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vol. 170, no. 6 the american naturalist december 2007
When to Care for, Abandon, or Eat Your Offspring: The
Evolution of Parental Care and Filial Cannibalism
Hope Klug
1,
*
and Michael B. Bonsall
2,†
1. Department of Zoology, University of Florida, Gainesville,
Florida 32611;
2. Mathematical Ecology Research Group, Department of Zoology,
University of Oxford, Oxford OX1 3PS, United Kingdom
Submitted November 8, 2006; Accepted July 20, 2007;
Electronically published October 1, 2007
Online enhancement: appendix.
abstract: Parental care and filial cannibalism (the consumption
of one’s own offspring) co-occur in many animals. While parental
care typically increases offspring survival, filial cannibalism involves
the killing of one’s young. Using an evolutionary ecology approach,
we evaluate the importance of a range of factors on the evolution
of parental care and filial cannibalism. Parental care, no care/total
abandonment, and filial cannibalism evolved and often coexisted over
a range of parameter space. While no single benefit was essential for
the evolution of filial cannibalism, benefits associated with adult or
offspring survival and/or reproduction facilitated the evolution of
cannibalism. Our model highlights the plausibility of a range of
alternative hypotheses. Specifically, the evolution of filial cannibalism
was enhanced if (1) parents could selectively cannibalize lower-qual-
ity offspring, (2) filial cannibalism increased egg maturation rate, (3)
energetic benefits of eggs existed, or (4) cannibalism increased a
parent’s reproductive rate (e.g., through mate attractiveness).
Density-dependent egg survivorship alone did not favor the evolution
of cannibalism. However, when egg survival was density dependent,
filial cannibalism invaded more often when the density dependence
was relatively more intense. Our results suggest that population-level
resource competition potentially plays an important role in the evo-
lution of both parental care and filial cannibalism.
Keywords: parental care, filial cannibalism, sexual selection, infanti-
cide, density-dependent egg survivorship.
Parental investment, and more specifically parental care,
* Corresponding author; e-mail: hklug@zoo.ufl.edu.
†
E-mail: michael.bonsall@zoo.ox.ac.uk.
Am. Nat. 2007. Vol. 170, pp. 886–901. 䉷 2007 by The University of Chicago.
0003-0147/2007/17006-42207$15.00. All rights reserved.
DOI: 10.1086/522936
is one of the most well-studied topics in life-history evo-
lution. Adaptive theories of evolution typically suggest that
parents should strive to increase offspring survival, and
parental care is one way in which parents are thought to
achieve this (reviewed by Clutton-Brock [1991]). Although
parental care is assumed to increase offspring survival, filial
cannibalism, the consumption of one’s own viable off-
spring, commonly co-occurs with parental care. Indeed,
filial cannibalism is prevalent in a range of taxa exhibiting
parental care (Polis 1981; Elgar and Crespi 1992). For
example, caring females consume some of their young in
the bank vole (Clethrionomys glareolus; Klemme et al.
2006), the house finch (Carpodacus mexicanus; Gilbert et
al. 2005), and the wolf spider (Pardosa milvina; Anthony
2003), and both parents of the burying beetle (Nicrophorus
orbicollis) are known to consume their offspring (Bartlett
1987). Filial cannibalism has been particularly well doc-
umented in fish species with paternal care during the egg
stage (reviewed in Manica 2002). Indeed, because of the
prevalence of filial cannibalism in fish systems, most the-
oretical and empirical work on the subject has focused on
fish (but see Bartlett 1987; Thomas and Manica 2003;
Creighton 2005). While early ethologists considered filial
cannibalism a social pathology with little or no evolu-
tionary significance, it is now typically thought to reflect
an adaptive trade-off between current and future repro-
ductive success (e.g., Manica 2002, 2004). However, de-
spite much theoretical development and empirical work
during the past few decades, the evolutionary significance
of filial cannibalism remains unclear in many systems.
The most widely accepted hypothesis of filial cannibal-
ism as an adaptive strategy suggests that energetic need is
the primary factor leading to filial cannibalism and that a
caring parent gains energy and nutrients from consuming
its offspring that are then reinvested into future repro-
duction, thereby increasing net reproductive success
(Rohwer 1978; Sargent 1992). Specifically, whole-clutch
cannibalism (i.e., the consumption of all offspring during
a given reproductive bout) is assumed to be an investment
in future reproduction, whereas partial-clutch cannibalism
(i.e., the consumption of only some offspring present) can
Evolution of Parental Care and Filial Cannibalism 887
represent an investment in either current or future repro-
duction. This energy-based hypothesis predicts that can-
nibalism will increase as food availability decreases and
when parental condition is poor (Rohwer 1978; Sargent
1992). While food availability and/or parental condition
affect the amount of cannibalism in some species (e.g.,
Stegastes rectifraenum [Hoelzer 1992], Pomatoschitus mi-
crops [Kvarnemo et al. 1998], and Abudefduf sexfasciatus
[Manica 2004]), it has no effect in others (e.g., Gasterosteus
aculeatus [Belles-Isles and Fitzgerald 1991] and Etheostoma
flabellare [Lindstro¨m and Sargent 1997]), and in two sys-
tems, cannibalism declines as male condition or food avail-
ability decreases (Jordanella floridae [Klug and St. Mary
2005] and Pomatoschistus minutus [Klug et al. 2006]).
Other studies have examined whether eggs can provide a
caring parent with sufficient energy to offset the costs of
care. Again, the evidence is mixed: two studies concluded
that energy attained from filial cannibalism is sufficient to
offset costs related to care (Kume et al. 2000; Thomas and
Manica 2003), while in another study, energy from eggs
was found to be insufficient (Smith 1992). Thus, parental
energetic need alone cannot explain the prevalence of filial
cannibalism.
Alternatively, Payne et al. (2002) and Klug et al. (2006)
suggested that filial cannibalism is mediated by density-
dependent egg survivorship and that by consuming some
eggs in their nests, caring parents can improve the sur-
vivorship of the remaining eggs and increase their net
reproductive success. Such density-dependent egg survi-
vorship is potentially related to the physical environment
(e.g., oxygen availability; Payne et al. 2002) or increased
benefits of parental care to the remaining offspring. The
hypothesis of filial cannibalism mediated by density-
dependent egg survivorship has received support in two
fish species (Stegastes leucostictus [Payne et al. 2002] and
P. minutus [Klug et al. 2006]) but has, in general, received
little further empirical or theoretical examination (but see
Payne et al. 2004). Likewise, some have suggested that filial
cannibalism is a mechanism by which parents reduce
brood size in response to anticipated resource competition
among their adult offspring (Bartlett 1987; Creighton
2005) or kill offspring of reduced quality (Forbes and
Mock 1998; see also Kozlowski and Stearns 1989). While
the former hypothesis has received some attention in the
burying beetle (Creighton 2005), neither of these hypoth-
eses of filial cannibalism has been explicitly evaluated.
Because of the mixed empirical support for the energy-
based hypothesis and the lack of empirical evidence re-
garding alternative hypotheses, filial cannibalism remains
an evolutionary conundrum. Indeed, previous work sug-
gests that a parent’s energetic need (Rohwer 1978; Sargent
1992; Manica 2002), expectations regarding offspring sur-
vival or reproductive value (Payne et al. 2002; Neff 2003;
Klug et al. 2006), competition for mates (Sikkel 1994;
Kondoh and Okuda 2002), and anticipated offspring re-
source competition (Creighton 2005) are potentially im-
portant factors for explaining the adaptive significance of
filial cannibalism. However, previous theory has tended to
focus on each of these factors in separate theoretical con-
texts (e.g., energetic benefits of consuming offspring
[Rohwer 1978; Sargent 1992], variation in offspring quality
[Forbes and Mock 1998], mate availability [Kondoh and
Okuda 2002], and expectations regarding offspring sur-
vivorship [Payne et al. 2004]), despite empirical evidence
suggesting that it is unlikely that any single factor alone
can explain the prevalence of filial cannibalism (e.g., Man-
ica 2004; Klug et al. 2006).
Here, we develop a model of parental care, total off-
spring abandonment (i.e., no care), and filial cannibalism
to begin to isolate the pivotal factors affecting the evolution
of care and filial cannibalism. First, we determine the gen-
eral conditions under which we would expect these strat-
egies (i.e., care, no care/total abandonment, filial canni-
balism) to evolve alone or in combination. We then
evaluate the plausibility of multiple alternative hypotheses
within a single theoretical context by assessing the im-
portance of a range of potential costs and benefits of care
and cannibalism. Specifically, we focus on costs and ben-
efits related to energetics, offspring survival and quality,
mate competition, and general resource competition.
Methods
The model is set up as an ecological problem in which a
rare mutant with a unique life-history strategy is allowed
to invade a resident population (e.g., Vincent and Brown
2005). Specifically, we assume that the resident strategy is
in equilibrium and that an alternative mutant strategy in-
vades from rare into the resident population. We assume
a system in which individuals develop through an egg stage
and a juvenile stage and then mature and reproduce as
adults. While in the egg stage, individuals can be aban-
doned by parents, receive parental care, suffer filial can-
nibalism, or receive parental care and suffer filial canni-
balism (fig. 1). We outline the dynamics of a system in
which a mutant with care and/or cannibalism invades a
resident population that either lacks or provides parental
care. We then use mutual invasion analysis to explore the
effects of costs and benefits of varying strategies on lifetime
fitness and the evolution (i.e., invasion from rare and sub-
sequent fixation) of parental care and/or filial cannibalism.
Model Dynamics
We consider a stage-structured system (which is appro-
priate for many fish, bird, and insect systems) in which
888 The American Naturalist
Figure 1: The model: individuals develop through an egg and a juvenile
stage and reproduce as adults. Eggs die (at rate d
E
), are consumed by
their parent(s) (at rate b), or mature into juveniles (at rate m
E
). Indi-
viduals survive and pass through the juvenile stage (at rate ), wherem 7 j
EJ
t represents the time spent in the juvenile stage. As adults, individuals
either die (at rate d
A
) or reproduce (at rate , where Kr[1 ⫺ (A(t)/K)]
represents the population carrying capacity).
individuals pass through egg (E), juvenile, and adult (A)
stages. Eggs increase as adults reproduce and decrease as
eggs mature and die, such that
dE A(t)
p r 7 A(t) 7 1 ⫺⫺d 7 E(t) ⫺ m 7 E(t), (1)
EE
()
dt K
where r is the rate of egg fertilization (i.e., mean repro-
ductive rate of adults), d
E
is the death rate of eggs, and
m
E
is the rate at which eggs mature. We assume logistic
population growth, where K represents population car-
rying capacity and density dependence associated with re-
source competition affects adult reproduction (i.e., the rate
of fertilization). Adults in the population increase as eggs
mature and survive the juvenile stage and decrease as
adults die, such that
dA
p m 7 E(t ⫺ t) 7 j ⫺ d 7 A(t), (2)
EJA
dt
where t is a time delay representing the juvenile stage, j
J
is survival rate through the juvenile stage, and d
A
is the
death rate of adults (fig. 1). The equilibrial densities are
thus
∗
d 7 A
A
∗
E p ,(3)
m 7 j
EJ
(d 7 d )/(m 7 j ) ⫹ (d /j )
EA EJ AJ
∗
A p K 7 1 ⫺ .(4)
{[ ]}
r
Resident and Mutant Trade-Offs
To explore the fixation of different strategies, we allowed
rare mutants with different life histories to invade a res-
ident population. Specifically, we considered the following
cases: (1) a rare mutant who provides parental care invades
a resident population with no care (and no cannibalism),
(2) a rare mutant who practices filial cannibalism invades
a resident population with no care (and no cannibalism),
(3) a rare mutant who provides parental care and practices
filial cannibalism invades a resident population with no
care and no cannibalism, and (4) a rare mutant who pro-
vides parental care and practices filial cannibalism invades
a resident population that provides parental care (but does
not cannibalize). The different life-history strategies are
represented through the incorporation of appropriate
trade-offs into the model (described in table 1), and the
model was analyzed using linear additive trade-offs and
nonlinear trade-offs (table 1).
In cases in which parental care was provided (either by
individuals in the resident population and/or by the rare
mutant), we assumed that parental care increases the sur-
vivorship of eggs (i.e., as d
E
decreases, parental care in-
creases) and that receiving parental care during the egg
stage increases an individual’s likelihood of surviving
through the juvenile stage (i.e., the level of care received
as an egg affects quality such that j
J
increases as d
E
de-
creases). Providing parental care is assumed to be costly
to the parent providing it, and thus, we assumed that the
reproductive rate of adults (i.e., their rate of producing
fertilized eggs) decreases and that the death rate of adults
increases as care increases (i.e., r decreases and d
A
increases
as d
E
decreases). Furthermore, in all cases in which care
is provided, a decrease in the maturation rate of the eggs
was associated with an increase in the reproductive rates
of adults: the less time that an individual has to spend
caring for a clutch of eggs, the greater that individual’s
reproductive rate will be (i.e., as m
E
decreased, r increased).
When we considered the case of a rare mutant practicing
filial cannibalism, we assumed some energetic benefit of
cannibalism, such that the death rate of adults decreased
and the reproductive rate of adults increased as canni-
balism increased (i.e., d
A
decreases and r increases as the
rate of cannibalism b increases). A further goal of these
analyses was to determine whether cannibalism could
evolve in the absence of a substantial benefit of canni-
balism. In these analyses, we assume no direct benefit of
cannibalism (i.e., there is no effect of b on d
A
or r) and
no indirect benefit of cannibalism in relation to canni-
balism freeing up other resources and reducing compe-
tition among adults (i.e., for these comparisons).K p K
m
We analyzed the model both assuming that egg survi-
vorship was density independent and for the case in which
Table 1: Trade-off functions
Parameter and trade-offs
Strategy
No parental care and no
filial cannibalism Parental care only Filial cannibalism only Parental care and filial cannibalism
Reproductive rate r and r
m
:
(1) Reproductive rate decreases as caring increases
(i.e., r or r
m
decreases as d
E
or d
Em
decreases); (2)
reproductive rate increases as maturation rate of
eggs increases (for a carer only; i.e., r or r
m
in-
creases as m
E
or m
Em
increases); (3) reproductive
rate increases as cannibalism increases (i.e., r
m
in-
creases as b increases)
Linear:
r p r
0
Nonlinear:
r p r
0
Linear:
r
m
p 7 (1 ⫹ d
Em
⫹ m
Em
)r
m
0
Nonlinear:
(d ⫹m )
Em Em
r p r 7
mm
0
(1⫹d ⫹m )
Em Em
Linear:
r
m
p 7 (1 ⫹ b)r
m
0
Nonlinear:
b
r p r 7
mm
0
(1⫹b)
Linear:
r
m
p 7 (1 ⫹ d
Em
⫹ m
Em
⫹ b)r
m
0
Nonlinear:
(d ⫹m ⫹b)
Em Em
r p r 7
mm
0
(1⫹d ⫹m ⫹b)
Em Em
Juvenile survival rate j
J
and j
Jm
:
Juvenile survival rate increases as care increases (i.e.,
j
J
or j
Jm
increases as d
E
or d
Em
decreases)
Linear:
j
J
p j
J
0
Nonlinear:
j
J
p j
J
0
Linear:
j
Jm
p j
Jm0
7 (1 ⫺ d
Em
)
Nonlinear:
(1⫹d )
Em
j p j 7
Jm Jm
0
d
Em
Linear:
j
Jm
p j
Jm
0
Nonlinear:
j
Jm
p j
Jm
0
Linear:
j
Jm
p 7 (1 ⫺ d
Em
)j
Jm
0
Nonlinear:
(1⫹d )
Em
j p j 7
Jm Jm
0
d
Em
Adult death rate d
A
and d
Am
:
(1) Adult death rate increases as caring increases (i.e.,
d
A
or d
Am
increases as d
E
or d
Em
decreases); (2)
adult death rate decreases as cannibalism increases
(i.e., d
A
or d
Am
decreases as b increases)
Linear:
d
A
p d
A
0
Nonlinear:
d
A
p d
A
0
Linear:
d p d 7 (1 ⫺ d )
Am Am Em
0
Nonlinear:
(1⫹d )
Em
d p d 7
Am Am
0
d
Em
Linear:
d
Am
p 7 (1 ⫺ b)d
Am
0
Nonlinear:
(1⫹b)
d p d 7
Am Am
0
b
Linear:
d
Am
p 7 (1 ⫺ d
Em
⫺ b)d
Am
0
Nonlinear:
(1⫹d ⫹b)
Em
d p d 7
Am Am
0
d 7b
Em
Note: The following trade-off functions were used to reflect the unique life histories of individuals who provide parental care and/or practice filial cannibalism. The death rate of eggs is assumed to be a
function of the parental care provided (i.e., as d
E
decreases, care is presumed to increase), and thus egg death rate is our proxy for care.
890 The American Naturalist
egg survivorship was density dependent. For the cases in
which egg survivorship was assumed to be density depen-
dent, the death rate of eggs follows an increasing function
in E, and we considered two functions,
2
d 7 E ,(5)
E
⫺1
d 7 (1 ⫹ q 7 E), (6)
E
where q is the strength of density dependence (following
Bellows 1981).
Invasion Dynamics and Fitness
Under the density-independent egg survivorship scenario,
the dynamics of the rare mutant are thus given by the
following equations and by incorporating relevant trade-
offs (table 1):
∗
dE A
m
p r 7 A (t) 7 1 ⫺⫺d 7 E (t)
mm Emm
()
dt K
m
⫺ m 7 E (t) ⫺ b 7 E (t) 7 A (t), (7)
Em m m m
dA
m
p m 7 E (t ⫺ t) 7 j ⫺ d 7 A (t), (8)
Em m Jm Am m
dt
where E
m
and A
m
are the egg and adult strategies for the
invading strategy; m
Em
, d
Em
, j
Jm
, and K
m
are the egg mat-
uration rate, egg death rate, juvenile survival rate, and
carrying capacity for the alternative invading strategy, re-
spectively; and b is the mutant’s rate of cannibalism (b
equals an average rate of cannibalism, which could rep-
resent some combination of whole- and partial-clutch can-
nibalism; if the mutant does not cannibalize). Theb p 0
mutant is assumed to be rare in the population, and thus,
density dependence operating on adult mutant reproduc-
tion occurs through competition with the resident. The
lifetime fitness of the mutant can then be found from the
determinant of
∗
A
l ⫹ d ⫹ m ⫹ b ⫺r 7 1 ⫺
Em Em m
()
K
m
.(9)
⫺m exp (⫺l7 t) 7 jl⫹ d
Em Jm Am
Hence, while some life-history parameters (e.g., fertil-
ization rate of eggs r) will be correlated with lifetime fitness
under some scenarios, the real measure of fitness in this
model is the eigenvalues of the invasion matrix. To evaluate
the invasion and replacement dynamics of a rare mutant
that provides parental care and/or practices filial canni-
balism, we used the fitness function of the mutant to cal-
culate the evolutionarily stable state(s) (i.e., when the rate
of change in fitness is 0). We then performed mutual in-
vasion analyses by evaluating when the fitness function is
10 (using a Newton-Raphson algorithm with the resident
dynamics A
∗
set at equilibrium) for different values of a
life-history trait (see table 1; appendix in the online edition
of the American Naturalist). We evaluated and present
pairwise invasion boundaries for different values of the
maturation rate of eggs. Comparing the invasion potential
with regard to the maturation rate of eggs is ideal because
(1) it allows us to represent a wide range of life-history
strategies, including faster and slower reproducers, and (2)
lifetime fitness is highly sensitive to maturation rate. Spe-
cifically, we illustrate (1) the conditions for which the mu-
tant would invade and outcompete the resident; (2) the
boundaries for which the resident would invade and out-
compete the mutant; (3) the putative coexistence range,
in which the strategies have the potential to coexist; (4) a
region of nonpersistence, where neither strategy will per-
sist; and (5) a region in which neither strategy will persist
or initial conditions of the model determine the strategy
that invades. Local stability analyses were performed and
are described in the appendix; for the parameter ranges
considered, the invasion dynamics were always stable. Nu-
merical simulations confirm that strategy coexistence oc-
curs when the dynamics are stable and that regions of
parameter space exist (labeled NP/IC and NP in figs. 2–
6) where neither strategy persists or where the outcome
is based on initial conditions. We evaluated the invasion
potential of the rare mutant for several biologically relevant
scenarios by changing the value(s) of a single life-history
parameter of interest for the mutant and/or resident
populations.
Biologically Relevant Comparisons
In addition to the fixed trade-offs reflecting varying life-
history strategies (table 1), we explicitly considered the
effects of varying selective regimes (e.g., differential mating
success associated with a particular strategy, effects of care
or cannibalism on population resources and, hence, car-
rying capacity) on the invasion dynamics. To do this, we
used pairwise comparisons in which we altered the mag-
nitude of one (or more) parameter(s) to reflect a biological
scenario of interest.
First, we evaluated the importance of offspring survival
benefits of care on the invasion patterns of varying strat-
egies. Empirically, parental care has been shown to reduce
the death rate of offspring (discussed in Clutton-Brock
1991), and thus, we compared the invasion patterns of the
caring mutant for a range of cases in which care was ef-
Evolution of Parental Care and Filial Cannibalism 891
Figure 2: Invasion of parental care. Parental care invades and/or coexists with no care more often (A) when parental care increases the maturation
rate of eggs, (B) when parental care increases parental reproductive rate ( , ), (C) when parental care is associated with a decreasedr p 1.0 r p 1.2
m
carrying capacity ( , ), and (D) when the caring mutant is able to cannibalize (i.e., for the mutant, for the residents).K p 20 K p 15 b p 0.01 b p 0
m
Invasion boundaries are shown for the maturation rate of the eggs, and unless noted above, , , ,r p r p 1.0 d p d p 0.9 d p d p 0.5
mEEmAAm
, , , . The mutant invades the resident in the regions labeled “care” (A–C) or “care & cannibalism” (D),j p j p 0.5 K p K p 20 b p 0 t p 0.1
JJm m
the resident invades the mutant in the region labeled “no care” (A–C) or “no care or cannibalism” (D), and both strategies coexist in the region
labeled “coexistence.” Neither strategy will persist in the region labeled “NP.” The region labeled “NP/IC” is a region in which neither strategy will
persist or where the outcome is dependent on initial conditions of the model. See main text for definition of variables.
fective (i.e., ) with those in which it was ineffectived ! d
Em E
(i.e., ).d p d
Em E
Second, sexual selection has been hypothesized to be a
major force in the evolution and fixation of parental care
(Baylis 1981; Andersson 1994). In some systems, mate
choice for a partner who will provide care is thought to
affect the reproductive rate of the nonlimiting sex (e.g.,
the number of eggs a caring parent receives per repro-
ductive bout or over the course of the breeding season is
correlated with parental care: Jordanella flordiae [St. Mary
et al. 2001] and Pomatoschistus minutus [Pampoulie et al.
2004]). Likewise, filial cannibalism has been shown to in-
crease the attractiveness of a caring parent’s nest in some
cases (e.g., Sikkel 1994) and might be preferred during
mate choice if there are benefits of cannibalism to re-
maining offspring (e.g., through density-dependent egg
survivorship). With regard to the model, if care or filial
cannibalism is a trait that is preferred by one sex, we would
expect the mutant exhibiting care or cannibalism to receive
more fertilizations per time period (e.g., a breeding season
or lifetime) than a resident who does not exhibit care. In
this sense, r
m
(i.e., egg fertilization rate in eq. [7]) of a
mutant who exhibits a preferred trait would be expected
to be, on average, greater than that of a resident who does
not exhibit the preferred trait. To incorporate this aspect
of sexual selection, we compared invasion patterns for
cases in which caring and/or cannibalism increased the
reproductive rate of the caring mutant relative to the res-
ident (i.e., ) with those in which the magnitude ofr
1 r
m
the reproductive rate did not differ between the mutant
and the resident (i.e., ).r p r
m
Likewise, it is possible that filial cannibalism creates
892 The American Naturalist
Figure 3: Invasion of filial cannibalism. A mutant that provides parental care and practices filial cannibalism invades and coexists with a resident
that provides care only (A) more often when cannibalism increases the maturation rate of eggs, (B) more often when cannibalism increases the
parent’s reproductive rate ( , ), (C) less often when cannibalism decreases the parent’s reproductive rate ( , ), andr p 0.5 r p 0.6 r p 0.6 r p 0.5
m m
(D) more often when parents are able to selectively cannibalize offspring with reduced future survival ( , , , ).d p 0.2 j p 0.9 d p 0.1 j p 0.95
EJEmJm
Invasion boundaries are shown for the maturation rate of the eggs, m
E
and m
Em
, and unless otherwise noted, , ,r p r p 0.5 d p d p 0.2
mEEm
, , , , . The mutant invades the resident in the region labeled “care & cannibalism,”d p d p 0.5 j p j p 0.9 K p K p 20 b p 0.015 t p 0.1
AAm JJm m
the resident invades the mutant in the region labeled “care only,” and both strategies coexist in the region labeled “coexistence.” Neither strategy
will persist in the region labeled “NP.” The region labeled “NP/IC” is a region in which neither strategy will persist or where the outcome is dependent
on initial conditions of the model. See main text for definition of variables.
reproductive conflict between parents. In this case, one
would expect noncannibalistic individuals to be favored
during mate choice (Kraak 1996; Lindstro¨m 2000). To
assess the importance of mate preference for a noncan-
nibalistic partner, we compared cases in which the mutant
has a reduced reproductive rate relative to the resident
(i.e., ) with the case in which the mutant and residentr
! r
m
have equal reproductive rates (i.e., ).r p r
m
Parental care, no care, and filial cannibalism might affect
population-level resources in different ways. For example,
providing care might necessitate greater per capita re-
sources (e.g., increased energetic need and nesting re-
sources per individual) than not providing care (i.e, K
1
for a caring mutant invading a resident without care).K
m
Similarly, the ability to cannibalize while providing care
might, in a sense, free up other resources and increase the
carrying capacity of a population (i.e., for a mutantK
! K
m
who can cannibalize and care invading a resident who can
only care). To begin to evaluate the importance of such
resource-related effects at the population level, we com-
pared cases in which carrying capacity varies between the
mutant and the resident population. Likewise, for cases in
which the mutant and residents have equal carrying ca-
pacities, we compared patterns for a range of carrying
capacities to determine whether a relatively productive
ecosystem (i.e., system with a large carrying capacity) or
unproductive ecosystem (i.e., system with a relatively small
carrying capacity) favors the invasion of a particular
strategy.
Finally, to further evaluate patterns of cannibalism evo-
lution, we compared the fitness boundaries for cases in
which parents were allowed to selectively cannibalize eggs
Evolution of Parental Care and Filial Cannibalism 893
Figure 4: Effect of density-dependent egg survivorship on the evolution of parental care and filial cannibalism. Parental care and no care invade
and/or coexist over a large range of parameter space when (A) the rare mutant does not cannibalize ( ). The range over which parental careb p 0
invades decreases when (B) the rare mutant cannibalizes ( ). However, increasing the strength of density dependence (q in eq. [6]) increasesb p 0.01
the range over which care and cannibalism invades: care with cannibalism invades more often when (C) the strength of the density dependence is
greater ( ) in comparison to (B) the case in which it is relatively weak ( ). Invasion boundaries are shown for the maturation rateq p 0.9 q p 0.6
of the eggs, m
E
and m
Em
, and unless otherwise noted, , , , , , ,r p r p 3 d p 0.9 d p 0.3 d p d p 0.5 j p j p 0.5 K p K p 20 b p
mE Em AAm JJm m
, , . The mutant invades the resident in the region labeled “care” (A) or “care & cannibalism” (B, C), the resident invades the0.01 t p 1 q p 0.6
mutant in the region labeled “no care” (A) or “no care or cannibalism” (B, C), and both strategies coexist in the region labeled “coexistence.” The
region labeled “NP/IC” is a region in which neither strategy will persist or where the outcome is dependent on initial conditions of the model. See
main text for definition of variables.
with reduced future survivorship (i.e., andd ! d
Em E
) with those of parents that could not selectivelyj 1 j
Jm J
cannibalize (i.e., and ).d p d j p j
Em E Jm J
Results
All of the strategies considered (i.e., parental care, no care/
total offspring abandonment, filial cannibalism) evolved
over a range of parameter space in all analyses. While the
evolution of parental care and/or filial cannibalism was
favored by benefits to adults and/or offspring, such benefits
were not essential for the invasion of a particular strategy,
highlighting the plausibility of a range of non–mutually
exclusive alternative hypotheses (table 2). In all cases con-
sidered, the coexistence dynamics were stable (appendix).
While incorporating nonlinear trade-off functions (table
1) into the model altered the results quantitatively, there
were no qualitative effects of these functions (i.e., the pat-
terns were the same), and thus, we present only results in
which linear trade-offs were used.
Invasion of Parental Care
Effects of Egg Maturation Rate, Egg Death Rate, Adult Re-
productive Rate, and Carrying Capacity. A mutant with
parental care invaded or coexisted with a resident popu-
lation lacking care over a wide range of life-history pa-
rameters (fig. 2A), particularly when care was effective at
decreasing the death rate of eggs (i.e., when ),d
! d
Em E
when it increased survivorship through the juvenile stage
(i.e., when ), and when caring increased maturationj
1 j
Jm J
rate of the eggs (i.e., when ; fig. 2A). Similarly,m 1 m
Em E
894 The American Naturalist
Figure 5: Effect of energetic benefits on the evolution of filial cannibalism. Parental care with filial cannibalism is more likely to invade and coexist
with no care if filial cannibalism is (A) beneficial to a parent’s survival and reproduction versus (B) the case where there are no benefits of cannibalism.
Likewise, care with cannibalism is more likely to invade a state of only care when (C) adult survival and reproductive benefits of egg eating exist
versus (D) the case where such benefits are absent. Invasions boundaries are shown for the maturation rate of the eggs, m
E
and m
Em
. Unless otherwise
noted, , , , , , , for A and B, and ,r p r p 1.0 d p d p 0.9 d p d p 0.5 j p j p 0.5 K p K p 20 b p 0.01 t p 0.1 r p r p 0.5 d p
mEEmAAmJJm m mE
,,,,,forC and D. The mutant invades the resident in the region labeledd p 0.2 d p d p 0.5 j p j p 0.9 K p K p 20 b p 0.015 t p 0.1
Em A Am J Jm m
“care & cannibalism,” the resident invades the mutant in the region labeled “no care or cannibalism” (A, B) or “care only” (C, D), and both strategies
coexist in the region labeled “coexistence.” Neither strategy will persist in the region labeled “NP.” The region labeled “NP/IC” is a region in which
neither strategy will persist or where the outcome is dependent on initial conditions of the model. See main text for definition of variables.
the range over which care invaded or coexisted with no
care increased when parental care was associated with an
increased rate of egg fertilization (e.g., if it was a preferred
trait, such that ; fig. 2A vs. fig. 2B) and when carer
1 r
m
was associated with a decreased carrying capacity relative
to the resident population (fig. 2A vs. fig. 2C). The evo-
lution of parental care was relatively insensitive to changes
in carrying capacity for cases in which the resident and
mutant had equal carrying capacities.
Effect of Cannibalism on the Evolution of Care. To evaluate
whether the ability to practice filial cannibalism affects the
evolution of parental care, we compared the case in which
a mutant with only parental care was allowed to invade a
resident population with no parental care and no canni-
balism with the scenario in which the mutant could care
and cannibalize (fig. 2A vs. fig. 2D). Indeed, filial canni-
balism facilitated the evolution of care. When the caring
mutant was allowed to cannibalize (fig. 2D), parental care
(and filial cannibalism) evolved over a wider range of pa-
rameter space and coexisted more often with no care than
when the mutant was not allowed to cannibalize (fig. 2A).
Invasion of Filial Cannibalism (with and
without Parental Care)
Effects of Egg Maturation Rate, Reproductive Rate, and Se-
lective Cannibalism. Parental care with filial cannibalism
was more likely to invade and/or coexist with a state of
only care when it increased the maturation rate of eggs
(i.e., when ; fig. 3A) and when filial cannibalismm
1 m
Em E
allowed a parent to improve the quality of care provided
for remaining offspring (i.e., cannibalism decreases d
Em
relative to d
E
). Care and cannibalism invaded and coexisted
Evolution of Parental Care and Filial Cannibalism 895
Figure 6: Effect of carrying capacity on the evolution of filial cannibalism. Parental care with filial cannibalism invades and coexists with care over
a range of parameter space when (A) cannibalism does not affect carrying capacity ( ). However, cannibalism invades over a greaterK p K p 20
m
range of parameter space when (B) cannibalism increases carrying capacity ( , ). Likewise, care with cannibalism invades no careK p 10 K p 20
m
over a range of parameter space when (C) care and cannibalism do not affect carrying capacity ( ), but it invades or coexists moreK p K p 20
m
often when (D) care and cannibalism decrease carrying capacity ( , ). For cases in which the resident and mutant have equal carryingK p 20 K p 10
m
capacities, parental care and cannibalism are more likely to invade when (E) carrying capacity is relatively small ( ) versus the case inK p K p 10
m
which it is relatively large (A). In contrast, care with cannibalism is more likely to invade no care/no cannibalism when (F) carrying capacity is
relatively large ( ) versus (C) the case in which it is relatively small. Invasions boundaries are shown for the maturation rate of theK p K p 50
m
eggs, m
E
and m
Em
. Unless otherwise noted, , , , , , for A, B, and E, andr p r p 0.5 d p d p 0.2 d p d p 0.5 j p j p 0.9 b p 0.01 t p 0.1
mEEmAAmJJm
,,,,,forC, D, and F. The mutant invades the resident in the regionr p r p 1.0 d p d p 0.9 d p d p 0.5 j p j p 0.5 b p 0.01 t p 0.1
mEEmAAmJJm
labeled “care & cannibalism,” the resident invades the mutant in the region labeled “care only” (A, B, E) or “no care or cannibalism” (C, D, F),
and both strategies coexist in the region labeled “coexistence.” Neither strategy will persist in the region labeled “NP.” The region labeled “NP/IC”
is a region in which neither strategy will persist or where the outcome is dependent on initial conditions of the model. See main text for definition
of variables.
more often when filial cannibalism increased the repro-
ductive rate of the caring parent(s) (e.g., care and/or can-
nibalism are preferred in mate choice, such that ;r
1 r
m
fig. 3A vs. fig. 3B) but invaded less often if it decreased
the reproductive rate of adults (e.g., noncannibalism is
preferred, such that ; fig. 3A vs. fig. 3C). Similarly,r
! r
m
care with cannibalism evolved more often when parents
could selectively cannibalize their offspring. Specifically, if
parents cannibalized offspring with a higher egg death rate
and a lower juvenile survival rate (fig. 3D), cannibalism
invaded more often than in cases in which parents were
not capable of selectively cannibalizing (fig. 3A). These
patterns were consistent when we considered parental care
and filial cannibalism evolving from a state of care or a
Table 2: Alternative hypotheses regarding the evolutionary significance of filial cannibalism (FC)
Hypothesis Description Model findings Related previous findings
1. Selective filial cannibalism Offspring with particular characteristics
(e.g., reduced survival, decreased
maturation rate) are preferentially
consumed
Evolution of FC facilitated by selective
cannibalism of offspring with lower
maturation rates, lower egg survival,
and/or lower juvenile survival
FC affected by certainty of paternity in
some systems (Neff 2003; Frommen
et al. 2007; Gray et al. 2007) but not
in others (Svensson et al. 1998); ef-
fect of other aspects of offspring
quality on FC largely unknown
2. Filial cannibalism speeds up egg
development By increasing costs associated with re-
maining in the egg stage, FC is a
mechanism by which parents increase
maturation rate of eggs (i.e., FC de-
creases the time it takes for eggs to
develop)
Evolution of FC more likely if cannibal-
ism increases egg maturation rate
Not previously examined, to our knowl-
edge; potentially relevant for systems
in which parent-offspring conflict ex-
ists over the optimal amount of care
provided/received
3. Energy-based filial cannibalism FC provides energy that offsets costs of
care and is reinvested into current
and/or future reproduction
Energetic benefit of eggs facilitated evo-
lution of FC
Substantial energetic benefit of FC and/
or effect of energetic need on FC
found in several systems (reviewed in
Manica 2002)
4. Density-dependent egg-survivor-
ship-mediated filial cannibalism Density-dependent egg survival medi-
ates FC; by consuming some young,
parents increase survival of remaining
offspring
Density-dependent egg survival alone
did not facilitate the evolution of FC;
more intense density dependence fa-
cilitated evolution of FC in compari-
son to weaker density dependence
FC is affected by density-dependent egg
survivorship in two species (Payne et
al. 2002, 2004; Klug et al. 2006)
5. Mate-choice-mediated filial
cannibalism FC is preferred in mate choice, thereby
increasing relative reproductive rate
If FC increases relative reproductive rate
of cannibals, FC evolves more often
FC increases nest attractiveness (and
consequently eggs received) in some
cases (Sikkel 1994)
6. Sexual-conflict-mediated filial
cannibalism FC is a nonpreferred trait and decreases
relative reproductive rate
If FC decreases reproductive rate, FC
evolves less often
Sexual conflict can inhibit FC in some
cases (Lindstro¨m 2000); sexual con-
flict regarding FC not well studied
empirically (but see Kraak 1996)
7. Filial cannibalism driven by re-
source competition FC is driven by population-level re-
source competition among adults
Evolution of FC is sensitive to popula-
tion-level carrying capacity
Mate availability (Kondoh and Okuda
2002) and other resource competition
(Creighton 2005) affects FC in some
cases; effects of general resource
availability on FC not well known
Note: We present several nonmutually exclusive hypotheses and briefly describe the findings of our model, and those of some previous work, in relation to these hypotheses.
Evolution of Parental Care and Filial Cannibalism 897
state of no care. Likewise, filial cannibalism (without care)
invaded and/or coexisted with no care/no cannibalism over
a greater range of parameter space if filial cannibalism
improved offspring survival (i.e., or ) ord
! d j 1 j
Em E Jm J
increased the maturation rate of eggs ( ) or whenm 1 m
Em E
parents were able to practice selective filial cannibalism of
offspring with reduced survival during the egg and/or ju-
venile stage.
Effects of Density-Dependent Egg Survivorship. When egg
survivorship was density dependent, parental care and/or
filial cannibalism evolved and coexisted over a wide range
of parameter space. However, in the absence of any other
benefits of filial cannibalism, density-dependent egg sur-
vivorship alone did not facilitate the evolution of canni-
balism. In fact, allowing a mutant that provides care to
cannibalize decreased the range over which care and can-
nibalism evolved, in comparison to the case in which the
mutant could not cannibalize (fig. 4A vs. fig. 4B). However,
when the mutant provided care and cannibalized, parental
care and cannibalism invaded over a greater range of pa-
rameter space as the strength of density dependence (i.e.,
q) increased (fig. 4B vs. fig. 4C). In other words, the evo-
lution of filial cannibalism was not facilitated by density-
dependent egg survivorship per se, but relatively intense
density dependence allowed cannibalism to evolve more
often in comparison to weaker density dependence (i.e.,
as q in eq. [6] increased, the range over which care with
cannibalism and no care could invade and/or coexist in-
creased). These patterns were the same for both density-
dependent functions considered.
Effects of Energetic Benefits of Consuming Offspring. En-
ergetic benefits of filial cannibalism (i.e., benefits associ-
ated with d
Am
and r
m
) increased the range over which care
and cannibalism could invade and coexist with no care
(fig. 5A vs. fig. 5B). However, in this scenario (i.e., con-
sidering care and cannibalism invading from a state of no
care or cannibalism), it is possible that cannibalism could
be thought of as simply hitchhiking in with care. Thus,
we considered the case in which cannibalism and care
invaded a resident who already provides care. While en-
ergetic benefits of cannibalism to adult reproduction and
survival increased the range of invasion and coexistence
(fig. 5C), filial cannibalism (which, in this case, is equiv-
alent to simply killing offspring, or abandoning offspring
that have no chance of surviving alone, during the course
of care) was still able to invade in the absence of benefits
to adults (fig. 5D).
Effects of Carrying Capacity. For cases in which cannibal-
ism alters the population carrying capacity, filial canni-
balism with parental care was more likely to evolve from
a state of only care if cannibalism increased the carrying
capacity. In other words, filial cannibalism was more likely
to invade if it somehow increased the productivity of the
system (fig. 6A vs. fig. 6B). In contrast, for the case of filial
cannibalism with parental care evolving from a state of no
care, cannibalism and care were more likely to evolve if
they were associated with a decrease in the population
carrying capacity (i.e., if care with cannibalism decreased
the productivity of the system; fig. 6C vs. fig. 6D). Likewise,
for the case of filial cannibalism (without parental care)
invading a resident state of no care/no cannibalism, filial
cannibalism invaded and coexisted over a greater range of
parameter space if it increased carrying capacity.
If carrying capacities were equal for the resident pro-
viding parental care and the mutant providing care and
practicing filial cannibalism (i.e., productivity was equiv-
alent for mutant and resident), cannibalism invaded and/
or coexisted more often when carrying capacity was rel-
atively low (i.e., in relatively unproductive systems; fig. 6A
vs. fig. 6E). This trend (i.e., more invasion and coexistence
of the mutant at lower carrying capacities) was consistent
for the case in which filial cannibalism (with no parental
care) invaded a resident with no care/no cannibalism.
However, when we considered the case in which canni-
balism with care invaded a state of no care (and assumed
that the carrying capacities were equal for resident and
mutant), cannibalism and care were more likely to invade
or coexist with no care/no cannibalism when carrying ca-
pacity was relatively large (i.e., when the system was rel-
atively more productive; fig. 6C vs. fig. 6F).
Discussion
We have shown that parental care, filial cannibalism, and
no care/total offspring abandonment can evolve over a
wide range of life-history parameters. Our results suggest
that the ability to abandon or consume offspring during
the course of parental care can actually facilitate the evo-
lution of parental care and that offspring abandonment/
no care, parental care, and filial cannibalism often have
the potential to coexist. Even in the absence of direct ben-
efits of filial cannibalism, such as energetic gain or in-
creased survival of remaining offspring, filial cannibalism
invaded (and coexisted with) noncannibalistic strategies
in multiple contexts (i.e., with or without care, across vary-
ing resident strategies, over a range of life-history param-
eters). In the absence of such benefits, cannibalism is
equivalent to simply killing (or abandoning offspring that
will subsequently die) during the course of care. Our re-
sults suggest that the evolutionary dynamics of filial can-
nibalism are likely comparable to those of simple offspring
abandonment (which provides no immediate benefits,
898 The American Naturalist
such as energetic gain, to parents). However, our results
suggest that the evolution and fixation of filial cannibalism
are favored by a variety of evolutionary and ecological
factors. While no single benefit of consuming eggs was
essential for the invasion of filial cannibalism to occur,
several potential benefits facilitate the evolution of filial
cannibalism.
In particular, our model highlights the plausibility of
several non–mutually exclusive alternative hypotheses fa-
voring the evolution of filial cannibalism (table 2). The
ability to selectively cannibalize eggs facilitated the evo-
lution of cannibalism in all contexts. The ability to selec-
tively cannibalize offspring allows parents to alter the phe-
notypes of the offspring they produce after fertilization
and on a relatively fine timescale, which might be partic-
ularly beneficial in a variable environment. Selective can-
nibalism of clutches of lower reproductive value has been
demonstrated in relation to uncertainty of paternity (i.e.,
possible cuckolding events) in some fishes (Lepomis ma-
crochirus [Neff 2003], Telmatherina sarasinorum [Gray et
al. 2007], and Gasterosteus aculeatus [Frommen et al.
2007]), and the elimination of low-quality offspring has
been a focus in other contexts (e.g., allowing lower-quality
offspring to be eliminated by siblicide [Stearns 1987] and
spontaneous and selective abortion in humans and sex
ratio adjustment in red deer [Stearns 1987; Kozlowski and
Stearns 1989]). However, selective filial cannibalism of vi-
able offspring in relation to other aspects of offspring qual-
ity has received little empirical attention. In particular, we
hypothesize that, in some contexts, filial cannibalism of
offspring with (1) reduced expected future survival or (2)
slower maturation rates during the period in which care
is being provided can be an adaptive strategy.
Alternatively, it is possible that filial cannibalism itself
increases the development rate of eggs. Our model suggests
that parental fitness is highly sensitive to the maturation
rate of eggs. If filial cannibalism increases the maturation
rate of eggs relative to those of noncannibalistic parents,
filial cannibalism evolves over a greater range of parameter
space (fig. 3A). Indeed, for cases in which parent-offspring
conflict exists over the optimal duration of parental care,
filial cannibalism might be a way in which parents speed
up the developmental rate of their eggs, thereby allowing
them to reduce per offspring costs of care or reenter the
mating pool faster. According to this hypothesis, caring
parents potentially benefit by providing care for a shorter
duration of time if filial cannibalism creates an environ-
ment in which offspring are eager to escape the egg stage
(e.g., because of increased risk of death). To our knowl-
edge, this idea of filial cannibalism speeding up egg de-
velopment has not previously been considered and, as
mentioned, is likely to be relevant for cases in which par-
ents and offspring differ in the optimal amount of care
they provide/receive.
Incorporating an energetic benefit of cannibalism fa-
cilitated the invasion of filial cannibalism. This finding is
consistent with previous theoretical and empirical work
suggesting that energetic need affects filial cannibalism
(e.g., Rohwer 1978; Sargent 1992; Kraak 1996; reviewed
by Manica [2002]). However, some empirical work sug-
gests that the effects of energetic need on filial cannibalism
are not always straightforward: in some species, canni-
balism increases as parental energetic need increases (e.g.,
Thomas and Manica 2003), whereas in other species, an
opposite pattern is observed (e.g., Klug et al. 2006). More-
over, in other systems, there appear to be no effects of
parental condition on filial cannibalism under some con-
ditions (e.g., Lindstro¨m and Sargent 1997), and in other
species, the relationship between energetic need and can-
nibalism differs in varying contexts (H. Klug and K. Lind-
stro¨m, unpublished data). Furthermore, some have sug-
gested that the energetic benefits of cannibalism are not
sufficient to explain the prevalence of filial cannibalism
(Smith 1992). In our model, filial cannibalism invaded
over a range of parameter space even when we removed
the benefits of cannibalism, suggesting that substantial en-
ergetic benefit of cannibalism is not necessarily essential
for the evolution of cannibalism. That said, there is no
doubt that filial cannibalism provides a caring parent with
energy and/or nutrients and such benefits are critical in
systems where parents are unable to feed during the course
of providing parental care (Manica 2002, 2004). Indeed,
energetic benefits certainly favor the evolution of filial can-
nibalism (fig. 5; previous work by Rohwer [1978] and
Sargent [1992]; reviewed in Manica 2002).
Likewise, increasing the strength of density-dependent
egg survivorship increased the parameter space over which
filial cannibalism evolved. However, density-dependent
egg survivorship alone did not facilitate the evolution of
filial cannibalism. Indeed, it seems unlikely that density-
dependent egg survivorship per se would lead to the evo-
lution of filial cannibalism in the absence of other trade-
offs associated with egg number. If animals can track their
environment, they would simply be expected to adjust the
number of eggs they produce according to expected egg
survivorship (i.e., they should lay at densities that maxi-
mize survival). Further work is needed to evaluate the
importance of density-dependent egg survivorship when
other trade-offs are associated with the number of off-
spring produced or when the environment is variable. Spa-
tial and temporal variation in the environment has been
hypothesized to influence patterns of cannibalism ob-
served in nature (e.g., Payne et al. 2004), but additional
work is needed to understand more fully the importance
of such stochasticity at varying scales.
Evolution of Parental Care and Filial Cannibalism 899
Sexual selection via mate choice and/or sexual conflict
also affected the invasion and fixation of filial cannibalism
and/or parental care. Our model suggests that the evo-
lution and fixation of parental care from a state of no care
can be facilitated by differential reproductive success if
parental care or filial cannibalism increases the reproduc-
tive rate of individuals exhibiting care or cannibalism (e.g.,
if parental care or filial cannibalism is preferred during
mate choice). This finding is consistent with some previous
work. For example, Pampoulie et al. (2004) and Lindstro¨m
et al. (2006) recently demonstrated mating preferences for
parental care, suggesting a potentially larger role for sexual
selection in the evolution of care than previously thought.
Additionally, filial cannibalism is possibly favored by sex-
ual selection if cannibalism directly benefits a choosing
mate or when it makes a caring parent more attractive in
some other way (Sikkel 1994; Lindstro¨m 2000). Likewise,
if a mating preference exists for noncannibals, the param-
eter space over which filial cannibalism evolves decreases.
Interestingly, the role of sexual conflict has received rel-
atively little theoretical or empirical attention previously
(but see Kraak and van den Berghe 1992; Kraak 1996;
Lindstro¨m 2000). In fishes, where filial cannibalism is typ-
ically practiced by caring fathers, the focus of almost all
work has been on costs and benefits of cannibalism to
caring males. One must also wonder whether benefits to
noncannibalistic females exist, and if such benefits are ab-
sent, why do females tolerate filial cannibalism? More em-
pirical work is needed to better understand the costs and
benefits of filial cannibalism to a parent who does not
practice filial cannibalism but has a mate that does.
Finally, population-level resource competition likely
plays a role in the evolution of both parental care and
filial cannibalism. When care and/or cannibalism affected
population carrying capacity in our model, parental care
was more likely to evolve if caring was associated with a
reduction in the carrying capacity (e.g., if productivity was
decreased), whereas filial cannibalism was more likely to
invade if it increased the carrying capacity (e.g., if can-
nibalism increased the productivity of the system). Ad-
ditionally, the evolution of filial cannibalism (with or with-
out parental care) was affected by population carrying
capacity, even for the case in which the carrying capacities
of the mutant and residents were equal. It is unclear how
parental care and filial cannibalism potentially alter
population-level dynamics and resulting carrying capaci-
ties in nature, but this idea warrants further attention. For
example, it is possible that the ability to cannibalize in-
creases resource availability to caring parents, thereby free-
ing up other resources and increasing the productivity of
a system. Regardless, understanding the ecological dynam-
ics of a system (i.e., intensity of resource competition and
population growth parameters such as carrying capacity)
is likely to be critical for understanding the evolution of
parental care and filial cannibalism across animal taxa.
While previous work has sometimes incorporated popu-
lation-level growth dynamics in parental care theory (e.g.,
McNamara et al. 2000), this is not a common approach.
In summary, our results suggest that parental care and
filial cannibalism can evolve over a range of life-history
patterns and ecological conditions and that multiple strat-
egies often have the potential to coexist. Coexistence, while
not well studied (but see Webb et al. 1999), is prevalent
in nature (e.g., maternal- or paternal-only care in many
taxa [reviewed in Clutton-Brock 1991], care and no care
with total offspring abandonment following egg fertiliza-
tion: Jordanella floridae [R. E. Hale, personal communi-
cation], the white stickleback Gasterosteidae spp. [Blouw
1996]; care and care followed by abandonment: Hypop-
tychus dybowskii [Narimatsu and Munehara 2001]). Like-
wise, there are many cases in which caring parents never
or rarely consume or abandon their offspring. Even in
fishes, where care with filial cannibalism has been well
documented, there are still many species exhibiting pa-
rental care in which filial cannibalism is absent (e.g., Mi-
cropterus dolomieui; Gillooly and Baylis 1999). For species
exhibiting filial cannibalism, there is a great deal of vari-
ation in the patterns of cannibalism observed among spe-
cies and within and between individuals (e.g., how many
eggs are consumed, who practices cannibalism and when;
Petersen and Marchetti 1989; Okuda and Yanagisawa 1996;
Lindstro¨m and Sargent 1997; Lissa˚ker et al. 2002; Klug
and St. Mary 2005; Klug et al. 2005). Understanding such
within- and between-species variation in filial cannibalism
and parental care will require more detailed theoretical
and empirical work that simultaneously considers multiple
factors (such as variation in offspring quality, energetic
needs of parents, mating preferences and sexual conflict,
general resource competition). Additionally, it will also be
important to assess the importance of environmental het-
erogeneity in the evolution of filial cannibalism. From this
study, our approach and results provide a novel basis for
further developing this theme of whether to care for or
consume one’s own offspring.
Acknowledgments
We are grateful to four referees, whose insightful com-
ments greatly improved this manuscript. Funding for this
work was provided by a National Science Foundation
(NSF) Graduate Research Fellowship and a NSF Doctoral
Dissertation Improvement Grant (Division of Integrative
Organismal Biology and the Office of International Science
and Engineering) to H.K. M.B.B. is a Royal Society Uni-
versity Research Fellow.
900 The American Naturalist
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Associate Editor: William G. Wilson
Editor: Michael C. Whitlock
