Metabolic Rate Limits the Effect of Sperm Competition on
Mammalian Spermatogenesis
Javier delBarco-Trillo*, Maximiliano Tourmente, Eduardo R. S. Roldan
Reproductive Ecology and Biology Group, Museo Nacional de Ciencias Naturales, Consejo Superior de Investigaciones Científicas, Madrid, Spain
Abstract
Sperm competition leads to increased sperm production in many taxa. This response may result from increases in
testes size, changes in testicular architecture or changes in the kinetics of spermatogenesis, but the impact of each
one of these processes on sperm production has not been studied in an integrated manner. Furthermore, such
response may be limited in species with low mass-specific metabolic rate (MSMR), i.e., large-bodied species,
because they cannot process energy and resources efficiently enough both at the organismic and cellular levels.
Here we compare 99 mammalian species and show that higher levels of sperm competition correlated with a) higher
proportions of seminiferous tubules, b) shorter seminiferous epithelium cycle lengths (SECL) which reduce the time
required to produce sperm, and c) higher efficiencies of Sertoli cells (involved in sperm maturation). These responses
to sperm competition, in turn, result in higher daily sperm production, more sperm stored in the epididymides, and
more sperm in the ejaculate. However, the two processes that require processing resources at faster rates (SECL
and efficiency of Sertoli cells) only respond to sperm competition in species with high MSMR. Thus, increases in
sperm production with intense sperm competition occur via a complex network of mechanisms, but some are
constrained by MSMR.
Citation: delBarco-Trillo J, Tourmente M, Roldan ERS (2013) Metabolic Rate Limits the Effect of Sperm Competition on Mammalian Spermatogenesis.
PLoS ONE 8(9): e76510. doi:10.1371/journal.pone.0076510
Editor: Wei Yan, University of Nevada School of Medicine, United States of America
Received June 10, 2013; Accepted August 29, 2013; Published September 19, 2013
Copyright: © 2013 delBarco-Trillo et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which
permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by the Spanish Ministry of Economy and Competitiveness (grants CGL-2011-26341 to ERSR and CGL2012-37423 to
JdT). JdT was supported by a Ramón y Cajal fellowship (RYC-2011-07943) whereas MT was supported by a "Juan de la Cierva" fellowship
(JCI-2011-10381), both from the Spanish Ministry of Economy and Competitiveness. The funders had no role in study design, data collection and analysis,
decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: delbarcotrillo@gmail.com
Introduction
seminiferous tubules consist of a seminiferous epithelium,
which lines the wall of the tubule, and the lumen, to which
mature sperm cells are released. The seminiferous epithelium
contains germ cells and their supporting and nourishing cells,
the Sertoli cells [13,14].
Several architectural features of the testis could influence
sperm production and thus may be affected by sperm
competition. On the one hand, an increase in the proportion of
testis occupied by seminiferous tubules should increase the
rate of sperm production per gram of testis [10,15-17]. On the
other hand, three architectural traits of the seminiferous tubules
could also impact on sperm production. First, a reduction in
tubule diameter could increase the number of tubules per gram
of testis, so we hypothesized that high levels of sperm
competition may favour a decrease in tubule diameter. Second,
a reduction in height of the seminiferous epithelium could result
in faster sperm production, so we also hypothesized that high
levels of sperm competition may favour a decrease in the
height of the seminiferous epithelium. Third, Sertoli cells
occupy a large proportion of the seminiferous epithelium, which
necessarily reduces the space occupied by germ cells; we
Sperm competition takes place when the sperm of two or
more males compete to fertilize the ova of a female [1-3]. Due
to the prevalence of female promiscuity [4], sperm competition
is a pervasive evolutionary force across taxa. In species that
experience high levels of sperm competition, males increase
their sperm production [5], which allows them to ejaculate more
sperm in competitive contexts [6]. Given the direct effect that
sperm competition has on male fitness, a general prediction is
that many morphological and physiological male traits will
evolve in response to the levels of sperm competition,
especially those traits that control sperm production [1]. The
most straightforward way to increase sperm production is by
augmenting the size of the testes [7-9], but males could also
maximize their rate of sperm production by adjusting either the
architecture of the testis or the kinetics of sperm formation
[10,11].
Sperm are produced in the testes through the process of
spermatogenesis that takes place within the seminiferous
tubules [12], which are separated by interstitial tissue [13]. The
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Evolution of Mammalian Spermatogenesis
hypothesized that high levels of sperm competition could lead
to a reduction in the number of Sertoli cells, which would lead
to a relative increase in the proportion of the seminiferous
epithelium occupied by germ cells (assuming that the size of
the remaining Sertoli cells does not increase) and thus to a
possible increase in sperm production.
The kinetics of spermatogenesis is species-specific and
determined by the seminiferous epithelium cycle length
(SECL). The SECL is the time period between two successive
occurrences of the same seminiferous stage [18,19]. A shorter
SECL leads to a shorter duration of spermatogenesis, and thus
to higher rates of sperm production [18]. Spermiogenesis is the
last phase of spermatogenesis in which spermatids
differentiate into spermatozoa [12]. A decrease in the duration
of spermiogenesis should also lead to an increase in sperm
production. Sperm production could also increase by elevating
the efficiency of Sertoli cells [14,20], defined as the number of
round spermatids (the haploid but still immature germ cells)
nurtured by each Sertoli cell [14,21]. Consequently, higher
levels of sperm competition might lead to a reduction in SECL
and spermiogenesis and an increase in the efficiency of Sertoli
cells [11,14,22].
Ultimately, the increase in sperm numbers in sperm reserves
(caudae epididymides in mammals) and in the ejaculate with
high levels of sperm competition [5,15,23] can be due to a
combination of responses, including increases in testis size,
changes in testis architecture and modifications in the kinetics
of spermatogenesis. Importantly, some of these responses
may also be affected by mass-specific metabolic rate (MSMR),
calculated as the ratio between basal metabolic rate and body
mass. For example, in a study comparing six shrew species, a
negative relationship was found between MSMR and SECL
[22]. Therefore, MSMR can limit the capacity of sperm
competition to affect reproductive traits that require a rapid
processing of energy and resources at the cellular level [24,25].
This “metabolic rate constraint hypothesis” has received recent
support in mammals: in species with high MSMR (e.g.,
rodents), high levels of sperm competition result in an
advantageous increase in sperm size, whereas species with
low MSMR (large-bodied mammals) exhibit a metabolic
constraint on the evolution of sperm size with high levels of
sperm competition [24,25].
Here we analyse for the first time a comprehensive series of
traits potentially influencing sperm production, including
aspects of testis architecture and kinetics of spermatogenesis
(percentage of seminiferous tubules, tubule diameter, height of
seminiferous epithelium, number of Sertoli cells, efficiency of
Sertoli cells, SECL and spermiogenesis), to assess which may
be affected by sperm competition and which by MSMR. We
also examined whether traits that require a fast turnover of
resources may be differentially affected by sperm competition
in species with higher or lower MSMR especially because of
metabolic rate constraints.
were available (n = 99). For each trait of interest we made
literature searches in Web of Knowledge and Google Scholar.
We only used results from studies in which males were adult
and reproductively normal. In experimental studies, we only
used data from the control groups. The methods used to
measure some traits may differ among studies (e.g., most
studies used tritiated thymidine to measure SECL, whereas
other studies have used bromodeoxyuridine (BrdU), with no
differences between methodological approaches, however,
being apparent [26]). We did not include two species for the
following reasons: a bat, Pteropus poliocephalus, due to the
unusual reproductive traits of Chiroptera which include sperm
storage for long periods of time in the female tract, and
possible effects of flight on the metabolic rate of this species;
and Dasypus novemcinctus, because its presence introduced
an extreme asymmetry at the base of the phylogeny (one
branch leading to Dasypus and the other branch leading to the
remaining 99 species) which could lead to spurious results in
phylogenetic analyses [27].
The variables that we analysed in this study and their
corresponding sample sizes were the following (see Dataset
S1): body mass (g; n = 99), testes mass (g; n = 99),
percentage of the testis occupied by seminiferous tubules (n =
62), diameter of the seminiferous tubules (µm; n = 66), height
of the seminiferous epithelium (µm; n = 32), relative number of
Sertoli cells (106 Sertoli cells / g testis; n = 35), efficiency of
Sertoli cells (number of round spermatids / Sertoli cell; n = 32),
seminiferous epithelium cycle length (SECL; days; n = 62),
duration of spermiogenesis (days; n = 25), daily sperm
production (106 sperm/g testis x day; n = 36), number of sperm
in the cauda epididymides (106 sperm; n = 43), number of
sperm in the ejaculate (106 sperm; n = 44), mass-specific
metabolic rate (ml O2/h x g; n = 67). When more than one value
was reported, we calculated an average value weighted by
sample size. It must be noted that daily sperm production is
normally calculated using a SECL value, and thus these two
variables can be partly autocorrelated. All variables were log10transformed to meet parametric assumptions, except for the
proportion of testicular tissue occupied by seminiferous
tubules, which was arcsine-transformed.
We tested the influence of sperm competition on the
architectural and kinetic variables mentioned above using
multiple regression analyses in which each variable of interest
was a dependent variable and body mass and testes mass
were the predictors (this is a more appropriate approach to
determine the effect of relative testes mass on a dependent
variable than using a "relative testes mass" measure, sensu
Kenagy and Trombulak 1986 [28], as the only predictor). We
also tested the influence of metabolic rate on those same
variables, using mass-specific metabolic rate as predictor. For
all these analyses we conducted phylogenetic generalised
least squares (PGLS) [29] models in R 2.13.0 [30]. The PGLS
estimates a phylogenetic scaling parameter lambda (λ), which
is then incorporated in the models to control for phylogenetic
effects. If λ values are close to 0, the traits are likely to have
evolved independently of phylogeny, whereas values close to 1
indicate strong phylogenetic association of the investigated
traits. The phylogenetic reconstruction used in the PGLS
Methods
The study sample includes all terrestrial eutherian mammals
for which information on the traits related to spermatogenesis
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variation of 1% in the range in the level 1 predictor. For
example, in Figure 1c it can be seen that an increase in MSMR
equivalent to 1% of MSMR’s range would produce a decrease
in SECL equivalent to 0.53% of SECL’s range, and that such a
0.53% decrease in SECL would produce a 0.25% increase in
the range of sperm numbers in cauda. All variations are
expressed as a percentage of the sample range.
analyses is included as Supporting Information (see Figure
S1).
We also performed six analyses in which for three dependent
variables (SECL, efficiency of Sertoli cells and sperm in the
cauda epididymides) we analysed separately species with high
and low MSMR. We ordered all species in each dataset by
MSMR and selected half of the species with the lowest MSMR
values for one analysis and half of the species with the highest
MSMR values for another analysis. If the number of species in
a dataset was odd, the one species in the middle of the dataset
was considered as a low MSMR species if its value was closer
to the highest value of the low MSMR species than to the
lowest value of the high MSMR species, or as a high MSMR
species if its value was closer to the lowest value of the high
MSMR species than to the highest value of the low MSMR
species. We used this approach to maintain similar sample
sizes between the analyses of low MSMR and high MSMR
species. The representation of species for each one of the six
analyses we performed is the following (for clarity, we use
these abbreviations: Art = Artiodactyla; Car = Carnivora; Eul =
Eulipotyphla; Lag = Lagomorpha; Per = Perissodactyla; Pri =
Primates; Rod = Rodentia): Low MSMR and efficiency of
Sertoli cells: 4 Art, 4 Car, 2 Per, 1 Pri, 2 Rod; Low MSMR and
SECL: 5 Art, 7 Car, 1 Eul, 2 Per, 5 Pri, 3 Rod; Low MSMR and
sperm in cauda: 4 Art, 1 Car, 1 Lag, 2 Per, 2 Pri, 6 Rod; High
MSMR and efficiency of Sertoli cells: 2 Car, 1 Lag, 3 Pri, 6
Rod; High MSMR and SECL: 1 Car, 6 Eul, 1 Lag, 3 Pri, 13
Rod; High MSMR and sperm in cauda: 6 Eul, 9 Rod.
In an effort to clarify the relative contribution of the variations
in testis architecture and kinetics of spermatogenesis (in
response to sperm competition or MSMR) to sperm production,
we calculated the impact that theoretical variations of the
predictor variables would have on the values of the dependent
variables. Thus, we identified three hierarchic levels of
variables based on the hypothetic relationships tested in the
models (see Figure 1): 1) sperm competition and MSMR; 2)
SECL, efficiency of Sertoli cells, and percentage of the testis
occupied by seminiferous tubules; and 3) daily sperm
production, and number of sperm in the cauda epididymides.
We then used the slopes and intercepts estimated by the
PGLS models to predict the relative influence of one level on
the successive levels. We applied two different approaches.
First, we used static variations: we introduced an increase in a
predictor variable at level 1 equivalent to 1% of its range (i.e.,
minimum to maximum), and the resulting variation on each
level-2 dependent variable was calculated (also as a
percentage of that variable’s range). For example, in Figure 1a
it can be seen that an increase in MSMR equivalent to 1% of
MSMR’s range would produce a decrease in SECL equivalent
to 0.53% of SECL’s range. We performed the same type of
calculations using the relationships between level 2 and level 3
variables: we introduced an increase in a predictor variable at
level 2 equivalent to 1% of its range, and the resulting variation
on the level 3 dependent variable was calculated. Second, we
used dynamic variations: the calculation procedure was similar
to the one used in static variations. However, for the
relationships between level 2 and 3, the variation (% of range)
introduced in the predictor variable at level 2 was the result of a
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Results
There was no significant relationship between MSMR and
relative testes size among species in this study (phylogenetic
generalised least squares, PGLS: P>0.05; Table S1).
Therefore, in those cases in which both MSMR and relative
testes size have significant effects on variables related to the
output of spermatogenesis, we consider these effects to be
mostly independent from each other.
We found that the percentage of testicular tissue comprising
seminiferous tubules increased with relative testes size (PGLS:
P=0.0002) and with MSMR (PGLS: P=0.0001) (Table 1).
However, neither relative testes size nor MSMR had any effect
on the diameter of seminiferous tubules, the height of the
seminiferous epithelium, or the number of Sertoli cells per gram
of testis (PGLS: P>0.05; Table 1).
We also found that SECL, and thus the duration of
spermatogenesis, decreased as relative testes size increased
(PGLS: P=0.023; Figure 1, Table 1) and when MSMR also
increased (PGLS: P=0.008). Neither relative testes size nor
MSMR were associated with the duration of spermiogenesis
(PGLS: P>0.05), despite the strong relationship between SECL
and the duration of spermiogenesis (PGLS: P<0.0001, R2
adjusted=0.9).
The efficiency of Sertoli cells increased in species with
higher relative testes size (PGLS: P=0.0004), but not in relation
to MSMR (PGLS: P=0.27). A higher efficiency of the Sertoli
cells was associated with shorter SECL (PGLS: P=0.014) and
shorter duration of spermiogenesis (PGLS: P=0.01; Table S3).
All the spermatogenic traits affected by high levels of sperm
competition had, in turn, positive effects on sperm production
after removing the effect of body size (Figure 1). That is, the
increased percentage of seminiferous tubules, the reduced
SECL, and the increased efficiency of Sertoli cells in response
to high levels of sperm competition, were all associated with a
higher daily sperm production per gram of testis (PGLS:
P<0.0005 for all analyses), and a higher number of sperm
stored in the caudae epididymides (PGLS: P<0.005 for all
analyses; Table S4).
Furthermore, when using phylogenetically-corrected linear
model equations to predict variation in sperm production
variables, we realised that there are important differences in
the relative degree in which the different traits underlying testis
architecture and kinetics of spermatogenesis influence total
sperm production (Figure 1). Sperm production and sperm
reserves were similarly affected by a static 1% increase in
SECL (-0.46%), in Sertoli cells efficiency (0.56%) and in the
percentage of tubules (0.61%; Figure 1a). However, a variation
of 1% in relative testes mass produced more than 1% variation
in seminiferous tubules (1.15%) and Sertoli cells efficiency
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Figure 1. Schematic representation of the relationships between sperm competition, mass-specific metabolic rate,
testicular architecture, kinetics of spermatogenesis, sperm production and numbers in sperm reserves in eutherian
mammals. a, Static 1% variations: numbers next to arrows are the relative variation in the dependent variable caused by a
variation of 1% of the sample range in the independent variable. For example, there is a -0.77% decrease in SECL when we
increase SC by 1%. b, c, Dynamic variations: numbers next to arrows between level 1 and level 2 variables are the relative variation
in the dependent variable caused by a variation of 1% of the sample range in the independent variable (thus these values are the
same as in panel a); numbers next to arrows between level 2 and level 3 variables are the relative variation in the dependent
variable (level 3) caused by the change in the independent variable (level 2) due to a 1% increment in level 1. All relative variations
are presented as percentages of the sample range. Relative variation percentages were calculated using the slopes and intercepts
estimated by PGLS models. Arrow widths are proportional to indicated magnitudes. Abbreviations: SC: sperm competition (relative
testes size); MSMR: mass-specific metabolic rate; SECL: seminiferous epithelium cycle length; ESC: efficiency of Sertoli cells; %
Tub: percentage of the testicular tissue occupied by seminiferous tubules; DSP: daily sperm production; Sperm Reserves: number
of spermatozoa in the caudae epididymides.
doi: 10.1371/journal.pone.0076510.g001
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Table 1. Effects of sperm competition and metabolic rate on spermatogenic traits.
Dependent variable
Predictor
Slope
F
P value
λ
r
CI
n
% of seminiferous tubules
RTSa
0.19
16.04
0.0002
0.79*, n.s.
0.46
0.25 to 0.76
62
MSMR
0.37
18.54
0.0001
0.25n.s., *
0.59
0.34 to 1.01
37
RTSa
0.03
1.45
0.23
0.71*, n.s.
0.15
-0.10 to 0.40
66
MSMR
0.04
0.96
0.33
0.24n.s., *
0.15
-0.16 to 0.46
43
RTSa
0.05
1.05
0.31
0.50n.s., n.s.
0.19
-0.17 to 0.55
32
MSMR
-0.08
3.80
0.067
<0.01n.s., n.s.
0.42
-0.03 to 0.92
20
Tubule diameter
Height of epithelium
Number of Sertoli cells
RTSa
-0.18
0.11
0.12
0.67n.s., n.s.
0.06
-0.29 to 0.41
35
MSMR
-0.14
0.91
0.35
0.23n.s., *
0.19
-0.21 to 0.59
27
32
RTSa
0.29
15.98
0.0004
<0.01n.s., *
0.60
0.32 to 1.05
MSMR
0.13
1.30
0.27
<0.01n.s., *
0.23
-0.18 to 0.65
25
RTSa
-0.07
5.48
0.02
<0.01n.s., *
0.29
0.05 to 0.56
62
MSMR
-0.10
7.59
0.008
0.76*, n.s.
0.38
0.10 to 0.70
47
Spermiogenesis
RTSa
-0.03
0.34
0.56
<0.01n.s., *
0.12
-0.29 to 0.54
25
MSMR
-0.06
1.31
0.27
<0.01n.s., *
0.26
-0.21 to 0.74
20
Daily sperm production
RTSa
0.26
8.58
0.006
0.999*, n.s.
0.45
0.15 to 0.83
36
MSMR
0.21
1.85
0.19
<0.01n.s., *
0.27
-0.14 to 0.70
25
RTSa
1.70
116.31
<0.0001
0.59n.s., *
0.86
0.99 to 1.61
43
MSMR
-0.99
2.94
0.097
0.999*, n.s.
0.30
-0.06 to 0.68
31
Efficiency of Sertoli cells
SECL
Sperm in cauda
Sperm in ejaculate
RTSa
1.58
32.21
<0.0001
<0.01n.s., *
0.66
0.49 to 1.11
44
MSMR
-1.22
3.48
0.071
0.999n.s., n.s.
0.30
-0.03 to 0.65
37
Phylogenetically controlled multiple regression analyses revealing the effects of relative testes mass (RTS) and mass-specific metabolic rate (MSMR) on spermatogenic
traits. a In the RTS analyses, we report the values for the second predictor (testes mass) after controlling for the effect of the first predictor (body mass; see Table S2 for the
values of body mass). All variables were log10 transformed (with the exception of the proportion of seminiferous tubules, which was arcsine transformed) prior to analysis.
The superscripts following the λ value indicate significance levels (n.s., p > 0.05; * p < 0.05) in likelihood ratio tests against models with λ = 0 (first superscript) and λ = 1
(second superscript). The effect size r was calculated from the F values; we also present the non-central 95% confidence interval (CI), an interval excluding 0 indicating
statistically significant relationships. The P values and CI that indicate statistical significance are shown in bold. Abbreviations: n: number of species in each analysis; SECL:
seminiferous epithelium cycle length.
doi: 10.1371/journal.pone.0076510.t001
(1.41%), but less than 1% variation in SECL (0.77%; Figure
1a). Consequently, the relative dynamic variation in daily sperm
production and sperm reserves caused by the effect of sperm
competition on testicular architecture variables (0.79% for
Sertoli cells efficiency; 0.70% for tubule percentage) would at
least double that caused by the effect of sperm competition on
kinetics of spermatogenesis, i.e., SECL (0.36%; Figure 1b). A
similar pattern was detected when analysing the effect of
MSMR as the cause of variation in the percentage of tubules
(architecture) and SECL (kinetics) (Figure 1c).
Species with higher daily sperm production have more sperm
stored in the caudae epididymides (PGLS: P=0.039), and, in
turn, more sperm stored in the caudae leads to a higher
number of sperm in the ejaculate (PGLS: P<0.0001; Table S4).
On the other hand, MSMR was not directly associated with
daily sperm production, sperm in the caudae, or sperm in the
ejaculate (PGLS: P>0.05).
Finally, we analysed separately species with low and high
MSMR to assess how species may adjust their spermatogenic
traits in response to sperm competition. In species with high
MSMR, increased sperm competition was associated with
shorter SECL (PGLS: P=0.002) and a higher efficiency of the
Sertoli cells (PGLS: P=0.005). This was not the case for
species with low MSMR (PGLS: P>0.05 for all analyses; Table
2). Higher levels of sperm competition ultimately led to greater
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numbers of sperm in the caudae regardless of whether species
had low MSMR (PGLS: P=0.0002) or high MSMR (PGLS:
P<0.0001).
Discussion
The overall result of our study is that sperm competition
leads to an increase in sperm production by affecting several
testicular traits. Therefore, high levels of sperm competition
promote a higher production of sperm not only by increasing
the size of the testes [8,28], but also by affecting morphological
and kinetic traits within the testes that have a direct influence
on sperm production. These traits are the percentage of
testicular tissue occupied by seminiferous tubules, the
efficiency of the Sertoli cells, and the seminiferous epithelium
cycle length (SECL) and thus the duration of spermatogenesis.
First, we found that high levels of sperm competition are
associated with an increase in the percentage of seminiferous
tubules in mammals. This expands earlier observations in
rodents [Gómez Montoto L, Arregui L, Roldan ERS;
unpublished data] and birds [10,15], which strengthens the
straightforward argument that an increase in the proportion of
sperm-producing tissue will lead to an increase in the numbers
of sperm produced.
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Table 2. Effects of sperm competition on spermatogenic traits depending on metabolic rate.
MSMR
Dependent variable
Predictor
Slope
F
P value
λ
r
CI
n
Low
ESC
body mass
-0.17
0.20
0.66
<0.01n.s., n.s.
0.14
-0.48 to 0.76
13
testes mass
0.27
2.86
0.12
0.47
-0.11 to 1.13
Low
SECL
body mass
0.07
4.25
0.053
testes mass
-0.06
2.10
0.16
body mass
-0.61
0.41
0.53
testes mass
1.61
26.29
0.0002
Low
High
High
High
SpCauda
ESC
SECL
SpCauda
body mass
-0.29
7.53
0.023
testes mass
0.37
14.02
0.005
body mass
0.15
7.00
0.015
testes mass
-0.13
12.19
0.002
body mass
-0.54
36.52
<0.0001
testes mass
1.67
39.7
<0.0001
<0.01n.s., n.s.
0.84n.s., n.s.
<0.01n.s., *
0.94*, n.s.
<0.01n.s., n.s.
0.42
0.01 to 0.88
0.31
-0.12 to 0.76
0.18
-0.37 to 0.72
0.82
0.61 to 1.69
0.68
0.17 to 1.47
0.78
0.39 to 1.70
0.50
0.12 to 0.98
0.61
0.28 to 1.13
0.87
0.76 to 1.89
0.88
0.79 to 1.93
23
16
12
24
15
Phylogenetically controlled multiple regression analyses revealing the effect of relative testes mass on spermatogenic traits in species with low and high mass-specific
metabolic rates (MSMR). All variables were log10-transformed prior to analysis. The superscripts following the λ value indicate significance levels (n.s., p > 0.05; * p < 0.05)
in likelihood ratio tests against models with λ = 0 (first superscript) and λ = 1 (second superscript). The effect size r was calculated from the F values; we also present the
non-central 95% confidence interval (CI), an interval excluding 0 indicating statistically significant relationships. The P values and CI that indicate statistical significance are
shown in bold. Abbreviations: n: number of species in each analysis; ESC: efficiency of Sertoli cells (number of round spermatids / Sertoli cell); SECL: seminiferous
epithelium cycle length; SpCauda: number of sperm in the cauda epididymides.
doi: 10.1371/journal.pone.0076510.t002
Second, we also found that another route to increase sperm
production in response to sperm competition is to increase the
efficiency of Sertoli cells. In fact, increasing the efficiency of the
Sertoli cells is as effective in augmenting sperm production as
increasing the percentage of seminiferous tubules. An increase
in the efficiency of Sertoli cells in relation to high levels of
sperm competition has also been shown in birds [14].
Third, another way to increase sperm production in response
to sperm competition is decreasing SECL and thus the duration
of spermatogenesis [11]. Our data supports this relationship
and thus agree with previous studies that compared a small
number of species, e.g., six shrew species [22] and two rodent
species [23]. We also found that higher efficiencies of the
Sertoli cells are associated with a decrease in SECL. It is thus
possible that the reduced SECL in response to high levels of
sperm competition is due to the concomitant increase in the
efficiency of the Sertoli cells.
Our results indicate that the contributions of (a) the
proportion of seminiferous tubules, (b) the efficiency of Sertoli
cells and (c) SECL to sperm production in response to the
same level of sperm competition are not symmetrical. Similar
changes in the proportion of seminiferous tubules and the
efficiency of Sertoli cells resulted in twice as much increase in
sperm production compared to SECL. This may be the case
because an increase in both the proportion of seminiferous
tubules and the efficiency of Sertoli cells would result in a direct
increase in the number of germ cells produced per gram of
testis. In contrast, the reduction in SECL may result in only a
partial
acceleration
of
spermatogenesis
because
spermiogenesis (the post-meiotic stage of spermatogenesis) is
not significantly affected by sperm competition.
All the spermatogenic traits affected by sperm competition
(percentage of seminiferous tubules, efficiency of Sertoli cells
and SECL) were associated with a higher daily sperm
PLOS ONE | www.plosone.org
production per gram of testis. Consequently, a combination of
increasing the percentage of sperm-producing tissue and
increasing the efficiency and speed of the sperm-producing
process (spermatogenesis) offers more potential and flexibility
for increasing sperm production when required, for example in
contexts with high levels of sperm competition. The importance
of the increase in daily sperm production in species with high
levels of sperm competition is illustrated by the fact that
species with higher daily sperm production had more sperm
stored in the caudae epididymides, which, in turn, allows males
to allocate a higher number of sperm in the ejaculate, as also
suggested in previous studies [5].
It is also important to consider which traits were not
associated with different levels of sperm competition. These
traits were the diameter of the seminiferous tubules, the height
of the seminiferous epithelium, and the number of Sertoli cells
per gram of testis. One explanation is that these traits may not
be able to evolve readily in response to external factors without
impairing the normal process of spermatogenesis.
MSMR and relative testes size were not associated with
each other, which suggests that the effects of MSMR on
testicular traits may be independent from those due to sperm
competition. Our results agree with the overall hypothesis that
sperm competition is the ultimate factor affecting the up or
down-regulation of sperm production, whereas MSMR would
be a proximate factor that would constrain such regulations
[22]. It must be noted that even though MSMR is associated
with some traits that can affect sperm production, MSMR itself
is not directly associated with daily sperm production, sperm in
the caudae, or sperm in the ejaculate. Species with low MSMR
(i.e., with large body sizes), have a lower proportion of
testicular tissue occupied by seminiferous tubules and longer
SECL. Large-bodied animals have larger testes, independently
of sperm competition levels, simply due to allometric reasons
6
September 2013 | Volume 8 | Issue 9 | e76510
Evolution of Mammalian Spermatogenesis
[13]; given that the interstitial tissue contains the blood and
lymph vessels, macrophages, and Leydig cells, an increase in
the volume of the testes may require, in turn, an increase in the
percentage of interstitial tissue to guarantee normal testicular
function [12,13]. The negative association between MSMR and
SECL is even more straightforward. In species with high
MSMR, the metabolism of all cell types involved in
spermatogenesis will be increased, which will facilitate a faster
activity of the spermatogenic processes and thus shorter SECL
and spermatogenesis duration [22].
Finally, we found that increased sperm competition was
associated with shorter SECL and a higher efficiency of the
Sertoli cells in species with high MSMR, but not in those with
low MSMR. Nevertheless, higher levels of sperm competition
ultimately led to greater numbers of sperm in the caudae
regardless of whether species had high or low MSMR. These
results suggest that all species, regardless of their MSMR,
have the potential to adjust sperm production in response to
sexual selection. However, MSMR determines the pathway that
different species take to increase sperm numbers under sexual
selection. Species with low MSMR are constrained in their
ability to respond to sexual selection when such a response
involves fast processing rates of energy and resources [25]; in
contrast, species with high MSMR are not subjected to such
constraints and have the potential to adjust more traits, e.g.,
decreasing SECL and increasing the efficiency of Sertoli cells.
Overall, our results generalize the “metabolic rate constraint
hypothesis” [25] to sperm production and show that MSMR
limits how increases in sperm production can be attained under
sexual selection.
(DOC)
Table S1. Relationship between MSMR and relative testes
size.
(DOC)
Table S2. Effects of sperm competition on spermatogenic
traits.
(DOC)
Table S3.
Relationships
spermatogenic traits.
(DOC)
among
testicular
and
Table S4. Effects of spermatogenic traits on indicators of
sperm production.
(DOC)
Dataset S1. The complete dataset used in all the analyses:
morphological, reproductive, and metabolic data.
(XLS)
Acknowledgements
We thank Montserrat Gomendio for comments and discussions
during early stages of this study. We also thank two
anonymous reviewers for their comments.
Author Contributions
Conceived and designed the experiments: JdT MT ERSR.
Performed the experiments: JdT MT. Analyzed the data: JdT
MT. Wrote the manuscript: JdT MT ERSR.
Supporting Information
Figure S1.
Phylogenetic reconstruction for the 99
eutherian mammal species utilised in the PGLS analyses.
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