Bolitotherus Cornutus. B. Cornutus Is a Te 1984; Price Et Al., 1984)
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Evo/utwn, 42(4), 1988, pp. 736-749 FIELD MEASUREMENTS OF NATURAL AND SEXUAL SELECTION IN THE FUNGUS BEETLE, BOLITOTHERUS CORNUTUS JEFFREY CONNER 1 Section of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853 Abstract. -Selection on three phenotypic traits was estimated in a natural population of a fungus beetle, Bolitotherus cornutus. Lifetime fitness of a group of males in this population was estimated, and partitioned into five components: lifespan, attendance at the mating area, number of females courted, number of copulations attempted, and number of females inseminated. Three phenotypic characters were measured-elytral length, horn length, and weight; there were strong positive correlations among the three characters. Selection was estimated by regressing each component of fitness on the phenotypic traits. Of the three traits, only horn length was under significant direct selection. This selection was for longer horns and was due mainly to differences in lifespan and access to females. The positive selection on horn length combined with the positive correlations between horn length and the other two characters resulted in positive total selection on all three characters. Received November 10, 1986. Accepted February 5, 1988 An important goal of current evolution selection for an increase in the value of char ary biology is to study selection occurring acter x, then an individual with a high value in natural populations. Many studies have of x will tend to have high fitness. If char demonstrated selection in nature, but very acter y is positively correlated with char few of these studies have measured lifetime acter x, then that same individual will have fitness, separated direct selection acting on a high value of y, thus creating indirect se a character from indirect selection on cor lection for an increase in y. The total selec related characters, or identified selective tion acting on a character is the sum of direct agents (Endler, 1986). The first shortcoming and indirect selection. can only be overcome by long-term field Second, identifying selective agents can studies; in animals, lifetime reproductive be facilitated by dividing lifetime fitness into success of individuals has just begun to be biologically meaningful components (e.g., estimated (Clutton-Brock et al., 1982; lifespan, access to females, insemination Fincke, 1982; Banks and Thompson, 1985; success) that can be more easily linked to Hafemik and Garrison, 1986; Koenig and the agents responsible for fitness differences Albano, 1987). Two recent theoretical ad (e.g., predation, male competition, female vances make the second and third problems choice) (Arnold and Wade, 1984a; Koenig easier to resolve. and Albano, 1986). Thus, by estimating as First, the methods of Lande and Arnold many components of fitness as possible over (1983) allow one to estimate selection acting the lifetime of an organism and relating on several correlated traits simultaneously, variation in these components of fitness to separating direct from indirect selection. several different characters simultaneously, Direct selection for an increase in character the specific agents that cause direct selection value (i.e., magnitude of a quantitative on individual target characters can often be character) results from a positive relation identified. ship between that character and fitness, when In the study reported here, I estimated other correlated characters are held con direct and total selection on three pheno stant. A character under direct selection can typic traits during the adult lifetime of male be called the "target" of selection (Price, Bolitotherus cornutus. B. cornutus is a te 1984; Price et al., 1984). Indirect selection nebrionid beetle that lives throughout its can be explained as follows. If there is direct life cycle on polypore shelffungi (principally Ganoderma applanatum). Eggs are laid sin gly on the surface of the fungi, and larvae 1 Current address: Department of Entomology, Cor feed by tunneling through the interior (Liles, nell University, Ithaca, NY 14853. 1956). After pupation (about three months 736 NATURAL AND SEXUAL SELECTION IN A BEETLE 737 after oviposition), adults emerge and are ac mate (Conner, 1987). Thus, female coop tive at night on the surface of the fungi, eration is necessary for mating to occur, be spending the daylight hours behind bark or cause males cannot force entry. After a suc in holes in the fungi. Eggs are laid from late cessful mating, a male guards the female for May through August. Adults may emerge two to five hours; this distinct behavior is late in the summer from eggs laid in the a reliable indicator of successful mating and spring, but most larvae overwinter and spermatophore transfer (Conner, 1987). emerge as adults the next year (Liles, 1956). A large natural population of B. cornutus Adults can live for several years (Pace, 1967). near Ithaca, NY, was studied for two mating Individual beetles do not come out on the seasons (1984 and 1985). This species surface of the fungi (where virtually all mat proved to be an excellent subject for studies ings occur) every night; thus only a subset of selection, since all of the biologically rel of males are active on any given evening. evant components of adult male mating The remainder spend the night behind the success were measurable (see below). Since bark or in holes in the fungi. Males appar growth does not occur during the adult stage ently come out on the surface of the fungi of B. cornutus, there were no complications solely for mating, since they cannot feed on due to correlations between the characters the upper surface of the fungi and do noth measured and age. One potential problem ing on the surface other than mating and was that generations overlap in this species. aggressive behavior (pers. observ.). Male Cross-sectional (short-term) studies on pop B. cornutus possess pronotal horns, which ulations with overlapping generations that vary allometrically with body size (Brown are increasing or decreasing in size can lead and Siegfried, 1983). The horns are used in to erroneous estimates of the magnitude and aggressive encounters between males. Fe even the direction of selection (Travis and males lack horns and show little aggressive Heinrich, 1986). In the present study, how behavior. ever, fitness was measured over all or most Courtship begins when a male climbs onto of the adult lifespan, and the population size the back of a female. Females appear to was stable throughout the study (see below). have little control over which males court them. A male's courtship frequency is de MATERIALS AND METHODS termined at least in part by male competi Field Observations tion, since males often aggressively chase The study population of Bolitotherus cor each other away from fungi containing fe nutus was located on a dead tree with 15 males (pers. observ.). After ten minutes to living fungi (Ganoderma applanatum). Bee several hours of courtship, the male at tles were collected when they were first seen tempts to mate. The female has very little and taken to the laboratory for marking and control over this stage, since she is rarely measuring; every adult beetle in the popu able to prevent a courting male from at lation was marked. The lengths of the tempting to copulate. The number of at horns and elytra (wing covers) of all males tempts a male makes is determined by at in this population were measured (to ±0.2 least two factors: male competition (since mm) with an ocular grid on a stereo micro males disrupt the courtships of other males scope. All beetles were also weighed (to ±0.2 before the courting male can attempt to mg) with a Mettler® balance and given an mate) and, possibly, male choice (since some individual paint mark on the elytra (with courtships are abandoned by males without Testors ® white enamel). This treatment did an attempt [pers. observ.; Pace, 1967; Brown not seem to disturb the beetles, and 90% of et al., 1985]). marked beetles were observed at least once While females probably have little con after release. trol over which males attempt to copulate The population was first marked in late with them, they have complete control over July 1983. To estimate lifespans, the beetles which males successfully mate and transfer were censused from one to three times per a spermatophore. Females have a heavy night on a total of 190 nights. Censuses were plate at the abdominal tip that acts as a trap performed on 3 5 nights from 22 July to 21 door; it must be opened before a male can September 1983, 77 nights between 19 May 738 JEFFREY CONNER and 28 August 1984, 59 nights between 16 A total of 134 males were present during May and 27 August 1985, and 19 nights the 1984-1985 observation period; thus, the between 27 May and 28 July 1986. study cohort represents half of the beetles To estimate mating success, the popula present. The other 67 males were first ob tion was observed for approximately 1,000 served either before 25 September 1983 or hours on 136 nights throughout the 1984 after 6 June 1985. The direct-selection anal and 1985 mating seasons (the same nights yses were performed separately on the co on which censuses were conducted in those hort and on all 134 males; both sets of anal two years; see above). Since the summer of yses are presented here. The population size 1985 was colder than 1984 (mean nightly was stable from 1984 to 1985; of the 134 low temperature 10.9°C vs. 12.3°C, t = 2. l, total males, 94 were present in 1984 and 93 P < 0.025) and beetle activity was sharply in 1985, with 53 males present in both sea reduced at temperatures below 10°C (pers.