Great Basin Naturalist Volume 45 Number 4 Article 6 10-31-1985 Sexual selection and mating system variation in anuran amphibians of the Arizona–Sonoran Desert Brian K. Sullivan Arizona State University, Tempe Follow this and additional works at: https://scholarsarchive.byu.edu/gbn Recommended Citation Sullivan, Brian K. (1985) "Sexual selection and mating system variation in anuran amphibians of the Arizona–Sonoran Desert," Great Basin Naturalist: Vol. 45 : No. 4 , Article 6. Available at: https://scholarsarchive.byu.edu/gbn/vol45/iss4/6 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]. SEXUAL SELECTION AND MATING SYSTEM VARIATION IN ANURAN AMPHIBIANS OF THE ARIZONA-SONORAN DESERT Brian K. Sulli\an' Abstract—Mating system variation in anuran amphibians of the Arizona-Sonoran Desert was reviewed. Male density and breeding period duration were negatively correlated in seven bufonids and pelobatids. Variation in male mating behavior and ability of females to freely select their mates unhindered by active-searching males also was related directly to male density. These observations support hypotheses relating ecological factors to mating system organization. It is suggested that male calling behavior, and anuran lek mating sytems in general, may be significantly influenced by predation on vocalizing males. The Arizona-Sonoran Desert boa.sts a surpris- ence opportunities for female mate selection ing diversity of frogs and toads: seven bu- and, hence, male behavior. Wells (1977) ar- fonids, three pelobatids, three ranids, two gued that for explosive-breeding anurans fe- hylids, one microhyhd, and one lepto- male choice should be restricted as a result of: dactyhd. As expected, the breeding biology of (1) temporal constraints limiting time avail- these anurans is significantly influenced by able for mate selection and (2) high male den- the xeric climate. Most of these species have sities and corresponding alternative male short, or "explosive," (Wells 1977) breeding mating tactics that typically reduce the ability periods during the summer rainy season, al- of females to move freely in breeding aggrega- though a few have relatively extended, or tions. Conversely, in species with prolonged "prolonged" breeding periods during the breeding periods and lower male densities, spring (Sullivan 1982a, 1983a, 1984, Sullivan females should be capable of freely selecting and Sullivan 1985). As suggested by Wells their mates and constitute a potentialK- signif- (1977), the mating behavior of anurans such as icant evolutionary force (e.g., Arnold 1983). those of the Arizona-Sonoran Desert appears The variation in breeding biology of anurans related to breeding period duration. For ex- of Arizona allows direct test of these hypothe- ample, Woodhouse's toad {Biifo woodhousei) ses. Here I summarize recent studies of sexual forms low-density breeding aggregations at selection in pelobatids and bufonids of Ari- sources of permanent water throughout the zona and, following Emlen and Oring (1977), spring, and all males within these choruses document the existence of predictable rela- call to attract females (Sullivan 1982a). In con- tionships between the ecology, mating sys-| trast, the Great Plains toad (B. cog,natus) terns, and reproductive behavior of these spe- forms high density aggregations at temporary cies. I also examine species recognition and rain pools for only a few nights each summer. acoustic competition among these anurans. Within these short-lived choruses some males Lastly, I discuss the evolution of anuran lek adopt satellite positions near vocalizing males mating sytems. and attempt to intercept females approaching the calling males (Sullivan 1982b). General Methods This diversity in breeding period duration and reproductive behavior provides an excel- Methods employed in the studies reviewed lent opportunity for evaluating hypotheses of here ha\ e been described in detail elsewhere sexual selection and mating svstem theorv (Sullivan 1983c, 1984, Sullixan and Sullivan (Trivers 1972, Emlen and Oring 1977, Wells 1985); a brief simimary follows. Observations 1977). It is generally hypothesized that diua- at anuran breeding aggregations in southern tion of breeding period should directly influ- Arizona were made with a six-\ olt headlamp Uepartini'iit of Zoolo sity, Tcnipc. Arizona H5'2H7 78712. 688 October 1985 SULLIVAN: SONORAN AMPHIBIANS 689 Table 1. Mean mule density (number of males) and brcedinj; period duration (days) for seven anurans (Bufo and Scap/^iopws) of southern Arizona (N ^ number of breeding aggregations). 690 Great Basin Naturalist Vol. 45, No. 4 Table 2. Relationships between male size and mating success five anurans (Bufo and Scaphiopus) of southern Arizona. Snout-vent length (mm) Mating males Nonmating males Positive assortative mating Source Species X N .X N P r N P S. coiichi 71.0 70.0 6 >.05* 0.07* 12 =.50 Sullivan and Sullivan, 1985 74.1 71.1 18 >.05 0.07* 20 =.43 74.4 69.2 18 >.05=* -0.40* 10 =.24 S. multiplicatus 50.9 51.3 28 >.05 0.05 21 >.05 Sullivan and Sullivan, 1985 51.2 50.0 15 >.05 48.0 48.0 11 >.05* B. cognatus 78.6 77.2 150 >.05 0.09 24 >.05 Sullivan, 1983a B. debilis 41.3 42.3 16 >.05* 0.20* 6 >.05 Sullivan, 1984 B. woodhousei 84.1 85.4 43 >.05 0.23 47 >.05 Sullivan, 1983b *For small samples (< 10) a Mann-Whitney U test or Kendal's tau was calculated anurans should select large males as mates if sei in Arizona male mating success is weakly size is an indicator of fitness (Wilbur et al. correlated (R" = — 25%) with participation in 1978, Fairchild 1981). Others have argued chorus activity (Fig. 1). However, this result that females might benefit from selecting a is expected since many males are only ob- male similar in size to themselves if fertiliza- served at a chorus for one night: a correlation tion success is related to the size difference occurs because any male who mates two or between mating males and females (Licht three times must be present at the chorus for 1976, Davies and Halliday 1979). Analysis of at least as many nights. Further, many males male size and mating success in four explo- who are unattractive as mates (low call rate sive-breeding anurans of Arizona has not sup- males) are active at choruses for many nights ported either of these hypotheses (Table 2.) without obtaining any matings. Hence, for Male size is positively correlated with mat- this anuran male persistance in chorus activity ing success in some explosive-breeding bu- is not an important determinant of mating fonids and pelobatids of the United States success. (Gatz 1981, Woodward 1982). However, in Female mate selection has been observed these forms large males might achieve greater in breeding aggregations of the three Arizona mating success as a result of size advantages bufonids with low-densitv choruses (Sullivan during direct struggles for possession of fe- 1983a, 1984). In all of these forms, females males (Sullivan 1983a). Davies and Halliday initiate amplexus by making physical contact (1979) and Lamb (1985) determined that large with a calling male and are able to move freely male bufonids are more successful in taking through breeding choruses to select their over females during male-male disputes. For mates from among the calling males. In B. these species a mating advantage for large woodhousei male mating success is correlated males may be independent of any female pref- with call rate, suggesting that females prefer erence. Woodward (1982) documented a high call rate males as mates (Sullivan 1983a). large-male mating advantage in one of four S. Males within a chorus retain a consistent, rel- couchi and one of two S. multiplicatus breed- ative ranking in call rate within and between ing aggregations in central New Mexico. This nights, and there is significantly more varia- interspecific variation suggests that a number tion in call rate between than within males of variables such as density and alternative (Sullivan 1983a, Sullivan and Leek in re\iew). mating tactics of males influence the outcome Females prefer speakers liroadcasting calls at of sexual selection in nature. the higher rate in simple two-speaker discrim- Male mating success in prolonged-breed- ination experiments, further supporting the ing anurans is often correlated with the num- female preference hypothesis. Proximately, ber of nights a male participates in chorus female choice of high call rate males could be activity (Kluge 1981, Woodward 1981, Ryan passive if they merely locate such males more 1983). For prolonged-breeding B. woodhou- readily, or active if genetically determined October 1985 SULLIVAN: SONOHAN AMPHIBIANS 691 1981-1 . , 692 Great Basin Naturalist Vol. 45, No. 4 would be determined by the heritability of patic species (Gerhardt 1982, Rose and male call rate and its relation to offspring sur- Capranica 1983, Sullivan and Leek, in re- vivorship and mating success. In any event, view). Discrimination experiments have re- mating is clearly nonrandom with respect to vealed that female tree frogs (Hylidae) can male phenotype and female behavior in B. discriminate between two acoustic stimuli dif- woodhousei fering only in pulse rate, and they prefer stim- These observations on male behavior and uli similar to advertisement calls of con- mate selection by females reveal that the pro- specific males (Loftus-Hills and Littlejohn longed-breeding species {B. piinctatiis and B. 1971, Straughan 1975). ivoodhotisei), and at least some of the explo- All pelobatids and most bufonids occurring sive-breeding forms (B. cognatiis and B. de- in southern Arizona have been observed in bilis), have lek-like mating systems (Sullivan mixed-species breeding aggregations, provid- 1983c). Females typically select their mates ing the potential for heterospecific interac- freely from among aggregations of displaying tions (Fig.