Ecology Letters, (2020) 23: 831–840
LETTER
Andres Hagmayer,1
Andrew I. Furness,2,3
David N. Reznick,4
Myrthe L. Dekker1 and
Bart J. A. Pollux1*
The peer review history for this article is available at https://publons.c
om/publon/10.1111/ele.13487
doi: 10.1111/ele.13487
Predation risk shapes the degree of placentation in natural
populations of live-bearing fish
Abstract
The placenta is a complex life-history trait that is ubiquitous across the tree of life. Theory proposes that the placenta evolves in response to high performance-demanding conditions by shifting
maternal investment from pre- to post-fertilisation, thereby reducing a female’s reproductive burden during pregnancy. We test this hypothesis by studying populations of the fish species Poeciliopsis retropinna in Costa Rica. We found substantial variation in the degree of placentation
among natural populations associated with predation risk: females from high predation populations had significantly higher degrees of placentation compared to low predation females, while
number, size and quality of offspring at birth remained unaffected. Moreover, a higher degree of
placentation correlated with a lower reproductive burden and hence likely an improved swimming
performance during pregnancy. Our study advances an adaptive explanation for why the placenta
evolves by arguing that an increased degree of placentation offers a selective advantage in high
predation environments.
Keywords
Life-history, live-bearing, matrotrophy, placenta, placentotrophy, Poeciliidae, predation, superfetation, Trexler–DeAngelis, viviparity.
Ecology Letters (2020) 23: 831–840
Understanding the origin and elaboration of complex traits is
of fundamental interest to evolutionary biologists. Placentas
are complex organs that have independently evolved many
times throughout the animal kingdom in widely divergent lineages (Wourms 1981; Wooding & Burton 2008; Blackburn
2015; Wake 2015; Ostrovsky et al. 2016). This repeated evolution points to a possible adaptive advantage (Losos et al.
1998), however, the nature of this advantage remains elusive.
To date, two adaptive hypotheses (termed the resource allocation and locomotor cost hypotheses) provide potential
explanations for why the placenta evolves (i.e. its selective
advantage). Both hypotheses treat the placenta as a life-history adaptation that evolves in response to ecological selection
pressures (Thibault & Schultz 1978; Trexler & DeAngelis
2003). Furthermore, both hypotheses posit that the ancestral
state is for all nutrients to be prepackaged in the form of egg
yolk (lecithotrophy) and that the derived state is for nutrients
to be supplied to embryos throughout pregnancy via a placenta (placentotrophy). This is presumably achieved via a
gradual shift in the timing of nutrient provisioning from preto post-fertilisation. If true, then the evolution of the placenta
should coincide with a reduction in egg mass at fertilisation
without affecting the total investment per neonate at birth
(Reznick et al. 2002; Trexler & DeAngelis 2003; Pollux et al.
2009). The live-bearing fish family Poeciliidae is a useful system in which to test these ideas, because it contains species
that span a continuum of pre- versus post-fertilisation nutrient
provisioning, ranging from strictly lecithotrophic to highly
placentotrophic. In addition, placentotrophy has evolved independently numerous times in this family (Reznick et al. 2002;
Pollux et al. 2014; Furness et al. 2019).
The resource allocation hypothesis (Trexler & DeAngelis
2003) posits that the evolution of the placenta permits females
to produce greater brood sizes and hence attain higher fecundity than lecithotrophic females when sufficient resources are
available to carry the developing embryos to term. Moreover,
placental females rely on a steady nutritional supply to provision embryos throughout pregnancy. Trexler & DeAngelis
(2003) therefore argued that the ability to selectively abort
embryos and recycle this investment, should resource conditions suddenly deteriorate, is a crucial preadaptation for the
evolution of placentation. Studies in the Poeciliidae have not
shown that they are able to do this, suggesting that the conditions under which the placenta might be favoured by natural
selection are restricted to environments characterised by high
and stable resource conditions (Reznick et al. 1996; Trexler
1997; Banet & Reznick 2008; Banet et al. 2010; Pollux &
Reznick 2011).
The locomotor cost hypothesis (Thibault & Schultz 1978;
Pollux et al. 2009; Pires et al. 2011) postulates that the placenta evolves to offset some of the locomotor costs associated
with a live-bearing mode of reproduction. The physical and
physiological burden of pregnancy negatively affects a
female’s locomotor performance in a broad range of live-
1
3
INTRODUCTION
Department of Animal Sciences, Wageningen University, 6708 WD, Wagenin-
gen, Netherlands
2
Department of Ecology and Evolutionary Biology, University of California,
Irvine, CA 92697, USA
Department of Biological and Marine Sciences, University of Hull, HU6 7RX,
Hull, UK
4
Department of Biology, University of California, Riverside, CA 92521, USA
*Correspondence: E-mail: bart.pollux@wur.nl
© 2020 The Authors. Ecology Letters published by CNRS and John Wiley & Sons Ltd
This is an open access article under the terms of the Creative Commons Attribution License, which permits use,
distribution and reproduction in any medium, provided the original work is properly cited.
832 A. Hagmayer et al.
bearing animals (Seigel et al. 1987; Plaut 2002; Noren et al.
2011; Fleuren et al. 2019). The evolution of the placenta
should allow females to attain higher fitness, because the production of smaller eggs at fertilisation reduces a female’s
reproductive burden during pregnancy. This improves body
streamlining and locomotor performance, notably without
sacrificing reproductive output. The presumed benefit of a
higher degree of placentation is that the improved swimming
performance offers a selective advantage to females in performance-demanding (e.g. high predation) environments because
it enhances survival probability (Pollux et al. 2009; Pires et al.
2011). Whereas the resource allocation hypothesis has already
been the focus of several empirical studies (Pires et al. 2007;
Banet & Reznick 2008; Banet et al. 2010; Pollux & Reznick
2011; Bassar et al. 2014), the locomotor cost hypothesis has
not yet been subject to similar systematic investigation.
Here, we quantify the degree of placentation in high and
low predation populations of the placental live-bearing fish
species Poeciliopsis retropinna (family Poeciliidae, Regan 1908)
in Costa Rica. We test a key prediction of the locomotor cost
hypothesis, which is that placentation evolves in performancedemanding (high predation) environments. If the placenta is
favored under high predation conditions, we should find a
higher degree of placentation in high predation populations.
Theory further predicts that this higher degree of placentation
should be due to a smaller egg size at fertilisation without
affecting the number, size, or quality of offspring at birth,
translating to a lower reproductive burden for females during
pregnancy (Thibault & Schultz 1978; Pollux et al. 2009; Pires
et al. 2011). Finally, we discuss the potential fitness advantage
of evolving a higher degree of placentation in high predation
populations.
Letter
location/snorkeler) at different positions along each river. In
one location (Rio Conte), water visibility was low and predator community was assessed using seine and cast nets. We
found 17 ‘high’ predation localities where piscivorous predator species were present, and 12 ‘low’ predation localities
where predators were absent (Table S1). At each location, 5–
37 adult females were collected, euthanised with an overdose
of MS-222, and preserved in 5% formaldehyde.
Laboratory measurements
Maternal standard length and the proportion of body fat were
measured using established protocols (Supporting Information
Methods 1.1). 28 of the 29 sampled populations contained
pregnant females (Table S1), therefore all subsequent anatomical and statistical analyses were carried out only with females
from 28 populations (npreg = 463). The ovaries were dissected
to count the total number of embryos (fecundity), regressors
(aborted embryos), broods at different developmental stages
(superfetation), embryos in a brood (brood size), and to determine the developmental stage and average dry mass of
embryos in a brood (Table 1). The developmental stages are
based on morphological criteria described in Haynes (1995)
and range from 0 (eggs at fertilisation, no development) to 45
(fully developed embryos). Fecundity was calculated by
excluding stage 0 embryos, because it was difficult to assess if
0-staged eggs were fertilised or not. Instead, embryos at developmental stage 2, rather than 0, were defined as ‘eggs at fertilisation’.
Quantification of the degree of placentation
The Matrotrophy Index (MI), calculated as the ratio of offspring mass at birth to egg mass at fertilisation, was used as
MATERIAL AND METHODS
Study species and collection sites
Poeciliopsis retropinna is found in freshwater streams of varying water velocity and predation pressure in Costa Rica and
Panama (Bussing 2002). They are observed in habitats with
low predation risk (no strongly piscivorous species present),
or co-occurring with one or more piscivorous predator species: Parachromis dovii, Eleotris picta, and Gobiomorus maculatus. During gestation, P. retropinna females transfer nutrients
to developing embryos via a placenta. The degree of post-fertilisation maternal provisioning in this species is extensive,
with offspring increasing in dry mass more than 100-fold during gestation (MI = 117) (Reznick et al. 2002). Moreover, P.
retropinna has superfetation, the ability to carry several
broods at different developmental stages.
During February and March 2013, 2017, and 2018, P. retropinna were collected at 27 different locations in the Rio Terraba and Rio Coto drainages in the province of Puntarenas,
Costa Rica (Table S1). Two of the locations were repeatedly
sampled resulting in 29 study populations. When water visibility was high (i.e. in all but one location), the occurrence of
piscivorous predator species was most effectively detected
using underwater visual census by three independent snorkelers. Censuses took place during daytime (c. 3–5 h per
Table 1 Summary of maternal life-history traits
Maternal life-history traits
Egg mass at
fertilisation
Offspring mass at
birth
Proportion egg fat
Proportion offspring
fat
Reproductive
allotment
Absolute
reproductive
allotment
Brood size
Fecundity
Superfetation
Abortion incidence
© 2020 The Authors. Ecology Letters published by CNRS and John Wiley & Sons Ltd
Dry mass of eggs at fertilisation (i.e.
developmental stage 2)
Dry mass of fully developed embryos (i.e.
developmental stage 45)
Egg fat at fertilisation divided by dry mass of eggs
at fertilisation
Offspring fat at birth divided by offspring dry
mass at birth
Proportion of the mother’s dry mass allocated to
reproduction (i.e. embryo dry mass, regressor dry
mass, and placental dry mass)
Total dry mass allocated to reproduction (i.e.
embryo dry mass, regressor dry mass, and
placental dry mass)
Number of embryos in a given brood
Number of embryos carried by a female counted
across all broods excluding stage 0 embryos
Number of broods at different developmental
stages
Number of regressors (i.e. aborted embryos)
divided by the sum of the number of regressors
and embryos
Letter
Predation drives placental evolution 833
an unbiased measure of the degree of placentation (Reznick
et al. 2002). Some live-bearing species allocate all resources to
eggs prior to fertilisation in the form of large fully-yolked
eggs (termed lecithotrophy). These embryos lose dry mass
over the course of gestation due to metabolic processes, leading to an MI < 1. Other species allocate nutrients to the
developing offspring post-fertilisation throughout pregnancy
(termed matrotrophy). Such species have an MI > 1, indicating embryos gain dry mass during pregnancy. Placentotrophy
represents one specific type of matrotrophy that is achieved
through a follicular placenta, roughly an analog to the mammalian placenta (Pollux et al. 2009). Because the MI is determined by both egg mass at fertilisation and offspring mass at
birth, an increase in the degree of placentation can be brought
about by an increase in offspring mass at birth and/or a
decrease in egg mass at fertilisation.
The MI for P. retropinna was estimated in relation to (1) a
given population in a specific year, and (2) high and low predation risk by using the Bayesian programming environment
JAGS (Plummer 2003) in R v 3.5 (R Core Team 2019) (Supporting Information Methods 1.2, 1.3).
In (1), ln-transformed embryo dry mass was fitted as a function of the developmental stage of embryos (stage), stage2,
proportion of maternal body fat (BF), maternal standard
length (SL), and BF 9 stage. For each population, the model
estimates year-specific intercepts and slopes on stage. This
allows for the prediction of MI for a given population in a
specific year. In addition, the model includes mother identity
as additional intercept to allow for variation among females
that is not accounted by maternal BF and SL. The population-specific MI’s were subsequently calculated by dividing
offspring mass at birth (stage 45) by egg mass at fertilisation
(stage 2) that were predicted for a given population in a specific year and for a female of overall average SL and BF. Since
the MI’s are predicted for a female of the same SL and BF,
the resulting MI’s are independent of these traits (Fig. S1).
In (2), ln-transformed embryo dry mass was fitted as a function of stage, stage2, predation, BF, SL, BF 9 stage and predation 9 stage. For each population, the model estimates yearspecific intercepts to account for systematic differences among
populations within a year. Moreover, the model includes
mother identity as additional intercept to allow for variation
among females that is not accounted for by maternal BF and
SL. The MI for high and low predation females was subsequently calculated by dividing offspring mass at birth by egg
mass at fertilisation that were predicted for a given predation
regime and for a female of overall average SL and BF.
Quantification of life-history variation among predation regimes
The effects of predation on life-history traits (egg mass at fertilisation, offspring mass at birth, proportion of egg and offspring
fat, reproductive allotment, brood size, fecundity, superfetation, and abortion incidence) were analysed by fitting each trait
as a function of (1) high and low predation risk, and (2) predator community in a series of (generalised) linear mixed effect
models in R v 3.5 (R Core Team 2019), using the package
lme4 (Bates et al. 2015). Considering Parachromis dovii (P),
Eleotris picta (E), and Gobiomorus maculatus (G) as predator
species, we observed six different predation categories in nature:
low (no strongly piscivorous species present), G, E, EG, PG,
and P. Except in the case of reproductive allotment, additional
fixed effects included the proportion of maternal body fat (BF)
and standard length (SL). Reproductive allotment is defined as
the proportion of the mother’s dry mass allocated to reproduction, and hence accounts for female dry mass, rather than BF
and SL. Maternal BF predictably responds to experimental
manipulation of food availability in the laboratory (Reznick
et al. 1996; Banet & Reznick 2008; Banet et al. 2010; Pollux &
Reznick 2011), and is believed to be a good indicator of fish
condition (Leips et al. 2013). Therefore, accounting for maternal BF may enable us to partly decouple the effects of predation
risk and food availability on life-history traits. The association
of life-history traits with maternal BF and SL is reported and
discussed in the Supporting Information (Table S2–S10;
Fig. S2). In the case of reproductive allotment, fecundity and
superfetation, the developmental stage of the most-developed
brood was fitted as an additional fixed effect to account for
females early in the reproductive cycle. Population, year, river,
and population 9 year were fitted as random intercepts
accounting for spatio-temporal non-independence of observations. Likewise, in the case of brood size, mother identity was
fitted as additional random intercept to correct for pseudoreplication, because brood size is measured multiple times in
females with superfetation.
To optimise normality and homoscedasticity of model residuals, reproductive allotment, abortion incidence, proportion
of egg and offspring fat and maternal BF were arcsine squareroot transformed. Fecundity, superfetation, and brood size
were fitted in generalised linear mixed effect models using a
log link for the Poisson-distributed responses.
Path analysis
Differences in reproductive allotment (RA) among populations could be due to effects on several life-history traits. For
instance, RA could be decreased by reducing brood size or
superfetation, which in turn decreases the number of embryos
(fecundity). Alternatively, RA diminishes when producing
smaller eggs at fertilisation or offspring at birth. We used a
path analysis to determine the contribution of each of these
life-history traits to differences in RA among predation
regimes. In total, three (generalised) linear mixed effect models, implemented in MCMCglmm (Hadfield 2010), were used
to estimate all paths (Supporting Information Methods 1.4).
Each model was refitted as a function of an intercept only
(null model) to compare the deviance information criterion
(DIC) of the full model against that of the null model
(DDIC).
In the first model, egg mass at fertilisation, offspring mass
at birth, average brood size for a given mother, and superfetation were fitted in a multivariate model as a function of high
and low predation risk allowing for the covariance between
the residuals of all responses (DDIC = 272.75). In the second model, maternal fecundity was fitted as a function of
superfetation and average brood size for a given mother, as
changes in both brood size and superfetation will affect fecundity (DDIC = 388.449). The third model subsequently
© 2020 The Authors. Ecology Letters published by CNRS and John Wiley & Sons Ltd
834 A. Hagmayer et al.
Letter
predicts absolute RA as a function of fecundity, egg mass at
fertilisation, and offspring mass at birth (DDIC = 135.094).
All three models included the proportion of maternal body
fat and standard length as fixed effects. In the case of superfetation and fecundity, the developmental stage of the most-developed brood was an additional fixed effect (see above). To
aid convergence, we did not fit a random year effect (which
was effectively zero), but otherwise the random effect structure was the same as above. Furthermore, all continuous
input variables were z-standardised to obtain standardised
partial regression coefficients (b*) that take values between 1
and 1 (Schielzeth 2010). In the case of Poisson-distributed
responses (fecundity and superfetation), b* was obtained retrospectively by dividing the estimated slope (b) by the phenotypic standard deviation of the response variable (rY).
However, rY was indirectly estimated using the variance of
the predicted model fits on the link scale (r2log(Ŷ)) and the
pseudo-R-squared (R2) (Menard 2011):
rY ¼
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
r2log Y^ þ R2 :
ð Þ
The effect of predation on RA, mediated through a specific
maternal life-history trait, is then given by the direct effect of
predation on the life-history trait and its contribution to the
RA. This effect is quantified by multiplying b* of predation
on the life-history trait with b* of the life-history trait on RA.
RESULTS
There was a more than two-fold range in the estimated degree
of placentation among the 28 natural populations of P. retropinna (MI ranging from 14.87 to 32.34; Table S1). Consistent
with the locomotor cost hypothesis, we found that P. retropinna females from high predation (HP) localities exhibit a
significantly higher degree of placentation (MIHP:
mean = 22.86, 95% CI