of the Field Museum (FMNH), and R. J. Baker and H. Garner
of Texas Tech University. We thank A. Burton for help with
language editing of earlier versions of this manuscript, and
S. Oviedo Rivera for her valuable and constant help. Also, we
sincerely thank the reviewers for their comments, which helped
us improve the quality of the manuscript.
Supplementary data are available at Journal of Mammalogy
Supplementary Data SD1.—Specimen information for spe-
cies used in this study.
Supplementary Data SD2.—MANOVA (permutated 10,000
times) pairwise comparisons between Cricetidae spermatozoa;
Wilks’ λ (above) and F-values with corresponding degrees of
freedom (below). All pairwise comparisons were signiﬁcant
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Submitted 21 December 2017. Accepted 24 October 2018.
Associate Editor was John Scheibe.