BEHAVIOR Sexual Behavior of Acanthocephala declivis guatemalana (: ) and the Allometric Scaling of their Modified Hind Legs

WILLIAM G. EBERHARD Smithsonian Tropical Research Institute, and Escuela de Biologfa, Universidad de Costa Rica, Ciudad Universitaria, Costa Rica

Ann. Entomol. Soc. Am. 91(6): 863-871 (1998) ABSTRACT The sexually dimorphic hind legs of Acanthocephala declivis guatemalana are used for different functions in males and females, and they scale differently on body size. The hind legs of males are used to deliver powerful squeezes to other males in battles at feeding and mating sites, and the diameter of the hind femur and the length of its large tooth are proportionally larger in large males compared with other body characters. The largest tooth on the female hind femur probably functions in the less forceful female-female battles, and it is also proportionally larger in large females. However, neither the female hind tibial plate, which is used as an aggressive signaling device, nor the diameter of the female hind femur are proportionally larger in large females. Wing wear and breakage of hind leg spines, combined with behavioral observations of marked bugs, indicate that the breeding system combines elements of resource defense polygyny and lek polygyny. Females mate more than once, despite the attendant cost of reduced feeding opportunities.

KEY WORDS Acanthocephala, Coreidae, sexual behavior, weapons, visual display traits

IT IS GENERALLY thought that both visual display struc- Materials and Methods tures and weapons under sexual selection have high All observations were made between 24 and 31 July allometric values (slopes of >1.0 in log-log regressions 1997 at the Chamela field station of the Universidad on indicators of body size) (Huxley 1972; Alatalo 1988; Autonoma de Mexico in Jalisco, Mexico (elevation =»50 Petrie 1988,1992; Green 1992; Burkhardt et al. 1994). m). Bugs were observed on the trunks and branches of The data supporting this idea are relatively sparse, young (=«20-30 cm diameter at breast height [dbh]) however (see summary in Eberhard et al. 1998, also Cordiaelaeagroidei (Borraginaceae) trees planted on the Wilkinson 1993, Goddard and Mathis 1997, Wilkinson station grounds. Bugs were given individually recognize- and Dodson 1997). In the case of visual display char- able marks on the pronotum with dots of colored fast- acters, the observations are largely limited to verte- drying paint while they rested unrestrained on the lower brates (mostly birds). It is thus of general interest to portions of trees. The trees were 10-15 m tall and bugs determine the allometric values of display structures sighted in the upper portions could not be checked for and weapons in other groups. marks (and sometimes could not be sexed). A. declivis is Many coreid bugs have modified hind legs that are large (body length «*3 cm) and moves relatively slowly thought to function in interspecific interactions. The and tolerates slow approaches of an observer to within most typical modification is an expansion of the hind ^30 cm. It was thus possible to observe many behavioral tibia into a more or less flattened and often brightly details in the field. colored plate or flag. The hind femora are also some- Surveys of bugs present on 4 adjacent trees (*=«5 m times thickened and adorned with spines (Schuh and apart) were made at irregular intervals separated by at Slater 1995) and serve as weapons in male-male com- least 2 h. There were 15-20 other trees on the station bat (Fujisaki 1980,1981; Mitchell 1980; Miyatake 1993, grounds; only 5 of these other trees had any bugs on 1995,1997). Males and females of some Acanthoceph- them during the study. The populations on the 4 study ala Lap. offer an interesting chance to study these trees were much larger than those on any other tree types of modifications: females have prominent tibial at the station. plates but relatively thin, toothed femora; whereas All measurements (Fig. 1) were of bugs collected at males have swollen hind femora with prominent teeth, the station, and include both specimens in the collec- but reduced tibial plates. The current study describes tion at the station and additional voucher specimens. the sexual behavior of Acanthocephala declivis guate- The ages of specimens were estimated according to malana Distant as it relates to hind leg use, and the the amount of forewing membrane torn by making a scaling of hind leg structures on body size. drawing of the outline of each front wing's margin and 864 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 91, no. 6

wear on the teeth on each hind femur was estimated by counting the numbers of teeth in which (1) the extreme tip was broken off, (2) a more sizeable por- tion of the tip was gone, or (3) at least half of the tooth was gone. A 3rd estimate of wear involved the per- centage of the small spines at the tips of the tibiae that were at least half missing. Four counts were made for each animal (prolateral and retrolateral views of each of the 2 tibiae), and the average of these 4 percentages Female was used as the hind tibia spine value.

Results Behavior. Bugs were found only on the trunks and branches of the C. elaeagroidei trees or on leaves and branches (but not trunks) of nearby vegetation. Sex- ual activity occurred only on C. elaeagroidei. Copu- lating pairs were noted at sunup (0730 hours), during the day, and up to 3 h after sundown (at 2400 hours). Fig. 1. Hind legs of a male and a female A. declivis, showing sexual dimorphism and the portions that were measured. Two copulations were observed to begin at 2230 hours. Copulation lasted up to ^90 min, so mating (and probably also fighting and feeding) continued rips in the membrane, labeling the backs of the draw- throughout the day and night. There was no obvious ings with specimen numbers, cutting out and then tendency for copulation to be especially common at mixing all the drawings, and then sorting them into any particular time of day. categories of wing damage (Fig. 2). The mean of the Both males and females inserted their mouthparts values for the 2 wings was calculated for each speci- for up to 30 min into the trunk of C. elaeagroidei trees. men. The estimates of forewing damage were thus In some cases a 1- to 2-mm-diameter circular area of obtained blind with respect to other traits. A femoral the bark around the point of insertion became wet spine wear value was also assigned to each hind leg, (Fig. 3), and often ants and occasionally vespid wasps and these were combined for each specimen. The clustered around such a feeding site (both while the

Fig. 2. Typical representatives of the 6 categories of wing damage of male A. declivis (black portions were missing). November 1998 EBERHARD: SEXUAL BEHAVIOR OF Acanthocephala declivis guatemalana 865

Females that had been resighted once were also less likely to remain for a further day after being resighted than were males. None of the 4 females was resighted on any further days, whereas 11 of the 17 resighted males were sighted at least 1 additional day (P = 0.0351, Fisher exact test). Summing all sightings of bugs during 28 surveys on 7 d, there were 659 sightings of males, and 552 of females on the 4 study trees (an additional 99 sightings involved bugs whose sex could not be determined because they were too high in the trees). Assuming that the bugs whose sex was not determined were a random sample of males of and females, the ratio of males to females was 1.19:1.0. Of the males and females sighted 31.0 ± 11.5% and 36.7 ± 10.4%, respectively, were copulating. The distribution of bugs on the study trees was strongly clumped. Thus, on 24 July there were at least 22 on 1 tree, and only 2 and 3 on the 2 adjacent trees (/ = 32.7, df = 2, P < 0.001). Relative population sizes on different trees varied over time. For instance, 4 d later the same 3 trees had 6, 23 and 1 bug respectively (G = 31.2, df = 2, P < 0.001 with respect to 1st counts). Courtship and Copulation. In 6 cases I saw a male perform series of brief (M3.5- to 1.0-s) bouts of quivering or body vibration while standing alongside but not touching a female, or facing her just before attempting to Fig. 3. A feeding female A. declivis raises her near hind mount. Bouts of vibration occurred both in rapid suc- leg in the warning posture as a male approaches her (drawn cession, and up to >30 s apart. In other cases a male from a photograph). mounted without any obvious preliminaries. Females often resisted mounting, by raising their hind legs, at- bug's proboscis was inserted and afterward). Several tempting to push the male away, or walking away. One "wet" feeding sites were used repeatedly by several male succeeded in squeezing past the female's raised different bugs over several days. Some feeding sites hind leg and onto her dorsum, but in 17 other cases in were at apparently undamaged sites of the trunk, but which female resistance was observed carefully, she suc- most occurred at fissures in the bark, or where the ceeded in leaving a male who attempted to mount her. outer bark had been recently scraped away. The bark Males sometimes pursued retreating females for up to of C. elaeagroidei is very thin, with <1 mm of outer 20-50 cm along the trunk, but none of these mounting layer covering the green inner layer which is also <1 attempts was successful. In no case did premount be- mm thick. Bugs sometimes inserted their mouthparts havior involve obvious display of the male's hind legs or 4 mm or more into the trunk, so their mouthparts must grasping of the female. have either passed through these layers and deeper Once a male had mounted, he moved around on the into the trunk or turned sharply. female with characteristic jerky movements until he Population Movements. Many marked bugs were was facing the same direction she faced. The male then resighted on several days, often on the same tree or, moved posteriorly and leaned laterally, rotated his less often, on trees up to ^10 m away. Males were seen genital capsule, and attempted to press it against the up to 7 d after being marked, and females up to 5 d after genitalia of the female. In 1 case the male vibrated his being marked. Males were resighted more often than body after mounting and before initiating genitalic females. For instance, 17 males and 23 females were contact. On 7 occasions a female lowered her abdo- marked during the evening of 25 July and the after- men to the surface of the trunk and extended her hind noon of 27 July; during the evening of 27 July and the legs posteriorly alongside her abdomen, forming a days of 28 and 29 July, 16 of the 17 males but only 4 of barrier to genital contact. When a male succeeded in the 23 females were resighted (/ = 23.0, df = 1, P < pressing his genitalia against those of the female, he 0.001). Even soon after, a bout of marking, however, remained immobile for 20-60 s, then dismounted and many unmarked males were seen. Thus, the day after turned to face away from the female, with his genitalia 12 males were marked, only 6 were resighted and at coupled to hers. Only 2 out of 26 males that had least 14 additional, unmarked males were present (this established genital contact with a female failed to number of unmarked males is probably an underes- couple. At no time did courting males perform any timate because it represents the maximum number behavior that displayed their hind legs to the female. confirmed to lack a mark; some of the bugs that were During copulation males were not aggressive. When too high in the tree to see if they had marks or were another bug approached or mounted a pair in copula, additional unmarked individuals). the copulating male generally moved away, or pressed 866 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 91, no. 6 his body to the surface of the trunk. Sometimes a often difficult to decipher, but they differed from male copulating male raised 1 hind leg as in female warning squeezing behavior (below) in 2 respects. Females did behavior (below); this was the only context in which not align their bodies with the tips of their abdomens males performed this behavior. Six copulations lasted pressing each other, and the hind leg squeezes were <10 min, 4 lasted 11-30 min, and 11 lasted >30 min. brief grasps, not sustained powerful squeezes as in The longest duration was 91 min. Both males and males. In 2 cases a female drove another female from females were generally quiet during copulation, and 1 a feeding site, and then fed there herself. or both often fed. Both sexes mated repeatedly. Three Male Aggressive Behavior. In contrast to females, ag- marked females and 2 marked males were seen in gressive males very seldom raised their hind legs. Low copula with >1 mate on the same day. level aggressive interactions involved moving jerkily Three male behavior patterns that may constitute cop- rearward or sideways toward another male, and opening ulatory courtship were noted. In several pairs the male and extending both hind legs laterally and rearward (Fig. buzzed his wings briefly while the female walked and 4). Usually the legs were held relatively low, near the pulled him, or immediately after being pulled. In 1 case trunk, and they often made tentative pawing motions a male buzzed once when the female was immobile and that may have served to locate the opponent. Mutually there were no other bugs nearby. Two males rhythmi- aggressive males backed and sidled toward each other, cally pulled and twisted on the female with their geni- gradually coming into tail-to-tail alignment. In at least 3 talia. One male performed periodic bouts of body vibra- battles the males' abdomens (or perhaps their entire tion similar to those seen before mounting for at least 45 bodies) vibrated rapidly at low amplitudes during these min., with each bout lasting <1 s. Quivering seemed to apparent attempts to align themselves. follow periodic attempts by the female to pull the male Many interactions broke off before alignment oc- or end the copulation. This behavior was only noted at curred. Further escalation generally involved more ste- the end of the study, so it was not certain whether similar reotyped behavior. The males first aligned themselves behavior occurred in other pairs. with their spread hind legs meshed. The tibiae crossed Copulation ended when the bugs pulled apart, ap- and the femoral-ubial joints were close to touching (Fig. parently with little force. Separation was often imme- 4). Then each male raised the tip of his abdomen and diately preceded by the female briefly tapping or moved it dorso-ventrally and laterally in an apparent scraping the male's dorsum with her hind legs 1 or attempt to contact the tip of the other male's abdomen. more times. Either the tips of her tibiae or their ventral 1 never noticed any movement or extension of the gen- surfaces briefly contacted the dorsal surface of the italia during this behavior. In the highest intensity inter- male's forewings (which rested on his abdomen). Usu- actions (N — 11), each male pressed the raised tip of his ally these female movements were brisk taps or abdomen against the tip of the other male's abdomen for pushes, but occasionally they were forceful, and may a second or 2 (Fig. 4), possibly forcefully. Then each have caused extension of the membranes basal to the male quickly reached dorsally with both of his hind legs, male's genital capsule. and gave his opponent's abdomen 1 or a few quick pow- In no case did a male appear to guard a female after erful squeezes. The ventral surfaces of both tibiae were copulation. In 4 carefully observed pairs the 2 bugs on the dorsal surface of the opponent's wings and the simply walked away from each other soon after sep- hind femora along the sides of his abdomen. In some arating. cases both males immediately fell from the trunk after Female Aggressive Behavior. Low intensity female squeezing, separated in mid-air, and then 1 or both flew "warning" behavior was generally elicited by the ap- back onto the trunk. In 1 case the alignment between the proach of another bug. Warning consisted of lifting the 2 changed during a flurry of squeezing, and 1 male ended hind leg on the side from which the other bug ap- up being clamped by the other's hind legs and held out proached (or, less often, both hind legs) and holding of contact with the trunk; the clamping male held him for it more or less extended dorsally, and often waving it a second or so, then released him and allowed him to fall. for up to 10-20 s (Fig. 3). Sometimes warning ap- The loser flew back to the trunk and landed ^1 m below. peared to be defensive, as the female leaned away This was the only fight in which I sensed the odor of the from the other bug. In other cases she swung both her bug's defensive substance. hind leg and her abdomen toward the other bug, and If 1 of the males was substantially smaller than the then struck downward with her hind leg, sometimes other, there was a variable amount of mutual jockeying softly tapping, and others forcefully swatting the other in which the smaller male failed to align himself pre- bug with the tip of her tibia. One female hit another cisely, and then moved away. Small males frequendy so hard she knocked her off the tree. If the other bug executed the initial stages of aggression, but were not was a male, he generally did not respond aggressively seen to squeeze. It was not determined whether such to female warning. If it was a female, she either moved aggressive harassment occurred near females with which away, often after raising her own hind leg, or tapped the small male had recently mated as if it was designed and kicked with her hind legs. to divert the larger male as seen in the neriid flies Glyphi- In high intensity female battles, each bug oriented dops and Nerius (Eberhard, in press). the rear of her abdomen toward the other and pawed Solitary males often spent many minutes "harassing" or swatted with her hind legs. In 4 cases, 1 or both copulating pairs. Commonly, the solitary male females briefly grasped the opponent's abdomen with mounted the female, tilted his body, moved rearward, her hind legs. The leg movements were rapid and and pressed his genitalia against her abdomen in an November 1998 EBERHARD: SEXUAL BEHAVIOR OF Acanthocephala declivis guatemalana 867

A

Fig. 4. Schematic drawings of high intensity male-male aggressive behavior in A. declivis (drawn from photographs and field notes). (A) Dorsal view of as each male backs and sidles toward the other with spread hind legs in an apparent attempt to align his abdomen with that of the other male. The males' abdomens and hind legs are close to the substrate. (B) Lateral view as both males raise their abdomens which are about to touch; their spread hind legs are in contact. Force may be exerted as rearward pushing, or ventral or dorsal pushing on the other male's hind legs once the tips of the abdomens touch. (C) Lateral view as each male has raised his hind legs to grasp and squeeze the other's abdomen. apparent attempt to copulate. Other males behaved most always directed toward other males, and was more aggressively with copulating pairs. In 1 case a consistently associated only with the presence of an- male mounted a copulating male, and gave his abdo- other male. Even when a female was nearby, her men a quick squeeze and a simultaneous sharp lateral presence seemed irrelevant to fighting males. For in- jerk with his hind legs, separating the copulating pair. stance, 1 male that had been courting a female was The attacking male immediately attempted to mount driven away by a larger male which then also moved the female as she moved away, and he chased but away, leaving the female alone. Winning males often failed to mount her. Most male aggression toward chased losers for up to a meter or more before re- copulating pairs was more difficult to interpret, and turning to their original positions. was not obviously designed to force pairs to separate. Male aggressive behavior thus seemed to represent In addition, to repeated mounting, it often involved attempts to maintain the area around the male free of sidling or backing toward the pair; occasionally the other males, and perhaps thus allow courtship and cop- male gave a sharp squeeze (at least once to the female) ulation to occur uninterrupted by other males. This tactic and immediately dismounted. was not very successful when there were numerous bugs In >10 cases male harassment appeared not to have on the trunk. Persistent small males often returned im- any effect on copulating pairs; in 21 others it appar- meditately after being chased away to again back ag- ently induced the pair to walk, sometimes leaving a gressively toward the large male, but then moved away female's feeding site. On 11 occasions such displace- again before he was able to align himself for a squeeze. ment was followed within <60 s by the female appar- Sometimes a male that was busy defending his region on ently attempting to terminate copulation by pushing the trunk failed to take advantage of the presence of or tapping repeatedly on the male's dorsum with her receptive females. The most dramatic illustration of in- hind legs. In 8 of these cases the pair then separated. effective male aggression occurred when 1 large male In at least 2 cases the harassing male was substantially spent much of 4 h threatening and attacking other males smaller than the copulating male, but nevertheless on an «=2 m segment of the trunk. Although this male apparently induced the pair to leave and separate. won all aggressive interactions, he failed to obtain a single Contexts of Aggressive Behavior. Females fre- copulation, even although ten copulations involving quently performed warning displays with their hind other males occurred on this portion of the trunk during legs, both toward other females and, probably less this time. often, to males. Female aggression usually occurred Morphology. The male's hind femur is thicker than near feeding sites. Male aggressive behavior was al- the female's, especially in lateral and ventral views 868 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 91, no. 6

^ a. front leg b. tibial plate E "

BE* "c 5^ O 9

C • • •

Width pronotum (mm) Width pronotum (mm) c. hind femur (lateral) d. hind femur (ventral) ":'ir 1! • . ^ Si , C "> c > •-C o 2 • • O 0 o»oo . ° £ > o o o

12 14 12 14 Width pronotum (mm) Width pronotum (mm) e. spine f. shape hind femur li . • * i- Z£ 2 9- "O d)

oo o oo o o%0o 12 14 Width pronotum (mm) Width hind femur in lateral view (mm) Fig. 5. Relations between different body dimensions of A. declivis (solid dots, males; open dots, females), illustrating lack of differences between males and females in a body trait not related to aggressive interactions (a), sexual dimorphisms in fighting (b, c, d), and signaling (e) structures. The relative proportions of the lateral and ventral widths of the male hind femur were the same as those of the female (f).

(Fig. 1). The teeth on the ventral surface are all pro- The relative sizes of different portions of the hind portionally smaller than those of the female except for legs of males and females differed. The slopes of log- 1, which protrudes substantially beyond the tips of the log regressions of other body parts, calculated using others. The female hind tibia, which has the same pronotum width as an indicator of body size, corre- brown color as the rest of her body, has planar ex- sponded to the values of a ("allometric values") in the pansions of both the dorsal and ventral edges of the allometric equation basal portion. The sharp angles of the dorsal and ven- y = b + mx" tral portions of the female tibia occur nearly opposite each other, and may delimit a visual "flag." The male (x is the width of the pronotum). When a for a par- hind tibia, in contrast, has a relatively small dorsal ticular character is >1, that character is disproportion- expansion and completely lacks a ventral expansion. ately large in large . Two male characters that are Small sections of the dorsal tibial plates were broken involved in generating force for squeezing with the hind or missing in 5 of 32 males and 6 of 27 females. The legs (width of hind femur in lateral and ventral view) had ventral tibial plate was not damaged in any female. allometric values substantially >1 (Fig. 5; Table 1). An- November 1998 EBERHARD: SEXUAL BEHAVIOR OF Acanthocephala declivis guatemalana 869

Table 1. Allomctric values (slopes of regression of log-transformed values) using 2 alternative indicators of body size, prothoracic widtli, and head width for different male and female body parts (JV = 30 males, 18 females)

Slope ± standard error Body structure On prothorax width On head width Male Female Male Female Width thorax 1.51 ± 0.20 1.37 ± 0.36** Width head 0.45 ± 0.06*** 0.34 ± 0.09** — — Width wing membrane 0.57 ± 0.10*** 0.5 ± 0.20* 1.07 ± 0.18*** 1.13 ± 0.39* Length last antenna] segment 0.17 ± 0.11 0.43 ± 0.19* 0.50 ± 0.19* 0.60 ± 0.41 Length front femur 0.52 ± 0.08*** 0.47 ± 0.17* 0.90 ± 0.16*** 0.98 ± 0.34* Width hind tibial plate 0.28 ± 0.13* 0.57 ± 0.34 0.62 ± 0.24* 1.12 ± 0.68 Diameter hind femur (lat.) 2.15 ± 0.26*** 0.48 ± 0.25 3.92 ± 0.48*** 3.06 ± 0.73*** (vent.) 1.98 ± 0.34*** 0.50 ± 0.37 4.12 ± 0.54*** 3.93 ± 1.27** Length spine hind femur 3.89 ± 0.32*** 0.38 ± 0.84 5.22 ± 0.66*** 3.26 ± 1.41* Length hind tibia 0.37 ± 0.09*** 0.304 ± 0.16 0.74 ± 0.15*** 0.65 ± 0.32

Significance of slopes are ***, P < 0.001; **, P < 0.01; *, P < 0.05. other male fighting trait, the large spine on the hind Because warning behavior involving dramatic hind leg femur, had an even higher allometric value. Females had displays was performed almost exclusively by females, lower allometric values for all 3 of these characters (P < the tibial expansions that occur only in females prob- 0.001, t-test of Zar 1984), although the value for the ably function as visual signaling devices, as probably femoral spine was also much higher than 1. also occurs in males of the coreid Leptoglossus australis The pattern of allometric values of tibial characters in F. (Miyatake 1993). The low allometric value of the males and females was different (Fig. 5; Table 1). The width of this flag means that its width is a relatively allometric value of the width of the female tibial flag, poor indicator of female body size. Given that a fe- which included both dorsal and ventral extensions of the male's opponent was only occasionally properly ori- tibia (Fig. 1), was very close to 1.0, while that of the male ented to view the flag laterally and thus appreciate its (which included only the dorsal extension) was dramat- true size, the low allometric value is perhaps not sur- ically lower (P < 0.001, t-test of Zar 1984). The length of prising. Threatening females generally failed to move the tibia of both sexes had an allometric value similar to before or during warning displays so as to bring their those of other body parts such as the width of the wing legs more broadside with respect to the opponent and and the length of the front femur. Use of an alternative thus provide a more lateral view of the flag. indicator of body size, head width, to scale these struc- The most intense fighting behavior of males differed tures gave generally higher allometric values but the from that of females in that it involved powerful same relative patterns (Table 1). squeezes of the opponent's abdomen. The thick femur Age Indicators. Mean wing wear values were greater and large femoral tooth of the male probably represent in males than in females (5.29 ± 3.54, N = 30 and 3.21 ± adaptations to deliver strong, punishing squeezes to 4.73, N = 18, respectively; P = 0.00197, Mann-Whitney other males. The wear and breakage on the femoral U test). The wear values for femoral teeth and tibial teeth of males suggests that the frequent forceful bat- spines, in contrast, did not differ between the sexes tles observed in this study are typical. My occasional (4.48 ± 3.56 versus 2.86 ± 3.86, P = 0.078 for femoral observations of males lacking 1 hind leg and the limp- teeth; 40.6 ± 31.1% versus 25.2 ± 18.9%, P = 0.145 for ing gait of 1 hind leg observed in 2 males may also have tibial spines, Mann-Whitney Utests). resulted from damage inflicted during battles. The There was a significant positive correlation between high allometric values of male hind leg traits suggest male size (as indicated by prothoracic width) and tibial that there has been strong selection favoring male spine wear (r = 0.574, P = 0.00174). There was also a fighting ability, as demonstrated in the coreid Margus positive but nonsignificant trend comparing male size obscurator (F.) (McLain et al. 1993). and wing wear (r = 0.29, P = 0.13). The corresponding The low allometric values for male tibial plates are values in females were 0.608 (P = 0.021) for tibial spine consistent with their lack of use in visual displays. Per- wear, and 0.072 (P = 0.771) for wing wear. Thus, large haps this tendency for large males to have dispropor- males and females both showed more tibial spine wear, tionately small tibial plates is related to the use of the hind but no significant differences in wing wear. legs as weapons. A hind tibia unencumbered by a large plate may be inserted more easily between the oppo- nent's legs, or between his legs and the dorsal surface of Discussion his abdomen to deliver a squeeze to the opponent. Much of the sexual dimorphism in Fig. 1 can be The shorter, but prominent teeth on the female understood in light of the behavior described above. hind femur probably function in grasping battles be- The male hind legs played no role in courtship; in- tween females, but their exact mechanical significance stead, male body vibrations, which also occur in male- is not clear. The common damage to female femoral male battles, may be important, as in the coreid Rip- teeth and tibial spines again indicates that intense tortus clavatus Thunberg (Numata et al. 1986). female battles occur frequently in nature. 870 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 91, no. 6

Some of the patterns of scaling on body size in A. legs, and partially spread them, possibly in the spread declivis are similar to those in another coreid, Lepto- position observed in A. femorata. glossus australis F., in which the male hind femora Both marked male A. femorata (Mitchell 1980) and (especially their width) were also disproportionally A. declivis tended to remain at particular sites. The large in large males but not females (Miyatake 1997), resident ratio of males to females in A. femorata was and in which males also use their hind legs as weapons 0.47, very similar to the value of 0.54 for A. declivis. in (Miyatake 1993). Female L. australis use their hind Male A. declivis were probably not defending sites legs to tap aggressively on other females in contests for that were attractive to females only because of the feeding sites. Hind leg design thus suggests that, as in possibility that the female could feed there, as seems A. declivis, powerful squeezes are more important in to occur in other coreids such as Margus obscurator male-male than in female-female battles in L. austra- (McLain et al. 1993). Females did not remain long on lis, although the behavioral descriptions mention only the study trees, the bugs were unequally distributed "fencing" and attempts to "wrap" the hind legs around among inhabited study trees, the distribution among the opponent (Miyatake 1993, 1997). trees changed sharply over only a few days, and most The high allometric values in A. declivis of those C. elaeagroidei trees observed had no bugs on their characteristics of the hind femur that are related to trunks. Short stays by females might be explained if fights with conspecifics resemble the similarly high only a brief period of feeding could satisfy their nu- values for weapons in other organisms such as horned tritional requirements (e.g., if C. elaeagroidei provides beetles (summary in Eberhard et al. 1998), fiddler some rare but essential nutrient [Belthoff and Ritchi- crabs (Huxley 1972), and horned ungulates (Lincoln son 1991]). The periodic aggressive interactions be- 1994). The low values for female hind femora widths tween females could be a reason for females to aban- are in accord with the absence of squeezing behavior. don inhabited trees, but if aggression were the only The high allometric values for female hind femoral reason for abandonment, the females would have been teeth suggest that they are important during fights. expected to move onto nearby, uninhabited trees, The lack of male guarding behavior after copulation which generally did not occur. More likely, volatile was unexpected, because females mated more than once, attractants released by males induced bugs to aggre- as was predicted for A. femorata (F.) (Thornhill and gate, as occurs in other coreids (Aldrich et al. 1993, Alcock 1983). The inability of large males to drive other Blatt and Borden 1996). males from a portion of the tree trunk where several bugs Thus, females fed at the sites where males fought and were present may be offset by their ability to inflict where copulation occurred, but their attraction to these physical damage to opponents with the large femoral sites was probably also due in part to the presence of tooth, rather than simply displace them, as occurs in other bugs. Seen from the male perspective, the mating many horned beetles (Eberhard 1979). Males of A. femo- system seems to comprise both resource defense polyg- rata and Margus obscurator were relatively successful in excluding other males from small plants (Mitchell 1980, yny and lek polygyny, as also occurs in the coreid N. McLain et al. 1993), whereas Notobitiis meleagris F. males meleagris (Miyatake 1995) and perhaps Galonus tasmani- on bamboo shoots were less successful, especially at high cus (Le Guillou) (Steinbauer 1997). By mating at sites population densities (Miyatake 1995). with other bugs present, females may be exercising "in- My observations suggest that the aggressive behav- direct" choice favoring males better able to defeat other ior of A. declivis is similar to that of A. femorata (Mitch- males (Wiley and Poston 1996). Similar clustering of ell 1980), Acanthocoris sordidus Thunberg (Fujisaki both males and females on particular host plants occurs 1981), L. australis (Miyatake 1993), and N. meleagris in other such as acridid grasshoppers (Greenfield (Mitatake 1995) in that the large hind legs of males are 1997), the anthophorid bee Centris adoni Fox (Thornhill used as weapons to squeeze other males near feeding and Alcock 1983), and the andrenid bee Perdita pipiyolin sites that are attractive to females, and that large males Snelling and Danforth (C. Dominguez, C. Cordero, and usually defeat smaller males. Some apparent differ- W. Eberhard unpub.). Leks in some insects are associ- ences in the details of the battles between the 2 species ated with nearby resources (Shelly and Whittier 1997). of Acanthocephalus may be due to different emphases Females rejected some mating attempts of males in the 2 studies. Mitchell (1980) described male-male present on the trees, although it was not clear whether "fencing" matches in which each male attempted to or not these rejections were biased against males with wrap his hind legs around the abdomen of the other particular traits. Because females mate with several bug and squeeze him, and illustrated a combat position males, cryptic female choice may also occur, and the with 1 male apparently being clamped and lifted by possible male copulatory courtship behavior I ob- the other, a position that would only occur at the end served supports this hypothesis (Eberhard 1996). Mul- of a fight in A. declivis. Perhaps this fencing corre- tiple mating by female A. declivis must confer advan- sponds to the jockeying for position in many A. declivis tages that compensate the feeding disadvantages battles involving a male's attempts to align his body (or documented here. The female cannot defend her ac- to avoid alignment) for a mutual squeeze. Threatening cess to a feeding site because she cannot swing her females of A. femorata were described by Mitchell abdomen toward other approaching females, the male (1980) as spreading their hind legs, in contrast to the sometimes pulls the female from a feeding site, and single leg raising behavior of female A. declivis (Fig. solitary males sometimes harass mating pairs and drive 3). Occasionally female A. declivis raised both hind them from feeding sites. November 1998 EBERHARD: SEXUAL BEHAVIOR OF Acanthocephala declivis guatemalana 871

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