Anim. Behav., 1997, 54, 1509–1515 Antennal asymmetry and sexual selection in a cerambycid beetle ANDERS PAPE MØLLER* & CARMEN ZAMORA-MUNx OZ† *Laboratoire d’Ecologie, CNRS URS 258, Universite´ Pierre et Marie Curie †Departamento Biologı´a Animal, Facultad de Ciencias, Universidad de Granada (Received 22 November 1996; initial acceptance 11 February 1997; final acceptance 30 April 1997; MS. number: 5394) Abstract. Cerambycid beetles have exaggerated antennae that are usually sexually size-dimorphic. We investigated the relationship between antenna morphology and sexual selection in the species Stenurella melanura (L.) in which males on average have antennae that are 13% longer than those of females. Males and females aggregate at flowers near oviposition sites for feeding during June–August. We sampled both copulating and single individuals at these sites. Fluctuating asymmetry (a measure of developmental instability) in antennae was considerably larger than in tibia and elytra and males had larger degrees of asymmetry in their antennae than females. Mated individuals did not differ from unmated individuals with respect to any of three size variables, but antennal asymmetry was smaller in mated individuals of both sexes. When two males were released with a female on a flower, males with symmetric antennae more often won the fight over the female than expected by chance. When two females and a male were released on a flower, the male more often preferred the female with more symmetric antennae than expected by chance. These results suggest that antennal symmetry, but not length, is currently under sexual selection. ? 1997 The Association for the Study of Animal Behaviour There has been a tremendous increase in our many cerambycid species, with males having the knowledge of certain aspects of sexual selection longest antennae. This obviously implies a role for over the last couple of decades (for a review see sexual selection, as already suggested by Darwin Andersson 1994). The general existence and (1871) and Linsley (1959) for this family. Surpris- importance of female choice and male–male com- ingly, there are, to the best of our knowledge, no petition have been repeatedly demonstrated. This previous studies of sexual selection with respect to is particularly the case for many insects. For insect this character in any cerambycid beetle. species with elaborate ornamentation, however, Our main aim in the present study was to there are few detailed studies of sexual selection, determine to what extent the morphology of the with horned beetles being a notable exception antennae and other morphological characters are (Brown & Balaton 1986; Conner 1988, 1989). subject to current sexual selection in the species Beetles of the family Cerambycidae have Stenurella melanura. More specifically, we investi- extremely elongated antennae, which are often gated the relationship between antennal length considerably longer than the body, and it is not and asymmetry and mating success. We did this difficult to imagine that such long antennae may by sampling mated and unmated individuals in be very costly for their bearers. These exaggerated the field, and by staging male–male competition structures can also be beneficial of course, when for a female and male choice of females in the used to detect potential predators. The length of field. the antennae is clearly sexually size-dimorphic in Stenurella melanura is a very common North European cerambycid beetle, occurring in conifer- Correspondence: A. P. Møller, Laboratoire d’Ecologie, ous forests as an adult from June to September. CNRS URS 258, Universite´ Pierre et Marie Curie, Oviposition takes place in dead wood, as is the Baˆt.A,7e`me e´tage, 7 quai St Bernard, F-75252 Paris case in most species of the family. Individuals of Cedex 5, France (email: [email protected]). C. Zamora-Mun˜oz is at the Departamento Biologı´a both sexes feed on flowers near oviposition sites, Animal, Facultad de Ciencias, Universidad de Granada, and these flowers also act as sites for mate search- E-2800 Granada, Spain. ing. Almost all the individuals we studied were 0003–3472/97/121509+07 $25.00/0/ar970565 ? 1997 The Association for the Study of Animal Behaviour 1509 1510 Animal Behaviour, 54, 6 observed and captured on the flowers of the Eppendorf vials with absolute alcohol until we composite Knautia arvensis. While feeding on could measure them. Only three individuals flowers, males also search for females. If a female escaped during capture attempts. is approached by a male, however, this does not necessarily imply the male will mate successfully, Experimental Interactions since the female may either leave the flower or resist the male’s approach. Interactions between During July 1993 single beetles were captured males and take-over attempts are frequent. Inter- and stored in Eppendorf vials before being used in actions between males consist of the individuals competition or mate-choice experiments. We grabbing each other’s antennae and limbs while staged male–male competition trials by simul- vigorously wrestling. This wrestling may some- taneously releasing two males on to a flower that times result in the amputation of an antenna or a already held a female. The release distance leg (Linsley 1959). Males of several cerambycid between each of the males and the female was less species have apparently evolved particularly than 1 cm and between the males less than 0.5 cm. powerful mandibles for this purpose (Thornhill When the Eppendorf vials were removed, the & Alcock 1983). Similarly, male–female inter- activities of the males were observed until one actions also involve frequent use of the antennae, male had started copulating with the female. We apparently for determining position and move- performed 25 trials, but no copulation occurred in ment of the other individual. Often antennal tips four and one male escaped in two. Both males appear to be used for determining the position of were re-captured in the remaining 19 trials. the tip of the abdomen of the other individual. In the second experiment two females were Females are able to terminate copulations by released on to a flower next to a male in the same simply flying away (Michelsen 1963). Copulatory way as described for the previous male–male mate guarding is common, as in other cerambycid competition experiment. We performed only 15 species (Parker 1970). Females regularly copulate trials owing to a male-biased operational sex ratio with more than one male (personal observation). in the population. One female escaped re-capture in two of these leaving 13 trials with re-captured females for analyses. MATERIALS AND METHODS Measuring Beetles Study Area Beetles were measured under a stereo micro- A.P.M. studied Stenurella melanura at a co- scope with a magnification of #12 for antenna niferous plantation near Kraghede (57 12 N, ) * and elytra and #25 for tibia, mainly by C.Z.M. 10)00*E), Denmark, regularly throughout July who was unaware of the origin of the different 1993. This area consists of 20–75-year-old stands samples that were identifiable only by identifi- of Picea abies with many fallen trunks which the cation numbers. The length of the three characters beetles use as oviposition sites. The beetles forage (antenna, tibia, elytra) was based on pre- on flowers, particularly of Knautia arvensis,in determined landmarks determined by the end open areas in the plantation, which have a very points of the characters. All characters were fully warm and sunny micro-climate. extended (antennae) and in the plane of the micro- scope field when measured. We measured 10 Sampling Beetles individuals on two subsequent days to estimate repeatability of character length and asymmetry A.P.M. walked along 300-m transects in open (Becker 1984; Falconer 1989). The measurements areas of the plantation between 0900 and 1500 from the first trial were covered when making the hours while inspecting all flowers for the presence second measurements. The average lengths of left of beetles. The individuals included in this study and right sides were highly repeatable (antenna: were all those recorded during the study period, in R=0.998, F9,10=849.79, P<0.001; tibia: R=0.985, total 67 males and 31 females. Males guarding F9,10=151.17, P<0.001; elytra: R=0.998, F9,10= females and individuals in copula we considered 1067.77, P<0.001). Similarly, repeatabilities of to be mated. Captured individuals were stored in asymmetries were also statistically significant, Møller & Zamora-Mun˜oz: Beetle antennal asymmetry 1511 Table I. Summary statistics for the size and asymmetry of three morphological characters in Stenurella melanura Antenna Tibia Elytra Males (N=67) Length (mm) 8.95 &0.07 1.66&0.01 6.12&0.05 Signed left"right value (mm) "0.001&0.01 "0.002&0.003 0.001&0.005 Kurtosis 0.521 "0.564 0.007 Skewness "0.324 0.172 "1.164 Absolute asymmetry (mm) 0.081&0.017 0.017&0.002 0.021&0.004 Relative asymmetry 0.011&0.002 0.010&0.001 0.003&0.001 Females (N=31) Length (mm) 7.89 &0.11 1.68 &0.02 6.35 &0.10 Signed left"right value (mm) "0.025&0.022 "0.005&0.003 0.007&0.006 Kurtosis 0.553 "0.520 1.000 Skewness "1.383 0.089 0.326 Absolute asymmetry (mm) 0.058&0.007 0.016&0.003 0.020&0.005 Relative asymmetry 0.007&0.001 0.010&0.002 0.003&0.001 Values are means&. Relative asymmetry is absolute asymmetry divided by length. with the exception of tibia, and measurement differences in variances of left-minus-right errors were smaller than the asymmetries for character values (Sokal & Rohlf 1995).