Behavioral Components of Sex Role Reversal in the Tettigoniid ...

Journal of Insect Behavior, Vol. 11, No. 4, 1998
Behavioral Components of Sex Role Reversal in the
Tettigoniid Bushcricket Ephippiger ephippiger
Michael G. Ritchie,1,2 David Sunter,1 and Leon R. Hockham1
Accepted January 28, 1998; revised February 16, 1998
Facultative sex role reversal is found in species of tettigoniid bushcrickets in
which males invest heavily in matings or offspring by producing large, nutritious
spermatophores. On high-quality diets such species show conventional sex roles,
but under low-quality diets males become the choosier sex. Comparative work
suggests that Ephippiger ephippiger (Tettigoniidae, Ephippigerinae) has one of
the largest described spermatophores (up to 40% of the male body weight).
Here we examine the behavior of this species under variable diet conditions in
the laboratory and find evidence of sex role reversal under poor-quality diet
conditions. We also examine the behavioral components of sex role reversal. In
the reversed condition, rejections of mating attempts are almost solely by the
male and contests are almost solely between females. Role-reversed males sing
less frequently and at a much reduced intensity. We use geographic variation
in the calling song of this species to assess the strength of female phonotactic
discrimination between races. This is not significantly reduced despite sex role
reversal. We therefore suggest that the male and female aspects of the acoustic
sexual communication system differ in that male components change most during
facultative sex role reversal.
KEY WORDS: sex roles; tettigoniidae; Ephippiger; spermatophore; calling song; phonotaxis.
INTRODUCTION
Which sexes display the conventional sex roles (coy and choosy versus competitive
and indiscriminate) is not always fixed but can vary according to environmental
conditions. In particular, the relative parental investment of the two
1Environmental and Evolutionary Biology, Bute Medical Building, University of St. Andrews, St.
Andrews, Fife, Scotland KY16 9TS.
2To whom correspondence should be addressed.
481
0892-7553/98/0700-048l $ I5.00/0 © 1998 Plenum Publishing Corporation

482 Ritchie, Sunter, and Hockham
sexes (Williams, 1966; Trivers, 1972), the operational sex ratio (OSR) (Emlen
and Oring, 1977), and the variance in quality of mating partner (Owens and
Thompson, 1994; Wearing-Wilde, 1996) can interact to determine which sex is
the choosiest. In most species females are the choosier sex; in others it is the
males [e.g., pipefish with male "pregnancy" (Vincent et al., 1994; for review
see Gwynne, 1991; Vincent, 1994)]. In some species sex roles are facultative
and manipulation of conditions or transplantation between field sites can cause
sex role reversal (Gwynne, 1985; 1993; Gwynne and Simmons, 1990; Schatral,
1993; Wearing-Wilde, 1996).
Several tettigoniid bushcrickets (katydids) display sex role reversal, for
example, Metaballus (Gwynne, 1981), Anabrus (Gwynne, 1984; 1993), Kawanaphila
(Gwynne and Simmons, 1990), Requena (Schatral, 1993; Simmons,
1994b), and possibly Conocephalus (Gwynne, 1982). The often large spermatophore
of male tettigoniids might predispose these species to sex role reversal,
at least when environmental conditions are poor, because the production of
spermatophores is costly. This results in an increase in the time it takes for a
male to produce a spermatophore, thus increasing male intercopulatory period
and decreasing reproductive rate. Also, females might "forage" for nutrientrich
spermatophores, further increasing the proportion of time males are unreceptive
(Gwynne, 1990). Under such conditions the OSR becomes biased toward
females and the relative value or cost of the spermatophore increases. Under
environmental conditions where food per se is not limited, the presence of
special nutrients in spermatophores (Gwynne, 1988b) may also predispose a
species to sex role reversal.
An example of sex role reversal was described by Gwynne and Simmons
(1990) in Kawanaphila nartee. Early in the season when pollen (the major
constituent of their diet) was scarce, this species showed sex role reversal with
females competing for matings. Later in the season, when pollen became more
abundant, typical courtship roles were resumed. Experimental manipulation has
confirmed the role of diet. Pollen is a limiting factor for males because they are
dependent on it as a source of protein for spermatophore production and mate
only when they have produced a spermatophore. Thus male investment (in mating
or offspring) is the main cause of sex role reversal in tettigoniid species,
though the ultimate cause might be operational sex ratio or mating partner quality
(Heller and von Helversen, 1991).
Not all tettigoniid bushcrickets are likely to show facultative sex role reversal.
Many have no or only small spermatophores (Wedell, 1993; Vahed and
Gilbert, 1996), and some large spermatophores may not represent a large investment
if they are not protein rich (Wedell, 1994). Will and Sakaluk (1994) describe
such spermatophores as "sham offerings." The bushcricket Ephippiger
ephippiger produces a large spermatophore that may represent up to 40% of

Sex Role Reversal 483
male body weight (Busnel and Dumortier, 1955), among the largest as a proportion
of body weight (Wedell, 1993; Vahed and Gilbert, 1996). Male investment
in spermatophores is not constant because spermatophore weight as a
proportion of male body weight can range from as little as 14 to 40% (personal
observation). It seems likely that E. ephippiger is a candidate for sex role
reversal, unless these large spermatophores are shams.
Previous studies (Schatral, 1993; Gwynne, 1981; 1984; 1985; 1990; 1993;
Simmons and Bailey, 1990) have identified sex role reversal by examining behaviors
such as approaches between sexes to initiate courtship, rejection of
courtship attempts, intrasexual fighting, and variance in mating success. Courtship
song and female phonotaxis are major components of the mating system
of tettigoniids yet have rarely been studied in a sex role-reversed system. Gwynne
(1984) found no obvious differences in song between normal and role-reversed
Anabrus simplex, whereas Metaballus litus changes the type of song used
(Gwynne, 1985). Here we have reared E. ephippiger in the laboratory and varied
the diet quality available to adults. We assess intrasexual encounters, courtship
encounters, and male calling song production and use geographic variation in
calling song to assess the strength of female phonotactic discrimination.
METHODS
The bushcricket E. ephippiger usually has a typical tettigoniid mating system
where males call and females approach and initiate mating. The song of E.
ephippiger varies geographically and "song races" have been defined on the
basis of changes in the number of syllables (wing movements) per chirp (Duijm,
1990; Ritchie, 1991; 1996). Usually E. ephippiger produces monosyllabic chirps,
but around the eastern Pyrenees and Mediterranean coast syllables per chirp increase
to an average of four or five. Individuals used here were polysyllabic,
from a laboratory stock originating from near the village of Sauto, de"partement
Pyre'ne'es Orientales, France. This is of the "cunii" form of the diurnus complex
of E. ephippiger (Oudman et al., 1989; Ritchie et al., 1997).
Nymphs were reared individually following the third molt. The normal diet
consisted of bees' pollen, flaked goldfish food, apple, lettuce, and cabbage ad
libitum. The pollen and the goldfish flakes were replaced weekly; the remainder,
every 2 days. Approximately 1 week before turning adult, half the nymphs were
randomly assigned to a low-quality diet group. They were given 12 pieces of
bees pollen (weighing approximately 0.007 g) per day and a small piece of
cabbage every other day.
Adult bushcrickets were transferred to large observation tanks (50 x 50 x
100 cm), with three bushcrickets of each sex per tank. Two cages were observed
per diet regime. Each tank contained bamboo canes for climbing and sand-filled

484 Ritchie, Sunter, and Hockham
tubs for oviposition and maintained on a 12:12 light cycle at 25 ± 2°C. Individuals
were regularly weighed (to the nearest 0.01 g) to assess the impact of
the diets.
Observations began 1 week after the experimental conditions had been
established and continued for 16 days. Using a focal sampling method, each
bushcricket was observed for 10 min per day in random order during their peak
period of singing activity (0900 to 1300). Approaches to the opposite sex,
premounting rejection by the opposite sex (judged to have occurred when one
bushcricket retreated from another attempting to initiate mating), intrasexual
contests (when two bushcrickets of the same sex engaged in a physical encounter),
and the calling frequency of the males were recorded.
Sound pressure levels (SPL) were measured using a Realistic sound level
meter (33-2050 on the "fast" setting). Males were suspended in a netting cage
within a sound-attenuated chamber and the meter was held approximately 15
cm dorsal to the male, oriented toward the elytra. Ten readings per male were
taken, following the period of behavioral observation. Some additional males,
which had been maintained as virgins, were also used. Their SPL did not differ.
The sound meter is flat to about 7 kHz (approximately -10 dB at 15 kHz, the
dominant frequency of Ephippiger song). Though not providing a reliable measure
of absolute SPL, the equipment nonetheless allows us to examine relative
levels. Female phonotactic discrimination was measured using a standard playback
protocol described in detail by Ritchie (1991) and Ritchie et al. (1995).
Briefly, synthetic song representing the mono- and polysyllabic song races was
played through ultrasonic loudspeakers which females approached on a T-shaped
pole, turning toward one. A trial consisted of playing the pair of songs to individual
females four times in succession, swapping the song between speakers
for each presentation. Four choices of one song therefore required three consecutive
changes of direction. Fourteen virgin females were given four trials
each on 4 separate days. Nine were raised under LD conditions, and none were
used in the observational experiment. All females readily completed all trials
and made an equal contribution to the levels of choice presented. This procedure
typically reveals a strong preference in favor of song of the appropriate race
(Ritchie, 1991, 1992; Ritchie etal., 1995).
RESULTS
Weight Gain
Figure 1 shows the weight of individuals from the two diet groups. An
analysis of covariance (time as covariate) shows that diet affected weight gain
(F 1,84 = 148.4, P < 0.0001) and an interaction between diet and sex (F 1 , 84 =
7.99, P = 0.006). Females are more severely influenced under these low-diet

Sex Role Reversal 485
Fig. 1. Adult weight under the two diet conditions. Each point is a mean from six
individuals. Standard errors bars are omitted for clarity; statistical analysis is presented
in the text. HD, high-quality diet; LD, low-quality diet.
quality conditions, as the normal sexual dimorphism in body weight is absent
under poor conditions (see also Simmons, 1994a).
Approaches, Rejections, and Competitions
Statistical analysis of the frequencies of these behaviors during the observational
period has been carried out using 2 x 2 x 2 contingency G tests with
diet, cage, and sex as the levels. This analysis is compromised because individuals
were not identified, so a more active individual will disproportionately
contribute to the incidence of behaviors. The analysis can therefore best be
considered a guide to the relative incidence of behaviors, rather than producing
exact probabilities (all "significant" results were very unlikely). The frequency
of approaches by one sex to the other was independent of diet and the observation
cage (tests for two-way interactions all produced P > 0.05). Males were rarely
observed to approach females. The sex which carried out most premounting
rejections varied between diets (Fig. 2a) [diet * sex G = 15.78, df = 1, P <
0.001; the strength of this effect differed between replicate cages cage * sex, G
= 9.59, df = 1, P = 0.002; other terms >0.05]. Under HD conditions females
rejected more often than males; under LD conditions males rejected most. There
was a large difference in the incidence of intrasexual competition between the
diet regimes. Under HD conditions, all contests were between males, whereas
under LD conditions almost all contests were between females (Fig. 2b) (diet
* sex G = 42.2, df = 1, P < 0.001; other terms >0.05). Few copulations
were seen under either diet condition during the periods of observation.

486 Ritchie, Sunter, and Hockham
Fig. 2. Premounting mate rejections (rejecting sex) (a) and intrasexual
contests (b) of males and females under the two diet conditions. HD,
open columns; LD, filled columns.
Acoustic Behaviors
LD males called significantly less frequently than HD males (Mann-
Whitney U test: LD, N = 6, median = 4.31; HD, N = 6, median, 33.69; P
=

Sex Role Reversal 487
Fig. 3. The frequency (a) and intensity (b) of male calls under the two diet
conditions. HD, open columns and circles; LD, filled columns and circles.
Intensity is plotted against male body mass.
facultative component of female preference. Females typically show a strong
preference for song of their own song race, and HD females examined here
showed an overall preference of 78% for the appropriate song model. LD females
showed a similar preference (Fig. 4), thus preferences were independent of diet
(x 2 = 1.99, df = 1, NS; analysis of deviance from a generalized logistic linear
analysis, chapter 8, Genstat 5 Committee, 1993).
DISCUSSION
In tettigoniids such as E. ephippiger there is no conventional parental care,
but males provide a nuptial gift (in the form of a large spermatophore) which

488
Ritchie, Sunter, and Hockham
Fig. 4. Preferences of females for synthetic song representing the
"song races" of this species. Columns are the proportion of choices
of polysyllabic song (of 16) per female. The solid line shows random
expectations; the dotted lines represent a probability level of 0.01
per individual female (see text for analysis across all females).
can represent paternal investment in some species (Gwynne, 1988a,b; Simmons,
1990; Wedell, 1994). Large spermatophores may predispose them to sex role
reversal if the spermatophore becomes a limiting factor, for example, when food
is scarce, as the operational sex ratio (Emlen and Oring, 1977) becomes biased
toward males. Our results clearly suggest that E. ephippiger shows sex role
reversal when diet is limiting (i.e., under LD conditions). Males rejected female
approaches significantly more often than females rejected males. This behavior
was also seen in other species by Gwynne (1981, 1985, 1994) and Wearing-
Wilde (1996). Under conventional sex roles (i.e., under the high-diet quality
conditions; HD), females reject males nearly twice as often as males reject
females. In contrast, in Requena verticalis, males reject females as often as vice
versa even under conventional sex roles (Schatral, 1993). This may arise because
male R. verticalis invest paternally in their offspring (Gwynne, 1988a,b). Apparently
conventional sex roles under a high-quality diet in E. ephippiger does not
exclude the possibility that males of this species invest in their offspring, because
parental investment theory predicts that conventional sex roles will exist unless
the male investment is equal to or greater than the female's investment in offspring
(Williams, 1966; Trivers, 1972). It is interesting to note that the number
of mating rejections by E. ephippiger males on the low-quality diet is substantially
greater than the number of mating rejections by females on the high-quality
diet (Fig. 2b). This may indicate that under nutrient stress, role-reversed males

Sex Role Reversal 489
are actually investing even more (and are hence more choosy) than females
under conventional sex roles. However, it could also occur because males are
less able to produce spermatophore material due to depletion or nutritive stress.
Females in the LD treatment were observed to take part in significantly
more intrasexual aggressive contests than males in the same group. Females in
the HD group were never observed to compete with each other. There were
almost twice as many contests between low-diet females as between high-diet
males. Female fighting under sex role-reversed conditions has also been observed
in Kawanaphila nartee by Gwynne and Simmons (1990) and in Metabalus litus
by Gwynne (1985). In both diet treatments females were more likely to approach
males. Schatral (1993) also found that diet did not affect the number of approaches
of female R. verticalis toward males even though sex role reversal
occurs in this species. This finding is probably not surprising since the mating
system of both species relies on phonotaxis.
Detailed studies of the phonotaxis system of tettigoniids under sex role
reversal are lacking. Schatral (1993) showed that there were no differences in
singing activity between normal and role-reversed Requena verticalis. In contrast
we find male E. ephippiger under LD conditions call less frequently and at a
significantly reduced intensity. A difference in overall calling activity was not
found in Anabrus simplex (Gwynne, 1984), but males at LD sites called for
significantly longer before moving to another perch than males at HD sites.
Interestingly, Gwynne (1985) found a difference in song type (rather than calling
rate or intensity) in Metaballus litus. In role-reversed populations males rarely
use continuous song, preferring to use short "zipping" song, possibly because
zipping males are harder to locate by predators (Rentz, 1973), and female eagerness
reduces the requirement for a conspicuous song. We cannot tell if the
difference in calling activity between the LD and the HD treatments described
in the present study arises because males actively reduce investment in energetically
expensive (Simmons, 1994a) singing behaviors because females are
more eager to mate, or they simply have less energy to expend on singing due
to nutrient stress. Arguably, if males are going to sing, they should do so at the
maximum intensity possible, suggesting that the latter explanation is more likely.
In addition to nutrient stress, males could be unreceptive due to an increased
female mating frequency under the LD conditions, increasing the proportion of
time that males were refractory. This could occur if females foraged for spermatophores.
The incidence of mating clearly has the potential to influence many
of the behaviors measured here, but these were so rarely observed that we cannot
infer if mating rates were higher in either group. Clearly matings remained rare
events under both conditions.
The phonotaxis experiment suggests that female discrimination between the
song types is not part of the facultative component of sex roles in this species,

490 Ritchie, Sunter, and Hockham
in that females were as discriminating in choice of song under role-reversed
conditions. The song models used in this experiment are "song races," geographic
variants of song known to influence genetically determined preferences
(Ritchie, 1996). It seems likely that female preference for song characteristics
has a hierarchical arrangement. For example, Ritchie et al. (1995) found that
females showed a strong, consistent preference against song traits reliably associated
with increasing male age. However, females preferred "old" song of the
appropriate song race over "young" song of the inappropriate song race, so
age effects must be lower in the preference hierarchy than song race. Characteristics
of song which are lower down in the female preference hierarchy might
still be part of the facultative component of sex roles (it would be interesting to
test this or to examine if the willingness to phonotax to less strongly preferred
song increases in the reversed condition). That discrimination between song
races is maintained despite sex reversal reinforces the suggestion that the song
races are very substantial changes in the mate recognition system of this species,
despite their recent evolutionary origin (Ritchie et al., 1997).
ACKNOWLEDGMENTS
This work was funded by the NERC (Advanced Fellowship GT5/92/TLS/
16 to M.G.R.). We thank William "Shakespeare" Snedden for improving the
English in the manuscript, Michelle Hockham, Karim Vahed, Jenny Gleason,
Henry Rae, and two referees.
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