Great Basin Naturalist Volume 59 Number 2 Article 11 4-30-1999 Assortative mating in soldier beetles (Cantharidae, Chauliognathus): test of the mate-choice hypothesis Ruth Bernstein University of Colorado, Boulder Stephen Bernstein University of Colorado, Boulder Follow this and additional works at: https://scholarsarchive.byu.edu/gbn Recommended Citation Bernstein, Ruth and Bernstein, Stephen (1999) "Assortative mating in soldier beetles (Cantharidae, Chauliognathus): test of the mate-choice hypothesis," Great Basin Naturalist: Vol. 59 : No. 2 , Article 11. Available at: https://scholarsarchive.byu.edu/gbn/vol59/iss2/11 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 59(2), ©1999, pp. 188-192 ASSORTATIVE MATING IN SOLDIER BEETLES (CANTHARIDAE: CHAULIOGNATHUS): TEST OF THE MATE-CHOICE HYPOTHESIS Ruth Bernsteinl and Stephen Bernsteinl ABSTH.v:r.-SoJdier beetles of 2 species, Ch(luliognathus basalis and C. deceptus, were examined to test the Crespi hypothesis that positive assortative mating by size is caused by mate choice. Specifically, we tested the prediction that if mate choice involves choosing the largest mate available, then mating individuals v!'ill be larger than nonmating individ­ uals. Four samples were taken, at different times during the mating season, from each of2 sites. Each sample consisted ofmating pairs, nonmating males, ancl nonmating females. Some ofthe samples contained beetles of both species; others contained beetles of a single specie.~_ For each gender elytron lengths of mating individuals were compared with elytron lengths of nonmating individuals. \Ve found no effect ofmating status (mating ys_ nomnating) on elytron lengths in sam­ ples that exhihited assortative mating (which occurs where 2 species coexist). Surprisingly, we found a consistent effect of mating status on elytron lengths in samples that did not exhibit a.ssortative mating (which occurs where only 1 species exists). Our results do not support the mate-choice hypothesis. Instead, mate choice and assortative mating appear to be alternative mating patterns in which mate choice occurs \'ihere a single species exists and assortative mating occurs where 2 species coexist. Key words: mate choice, (lSS01-tative mating, soldier beetles, Chauliognathus deceptus, Chauliognathus basalis. Positive assortative mating by size occurs a preference for larger males (Crespi 1989), when the body sizes of mating pairs are more \Vhen male choice occurs in soldier beetles, it similar than if they mated at random. This is most likely because larger females carry mating pattern has been observed in soldier more eggs (Ridley 1983), Wnen female choice beetles (Chauliognathus), as reported by Mason occurs, the apparent preference may be due to (1972), McCauley and Wade (1978), McLain the superior ability of larger males in over­ (1982, 1984, 1985), and Bernstein and Bern­ coming the higher "resistance to mating" of stein (1998), The ultimate cause of positive larger females (McCauley and Wade 1978, assortative mating by size may be sexual selec­ McCauley 1981), A prediction of the mate­ tion, in which differences in reproductive suc­ choice hypothesis is that the mean size ofmat­ ce!'iS, cau!'ied by competition over mates, are ing individuals will be larger than the mean related to body size (Andersson 1994), Alter­ size of nonmating individuals (Arnqvist et al. natively, positive assortative mating by size 1996), Most data on arthropods do, in fact, may, be an artif~lct of environmental factors, show this mating pattern, either for females or such as temporal or spatial covariance ofbody for both sexes (Crespi 1989), size among mating males and females. In the study described herein, we exam­ Many hypotheses have been offered to ex­ ined the mate-choice hypothesis in 2 species plain assortative mating in arthropods (Crespi of soldier beetles (Chauliognathus deceptus Le 1989), One ofthese is the mate-choice hypotlle­ Conte and C. basalis Fender), These beetles sis, hased on sexual selection, in which indi­ mate conspicuously, occur in large populations, viduals choose larger mates because they ben­ and remain coupled for many hours, (In a pre­ ellt reproductively and m'e differently capable liminary study vdth marked pairs, "ve found ofexercising choice (Darwin 1871, Ridley 1983), that 68% of mated pairs remained together for Male choice involves a large-male mating ad­ more than 5 hand 34% for more than 17 h), In vantage in male-male competition for females a previous study (Bernstein and Bernstein 1998) comhined with a preference for larger females; we found positive assortative mating by body female choice involves hlctors that increase size in some populations but not in others. large-female pairing probabihty combined with Arnqvist et aL (1996) lists, for the mate-choice II )qJaI'tI1Wl It or Environll"lental, I'rlllu!l1t(ml. and Or!(anhHlk l\iolo!!)'. (j niV(:'~ity ofColorado. Boulcl<;l; CO 80,109, 188 1999] MATE CHOICE IN SOWIER BEETLES 189 hypothesis, the following predictions: (1) the ba.salis, an unexpected result since the 2 form of assorlative mating will be true, (2) species cannot be distinguished in the Held. mating males will be larger than nonmating Males were identiHed to species by the shape males, and (3) mating females will be larger of the copulatory organ, using the key pro­ than noomating fema.les. The first prediction vided in Fender (1964); females were identi­ ha, been tested (Bernstein and Bel11stein 1998) fled by species-specific correlations between and found true: the correlation between body length of the posterior elytron spot and length sizes ofmates is true (linear) rather than appal"· of the elytron (Ilernstein and Bernstein 1998). ent (in which the variance in male size changes 11,e right elytron of each beetle was severed with increasing female size). Here, we test the from the body and its maximum length was remaining 2 predictions, specifically that mat­ measured to the nearest 0.001 mm, lIsing a ing individuals are larger than nonmating indi­ binocular microscope with an eyepiece viduals in the populations that exhibit assorta­ micrometer. tive mating but not in the populations that do The eflects of sampling time aod mating not exhibit assortative mating. status (mating versus nonmaling) on body size were analyzed by 2-factor analyses ofvariance. METHODS Sampling time represents both progression of the mating ~ea.son and variations in assortative We eolJccted mating and nonmating beetles mating, as only the first 2 samples at the from 2 sites, one on the plains and the other in canyon site exhibited this matin~ pattern a canyon, within 30 km of Boulder, Colorado. (Bernstein and Bernstein 1998). All samples The plains site is a meadow (elevation 1760 m) were redutrt'CI in size (hy random elimination) near Eldorado Springs, where beetles were to the smallest sample in order to meet the leeding and mating on sunllower (Helillnthus recommended equal sample sizes for the 2­ annum L.) blossoms. The canyon site is a lactor AN OVA. We hegan with 24 groups: a roadside in South St. Vrain Canyon (elevation male group and a female group fi'om each of 1830 m) near Lyons, where beetles were feed­ the 4 samples of C. deceptw! at the canyon ing and mating on blossoms of rabbitbrush site, Ii-om each of the 4 samples of C. ha.salis at [Chrysotharn",", TUtuseosus (Pall.) Britt.]. Eacll the canyon site, and from each of the 4 sam­ sampling site encompassed an area of less ples of C. ba.salis at the plains site. Five of the than 0.5 ha. At each site 4 samples were taken 24 groups were eliminated from analysis, 4 at 1-wk intervals during the approximately because they were too small (the male group month-long mating season. All beetles in each and female group from samples 3 and 4 of C. sample were collected on a single morning, ba.salis at the canyon site) and 1 because it was between 0900 hand 1030 h. At this time of so mnch smaller than ti,e other samples (sam­ day beetles are too cool for rapid locomotion ple 4 of C. ba.salis females at the plains site) and so are likely to have been coupled since at that we preferred to eliminate it rather than least the previous evening. The sluggish condi­ greatly reduce the other samples. Where sig­ tion also prevents sampling bias, as any beetle nificant eOects of sampling time were found, (regardless of size or mating statns) is easily the means of mating and of nonmating indi­ captured by sliding it li'om a blossom into a viduals were compared llsing the Newman­ collecting vial. Whenever possible, each sample Kenis multiple-comparison test (Zar 1996) with consisted of 40 mating pairs, 40 nonmating the level ofsignincancc set at 0.05. males. and 40 nOll11l3ting females. Samples of 10 presenting a summary of our results, we mating pairs are the same ones reported in an compare the pattern ofassortative mating with earlier publication in which patterns of assnr­ the pattern ofsize diflerences between mating tative mating were described (Bernstein and and nonmating individuals among the samples. Bernstein 1998). Part of this comparison involved testing for Beetles were frozen within a few hours after differences among the Pearson product corre­ capture and tben preserved in 70% alcohol. lation coefficients (r), our measure of assorta­ Plains samples consisted entirely of Chauliog­ tive mating. For these tests we lollowed the I'IlI1.htts ba.salUJ. Canyon samples consisted of 2 statistical prncedures descIibed by Zar (1996) species, ChauliogTUtthu., deceptus and C. with the level of significance set at 0.05.