Behavioral Ecology Vol. 8 No. 6: 605-611
Food affects the potential reproductive rates
of sand goby females but not of males
CharlotU Kvarnemo
Department of Zoology, Uppsala University, Villavagen 9, S-752 36 Uppsala, Sweden
The interspawning interval of female sand gobies, Pomatosdastus wtmutus, a batch-spawning fish with paternal care, was significantly shorter when the fish were fed daily than when they were fed every fourth day. The incubation time of males was not
affected by feeding, nor was the interbrood interval Males have an equal or higher potential reproductive rate than females.
As females reproduce more slowly when food is scarce than when it is abundant, and males do not, die difference between the
sexes in potential reproductive rate increases when there is food shortage. Because of this difference, both male bias in operational sex ratio and intensity in male-male competition for mates are predicted to increase as food availability decreases.
Furthermore, a tradeoff between current and future reproduction is demonstrated to operate only when resources are limited,
because die correlation between egg number of the first and second clutch was positive among high-food females but negative
among low-food females. The number of eggs per female clutch did, however, not differ between treatments in first or second
dutch. I conclude that operational sex ratio and sexual selection are expected to vary within and between sand goby populations
in accordance with prey availability. Key words: food availability, operational sex ratio, Pomatoschistus wnnutus, potential reproductive rate, sand goby, sexual selection. [Bthav Ecol 8:605-611 (1997)]
S
exual differences in potential reproductive rate (PRR; Le.,
the reproductive rate if not limited by mate availability)
are important factors affecting die operational sex ratio (i.e.,
ratio of sexually active males versus females, Emlen, 1976; Emlen and Oring, 1977), mating competition, and thua sexual
selection in many animal populations (Ahnesj6, 1989; Berglund et al., 1989; Qutton-Brock and Parker, 1992; QuttonBrock and Vincent, 1991; Gwynne, 1990; Kvarnemo and Ahnesjd, 1996; Parker and Simmons, 1996; Rosenqvist, 1990).
When one sex has the potential to mate at a higher rate than
die other, die operational sex ratio will be biased towards this
sex (Berglund and Rosenqvist, 1993; Chitton-Brock and Parker, 1992; Chitton-Brock and Vincent, 1991; Kraus, 1989; Kvarnemo, 1996). The importance of die operational sex ratio in
sexual selection is apparent: when diere is a shortage of sexually active individuals of one sex, members of die opposite
sex will have to compete for mating*. A skewed operational
sex ratio may affect both intra- and intersexual interactions
(Emlen and Oring, 1977), and the more competitive sex will
be under stronger sexual selection.
Chitton-Brock and Parker (1992) provided a mathematical
definition of operational sex ratio, where die reproductive cycle of an individual is mainly divided between being sexually
active ("time in") and inactive ("time out"; G). An individual
is not sexually active (Le., ready to mate) when engaged in
activities such as parental care or egg maturation, and will
remain unavailable for matings until die G is completed. The
operational sex ratio is die time fraction of a reproductive
cycle a male is sexually active, divided by die time fraction a
female is sexually active, multiplied by the adult sex ratio of
males to females. The reproductive cycles (i.e., die realized
reproductive rates) of males and females are equal, on average, in any sexually reproducing population with an unbiased
adult sex ratio. The operational sex ratio can then be calcu-
C Kvarnemo a currently at the Department of Zoology, University
of Western Australia, Nedlands, WA 6009, Australia.
Received 22 August 1996; revised 3 March 1997; accepted 10 March
1997.
1045-2249/97/$5.00 C 1997 International Society for Behavioral Ecology
lated as die average male "time in" divided by die average
female "time in."
As die operational sex ratio is based on "time in," die PRR
is based on "time out" Potential reproductive ratio is denned
as die inverse of G for males and females, respectively (Le.,
l/Gm and 1/G,; Chitton-Brock and Parker, 1992). Importantly,
die PRR, as opposed to die realized reproductive rate, may
differ between die sexes (Chitton-Brock and Parker, 1992;
Chitton-Brock and Vincent, 1991). Potential reproductive rate
can be measured experimentally as die reproductive rate of
each sex in a population where die availability of partners is
unlimited, while other physiological and ecological limitations
such as body size, food availability, temperature, and nest site
availability remain (Kvarnemo and Ahnesjo, 1996). If die
adult sex ratio is unbiased, then die relative PRR of males and
females will be die main determinant of die operational sex
ratio and of which sex competes most for mates (ChittonBrock and Parker. 1992).
Because die natural physiological and environmental constraints remain, it is important to understand how fluctuations
in these parameters will influence die PRR of each sex. PRR
can be affected by factors such as food (Gwynne, 1990), temperature (Ahnesjo, 1995; Ichikawa, 1993; Kraus, 1989; Kruse,
1990; Kvarnemo, 1994), nest site availability (Almada et al.,
1995; Forsgren et al., 1996b), and body size and/or age distributions (Ahnesjo, 1995). A sexual difference in PRR may
then appear when males and females are differentially affected by any of these factors.
In both field and laboratory studies on katydids (e.g., Anabrus simplex and Kawanaphila nartet), the operational sex
ratio and sex roles have been shown to shift widi food availability (Gwynne, 1984, 1990; Gwynne and Simmons, 1990;
Simmons, 1992, 1994; Simmons and Bailey, 1990). In diese
species males produce a spermatophore, which is an energetically expensive nuptial gift, as a nutritive donation to die female. In situations with food surplus, die operational sex ratio
was male biased, males competed for females, and females
were choosy. In contrast, under food shortage females became
die more competitive sex, as males took longer to produce
spermatophores and die operational sex ratio became female
biased, a situation reinforced by females "foraging" for spermatophores by repeated matings. In K. nartet female expen-
606
diture of energy on reproduction was higher than that of
males when fed ad libitum, while die reverse was true when
food was limited (Simmons, 1994, 1995).
In general, a differential effect of food availability on the
PRR of die sexes can be expected, since female egg production is dependent on food intake in many animal^ (fish: Wootton, 1979; Jobling, 1993; reptiles: Ford and Seigel, 1989; birds:
Arvidsson and Neergard, 1991), whereas male sperm production and parental duties (apart from feeding die young) may
be less resource determined.
In this paper, I present results from an experimental study
designed to assess die impact of food abundance on the PRRs
of male and female sand gobies (Pomatosdtisnu wonutus). My
hypothec* was that food availability affects the PRR of the
sexes differendy and consequendy influences die operational
sex ratio. Hence, my aim was to show experimentally die
mechanism of bow an environmental factor (food) can profoundly affect die process of sexual selection.
The sand goby is a small, batch-spawning marine fish with
one reproductive season (Fonds, 1973). It exhibits paternal
care, and males are more competitive dvan females for mates,
as is common among fish (Breder and Rosen, 1966). The
male builds a nest under a mussel shell, usually Mya artnaria
or Mjtihis tduUs, by shifting sand with his tail and digging
underneath, and then proceeds to attract females to spawn in
his nest. Male breeding coloration includes a blue and black
border on the anal fin, and a shiny bhie spot on die first
dorsal fin. This coloration is displayed during courtship, when
die male stands motionless with all fins erected, succeeded by
jerky swimming toward die female and a leading display back
to die nest. Females may also court males by bobbing up and
down in front of diem, snowing their extended bellies (Fonds,
1973; Guitel, 1892). A conspicuous pattern of black lines appears around die eyes and mouth of females that are ready
to spawn. During spawning, die female attaches die eggs to
die ceiling of die shell in a single layer (Fonds, 1973; Hesthagen, 1977). Paternal care includes fanning and nest defense until die eggs hatch (Guitel, 1892). A female lays several
consecutive dutches over die season in die nests of different
males, and a male can care for egg clutches from more dian
one female at a time (i.e., in one brood), as well as in subsequent broods (Fonds, 1973; Hesdiagen, 1977). An implication of male and female PRR for die operational sex ratio in
a sand goby population is that a female is not sexuaOy active
after she has laid a dutch of eggs until a new clutch of eggs
have matured. Similarly, a male with a nest full of eggs is not
sexually active until die eggs have hatched.
METHODS
I experimentally investigated die impact of food availability
on die PRRs of male and female sand gobies under high and
low food regimes. The study was carried out at Klubban Biological Station (58°15' N, 11°28' E) on die Swedish west coast
during May, June, and die beginning ofJuly 1991. Sand gobies
used in die aquarium experiment were caught in a shallow
sandy bay by a hand trawl. The sexes were segregated and
stored in tanks (100-150 1) for about 1 week, during which
they were fed daily with fresh meat of M. edutis. During thu
time die fish habituated to being in aquaria and to eating
chopped mussel meat. Both storage tanks and experimental
tanks were supplied with continuously renewed sea water, and
naatpal day light was supplemented with artificial light. I carried out die two experiments in 30-1 aquaria, supplied widi a
3-cm sand layer and one halved clay flower pot (diam 6 cm,
deptii 6 cm) as a standardized nesting site. Water temperature
followed die natural variation.
Behavioral Ecology VoL 8 No. 6
Food and reproductiv
I randomly assigned pairs of fish to one of two feeding treatments. Half die fishes were assigned to die high-food treatment and were fed once a day. The other half were assigned
to die low-food treatment and fed every fourth day. At each
feeding occasion, die content of one At tduUs mussel was
given per aquarium (range of maximum length of die shells,
25-35 mm). Any mussel meat not consumed by die next day
was removed. Left-over mussel meat in die experiment, collected at 10 occasions, showed dial, as a rule, there was food
left over on die day after feeding in both treatments. The fish
in die high-food treatment left more food (wet weight, mean
± SE, 0.2 ± 0.03 g/individual) dian did diose in die low-food
treatment (0.1 ± 0.03 g/individual), but die difference was
not significant (one-way ANOVA: /",.„ - 2.4, p - .13). Bottom
samples of their natural food (oHgochaetes, small crustaceans,
juvenile moDusks, etc) suggest that die biomass, in wet
weight, is similar in die wild to what I gave diem in die highfood treatment Thus, I manipulated die food availability, primarily by access over time, but also by total amount given.
New replicates were started successively diroughout die season. The feeding treatments started die day die pairs were
placed in die experimental tanks. All females were chosen to
be mature at start, with clearly rounded bellies, and spawned
their first clutch shortly after [within, on average, 2.8 ± 0.4
days (±SE)].
In each aquarium there was one nesting male and one female, weighed to nearest 0.01 g and measured to nearest 1
mm tool body lengdi before start. When die female had laid
a clutch of eggs, she was removed from die aquarium, reweighed and measured, and placed into a new aquarium together with mother male and an empty pot for nest building.
The pot containing die eggs was also removed briefly, and die
eggs in a predetermined 0.5-cm1 section were photographed
to estimate the density of eggs. Further, die contour of die
whole dutch was drawn and 20 eggs were carefully removed
from die brood. The pot was then returned and die male left
to care for die eggs until hatching. The removed eggs were
dried at least 24 h at 60°C and weighed on a micro-balance
to nearest 0.1 ug. The egg section was photographed a second
time shortly before hatching to see if eggs had disappeared,
and any decrease in die area covered by eggs was measured.
I followed each female for two consecutive spawnings to
determine die interspawning interval and followed die males
until die eggs hatched, giving a measure of die time of incubation. Sample size may differ slightly between comparisons
because for various reasons some females failed to spawn two
dutches, and a few males failed to complete dieir incubation
of die eggs. Also, occasional females which were paired with
an exceptionally inactive male, thereby considerably prolonging die interspawning interval of die female, were exduded
from die data set.
The females in die high-food treatment (n •> 21) did not
differ from die females in die low-food treatment (n « 12)
in body weight (mean ± SE, 1.5 ± 0.05 g), body lengdi (59.1
± 0.7 mm), or condition (weight/lengdi: 26 ± 0.6 mg/mm;
one-way ANOVA: p values 0.65-0.86), nor did die males in die
high-food treatment (n = 19) differ from die males in die
low-food treatment (n = 16) in weight (1.1 ± 0.1 g), lengdi
(53.6 ± 0.9 mm) or condition (19 ± 0.7 mg/mm; p values
0.13-0.39).
Food and male interbrood lutavml
In die second experiment, I investigated male interbrood interval with feeding regimes as above: half die replicates (n —
8) were fed every day and half every fourth day (n «• 7).
Kvarnemo • Food and potential reproductive rates in a goby
607
Intorvpcwnlng tntavsl
a)
20
n*
50
*
T
•
40
I"
•o
OB
CD
<D
n
20-
10
n.10
high
high food
b)
low food
lnt»rbrood Interval
UTTM of (noubsflon
20-
15-
CD
8"
n»7
1
high food
low food
1
high food
low food
Figure 1
The effect of feeding on reproductive rate (±SE) in the sand goby,
expressed as (a) intenpawning interval of females (one-way
ANOVA: f lja » 30.5, p<.00l) and (b) the number of days the
males spent incubating the eggs (left; FIJ} — 0.5, p - .49) and
interbrood interval (right; log-transformed: FlM - 0.9, p - .36),
when fed every day (high-food treatment) or every fourth day (lowfood treatment).
There was one male and one female per aquarium, and a pot
for spawning. The female was removed after spawning and
the male was left to care for the eggs. When die eggs reached
the developmental stage of dearly visible eyes (Le., close before hatching), I added a new gravid female, with a rounded
belly indicating mature eggs in the ovaries. The time was then
noted between the hatching of the first brood and the laying
of a second brood. Males did not differ in body length between treatments (one-way ANOVA: FlM - 3.2, p - .10). Body
weight was not measured.
Potential reproductive rate
As a rule, a female spawns her complete clutch in one male's
nest, and the spawning event determines if a female will be
counted as sexually active. I therefore based the estimate of
the female "time out" (Q) on die number of days between
consecutive spawnings, as measured when the females had
continuous access to males (Le., they were free to respawn as
soon as they were able). Thus, female PRR was calculated as
1/C,.
n-7
ft-10
n»7
low
high
low
1st clutch
2nd dutch
Figure I
Dry weight (iSE, ug) of 20 eggs collected shortly after spawning
from the sand goby females' first clutch (left; one-way ANOVA; FlM
- 2.0, p -.18) and second dutch (right; *",.„ - 6.0, /K.05), in the
high- and low-food treatments when only females spawning twice
were included. The same result recurred when all females that
spawned a first clutch were included (Flxl — 0.8, p - .58).
Measuring mala PRR is more complicated. I decided to estimate male PRR in terms of the incubation time divided by
two, since in my study population an average male is capable
of caring for two dutches simultaneously (Rvarnemo, 1994).
I then added the interbrood interval, as measured when the
males had free access to females and thus could spawn again
as soon as they wanted to after the first brood had hatched.
The same individual males did not recur in the measurements
of incubation time and interbrood interval, respectively.
Hence, I used the individual male values of the incubation
times (Bm) and added the mean value of the interbrood interval (Ij), specific for each treatment (Figure lb). I did this
for two reasons: (1) there was a larger sample size of incubation times and (2) despite the lower sample sizes of the
interbrood intervals, they showed the lowest variation (Figure
lb). Thus, male PRR was calculated as PRR^ « 1/G^, 1/(2^/2 + If). Water temperature was also taken into account as a covariate because temperature varied naturally, and
there is a stronger positive effect of temperature on male reproductive rate (Rvarnemo, 1994).
All values are given as means ± SE. Because the data were
normally distributed (after transformation where indicated),
I used parametric tests. In the statistical analyses of covariance,
nonsignificant interactions were always deleted from further
analysis.
RESULTS
Among females in the food and reproductive rate experiment,
there was a significant difference between treatments in number of days between first and second spawning (Figure la).
The mean water temperature prevailing during the female
intenpawning intervals did not differ between treatments
(11.7 ± CTC; one-way ANOVA, treatment: F IJ0 - 0.01, p .91). Low-food females lost body weight between their first
and second clutch (-0.10 ± 0.02 g), whereas high-food females
gained weight (0.02 ± 0.01 g; one-way ANOVA: Flaa « 23.7,
/KC.001), but growth measured as body length was not significantly affected by treatment (f, M = 2.5, p = . 12). Dry weights
of newly spawned eggs showed no difference between treatments in the first dutch, whereas in the second dutch, highfood females spawned heavier eggs as compared to low-food
females (Figure 2). NumbeT of eggs per dutch was not af-
60S
Behavioral Ecology VoL 8 No. 6
d>
0,15
"5
4000
©
3000
0,103
•o
2
2000
a.
<D
v
_
<d
1000
o
f
g
0
1000
1000
1000
4000
SO0O
6000
0,06 •
o
Q.
0JO0
female
number of eggs in 1st chiton
male
high-food
tch
4000
•s
3000
CM
c
03
•
2000
•
1000-
E
c
1000
2000
3000
male
low-food
Figure 4
The potential reproductive rate ( i S E , clutches/day) of male and
female sand gobies, in the high- and low-food treatments, showing
that males may reproduce at a dearly higher rate than females
when the food availability is low, but not when high (two-factor
ANCOVA, with mean water temperature of each reproductive
interval as a covariate; interactions between treatment and sex: FlJtl
- 30.41, p<.00\; temperature and sex: FlJtl - 6.29, p<.001; all
other interactions ns). The data were arcsin transformed for
normality.
b)
•o
female
4000
8000
6000
number of eggs In 1st clutch
Figure 3
Number of eggi in second clutch as related to number of eggs in
the first dutch spawned (a) by females fed daily (high-food
treatment) and (b) by females fed every fourth day (tow-food
treatment), demonstrating a clear trade-off between current and
future reproductive effort when food is in short supply (one-factor
ANCOVA: interaction between treatment and covariate, Le., number
of eggs in first clutch: FlJ7 m 20 J , /K.001, with number of eggs in
second clutch as dependent). This interaction remained significant
without the outlier (FlM - 13.1. p<.0l).
0.4 ± 0.2 mm; Fl-a - 10.1, p<.01). Egg dutches developed
equally well in both treatments. This was measured with the
initial egg number as covariate and proportion of eggs that
hatched (arcsin transformed for normality) as dependent variable in a one-factor ANCOVA (treatment Flit m 0.04, p •»
.84; covariate: Fijn " 0.64, p •» .43; ns interaction: Fxix m .01,
p " .84). Thus, males were not more prone to filial cannibalism in either treatment and were not influenced in this
aspect by the initial dutch size. On average, 69 ± 8.5 % and
72.4 ± 9.3% of the eggs hatched in dutches of high- and lowfood males, respectively.
Comparison of the effect of food availability on the male
and female PRR in a two-factor ANCOVA, with mean water
temperature of each reproductive interval as a covariate,
showed significant interactions between treatment and sex
and between temperature and sex (Figure 4).
DISCUSSION
fected by feeding in the first dutch, nor in the second, but
was strongly related to female body length in both (one-factor
ANCOVA, first clutch—treatment; F 1J9 - 0.40, p - .53; body
length: FIX " 13.3, p<.001; as interaction; second dutch:
treatment: Flxt - 0.004, p - .95; body length: FXXJ - 5.43,
p<.05, ns interaction). High-food female* that spawned many
eggs in the first dutch also spawned many in the second
dutch, while the reverse was the case among the lew-food
females (Figure 3a,b).
Among males, there was no effect of the treatment on the
interbrood interval. High-food males did not take on a new
brood earlier than low-food males (Figure lb). The same was
true for the number of days males spent guarding the eggs
before hatching (Figure lb). In this experiment, low-food
males grew less than well-fed males during the time of egg
guarding in terms of gained body weight (high-food: 0.10 ±
0.02 g, low-food: 0.01 ± 0.02 g; one-way ANOVA: Fljn - 24.7,
/K.001) and body length (high-food: 1.3 t 0.2 mm, tow-food:
My experiments show that the PRRs of male and female sand
gobies are differentially affected by food availability. I compared the impact of food on the PRRs of both sexes and
found that the interspawning interval of females was strongly
dependent on food availability, whereas incubation time and
interbrood interval of males were not When an environmental factor affect* the PRR of males and females differently, this
is predicted to affect the operational sex ratio and consequently affect patterns of mating competition and processes
of sexual selection (Qutton-Brock and Parker, 1992). I thus
predict more intense male-male competition for mates in situations when food is scarce than when it is abundant
If food availability fluctuates considerably over the breeding
season or differs between breeding areas, this will certainly
affect female PRR and thus presumably the operational sex
ratio, as less food should result in fewer females being ready
to mate at any specific time. Sand gobies feed on infauna (i.e.,
meiofauna and juvenile macrofauna, such as small crustaceans, potychaetes, oligochaetes and molhuks living in the up-
Kvarnemo • Food and potential reproductive rates in a goby
permost bottom substrate) (Aaraio, 1991; Aarnio et aL, 1991;
Healey, 1971a; He*thagen, 1977), which often differs in abundance and composition between sites and between season*
(Evans, 1983; Healey, 1971a; M&ller, 1986; Zander and Hagemann, 1986). Also, there is a gradual increase in water temperature over the breeding season in my study area, which
will result in increased metabolic rates and thus increased
food demands in the fish (Fond* and Verhuis, 1973; JobHng,
199S). Conceivably, therefore, sand goby females may become
limited by insufficient feeding success at higher temperatures
later in the season.
Earlier studies have investigated the importance of food ration for female egg production (in the three-fpined stickleback, Gatterostttis acuieatus. Fletcher and Wootton, 1995;
Wootton, 1973,1977; and in the guppy, PoedUa nticulata: Reznick and Yang, 1993). These results have been discussed further in comparison to male reproductive rate (Wootton et aL,
1995). Experimental comparisons between the sexes in relation to food availability, however, have not been conducted
earlier in fish.
Incubation time by males did not differ between the two
feeding regimes in my study. However, I do not exclude the
possibility that differences could still arise under even more
severe food shortages, especially if male sand gobies, like parental females in the convict dchlid, CUMasoma rugnfasdaturn, decrease the amount of fanning when fed reduced rations (Townshend and Wootton, 1985; see also Wootton et aL,
1995). Low food availability could then increase the time of
incubation by males because eggs of at least some fish species
are known to develop more slowly at low levels of oxygen (Hamor and Garade, 1977). Yet, even though males may fan less
when exposed to a food shortage, such a result could have
been masked by rather high levels of oxygen in the water
caused by the constant flow of new sea water through the
experimental aquaria.
In a previous study, the PRR of males was shown to be more
strongly affected by water temperature than the PRR of females (Kvarnemo, 1994). Combining those results with the
present study, I conclude that the PRR of females is more
dependent on food than on temperature, whereas the PRR
of males is more dependent on temperature than on food.
Furthermore, males are probably limited above all by female
availability, whereas females are limited by their own egg production. If the water is warm and/or the food is scarce, males
are expected to compete more strongly for matings. However,
the operational sex ratio probably will never be so strongly
female biased as to make females die more competitive sex:
the PRR of females did not exceed that of males, not even in
a situation with cold water and food in excess (Kvarnemo,
1994) . Only if other factors additionally affect the operational
sex ratio, such as size or availability of nest sites, which can
limit the reproductive success of males (Forsgren et aL,
1996b), or if the adult sex ratio is strongly female biased can
females be expected to become more competitive for mates.
Indeed, in an experimental study on adult sex ratio and sexual interactions, females competed more widi each other
when the adult *ex ratio was female biased than when it was
male biased (Kvarnemo et aL, 1995). Thus, these studies of
the sand goby unequivocally show that environmental factors,
such as food and temperature, can cause sexual differences
in PRR and as a result affect operational sex ratios and, potentially, patterns of competition for mates (Kvarnemo, 1994,
1996; Kvamemo et aL, 1995). Consequently, the inevitable dynamics of such environmental factors may result in fluctuating
strength and direction in mating competition and therefore
in sexual selection.
Clutch size was dearly related to body size in both treatments and both clutches, as is commonly found in fish (Bag-
609
enal and Braum, 1978; Healey, 1971b; Kynard, 1978; Svardson,
1949; Wootton, 1979). Lowfood females produced as many
eggs per dutch as high-food females, both in first and second
dutches. Interestingly, high-food females that spawned large
numbers of eggs in their first dutch repeated this achievement in their second dutch, whereas among low-food females
thi* relationship was negative. This dearly demonstrates that
a certain reproductive expenditure only becomes a significant
burden when resources are limited (Stearns, 1989; 1992), as
females then have to trade resources between present (first
dutch) and future reproduction (second dutch). Similar results have been predicted (e.g., Steam*, 1989; Tuomi et aL,
1983) and confirmed (reviewed by van Noordwijk, 1989; see
also Schwarzkopf, 1993; Verhulst, 1995). A mechanism for this
trade-off is indicated in a study on the three-spined stickleback, G. aadtatus, in which die number of eggs spawned by
a female is specifically determined by her body weight at the
start of each cycle (Wootton and Evans, 1976). A female, unable to meet die energetic demands during that cycle, transfers energy from body tissues into eggs, consequently loses
weight, and produces fewer eggs next time (Wootton and
Evans, 1976). The low-food females in my study lost weight,
whereas the high-food females gained weight, during the reproductive cyde.
Filial rannfhali«m is a prevalent phenomenon in die sand
goby (Forsgren et*aL, 1996a), as in many odier fish spedes
with parental care (e.g., Coitus gobia Marconato et aL, 1993;
Oxyltbhis pietus: DeMartini, 1987; Padogobius vtartsnsi Bisazza
et aL, 1989; AidabUnnius spkjnx. Kraak, 1996). Partial dutch
cannibalism is believed to be a measure to counter depleted
energy reserves in species with uniparental male care when
die male has restricted feeding opportunities. By rannihalJTing some of die eggs, die male would be able to remain in
good enough condition to raise additional broods (FitzGerald, 1992; Rohwer, 1978). Male hatching success was not affected by feeding treatment in my study, filial cannibalism occurring equally often in botil treatments. Similarly, in die
three-spined stickleback no relationship was found between
amount of food supplied and number of eggs eaten by parental males (Belles-Isle* and FitzGerald, 1991). In a recent model, Kraak (1996) suggested diat food availability will be important to whether a male will cannibalize healthy eggs only
when he guards a small brood, whereas in a larger brood his
energetic needs will be fulfilled by eating dead or diseased
eggs. However, my results do not support this hypodiesii, as
initial dutch size and feeding treatment did not interact in
determining die proportion of hatchable eggs. Another model predicted diat increased food availability would decrease
filial cannibalism, whereas increased egg quantity would increase it (Sargent, 1992). My study does not support dii* model either, as neidier feeding regime nor initial clutch size had
any impact on die propensity of males for filial cannibalism.
One could argue diat all males were well fed before die start
of die experiment and may thus have been in such a good
condition diat die feeding treatments had no effect on die
males. Yet, die treatments dearly differed in terms of male
growdi during die incubation period, whereas partial dutch
cannibalism did occur in bodi treatments. Thus, although
some odier studies have been supportive (e.g., Marconato et
aL, 1993), food availability alone may not be suffident to explain why partial clutch filial cannibalism occurs.
In summary, die PRR of females was found to depend
strongly on food availability. In contrast, die PRR of males was
unaffected by food, both widi respect to incubation time and
interbrood interval. Thus, my study dearly shows diat food
availability differentially affected die PRRs of males and females, and when food availability causes a sexual difference
610
in PRR, I predict the operational sex ratio and sexual selection to be affected accordingly.
I thank Ingiid Ahnesjo, Rebecca Fuller, Adam Jones, Kai lindstrom,
Carin Magnhagen, Gunina Rosenqvist, and Staffan Ulfstrand for commenting on and improving die paper. I also thank Khibban biological
station for excellentfiw'ii»"*«and Annika Bokstrdm.Johan Dannewitz,
Annika Fongren, Kerstin Johansson, Anna Karlsson, Par Kariston,
Sami Merilaita, Anders Nordlof, Barbro Nystrora, Cari-Gustaf Thulin,
and Sara Osdund for assistance. Financial support was given by the
Royal Swedish Academy of Sciences, the Scholarship Fund of Inez
Johansson, and the Zoological Foundation.
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