Ethology, 75, 265-284 (1987) 0 1987 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0179-1613

Zoologisches Institut ( I l l ) der Universitat Wiirzburg, Wiirzburg

Male Aggression, Female Mimicry and Female Choice in the Rove Beetle, Aleochara curtula (Coleoptera, Staphylinidae)

KLAUS PESCHKE

With 8figures

Received: Murch 27, 1986

Accepted: Ilecembrr j, 1986 (W. Wickler)

Abstract

Causes, consequences and pheromonal regulation of male contest and female choice in the staphylinid beetle, Aleochuru curtula (Goeze), have been investigated in the field and in the laboratory. At the feeding and mating site (carcass) the sex ratio is male biased. Polyandry is affected by prolonged copulations, spermatophore plugs and anti-aphrodisiac pheromones transferred from the male, and by female repulse bzhaviour as well. Aggression of competing males leads to expulsion of inferior wales from the carcass. Young, starved and multiply mated males, which need access to the food resource, produce the female sex pheromone. They release homosexual responses, but also avoid intrasexual aggression. O n the other hand, females behave aggressively towards individuals bearing the female sex pheromone o r repulse their copulatory attempts. Those males of insufficient physiolog- ical condition produce a lighter spermatophore and fertilize less eggs. The adaptive significance of feinile mimicry, male mating tactics, and female choice is discussed.

Introduction

In spite of a plenty of theoretical framework and experimental studies on insect mating strategies (for review see: BLUM & BLUM 1979; THORNHILL & ALCOCK 1983; SMITH 1984), only a few examples of reproductive behaviour mediated by chemical signals have been worked out with respect to the ultimate questions of evolutionary biology (e.g. ALCOCK 1982; B O P P R ~ 1984; CARIIE & BAKER 1984). O n the other hand, the physiological investigations of sex pheromones have produced the best examples of exact and unequivocal transmis- sion of information using chemical cues (for recent reviews see: BELL & CARDE 1984; HUMMEL & MILLER 1985; SCHNEIDER 1983). These studies focus principally on the location and identification of mates, whereas other essential factors of

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266 KLAUS PESCHKE

sexual strategies like regulation of intrasexual competition and mate choice by means of pheromones have not been considered.

The staphylinid beetle, Aleochuru curtulu (Goeze), seems to be a good model for the investigation of various counteracting selective pressures on odour com- munication. Males recognize their mates by means of a female aphrodisiac pheromone (PESCHKE 1978 a, b), a mixture of alkenes from the cuticular hydrocar- bons (PESCHKE & METZLER 1987), which releases the male sexual behaviour at close range (PESCHKE 1983). Freshly emerged males, however, also release homosexual responses by emitting the female sex pheromone and sex specificity is then attained on the ceasing of its production during maturation (PESCHKE 1986a). The adaptive significance of this chemical mimicry has been demonstrated in a preliminary study (PESCHKE 1985): young males are protected from being aggres- sively attacked by mature males, whereas females prefer mature males without the female pheromone. In a further publication (PESCHKE 1987) the chemical identity of male releasers of homosexual responses and modulators of intermale aggression and female choice with the female sex pheromone blend is demonstrated.

In the present paper, the adaptive significance of male competition, chemical mimicry and female choice is investigated in the ecological context. The beetles live on carcasses, a rare resource unpredictable in time and space, where the beetles feed on blow fly maggots and search for mates (PESCHKE et al. 1987a). Females deposit their eggs in the surroundings of the carrion, where the para- sitoid larvae search for blow fly pupae as hosts (PESCHKE & FULDNER 1977; PESCHKE et a]. 1987b).

Here we investigate first the ecological factors causing the aggression among A. curtulu males. Are their females a particularly limited resource and for what reasons? The operational sex ratio of this species may be affected by the differential abundance of sexes at the place of rendezvous and/or by a reduced receptivity of females (EMLEN & ORING 1977). What are the consequences of male aggression? Competition in general should reduce access of competitors to limited resources. Are A . curtulu males excluded from the mating site and - since the place of rendezvous is identical with the small habitat for food allocation - are competing males also influenced in their access to nutrition?

The correlation of the release of homosexual responses with the avoidance of intermale aggression is also investigated for other physiological types of the long- lived A . curtulu males of different nutritive status and mating history. Investiga- tions of female mimicry in vertebrates using chemical (MASON & CREWS 1985) or behavioural and morphological cues (for review see WELDON & BURGHARDT 1984) mainly focus on the benefit of stealing sneaky copulations; but also the regulation of aggression in social groups (WICKLER 1967, 1968) or access to food resources within males’ territories (e.g. ROHWER et al. 1980) have been considered. Behavioural transvestism in insects has only been described for male scorpion flies stealing nuptial gifts (THORNHILL 1979). What is the adaptive significance of female mimicry in A . curtula males?

We also investigated how females respond to males of various pheromonal status. Are females able to choose among males according to chemical criteria and what information do these criteria offer about the physiological or genetic quality of males?

Male Aggression, Female Mimicry and Female Choice in the Rove Beetle 267

neutral a

repulse of grasping

Fig. I: Behavioural responses of A . curtulu males and females in homo- and heterosexual encounters. a. Neutral encounter without any conspicuous behavioural response except of slight abdominal bending o r antennal movements. h. grasping of males with tong-like parameres towards the females’ abdomen; c. copulation with male (left) attached to the female (right) in the bent over position; d. aggression between two males with drumming abdomina; e. females’ repulse of male grasping by

oscillating her abdomen

Materials and Methods

1. Field Collections

A . currulu was baited during the reproductive period from May to July with rabbit or rat carcasses in deciduous forests near Ochsenfurt (Bavaria). For registration of beetles in the surround- ings of carrion, we installed pitfall traps or trapping stones at distances of 0.5 to 4 m (for details see PESCHKE et a!. 1987a).

2. Laboratory Cultures

‘4. curtulu was continuously reared in the laboratory according to FULDNER (1968) and PESCHKE (1986a). Exactly 4 weeks after parasitation of the host fly pupae (Culliphoru etythrocephulu Meig.) by the Alcochuru larvae, the beetles emerge from their pupal cocoons (22°C). Immediately after emergence, the sexes were separated and the males were kept in groups of 10 in boxes of 10 x 10 x 7 cm. O n e month later, they could be used as tester males in the model bioassay. Females were kept individually in boxes of 5.5 x 3.5 x 1.5 cm. Three times a week beetles were transferred to freshly prepared boxes with moist filter paper and cut third instar maggots of Culliphoru as food. Sexually isolated and well fed beetles of both sexes of an age of 3 weeks were termed as “standard laboratory beetles”. The normal temperature and light regime were 22°C und LD 16/8 h. Starved males were kept individually, on moist filter paper only, in order to avoid cannibalism.

3. Criteria for Behavioural Responses

Nerrtrul encounters. In many encounters of two individuals, n o conspicuous behaviour could be observed. Only slight antennal movements or a bending of the abdomen without protruding of male genitalia were seen when the beetles passed each other (Fig. 1 a).

19::.

268 KLAUS PESCHKE

18 h white light red-sensitive - 3 min recordings at 60 min 6 h dim red light camera intervals for 2L h 220 c

red glass ( # 11.5 cml

nylon grid

A. cortulo marked by dots of enamel paint \

decaying meat large arena with maggots iL7 x L1 cm)

Fig. 2: Laboratory simulated carcass for video observations

Mule grasping response. At a distance of about 2.2 mm from the female, the male bends its mobile abdomen over the head and protrudes the genitalia with the tongs-shaped parameres (Fig. 1 b). This behaviour is easily observed as a criterion of male copulatory response just before contact with the females (PESCHKE 1978a, b, 1983).

Copulation. Upon contact with the female, the male works his parameres along to the tip of her abdomen. The male genitalia are then intromitted into the females’ genital pouch (Fig. 1 c). In this tandem position of copulation (PESCHKE 1978b) the male is attached to the female in a totally bent over posture. The pair is able to run in a well coordinated manner.

Aggression. Male fighting behaviour is characterized by pushing the opponent with head and mandibles, chasing him and drumming with the mobile abdomen on to the other individual (Fig. 1 d, see also chap. 11).

Females’ repulse of mule grasping. Females apparently unwilling to copulate oscillate their mobile abdomen, thereby avoiding the fixation of the male genitalia (Fig. 1 e).

4. Bioassays

Model bioassay. For quick quantification of the release male copulatory responses (grasping), the model bioassay was used as a standard test (PESCHKE 1978a, 1983, 1986a). Males or females were freshly killed by freezing (45 min at -17°C) and glued to the tips of glass needles. The number of standard tester males responding to these models with the grasping response was specified in percentiles.

Observation of single pairs. Beetles of different sex and various physiological status were observed as single pairs in plastic boxes of 5.5 x 3.5 x 1.5 cm for a period of 15 or 30 min. The individuals were marked by dots of enamel paint on pronotum and both elytra. We recorded all encounters oi beetles and specified the percentage of encounters in which a certain behaviour was shown. Some single pairs were continuously observed by a red sensitive video camera in a natural cycle of white and dim red light (16/8 h). A recorder with time lapse setting (8 h) was used for registration (National N V 8030).

Presence at the carcass. A plastic box of 20 x 20 x 7 cm was filled with moist plaster (2 cm height), which was covered with a thin layer of soil. In the centre a replaceable petri dish of 5.5 cm diam. was situated so that its top was level with the plaster ground (Fig. 2). The dish was also filled with plaster leaving on top an open space of 3 mm height. Here a flat piece of decaying liver together with 10 third instar maggots of Calliphora was deposited. The dish was covered by a lid of nylon net (1 mm width of mesh). Three maggots were cut and smeared over the grid. A disk of red glass (5.5 cm diam.) was supported by small rubber feet and covered the artificial carcass leaving a free space of 4 mm above the grid.

Increasing numbers of standard males or females (5, 10, 15, 20) were given into the arena, and after a period of 1 h for settling down, we recorded in 5-min intervals for a period of 1 h the number

Male Aggression, Female Mimicry and Female Choice in the Rove Beetle 269

Tuble 1; Frequencies of copulations of A . curtulu (% beetles found in copula). a) Field observations at rabbit carcasses in the reproductive period from May to July 1980 directly at the carrion and in its surroundings (pitfall traps or trapping stones at distances of 0.5 to 4 m). b) Video observations at the

laboratory simulated carcass with beetles collected in the field or from laboratory cultures

% N N beetles copulations samples

copulating (beetles) (carcasses)

a) Field observations at the carcass 4 29 (1396) 218 (16) in pitfall traps 0 0 (287) 127 (4) under stones 0 0 (137) 37 (6)

b) Video observations of field collected beetles 5 30 (1308) 264 ( I 1) of laboratory reared beetles 3 10 (699) 50 (5)

of individuals present at the carcass. In other experiments, 5 standard males were observed together with 5 other A . curtulu individuals of various sex and physiological status.

Video observation in the laboratory simulated currzon. A similar artificial carcass as above (Fig. 2; red cover plate 11.5 cm diam.) was put into a larger arena (50 x 50 cm). Marked beetles of various sex and physiological status were added in groups of five. The area of the carcass was observed by a red sensitive camera in a natural rhythm of white and dark red light (LD 1816 h). The events at the carcass were recorded for 3 min every 60 rnin at normal speed over a period of 24 h. For each encounter we recorded whether aggression, grasping or neutral behaviour was exhibited. Every h the number of beetles present at the carcass was recorded too.

In a simulation of the natural succession, we transferred 5 mature A . curtula individuals of each sex, which had been starved for a period of 5 d, to an arena with fresh liver only. One day later, the carrion was replaced by liver incubated with ten freshly emerged first instar maggots. The carrion was then replaced daily by a new one, comprising the following successional stage. At the 6th day the fully grown maggots leave the decaying meat for pupation and the old carcass was left in the experiment until the 14th day of succession.

5. Statistics

For statistical evaluation of relative frequencies the fourfold table Xz-test or the exact test of Fisher was used. Confidence limits were given at the 95 % level. Cumulative frequencies were compared by the Kolmogoroff-Smirnoff test. The Mann-Whitney U test was used for the comparison of rank sums (SACHS 1984).

Results

I. Causes of Competition: Females as the Limited Resource

1. The Mating Site and Frequency of Copulations

As early as 1926 KEMNER stated that copulations of A . curtulu occur at carcasses. U p to 260 individuals may gather at a rabbit corpse, the cavities under which are the preferred microhabitat of the beetles (PESCHKE et al. 1987a). It should be possible to register copulations by sampling A . curtulu out of carcasses, since the long lasting coupling (22-160 min) is very tight due to the fixation of complex genital organs (PESCHKE 1978 a). However, we only observed 4 YO of the

270

a '"1

12 14 16 18 20 22 days of succession

b 1001

Fig. 3: Sex ratio (Yo males) of A . curtulu during the succes- sion at carrion. a. Natural succession at rats exposed on May 24, 1980 in deciduous forests in northern Bavaria; b. laboratory simulated carcass (see text). Significant devia- tion from the 50 : 50 % ratio (p < 0.05, Xz-test) is indicated by asterisks. H: hatching of calliphorid maggots; M: emigration of postfeeding

maggots

* X a I VI H M

0 2 4 6 8 1 0 1 2 1 4 days of succession

individuals per sample to be coupled (Table I). 69 % of the copulations (N = 29) were registered prior to emigration of postfeeding maggots ready to pupate in the carcass vicinity; 31 % of the copulations were seen after that decisive event in the succession of the carrion insect community (PESCHKE et al. 1987a, b). Under continuous video observations of field-collected and laboratory-reared beetles on the simulated carcass we again registered only 5 % or 3 Yo of the beetles, respectively, to be coupled (Table 1).

Are copulations that rare or do the beetles copulate at other sites? Females deposit their eggs in the surroundings of carrion at a distance up to 4 m thus easing the finding of the hosts by the parasitoid larvae (PESCHKE et al. 1987a, b). This may be an appropriate place for males to seek mates, however, we could not observe copulations in pitfall traps or under stones in the carrion surroundings (Table 1).

Male Aggression, Female Mimicry and Female Choice in the Rove Beetle 271

Table 2: Release and repulse of grasping and copulations by A . turtrrlu females of various mating historv during 30-min observations of single pairs

~~~~~~

N Mating history "/" grasping 7" repulse "0 cop. encounters of females of males by females (pairs)

Virgin 94 70 82 172 (34)

Immediatcly after first copulation

1 d after first copulation 94:':: .: 27:: 909 (37)

360 ( 7 2 ) 2 5::- ::. ::- 2:' :>::. o::.::.:'

94::. ::. ::-

: p < 0.05, ".:'~:: p < 0.001; x'-tcst: comparison to the preceding line

2. Differential Abundance of Sexes a t t h e Mat ing Site

Mark and recapture experiments in the field demonstrated that males and females of A . curtulu arrive at carcasses in a balanced sex ratio, a result which was also obtained in laboratory cultures (PESCHKE et al. 1987a). Females, however, leave the carcass soon after copulation and feeding for oviposition in the sur- roundings of carrion. This female behaviour results in a skewing of the sex ratio at the carcass during the succession (Fig. 3 a).

Since the beetles live hidden in cavities of the carcass, a further analysis of the sexual behaviour was only possible at a laboratory simulated carrion using long- term video observations. In 19 video experiments, each with 5 pairs of A . curtulu freshly collected in the field (May to July, 190 individuals, 2252 registrations), we observed within a 24-h period an average sex ratio of 64 Yo males (p < 0.001) directly at the artificial carcass.

In the laboratory simulation of the natural succession we registered a balanced sex ratio in early stages of maggot development, whereas after migration of postfeeding blow fly larvae, the sex ratio directly at the carcass became male biased (Fig. 3 b).

3. Polygamy

The scarcity of copulations might be due to the behaviour of either males o r females. The capability of males to copulate repeatedly was tested by observing individual males together with three females (standard beetles) over a period of 4 d (time lapse). The three females were replaced daily by virgin ones. O n e to three females copulated per day. In total, 31 copulations were observed at an interval of 8.70 k 4.44 h (min 1.40 h, max. 18.15 h).

If individual standard females were observed together with 2 males, which were daily replaced by new ones, the copulation interval was 32.50 k 20. 81 h (min 10.87 h, max. 63.94 h, N = 13). These intervals are significantly longer than those for males with multiple females (p < 0.001).

Mature virgin females d o not immediately allow copulation with the first male ready to grasp (30-min observations). In 70 Yo of the encounters with

2 72 KI AU5 PESC I Ihl

grasping males (Table 2 ) , they show the typical repulse behaviour. However, most of the virgin females did copulate within the 30-min observation period

Repeated copulations of A. curtulu females are prevented by different mechanisms of male mate guarding and repulse behaviour of females. We could never observe a take-over of a copulating female by another male. The males release the females with a large spermatophore filling the genital chamber like a plug (PESCHKE 1978b). A . curtulu females were never observed copulating again while a spermatophore was still in the genital tract. The spermatophore is ejected at a variable time after termination of copulation (30 min-24 h) and is then eaten by the female.

Males encountering a female which still bears a spermatophore initially show the grasping response as usual. After contact of parameres with the posterior abdominal region of the female, however, the male releases the females' abdomen and pays her no further attention. The percentage of graspings is drastically reduced (Table2) and no such pair copulated during the observation period. Virgin females, which were freshly killed and contaminated with a spermatophore removed from another just mated female, also elicited the grasping response in the first instance. However, contact with the contaminated region led to interruption of the further sequence of male copulatory behaviour.

In the first 2 h after the first copulation, the females exhibit a very low level of repulse per grasping. One day later females are still as attractive as virgin ones. However, male copulatory attempts are then vigorously repulsed by her oscillat- ing abdomen and therefore the rate of copulation is significantly reduced in comparison to virgin females (Table 2 ) . In small boxes with single pairs (female mated one day before, virgin male), the female spends 60 k 21 70 (N = 14) of the observation time (15 min) in repulsing. During that time she is not able to feed.

(82 Y o ) .

11. Manifestations and Consequences of Competition

The probability of male-male encounters at the mating site of A . curtulu is very high and the males behave very aggressively towards each other. They bite at legs and antennae and beat their mobile abdomens on to the opponent. Two males observed for 15 min spent 18 k 9.4 % of the observation time with fighting (N = 12). Occasionally, legs or antennae are lost. Other injuries like crippled antennae may be due to the use of the defensive secretion from the abdominal tergal gland, which is transferred to the opponent by abdominal drumming. The secretion comprises noxious quinones together with aliphatic aldehydes and hydrocarbons (PESCHKE & METZLER 1982). The chemical quantification of the use of defensive secretions in male agonistic behaviour will be described elsewhere (PESCHKE & METZLER in prep.).

The most severe consequence of male agonistic behaviour seems to be the expulsion of the inferior from the carrion. At the laboratory simulated carcass we observed some males, which continuously patrol the carrion and try to expel every other male by the aggressive behaviour described above. With an increasing density of males in the total arena, the number of individuals directly at the

Male Aggression, Female Mimicry and I-emale Choice in the Rove Beetle 273

0 .

Fzg. 4: Presence of A . crrrtulu indi-

carcass depending on the total number of individuals in the arena (see text). Separate experiments for males and females; vertical bars: SD

of means (25 obs. per experiment)

vidunls at the laboratory simulated 15

,,, (3

2 s 10. 5 a,

c. 0 u) 0 3

> -0 C

- n 5 .- .-

T

I I I 1

carcass is not significantly increased. Only 3 - 4 males stay at the carcass at a time (Fig. 4). O n the other hand, females of increasing number in the total arena are allowed to crowd at the carcass.

111. Regulation of Competition

1. Variation in the Release of Aggression and Homosexual Grasping Responses

Intermale aggression was also demonstrated for field collected beetles. Individuals randomly sampled at carcasses exposed during the reproductive period from May to July (1980) were observed in the video setup at a laboratory simulated carrion (12 repeats with 5 males and 5 females). The percentage of encounters involving male agonistic behaviour varied considerably between 54 7'0 and 93 (Yo, and in 15 % of the male-male encounters homosexual grasping responses were registered.

For quantitative evaluation of the female sex pheromone titre we used the standard model bioassay with freshly killed beetles. A . curtula males and females (N = 158) were collected at 11 carcasses in the period from April to October (1978), immediately killed by freezing, and tested for release of the grasping response of laboratory tester males. With females as a control, a constant rate of grasping was obtained over the whole year (Fig. 5). Males from the field also released a median rate of 80 % homosexual grasping with a conspicuous interin- d i d i i a l variance. however For examnle. we collected at one carcacc (Mav 7) one

yielded a rate o f 93 YO.

274 KLAU PESCH~LI

0 .

0 C a .-

* 100 !?

", 80

< O 60

40

20

m ul

F

I P f Q

Fig. >; Release of copulatory responses ( % grasping) of standard A . curtiila males towards females and males col- lected in the field from carcasses exposed from Apr. to Sept. in deciduous forests in northern Bavaria. Each vertical line rep- resents collections from one carcass at one date with min. and max. response rates towards individual beetles (each tested with about 30 standard tester males); dots represent

medians

v Sept

2. Physiological Types of Males Releasing Homosexual Response

In the following experiments we investigated the various physiological types of males, which might release homosexual grasping responses by controlled variation of laboratory conditions.

Development and Nutrition

Old standard males which were sexually isolated and fed on blow fly maggots ad libitum do not release male sexual behaviour (Fig. 6a). As has been shown in previous studies (PESCHKE 1985), however, freshly emerged males of A . curtufa produce the female sex pheromone (Fig. 6b). Its titre is gradually reduced within the first two weeks of adult life (PESCHKE 1986a). O n the other hand, there was no difference in the decline of the rate of homosexual responses released by young males during development, if they were fed on maggots or on decaying liver (Fig. 7a). However, males which were starved from emergence on, released the grasping response of tester males at a significantly higher rate even 10 d after emergence (Fig. 7c). By the 14th day, all starved beetles were dead.

Thus, nutrition plays an essential role in the development of the pheromone content of young males. If old standard males were starved over a period of 10 d, they again release the homosexual response in tester males (Fig. 6 c). Control males which were fed up to the test date did not show any sex pheromone activity.

Copulations

If standard males were kept together with mature females for a period of 5 d, they released a high homosexual grasping response rate of tester males (61 %, Table 3). The mere presence of females or copulations may accelerate the

M a l e Aggression, Female Mimicry and Female Choice in the Rove Beetle 275

be havioural

dd - YY comparison

dd development

age (d)

d d nutrition

period of starvation (d I

66 previous copulations

Ncop -

N

release of grasping response

model bioassay ( v ~ ddgrasping~

IGR)

1001 ***

grasping response release of male stay at the towards pp aggression carcass

model bioassay video observations arena observations dd grasping) (% aggression/ Nindividuals at

G R I encounter carcass

I . .

release of female aggression

30 min-observations (%aggression / encountei % repulse/grasping)

9 100 ***

I241

100, ***

Nencounters "pol

Fig. 6: Various behavioural responses of A . curtula males and females towards other individuals of various sex and physiological status (for further explan. see text). For stay at the carcass: open columns: number of standard laboratory males; black columns: number of individuals of various physiological status. Significant differences to control: ':.''.':- p < 0.001, p < 0.05; Xz-test and Mann-

Whitney U-test

production of the female sex pheromone by males, on the other hand, contamina- tion by surface contact with females is also likely.

If virgin males (> 14 d old) were kept for 5 d in a box separated from females by a nylon screen (width of mesh 1 mm) not allowing physical contact but circulation of odours, the rate of release of homosexual grasping by these males is not increased (Table 3). Males, which were put together with rejecting females (first copulation one day before), exhibited numerous grasping responses provid- ing contact with females, but did not copulate within a 30-min period. After killing these males by freezing, they released the homosexual grasping response of tester males only at a rate of 6 YO (Table 3).

Immediately after their first copulation (30 min), the males elicited homosexual grasping in 38 % of the tester males (Table 3, Fig. 6d). Males, which had copulated twice within a 3-h period, released homosexual grasping in as many as 87 % of the tester males. The increase of pheromone titre in copulating males starts very soon: If a copulation was interrupted after 15 min, the males already released homosexual grasping responses at a rate of 19 %. Other males were

276

100-

80- m C 0. .-

; 60- m m a, 5 LO- € s

20-

0-

KLAUS PESCHKE

-

a homosexual release

of grasping response

100

80

e 60

.- F a m

m m 0)

5 40 E s

20

0

b grasping of young males

towards females

100

m 80 a

2 60

0 40 E s-

20

C

m

m m W

.-

n. Y

A 5 10

5 10

100

m a, - 40 z I

20

94

5

60 121

5

10

61 121

T

i0 age of males (d 1 age of males (d)

Fig. 7: Homosexual grasping responses (a, c) and grasping toward females (b, d) of A. curtulu males of various age (5-10 d postemergence) and physiological status in the model bioassay. a, b: Males fed on cut Culliphoru maggots or on beef liver; c, d: males fed on Culliphoru maggots or starved from emergence on. O n top of each column: N of tester males, o r (in brackets) N of freshly killed beetles tested; vertical bars: 95 YO confidence limits. Significant differences to the neighboring column: ’w* p

< 0.001; $test

decapitated immediately after genital linkage by pulling a noose of thin nylon thread tight around their neck and cutting off the head. Copulation is then terminated after a period of 30 min as usual, and a normal spermatophore is transferred to the females’ genital chamber. These decapitated males release the homosexual grasping response right after copulation at a significantly lower rate than control males. This experiment demonstrates that not a contamination via contact with the female causes the release of homosexual response by copulating males, but the male triggers neuronally or hormonally his own pheromone production or release.

Male Aggression, Female Mimicry and Female Choice in the Rove Beetle 277

Table 3: Release of homosexual grasping responses (GR) of standard A. curtulu males towards males of various mating history (freshly killed, model bioassay)

Freshly killed males

N encounters

% G R (pairs)

Males in scxual isolation Odour of females Mixed with females With repulsing female 1 copul., male head amputated 1 copul., artificially

interrupted after 15 min 1 copul., immediately after

termination (30 min) 2 copul. within 3 h

1 7 L

61 6 8

19

38 87

606 (22) 150 (5) 154 (5)

178 (6)

118 (4)

680 (23) 206 (7)

210 (7)

3. Male Sexual Activity

In order to prove whether the sexual activity of males is correlated with their own production of the female sex pheromone, we tested males of various physiological status for their grasping response towards standard females freshly killed by freezing.

Newly hatched males did not respond sexually towards females in the model bioassay (Fig. 6e). Their grasping rate increases within the first two weeks of adult life (PESCHKE 1986a). Feeding exclusively on blow fly maggots or on decaying liver did not cause differences in the development of sexual activity (Fig. 7b). However, males starved from emergence on showed only a few grasping responses even at the 10th day postemergence (Fig. 7d). If old standard males were starved for a period of 10 d, a significantly reduced number of them exhibited the grasping response towards females (Fig. 6 f). Males tested immedi- ately after one or two copulations within a 3-h period also showed the grasping response at a significantly reduced rate (Fig. 6 g). In summary, in all physiological conditions at which males were shown to produce the female sex pheromone their own sexual activity is reduced.

4. Males Avoiding Intrasexual Aggression

Do all types of A . cuvtula males which produce the female sex pheromone avoid intrasexual aggression, as was demonstrated for freshly emerged beetles (PESCHKE 1985)? The agonistic behaviour was analysed by video observations in the laboratory simulated carcass. Five mature and well fed males and females, formerly sexually isolated, were put together with 5 males of various physiologi- cal status. For each encounter with another individual, the response of the standard male was recorded.

Standard males behaved very aggressively towards each other but not towards females (Fig. 6 h). Freshly emerged males did not release agonistic behaviour of old males either (Fig. 69 . Mature males, which had been starved for

278 KLAUS PESC€lhL

a period of 10 d, released a significantly lower rate of aggression (Fig. 6k) as did males after one or two copulations (Fig. 61). Thus, all those males triggering homosexual grasping avoid intrasexual combats.

Is the expulsion from the carcass moderated by producing the female sex pheromone? We gave 5 marked standard males together with 5 other males of various physiological status into an arena for the usual density experiment. As a control, 10 mature, sexually isolated and well fed males were observed to yield a constant rate of 3 - 4 individuals staying at the carcass. The addition of 5 mature females, however, significantly increased the total number of beetles at the carrion to about 9, half of them being females (Fig. 6m). Freshly emerged males were also allowed to stay at the carcass (Fig. 6n). Standard males, which had been starved for a period of 10 d (Fig. 60), as well as multiply mated males (Fig. 6p), also increased the total density of A . curtula individuals at the artificial carrion. Thus, a reduced rate of release of intrasexual aggression along with the produc- tion of the female sex pheromone allowed these males to stay at the carcass without being expulsed.

IV. Female Choice

Females show aggressive behaviour towards other females: They push with their head and mandibles and chase the other individual over a distance of a few mm. Female aggression, however, never attains the intensity of male agonistic behaviour. We never observed emission of the defensive secretion (PESCHKE & METZLER in prep.) or physical injuries. Expulsion from the carcass was also not observed as demonstrated in the density experiments (see above).

In observations of single pairs for a period of 30 min, two females yielded a rate of 64 % aggression per encounter. In a heterosexual pair, females exhibited only little aggression against mature males (Fig. 6q). O n the other hand, old females chase freshly emerged males in 29 Yo of the encounters (Fig. 6r) . If young males show their first grasping response at the 5th day postemergence the mature females do not allow fixation of male parameres by aggression at this time. The repulse behaviour of females unwilling to copulate (abdominal oscillation) was also used as a criterion for female choice. Females respond with repulse to 32 % of grasping reactions of mature, virgin, well fed males. Males starved for 10 d, however, elicited a significantly higher rate of repulse (Fig. 6s), as was also demonstrated for males after previous copulations (Fig. 6 t). Thus, all those males releasing homosexual responses and avoiding intrasexual aggression, are more frequently rejected by females.

In order to evaluate the ultimate factors for the repulse of these types of males, we first investigated the effects of multiple copulations on the transfer of spermatophores and fertilization of eggs. The spermatophore of A . curtula con- sists of a sperm sac with a tube, which is inserted into a the spermathecal duct of the female (PESCHKE 1978b). This sperm sac is embedded in a gelatinous matrix. During the third copulation of a male within a few h in significantly less cases spermatophores are transferred (Fig. 8 a) and the weight of the spermatophores is decreased (Fig, 8b) due to the reduced amount of the gelatinous matrix. A

Male Aggression, Female Mimicry and €emale Choice in the Rove Beetle 279

significantly lower number of eggs is produced by females copulating with males, which have already mated twice (Fig. 8c). Especially the rate of hatching larvae is drastically reduced.

Similar tendencies are observed in copulations of young males (5 d old) and mature males after starvation for a period of 10 d. The percentage of beetles transferring spermatophores is not affected (Fig. 8 d), but the spermatophore weight is lower than in control males (Fig. 8e). The number of eggs and especially the rate of hatching larvae after copulation with young or starved males is significantly lower than in control experiments (Fig. 8 f).

Discussion

The mating system of the staphylinid beetle, Aleochuru curtulu, is strongly determined by the ecological features of the small place of rendezvous (PESCHKE 1986b). Individuals of both sexes are brought together in larger numbers by the odour of carrion. Here, the operational sex ratio (EMLEN & ORING 1977) is significantly skewed towards males, because (1 .) females leave the mating site within 1 day after feeding and copulation for egg deposition in the surroundings of the carcass, because (2.) males guard the females by prolonged copulations, spermatophores and anti-aphrodisiac pheromones, and (3.) because of the females’ reluctance. As a rough estimate, only 2 YO of the A . curtulu individuals at the mating site are receptive females (see also PESCHKE et al. 1987a).

As in many other insects (THORNHILL & ALCOCK 1983) the scarcity of receptive females seems to be the driving force of intermale competition. The benefit of male aggression in A . curtulu consists in the maintenance of a position at the mating site and the reduction of the number of would-be competitors for the scarce receptive females. O n the other hand, the costs of aggression in terms of time consumption, energy (abdominal drumming), material (defensive secre- tion), and risk of injuries seem to be very high (for review see also: THORNHILL & ALCOCK 1983).

As a further consequence, however, the access to the essential food resource is affected by male aggression as well, because in the carrion system mating and feeding site are identical (PESCHKE et al. 1987a). Freshly emerged beetles only mature when food is available and old ones do not survive a 10-d period of starvation. Thus, the long-lived beetles moving from one carcass to the other (PESCHKE et al. 1987a) need to replenish energy reserves. Since the weight of the spermatophores decreases after multiple copulations, intake of food is also necessary for these males.

The common trait of all males which need access to the food resource is their production of the female sex pheromone, which is positively correlated with the reduction of intermale aggression (for chemical data see PESCHKE 1987). In periods of nutritional stress and reduced sexual activity the males are able to settle at a carcass and restore their substance and energy resources for a forthcoming reproductive cycle. Males have to find an optimal balance of avoiding intermale aggression by producing the female mimicking pheromones, and on the other

eggs p b-h .*d

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Male Aggression, Female Mimicry and Female Choice in the Rove Beetle 28 1

hand, of being accepted by females by not producing the female sex pheromone. The fine tuning of this balance can be varied several times during the life history of the long lived individuals.

The absolute identity of the female sex pheromone and male chemicals (PESCHKE 1985, 1987), which release homosexual responses, allows the female- mimicking males to deceive their competitors successfully.

For and

Feeding and copulating are not temporally separated by sharp boundaries. example, starved o r multiply mated males, which have reduced sexual activity which are more likely to be rejected by females, are occasionally observed to

gain some sneaky copulations. Thus, female-mimicking males, which steal in the carrion system have also some immediate reproductive success and thus perform an alternative mating tactic in the sense of DOMINEY (1984).

Males have various options when arriving at a carcass (see also dung flies at a resource of transient nature: PARKER 1970a; PARKER & STUART 1976). They may stay ( I ) at the carrion itself, ( 2 ) in its close surroundings (small male strategy: PIXHKE in prep.), o r (3) leave the carcass to search for another one. In the last case, the decay process at the carcass may have proceeded too far to provide blow fly pupae suitable as hosts for parasitation by Aleochara larvae. Males, however, which copulate with transitory females in late successional stages, may gain reproductive success when the female uses some of his sperm to fertilize eggs at the next carcass to be occupied.

In addition to competition for females prior to copulation, A . curtula males also assure paternity by three mechanisms of guarding. Prolonged copulations protect the female for at least half an hour (see also: PARKER 1970b; THORNHILL 1976; MCLAIN 1981 ; SILLEN-TULLBERG 1981). A plug-like spermatophore physi- cally prevents further copulations for several h (for review see THORNHILL & ALCOCK 1983). Anti-aphrodisiac pheromones transferred by the spermatophore, make the female chemically less attractive for about 1 day (see also GILBERT 1976; FRANKII et a]. 1980; KUKUK 1985; MAUSER & PESCHKE 1986).

During the first day after copulation, when the females’ own time-consum- ing repulse behaviour is reduced, she is able to feed at the carcass without being pursued by other males. After leaving the carcass for its surroundings, the probability of encountering a male is drastically reduced and the females have then developed their own rejection response. According to mark and recapture experiments (PIXHKE et al. 1987a) some of them occasionally return to the carcass, feed and produce a further batch of eggs (ca. 30/d). Since the effect of the anti-aphrodisiac pheromone is already gone then, the females have the option to mate again. However, as a maximum only 2-3 copulations of a female may occur at one carcass.

Reluctant females especially reject young, starved or multiply mated males which bear the female sex pheromone. All these types of males transfer signifi- cantly smaller spermatophores and need to replenish energy and substance. As in other insects (for review see: MARSHALL 1982; GWYNNE 1984) the large sper- matophores of A. curtula (PESCHKE 1978 b) probably provide nutrients because they are eaten by the females after reception of sperm. Spermatophores also

282 KLAUS PESCHKE

provide a plug and anti-aphrodisiac pheromones. In the case of young, starved or multiply mated males, the number of eggs or hatching larvae is significantly reduced, which may be caused by less sperm transferred or by reduced accessory gland products. It also can not yet be excluded that females limit the number of eggs laid or being fertilized by sperm of inferior males.

O n the other hand, males which don’t bear the female sex pheromone are mature, well fed and have not recently copulated and thus have stores of material for a large spermatophore. Additionally, these males have succeeded in numerous aggressive interactions with other males. By this pheromonal criterion of mate assessment the females do not only choose a physiologically competent male but probably also exclude genetically deficient males which are not able to populate the carrion system.

Acknowledgements

I am very grateful to Prof. Dr. K. E. LINSENMAIR for critical discussion and I wish to thank Prof. Dr. D. FULDNER for cooperation in the study of the ecology of Aleocbara species. The quantity of behavioural data could only be obtained with the skillful technical assistance of Mrs. C. GANTERT over several years. I also gratefully acknowledge the help of Mrs. J. HOFMANN with fly and beetle cultures. Thanks are due to Mr. J. MAUSER for statistical analysis. I am very grateful to Dr. A. E. JURSS for reading the English manuscript. The Deutsche Forschungsgemeinschaft continuously provided financial support of the long termed projcet (SFB 4/B5 and Pe231/4, 1 4 ) .

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Author’s address: Dr. Klaus PESCHKE, Zoologisches Institut 111, Lehrstuhl fur Tierokologie, Universitat Wiirzburg, Rontgenring 10, D-8700 Wurzburg.