Postcopulatory sexual selection influences baculum evolution in primates and carnivores

Abstract

The extreme morphological variability of the baculum across mammals is thought to be the result of sexual selection ( particularly, high levels of post- copulatory selection). However, the evolutionary trajectory of the mammalian baculum is little studied and evidence for the adaptive function of the baculum has so far been elusive. Here, we use Markov chain Monte Carlo methods implemented in a Bayesian phylogenetic framework to reconstruct baculum evolution across the mammalian class and investigate the rate of baculum length evolution within the primate order. We then test the effects of testes mass (postcopulatory sexual selection), polygamy, seasonal breeding and intromission duration on the baculum in primates and carnivores. The ances- tral mammal did not have a baculum, but both ancestral primates and carnivores did. No relationship was found between testes mass and baculum length in either primates or carnivores. Intromission duration correlated with baculum presence over the course of primate evolution, and prolonged intro- mission predicts significantly longer bacula in extant primates and carnivores. Both polygamous and seasonal breeding systems predict significantly longer bacula in primates. These results suggest the baculum plays an important role in facilitating reproductive strategies in populations with high levels of postcopulatory sexual selection.

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Cite this article: Brindle M, Opie C. 2016
Postcopulatory sexual selection influences
baculum evolution in primates and carnivores.
Proc. R. Soc. B 283: 20161736.
http://dx.doi.org/10.1098/rspb.2016.1736
Received: 5 August 2016
Accepted: 4 November 2016
Subject Areas:
evolution
Keywords:
baculum, postcopulatory sexual selection,
prolonged intromission, primates, carnivores,
Bayesian phylogenetics
Authors for correspondence:
Matilda Brindle
e-mail: matilda-jane.brindle.14@ucl.ac.uk
Christopher Opie
e-mail: kit.opie@ucl.ac.uk
Electronic supplementary material is available
online at http://dx.doi.org/10.6084/m9.fig-
share.c.3583481.
Postcopulatory sexual selection influences
baculum evolution in primates and
carnivores
Matilda Brindle and Christopher Opie
Department of Anthropology, University College London, 14 Taviton Street, London, WC1H 0BW, UK
CO, 0000-0002-3379-4703
The extreme morphological variability of the baculum across mammals
is thought to be the result of sexual selection (particularly, high levels of post-
copulatory selection). However, the evolutionary trajectory of the mammalian
baculum is little studied and evidence for the adaptive function of the baculum
has so far been elusive. Here, we use Markov chain Monte Carlo methods
implemented in a Bayesian phylogenetic framework to reconstruct baculum
evolution across the mammalian class and investigate the rate of baculum
length evolution within the primate order. We then test the effects of testes
mass (postcopulatory sexual selection), polygamy, seasonal breeding and
intromission duration on the baculum in primates and carnivores. The ances-
tral mammal did not have a baculum, but both ancestral primates and
carnivores did. No relationship was found between testes mass and baculum
length in either primates or carnivores. Intromission duration correlated with
baculum presence over the course of primate evolution, and prolonged intro-
mission predicts significantly longer bacula in extant primates and carnivores.
Both polygamous and seasonal breeding systems predict significantly longer
bacula in primates. These results suggest the baculum plays an important
role in facilitating reproductive strategies in populations with high levels of
postcopulatory sexual selection.
1. Introduction
The morphology of male intromittent organs is argued to be subject to more
rapid divergent evolution than any other form in the animal kingdom [1].
The baculum, or penis bone, does not buck this trend and has been described
as ‘the most diverse of all bones’ ([2], p. 1), varying dramatically in length,
width and shape across the Mammalia.
The baculum is not uniformly present across mammals. It was thought only to
exist in eight of the mammalian orders: Afrosoricida, Carnivora, Chiroptera,
Dermoptera, Erinaceomorpha, Primates, Rodentia and Soricomorpha [3,4]. How-
ever, it has recently beendiscoveredthat a Lagomorph, the American pika (Ochonta
princeps), also has a small baculum [5]. This discovery suggeststhat baculum pres-
ence may be more prevalent across mammals than historically assumed. Certain
orders have a mixture of baculum presence and absence across species; these are
the Carnivora, Chiroptera, Primates and Rodentia. In Primates, for example,
humans, tarsiers and several Platyrrhines lack a baculum. The Lagomorphia
may be similarly divided. Aside from documenting the presence and absence of
the baculum across the mammalian orders,the evolutionary history of thebaculum
had not been studied until recently, leaving many questions unanswered.
Genital (and hence bacular) morphology is suggested to be subject to sexual
selection [6]. The few empirical tests conducted to date may support this hypoth-
esis. Stockley et al. [7] found that baculum width in polygamous house mice (Mus
domesticus) was a significant predictor of male reproductive success. Simmons &
Firman [8] were able to manipulate baculum width experimentally by altering
&2016 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution
License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original
author and source are credited.

the level of sexual selection pressure in populations of house
mice. After 27 generations, populations with artificially
enforced high levels of postcopulatory sexual selection
pressure had significantly thicker bacula than populations in
which monogamy was enforced and sexual selection pressure
was therefore absent. These studies indicate that intra-sexual
selection, in particular postcopulatory sexual selection
pressure, may be driving bacular evolution. If this is the case,
the bacula of populations under high levels of intra-sexual
selection pressure, such as those with polygamous (multi-
male, multi-female) or seasonal mating systems, should be sub-
ject to stronger evolutionary forces.
Different mating systems generate variation in levels of
postcopulatorysexual selection pressure and therefore morpho-
logical variability; for example, species with high levels of
sperm competition tend to have large testes relative to their
body mass [9]. Residual testes mass is thus considered to be a
reliable measure of the mating system of a population and
therefore the degree of sexual selection pressure [9]. Orr &
Brennan [10] found that relative testes mass was a significant
predictor of baculum presence across Chiroptera, Eulipotyphla,
Primates and Rodentia. However, the same study found no
relationship between baculum presence or width and mating
system, indicating that a third variable may be at play. Ramm
[11] tested for a relationship between testes mass and baculum
length in four orders by first establishing the level of phylo-
genetic dependency between species and then conducting
appropriately corrected regressions. A positive relationship
between testes mass and baculum length was noted in Rodentia
and Carnivora, but the same test found no relationship in
Primates or Chiroptera.
The adaptive function of the baculum, under high levels
of intra-sexual selection, is yet to be established. A potential
strategy by which a male could increase their reproductive
success, by outcompeting rival males, is through prolonging
intromission and consequently delaying a female mating
with another male [12]. The prolonged intromission hypoth-
esis argues that the baculum helps to facilitate this prolonged
duration of intromission by supporting the penis [13].
In this context, the proximate mechanism of the baculum
is to act as a supportive rod, strengthening the penis and
protecting the urethra during prolonged intromission [12].
A recent study on three different species of bat found that
the baculum formed a functional unit with the corpora caver-
nosa, which protected the glans tip and the shaft of the
penis when erect [14]. The authors posit that the baculum
also helps to limit constriction of the distal urethra and ure-
thral opening in the erect penis during intromission,
facilitating sperm flow. In many species of primate in
which the baculum is elongated, the distal end of the bone
projects slightly from the urethra while the penis is erect
[15]. This could bring the baculum into contact with a
female’s cervix during intromission, facilitating the transfer
of semen into the cervical canal [12,15].
Evidence for the prolonged intromission hypothesis has
so far proven controversial. Early studies found that pro-
longed intromission was correlated with elongated bacula
in primates and carnivores [13,16]; however, these studies
did not take into account the statistical non-independence
of data that arises due to a shared evolutionary history
between species. A later study, corrected to account for
phylogeny, tested for a correlation between prolonged intro-
mission and baculum length in North American carnivores,
but did not find support for the hypothesis [17]. However,
data to test this hypothesis were only available for 18 species,
of which only two were characterized as having short intro-
mission duration. Dixson et al. [18] argue that this sample is
not representative enough to decisively refute the prolon-
ged intromission hypothesis, and carried out their own
phylogenetically corrected analysis in a sample of 57 species
of mammal. This time, a significant correlation was found
between the two variables. Although both studies were cor-
rected for phylogeny, if the degree of phylogenetic
dependency is not established before an analysis is adjusted,
correcting for phylogeny can produce misleading or incorrect
results, as the level of relatedness between species varies
across a phylogeny [19]. Bayesian Markov chain Monte Carlo
(MCMC) analyses enable species’ phylogenies to be incorpor-
ated into an analysis, rather than simply correcting for
phylogeny, and can thus produce more reliable results [20,21].
In a new study, Schultz et al. [22] used the earlier phylogenetic
methods of stochastic mapping to model the presence and
absence of the baculum in 954 mammalian species, and argue
that the baculum independently evolved a minimum of nine
times in mammals. However, this sample is unlikely to reflect
the course of evolution across the entire mammalian class,
particularly as baculum absence was only noted in 103 species.
Here, we use phylogenetic comparative methods within a
Bayesian MCMC framework [23] to examine the evolutionary
history of the baculum and investigate the hypothesis
that increased levels of intra-sexual selection affect baculum
evolution. We first reconstructed the evolutionary trajectory
of the baculum across the entire mammalian class and exam-
ined the rate of bacular evolution in the primate order. Then,
we tested for a relationship between baculum length and
testes mass in both primates and carnivores. We then further
tested for correlated evolution between baculum presence
and intromission duration in primates, before conducting
phylogenetic t-tests to establish whether primates and carni-
vores with prolonged intromission durations have longer
bacula than those with short intromission durations. Finally,
we used the same tests to examine whether increased levels of
postcopulatory sexual selection pressure caused by (i) poly-
gamous mating systems and (ii) seasonal breeding patterns
led to an increase in baculum length in primates. Primates and
carnivores are likely to be particularly rewarding groups to
study, because there is a mixture of baculum presence and
absence within each order. This means that differences
between those with and without bacula can be tracked at
the species level, rather than across orders. Furthermore, as
these orders are arguably more extensively studied than
other mammalian groups, there are more data available.
If the baculum facilitates prolonged intromission and
increased proximity to the cervix in order to reduce the level of
sperm competition and increase reproductive success, then sev-
eral predictions can be made and tested. It would be expected
that there would be a relationship between baculum length
and testes mass, which can be used as a proxy for the level of
postcopulatory sexual selection pressure in a population. Intro-
mission duration would be expected to correlate with baculum
presence across the course of evolution. Species with prolonged
intromission durations should have elongated bacula. Finally,
groups in which postcopulatory sexual selection pressure is
highest, such as those in which mating is polygamous or
occurs seasonally, should have longer bacula than groups
with lower levels of postcopulatory sexual selection pressure.
rspb.royalsocietypublishing.org Proc. R. Soc. B 283: 20161736
2

2. Material and methods
Ancestral state reconstructions, tests of correlated evolution, tests
of trait relationships and phylogenetic t-tests were all conducted
using BAYESTRAITS (v. 2) [24]. Trait data were compiled from the
literature (electronic supplementary material).
A supertree phylogeny of 5020 extant mammals was used to
reconstruct the ancestral states of baculum presence across the
mammalian order [25]. All analyses of the primate and carnivore
orders were conducted on a posterior distribution of 10 000 mol-
ecular phylogenies inferred using Bayesian MCMC methods [26].
Chronograms were used in all ancestral state reconstructions and
tests of correlated evolution, whereas phylograms were used in
tests of trait relationships and phylogenetic t-tests.
A reversible-jump MCMC method with an exponential hyper-
prior ranging between 0 and 0.07 was used to estimate discrete
ancestral states [27]. Each chain was run for 5 million iterations with
a burn-in of 50 000 iterations; this was the case for all analyses aside
from the variable-rates model. We were interested in inferring seven
key nodes across the mammalian phylogeny, as well as the ances-
tral state of baculum presence for both the primate and carnivore
orders (electronic supplementary material, figure S1). Nodes were
constructed using the ‘add MRCA’ procedure in BAYESTRAITS [24].
A variable-rates model was used to reconstruct the course of
baculum length evolution across the primate order (following
Venditti et al. [28]). The model allows the rate of evolution to
change across a phylogeny over time, identifying when and
where evolutionary rates have differed without prior knowledge.
Stretched branches of the phylogenetic tree indicate that a trait
has evolved quickly and compressed branches indicate slow
rates of trait change. The model was run for 10 million iterations,
with a burn-in of 100 000 iterations.
Baculum length and testes mass were tested for a relationship
using a multiple regression between baculum length, testes mass
and adult male body mass. The model was run three times and
the chain with the median log marginal likelihood was chosen;
this approach was also taken when conducting correlated evolution
and hypothesis tests.The proportion of the slope parameter (
b
)that
crossed zero was used to establish the p-values (following Organ
et al.’s [29] method for phylogenetic t-tests).
To test for correlated evolution between baculum presence and
intromission duration, we compared the log marginal likelihood of
independent (traits constrained to evolve separately) and depen-
dent models. An exponential hyperprior with a range of 0–0.05
was used. The two models were compared using log natural
Bayes factors (BFs), calculated as two times the difference in log
marginal likelihood between the models [24]. BFs were interpreted
following Kass et al. [30]: 0–2, minimal support; 2–6, positive sup-
port; 6– 10, strong support; more than 10, very strong support.
Recent literature has highlighted several issues with using the
harmonic mean as a measure for estimating the log marginal like-
lihood of a model and the relative merits of the stepping-stone
sampling method, which is argued to be more accurate [31,32].
We therefore used the stepping-stone sampling method to esti-
mate the log marginal likelihood. One hundred stones were used
per 10 000 iterations of the Markov chain. Following Xie et al.
[32], a beta (
a
, 1.0) distribution was employed and
a
was set at
0.3. Stepping-stones were used in this way for all tests of trait
relationships, correlated evolution and phylogenetic t-tests.
MCMC phylogenetic t-tests were used to test hypotheses
accounting for the statistical non-independence of the data, due
to shared evolutionary history (following Organ et al. [29]).
3. Results
AmultistateanalysisinBAYESTRAITS [24] (n¼1818) indicates
that the ancestral mammal did not possess a baculum (baculum
absence, mean probability ¼0.98). Ancestral state reconstruc-
tions across six nodes (table 1; electronic supplementary
material, figure S1) suggest that the baculum first evolved
after non-placental and placental mammals split (baculum
absence, mean probability ¼0.93), but before the most recent
common ancestor (MRCA) of primates and carnivores evolved
(baculum presence, mean probability ¼0.99). The ancestral pri-
mate and carnivore both had a baculum (baculum presence,
mean probability ¼1.00 and 1.00, respectively).
The evolutionary trajectory of baculum length was visual-
ized using the variable-rates model, which stretches or
shrinks the branches of a phylogenetic tree according to
differing rates of trait evolution (figure 1; electronic sup-
plementary material, figures S2– S4). Primates show marked
variation in baculum length and this is reflected in their
evolutionary history. Very little change has occurred in
platyrrhines and tarsiers, which tend to have small or
absent bacula, indicating that the baculum of the ancestral
primate was fairly small. By contrast, strepsirrhines and
catarrhines have undergone relatively fast evolutionary
change, and generally have longer bacula than the platyr-
rhines and tarsiers. The apes represent an interesting group
as they have undergone high rates of change, yet have very
small or absent bacula. This suggests that after the
platyrrhine and catarrhine lineages split, the baculum of the
ancestral catarrhine underwent a high rate of evolution and
became a lot longer. When apes subsequently split from
Old World monkeys this trend reversed and the ape
baculum underwent further high rates of evolution, this
time reducing in length. (See electronic supplementary
material for variable-rates trees depicting carnivore baculum
length evolution and primate and carnivore testes mass
evolution, figures S2– S4.)
Baculum length could not be predicted from testes mass
in either primates (n¼46, p¼0.139, R
2
¼0.03) or carnivores
(n¼32, p¼0.231, R
2
¼0.37) (electronic supplementary
material, table S2).
Table 1. Probability of baculum presence or absence at the root and six
nodes of the mammalian phylogeny (see electronic supplementary material,
figure S1, for nodes). Probabilities of baculum presence or absence of
ancestral primates and carnivores are also given.
baculum absent baculum present
mean
probability s.e.
mean
probability s.e.
root 0.98 0.0011 0.02 0.0011
node 1 0.93 0.0020 0.07 0.0020
node 2 0.51 0.0016 0.49 0.0016
node 3 1.00 0.0000 0.00 0.0000
node 4 0.01 0.0000 0.99 0.0000
node 5 0.3 0.0010 0.7 0.0010
node 6 0.01 0.0000 0.99 0.0000
primates 0.00 0.0000 1.00 0.0000
carnivores 0.00 0.0000 1.00 0.0000
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3

We found positive evidence for correlated evolution
between baculum presence and intromission duration in
primates (n¼299, log BF ¼4.78; table 2). Ancestral state recon-
structions and model rates indicate that baculum presence and
short intromission durations (mean probability ¼0.73) pre-
ceded a shift to prolonged intromission (figure 2). After long
intromission durations had evolved, the baculum was rarely
lost. However, the baculum was often lost if intromission dur-
ation remained short. Long intromission durations rarely
became short again when a baculum was present. By contrast,
when a baculum was absent, intromission duration switched
frequently between being long and short.
The hypothesis that postcopulatory sexual selection influences
baculum length was tested through a series of phylogenetic
t-tests (table 3). In line with our predictions, we find that species
in both the primate and carnivore orders in which intromission
is prolonged have significantly longer bacula than species in
which intromission is short (n¼53, p¼0.000 and n¼41, p¼
0.018, respectively). Primates in polygamous mating systems
werefoundtohavesignificantlylongerbaculathanthosein
Tarsius_lariang
Galagoides_demidoff
Tarsius_bancanus
Cheirogaleus_medius
Varecia_variegata_variegata
Trachypithecus_francoisi
Hapalemur_griseus
Microcebus_murinus
Pan_troglodytes_troglodytes
Lemur_catta
Ateles_paniscus
Piliocolobus_badius
Erythrocebus_patas
Colobus_guereza
Tarsius_syrichta
Alouatta_pigra
Alouatta_seniculus
Macaca_sinica
Daubentonia_madagascariensis
Otolemur_crassicaudatus
Mandrillus_sphinx
Colobus_polykomos
Homo_sapiens
Nasalis_larvatus
Papio_papio
Cercocebus_torquatus
Chiropotes_satanas
Alouatta_palliata
Alouatta_sara
Rhinopithecus_roxellana
Theropithecus_gelada
Saguinus_bicolor
Pan_paniscus
Miopithecus_talapoin
Loris_tardigradus
Perodicticus_potto
Saimiri_boliviensis
Papio_cynocephalus
Saguinus_fuscicollis
Cercopithecus_mona
Presbytis_comata
Callithrix_jacchus
Chlorocebus_aethiops
Callithrix_pygmaea
Saguinus_mystax
Ateles_fusciceps
Lagothrix_lagotricha
Alouatta_belzebul
Cercopithecus_solatus
Macaca_nemestrina
Cebus_apella
Macaca_fuscata
Papio_hamadryas
Pygathrix_nemaeus
Cacajao_calvus
Cercopithecus_mitis
Lophocebus_albigena
Cebus_capucinus
Eulemur_fulvus_fulvus
Callimico_goeldii
Macaca_thibetana
Papio_ursinus
Macaca_arctoides
Saguinus_oedipus
Propithecus_verreauxi
Aotus_lemurinus
Macaca_fascicularis
Alouatta_caraya
Tarsius_dentatus
Alouatta_guariba
Gorilla_gorilla_gorilla
Macaca_assamensis
Pongo_pygmaeus
Macaca_nigra
Ateles_geoffroyi
Leontopithecus_rosalia
Cacajao_melanocephalus
Galago_senegalensis
Ateles_belzebuth
Macaca_mulatta
Cercopithecus_neglectus
Saguinus_midas
Callithrix_humeralifera
Callithrix_argentata
Papio_anubis
Mandrillus_leucophaeus
Procolobus_verus
Hylobates_lar
Figure 1. A primate phylogeny scaled to reflect the rate of bacular evolution. Darker red branches indicate lower rates of evolution; blue branches indicate
particularly high rates of evolution.
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4

other mating systems (n¼65, p¼0.032). Finally, seasonally
breeding primates have significantly longer bacula than primates
that do not breed seasonally (n¼63, p¼0.045).
4. Discussion
Our results have uncovered the evolutionary trajectory of the
baculum across the mammalian class, showing that the bacu-
lum first evolved after placental and non-placental mammals
split around 145 million years ago (Ma), but before the
MRCA of primates and carnivores evolved around 95 Ma
[25]. We show for the first time that both the ancestral
primate and the ancestral carnivore had a baculum, a result
bearing important implications for how the baculum should
be studied within these orders. Analyses should focus on
examining why the baculum was retained in certain species
and lost in others, not why the baculum might have evolved;
it was already present in their ancestors.
We found no relationship between baculum length and
testes mass in primates or carnivores, supporting previous find-
ings in primates, but not carnivores [11]. This discrepancy
is probably explained by the use of a Bayesian phylogenetic
framework for our analyses; our finding suggests that any
observed relationship between baculum length and testes
mass could have evolved by chance. Although these results
do not provide support for the hypothesis that baculum
length is sexually selected, they do not refute it. It is possible
q12
z: 0.00%
baculum,
long intromission
no baculum,
short intromission
baculum,
short intromission
no baculum,
long intromission
q31
z: 62.24%
q21
z: 80.72%
q43
z: 0.46%
q13
z: 0.02%
q24
z: 98.18%
q34
z: 0.16%
q42
z: 0.02%
root
Figure 2. Coevolution between primate baculum presence and intromission duration. zpercentages show the posterior probability that a transition rate from one
state to another is zero (i.e. how often a given transition does not occur). Thick black arrows indicate that a transition happened frequently; thinner or absent arrows
indicate that a transition was rare or practically non-existent. (Online version in colour.)
Table 2. Likelihood of dependent and independent models of correlated evolution between baculum presence and intromission duration in primates. The Bayes
factor indicates positive support for the dependent model of evolution over the independent model. Bayes factors were interpreted following Kass et al. [30]:
O–2, minimal support; 2– 6, positive support; 6 –10, strong support; more than 10, very strong support.
coevolution analysis
dependent model independent model
log likelihood log likelihood log natural Bayes factor
intromission duration 245.77 248.16 4.78
Table 3. Phylogenetic t-tests of baculum length and intromission duration in primates and carnivores, and mating system and breeding seasonality in primates.
model
primates carnivores
b
s.e.
b
p-value
b
s.e. bp-value
baculum length and intromission duration 16.64 +0.05 p¼0.0000 64.90 +0.41 p¼0.018
baculum length and mating system 23.45 +0.03 p¼0.0318 — — —
baculum length and breeding seasonality 23.55 +0.03 p¼0.0448 — — —
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5

that aspects of baculum morphology, such as width or shape,
are more likely to vary with testes mass. For instance, baculum
shaft width is a significant predictor of the number of offspring
sired by male house mice [7]. Indeed, Orr & Brennan found
that testes mass predicted baculum width in four orders of
mammal; however, when this relationship was tested using a
phylogenetic model, baculum width was no longer a significant
predictor [10]. It is possible that these results would have
remained significant if the relationship had been examined at
the order level, as bacular function may vary from order to
order. Our results serve to highlight the importance of using
full phylogenetic methods when examining trait evolution.
This study has been the first to demonstrate that baculum
presence has correlated with intromission duration over the
course of primate evolution. The result highlights the inter-
play between morphological and behavioural phenotypes
over evolutionary time. The baculum physically supports
and protects the male’s penis [12,14], and assists the transfer
of semen towards a female’s cervix [12,15]. However, it also
plays an important role in facilitating prolonged intromission,
which itself may be a sexually selected behaviour, aimed at
increasing reproductive success by delaying females from
re-mating [12].
Our results confirm that the prolonged intromission
hypothesis remains robust when analysed within a rigorous
phylogenetic framework. Phylogenetic t-tests show that the
baculum is significantly longer in both primate and carnivore
species in which intromission is prolonged. This suggests that
the elongation of the baculum over the course of mammalian
evolution was probably driven by its utility in prolonged intro-
mission. Two more phylogenetic t-tests showed that primates in
polygamous mating systems and seasonally breeding primates
had significantly longer bacula than primates in other mating
systems and those without a seasonal breeding pattern, high-
lighting the importance of postcopulatory sexual selection as
a driver of bacular evolution.
The finding from the test of correlated evolution, coupled
with the results of the phylogenetic t-tests, allows us to begin
piecing together the proximate and ultimate functions of the
baculum. Polygamous mating systems and limited breeding
seasons create high levels ofpostcopulatory sexual competition.
In this environment, prolonging intromission could delay a
female from re-mating, thus increasing a male’s chance of suc-
cessfully fertilizing her under competitive conditions. Ensuring
that the urethra is unrestricted and there is as little distance as
possible for sperm to travel is a way of increasing the amount
of sperm transported to the cervical canal. The baculum
serves as a supportive structure duringprolonged intromission,
both protecting the urethra and preventing it from being
constricted [14].
These results do not necessarily apply to other orders within
the mammalian class, but they do highlight potentially reward-
ing lines of enquiry. It is also important to note that, even within
the primate and carnivore orders, other factors are likely to
influence whether a baculum is retained or not and how its
morphology evolves. Studies of bacular evolution tend to
focus on why it is present in certain species, or why it might
have increased in length or width; factors driving the reduction
or disappearance of bacula have largely been ignored.
Ingenious studiesare beginning to pick apart the proximate
mechanism of the baculum, as well as some of the factors
driving its evolution in different mammalian orders [8,14].
By comparing these findings across orders and examining
the baculum through a phylogenetic framework, we can
begin to build a more comprehensive picture of the proximate
and ultimate functions of the baculum, and how it evolved in
extant species and their ancestors.
Data accessibility. Data are available from Dryad Digital Repository [33].
Authors’ contributions. M.B. and C.O. designed research; M.B. and C.O.
performed research; M.B. and C.O. analysed data; M.B. and C.O.
drafted the manuscript. All authors gave final approval for
publication.
Competing interests. We have no competing interests.
Funding. M.B. is supported by a NERC Doctoral Training Studentship,
and C.O. is supported by a Leverhulme Early Career Fellowship.
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