Seasonal Differences in Aggression and Site Tenacity in Male Green Frogs, Rana Clamitans

Copeia, 2004(1), pp. 159–164

Seasonal Differences in Aggression and Site Tenacity in Male Green Frogs, Rana clamitans

DONALD B. SHEPARD

Many anurans are thought to be territorial only during breeding; thus these species should exhibit seasonal differences in social behavior. Aggressive behavior and site tenacity, two components of territoriality, were examined in male Rana clamitans (Green Frogs) to test for differences between breeding and nonbreeding seasons. I predicted that aggression and site tenacity should be increased during the breeding season and reduced during the nonbreeding season. Aggression (male wrestling) was observed only during the breeding season. Males moved at a constant rate regardless of size (snout–vent length) or season (breeding or nonbreeding); however, males moved greater total distances during the breeding season but restricted movements to small areas throughout the nonbreeding season.

ERRITORIALITY has been classically de- and Savitzky, 1976; Howard, 1978a; Fellers, T fined as ‘‘any defended area’’ and is typi- 1979). Alternatively, aggression may function in cally manifested by aggression toward intruders the maintenance of an individual distance (Noble, 1939). The term was originally coined (Pierce and Ralin, 1972; Whitney and Krebs, to describe the behavior of male birds (Howard, 1975; Dyson and Passmore, 1992). 1920), but debate has arisen more recently con- Rana clamitans (Green Frogs) are common cerning the definition of and criteria to dem- throughout much of eastern North America onstrate territoriality (see Kaufmann, 1983; (Conant and Collins, 1991). Their breeding sea- Maher and Lott, 1995). For anurans, Mathis et son extends several months (Wright, 1914; al. (1995) proposed three necessary compo- Wells, 1977b). Male R. clamitans have been ob- nents of territoriality: (1) resource limitation, served in physical encounters (Brode, 1959; (2) site tenacity, and (3) resource defense. Fol- Schroeder, 1968; Wells, 1978) and are often lowing this framework and recommendations found calling from the same site on successive from Maher and Lott (1995), territoriality is nights (Brode, 1959; Wells, 1977b). Previous re- best defined as the extended defense of an area search on territoriality in R. clamitans differs (through advertisement or aggression) that con- with respect to aggression and site tenacity. Mar- tains some limiting resource required for sur- tof (1953a) observed no active defense of areas vival or reproduction. It is important to note and found that males did not restrict their re- that advertisement and aggression alone do not productive activities to spatially fixed areas, fre- constitute territoriality, nor is the defense of a quently moving between ponds Ͼ 100 m apart. resource sufficient evidence to qualify it as lim- In contrast, Wells (1977b, 1978) observed a iting (Mathis et al., 1995). Further, the defense large number of male wrestling bouts (105 over of an area without site tenacity should be con- three years) and found that males were relative- sidered the maintenance of an individual dis- ly stationary. Because male R. clamitans are ter- tance and not territoriality (Wells, 1977a). ritorial only during the breeding season (Wells, Aggressive behavior has been reported in 1977b, 1978), they should exhibit seasonal dif- many anuran species (reviewed by Wells, 1977a) ferences in behavior. The objective of this study and in most circumstances occurs between was to test whether aggression and site tenacity males during the breeding season. Aggressive in male R. clamitans differ between breeding behavior in anurans consists of aggressive call- and nonbreeding seasons. I predicted that ag- ing, charging, head-butting, shoving, splashing, gression and site tenacity should be increased jumping on, dunking, kicking, biting, or wres- during the breeding season and reduced during tling. Such behavior can be energetically costly, the nonbreeding season. physically damaging, and sometimes fatal (Lutz, 1960; Kluge, 1981; Bolivar-G et al., 1999). What MATERIALS AND METHODS appears to be aggression may be misdirected attempts at amplexus (Whitford, 1967; Crawford, Study site.—My study area was a approximately 2000) or feeding (Boice and Williams, 1971), 180 ϫ 80 m wetland located approximately 3 but most often, aggressive behavior has been in- km east of the Illinois River near the junction terpreted as defense of a territory (Duellman of Illinois Highway 26 and the Atchison, Tope-

᭧ 2004 by the American Society of Ichthyologists and Herpetologists 160 COPEIA, 2004, NO. 1 ka, and Sante Fe Railroad, Marshall County, Il- Data analysis.—I used mean Distance Moved per linois (40Њ55’45’’ N, 89Њ25’39’’ W). Emergent Day (DMD) and Net Displacement (ND) as in- vegetation was most concentrated in the north- dicators of site tenacity. Net Displacement (ND) ern and southern ends and consisted predomi- for an individual was calculated by measuring nately of arrowhead (Sagittaria latifolia), willow the distance between the initial capture point (Salix sp.), bulrushes (Scirpus spp.), common and final capture point within the season cattail (Typha latifolia), narrowleaf cattail (Typha (breeding or nonbreeding). High site tenacity angustifolia), bur reed (Sparganium eurycarpum), should consist of a low DMD and a small ND. sedges (Carex spp.), reedgrass (Phragmites sp.), Although distances between capture points may arrow arum (Peltandra virginica), and water not represent the total distance traveled, they plantain (Alisma subcordatum). Duckweed (Lem- are the minimum distance moved and thus are na sp.) covered much of the water surface. Wa- a conservative estimate of movement. Addition- ter depth varied from 20–80 cm depending on ally, because an individual’s size may relate to location, precipitation, and time of year, but ar- movement and individuals might move greater eas used by R. clamitans were typically 20–50 cm distances over longer periods of time, it was nec- deep. essary to account for these potential influences. Therefore, I included SVL and the Time Inter- val between captures (TI) as covariates in anal- Methods.—I made behavioral observations and yses. I used ANCOVA (Sokal and Rohlf, 1995) hand-captures of R. clamitans on 30 nights be- with SVL and TI as covariates to test if calling tween 1 May and 7 September 1999 with the aid males had lower DMD and smaller ND than of a headlamp. Observations began shortly after noncalling males during the breeding season. I dusk and continued through 2400 h CST or lat- also used ANCOVA with SVL and TI as covari- er depending on activity level (typically 3–4 h ates to test whether males had lower DMD and per night). Prior to capture, I made observa- smaller ND during the breeding season than tions at a suitable distance away from the frog the nonbreeding season. To ensure indepen- (approximately 2–3 m) and recorded any ag- dence of breeding and nonbreeding seasons, gressive behavior or social interactions. Dura- only movement data during the breeding sea- tion of observation depended on activity but son was used for individuals captured during was always Ն 5 min. I individually marked frogs both periods. Data were analyzed using SAS by toe-clipping but did not clip the thumbs of (vers. 8.01, SAS Institute, Inc., Cary, NC). Trans- males because of their potential role in amplex- formation was necessary in some cases when us and fighting. My activities did not appear to raw data did not meet parametric assumptions. affect frog behavior, and I observed no adverse Means are reported Ϯ 1 SE, and ␣ was set at reaction to toe-clipping. For each frog, I mea- 0.05. sured snout–vent length (SVL) to the nearest 1 mm using a hand ruler and noted whether or RESULTS not males were calling. Frogs were sexed by Male population characteristics.—In 1999, the comparing relative sizes of the eye and tympa- breeding season spanned from 18 May to 27 num (Conant and Collins, 1991) or by other July; the nonbreeding season was classified as 28 external characteristics such as venter colora- July to 7 September. A total of 115 males was tion. captured throughout the study with a recapture I constructed a grid system consisting of num- rate of 27% (31 of 115 recaptured at least once, bered wooden stakes at 4-m intervals. Upon cap- mean ϭ 1.4 recaptures, range ϭ 1–3). Overall turing a frog, I marked the site with flagging mean SVL for males was 80.5 Ϯ 0.6 mm (n ϭ tape labeled with the date and the individual’s 115, range 68–95 mm). Mean SVL of males was ID number. On a subsequent day, I measured not significantly different between seasons the distance (to the nearest 0.1 m) and bearing (breeding mean ϭ 80.0 Ϯ 0.6 mm, n ϭ 93; non- to the site from the nearest stake using a mea- breeding mean ϭ 82.5 Ϯ 1.2 mm, n ϭ 22; t113 suring tape and compass. I converted these ϭϪ1.70, P ϭ 0.09). The small observed differ- measurements to X–Y coordinates then ob- ence is likely caused by growth over the course tained movement distances for individuals by of the year. During the breeding season, calling calculating distances between capture points. I males were significantly larger than noncalling classified the breeding season as the period be- males (calling mean ϭ 81.4 Ϯ 0.7 mm, n ϭ 70; tween the days the first and last egg masses were noncalling mean ϭ 75.9 Ϯ 1.2 mm, n ϭ 23; t91 deposited as determined by routine visual en- ϭ 3.97, P ϭ 0.0001). Smaller noncalling males counter surveys. were occasionally (n ϭ 4) observed in a low de- SHEPARD—GREEN FROG AGGRESSION AND SITE TENACITY 161

ﬂated posture Ͻ 1 m from a larger calling male, Noncalling males during the breeding season but I never observed any interactions among were not used in comparisons between the the males nor any attempts by the smaller males breeding and nonbreeding season because non- to intercept females or occupy vacated calling calling males were considered nonterritorial. sites. ANCOVA revealed DMD was not signiﬁcantly

influenced by either SVL (F1,22 ϭ 0.81, P ϭ 0.38) Aggressive behavior.—I observed no physical ag- or TI (F1,22 ϭ 2.07, P ϭ 0.16) so these factors gression during the nonbreeding season, but I were removed from the model. The resultant did observe wrestling bouts between males on ANOVA showed no significant difference in two occasions (20 May and 18 June) during the DMD between the breeding and nonbreeding breeding season. The manner of wrestling was seasons (breeding mean ϭ 0.95 Ϯ 0.16 m, non- similar in both cases; thus, I will describe only breeding mean ϭ 0.86 Ϯ 0.15 m; F1,24 ϭ 0.15, P the latter bout. On the night of 18 June, I first ϭ 0.70). ND was not significantly influenced by observed three males in tandem amplexus with SVL (F1,21 ϭ 0.34, P ϭ 0.56), but there was a no female present. The male on bottom was vis- significant interaction between season and TI ibly larger than the two males on top and was (F1,21 ϭ 22.34, P ϭ 0.0001). Thus, the relation- moving about while repeatedly emitting release ship between ND and TI was different in the calls and trying to dislodge the other males. breeding and nonbreeding seasons with a pos- This behavior persisted for approximately 3 min itive relationship (y ϭ 0.15x ϩ 1.09, R2 ϭ 0.73, after which all males separated and the two F1,10 ϭ 27.2, P ϭ 0.0004) during the breeding smaller males dispersed. The remaining large season and no relationship (y ϭϪ0.02x ϩ 2.46, 2 male (A) began calling after approximately 5 R ϭ 0.06, F1,12 ϭ 0.80, P ϭ 0.39) during the min, and another nearby calling male (B) im- nonbreeding season. mediately moved toward him and called from 25 cm away. Both males were in a high, inflated DISCUSSION posture and in clear view of one another. Next, male A called then jumped at and rammed into Territoriality is the extended defense of an male B whereupon the two males began to wres- area (through advertisement or aggression) that tle. The males grappled with their forelimbs contains some limiting resource required for and pushed against each other venter to venter, survival or reproduction. If male R. clamitans the force of which raised their bodies vertically were territorial during the breeding season, out of the water. After approximately 1 min of they should have exhibited aggression toward wrestling, male A overpowered male B and conspecific males and shown site tenacity pinned him on his back under water. Male B (Mathis et al., 1995). Further, there should be remained motionless until after approximately seasonal differences with increased aggression 2 min when male A released him and both and site tenacity during the breeding season males dispersed. compared to the nonbreeding season. Consis- tent with these predictions, male-male aggres- Site tenacity.—To meet assumptions of normality sion was only observed during the breeding sea- and homogeneous variance, it was necessary to son, although only on two occasions. The se- square-root transform the variables DMD and quence and manner of wrestling I observed ND. Within the breeding season, DMD was not generally followed published descriptions (Bro- significantly influenced by SVL (F1,13 ϭ 0.09, P de, 1959; Schroeder, 1968; Wells, 1978). How- ϭ 0.77) or TI (F1,13 ϭ 0.54, P ϭ 0.47), so they ever, there is variation in the frequency of male– were removed from the model simplifying it to male aggressive encounters between studies. a one-way ANOVA. DMD was significantly lower Wells (1978) observed 105 wrestling bouts over for calling males than noncalling males (calling three years, in contrast to no wrestling bouts ob- mean ϭ 0.95 Ϯ 0.16 m, noncalling mean ϭ 1.90 served by Martof (1953a). The large number of

Ϯ 0.53 m; F1,15 ϭ 5.36, P ϭ 0.04). ND was sig- wrestling bouts observed by Wells (1978) was nificantly influenced by TI (F1,13 ϭ 17.93, P ϭ likely an artifact of his study site design (a 21 ϫ 0.001) but not by SVL (F1,13 ϭ 0.05, P ϭ 0.82); 21 m enclosed human-made pond with 25 trans- thus only TI was retained as a covariate. AN- planted males). Male density at Wells’ (1978) COVA with a comparison of least squares means site was probably unnaturally high. If all areas showed that calling males had smaller ND than of his pond were equally used, the density noncalling males within the breeding season, would have been 5.7 males/100 m2, which is but the difference was not significant (calling similar to the average density of 5.5 males/100 2 lsmean ϭ 2.39 Ϯ 0.32 m, noncalling lsmean ϭ m observed during the breeding season in this 3.54 Ϯ 0.49 m; F1,14 ϭ 3.77, P ϭ 0.07). study (Shepard, 2002). However, because some 162 COPEIA, 2004, NO. 1 areas of Wells’ pond were not used (Wells, gression. Likewise, individuals that were aggres- 1977b), the actual male density was several sive during the breeding season, but lacked site times higher. Increased male density results in tenacity, should not be considered territorial. more frequent male-male aggressive encounters Territoriality involves long-term site tenacity in anurans (Backwell and Passmore, 1990; Dy- (Wells, 1977a); however, the length of time at a son and Passmore, 1992) and may explain the site required to constitute long-term tenacity is difference in the number of wrestling bouts be- debatable. Because territoriality involves the de- tween studies. fense of a limited resource, the evaluation of In territorial anurans, site defense is often the site tenacity must consider what the resource is most conspicuous characteristic, but site tenac- and how resource quality, as measured by re- ity is also a critical component of territoriality productive success, varies spatially and tempo- (Mathis et al., 1995). Within the breeding sea- rally. Based on the observation that females ovi- son, site tenacity was higher for calling males posited at male calling sites, Wells (1977b, 1978) than noncalling males, which was expected be- hypothesized that oviposition sites were the re- cause only calling males are presumed to be ter- source that male R. clamitans defended. Ovipo- ritorial. However, a comparison of male site te- sition in R. clamitans is aquatic, and eggs take nacity between seasons produced conflicting re- 3–6 days to hatch, with length of time to hatch- sults. Overall, male site tenacity was higher in ing depending primarily on water temperature the nonbreeding season. DMD was indepen- (Wright, 1914; Moore, 1939). Neither parent dent of TI and season indicating that males provides care to the eggs or larvae (Wells, moved at the same rate throughout both sea- 1981), and the larvae are free to disperse from sons. ND was positively related to TI during the the oviposition site after hatching. Therefore, breeding season but unrelated during the non- the time period that site quality or male site te- breeding season. Thus, males moved greater nacity affects offspring survival and influences distances with increasing time during the breed- parental reproductive success does not extend ing season, but in the nonbreeding season, beyond the embryo stage. males remained close to their initial capture For species such as R. clamitans and Rana ca- points. Additionally, larger males should have tesbeiana with extended breeding seasons, the had higher site tenacity because they are less quality of particular oviposition sites likely varies likely to be displaced by competing males. Wells throughout the breeding season. Because de- (1977b) found that larger males had higher site velopment rate is positively related to water tem- tenacity; however, my results showed no signifi- perature (Moore, 1939; Howard, 1978b) and cant relationship between male size and site te- faster hatching time results in decreased expo- nacity in either season. sure of eggs to predation (Howard, 1978b), wa- Wells (1977b) found that males occupied an ter temperature may be an important determi- average of 3.7 sites during the breeding season, nant of territory quality. In R. catesbeiana, male and although most males spent Ͼ 1 week at chorus locations shifted during the breeding least one site, most residency periods were Յ 1 season in association with changing water tem- week. During the breeding season, the average peratures. Early in the season, choruses occu- distance between calling males was 3.4 m (Shep- pied warmer areas where development rate was ard, 2002). Most individual movements over accelerated, whereas later in the season, cho- time intervals Յ 7 days were within this dis- ruses occupied cooler areas presumably to avoid tance; thus, these individuals can be considered lethally high water temperatures (Howard, as having some degree of site tenacity. During 1978a,b). Additionally, shifting of male calling the nonbreeding season, site tenacity was con- sites might be in response to predator densities siderably higher because males on average re- (Howard, 1978b, 1981). Because male R. clami- mained within a 5.4-m radius over several weeks. tans do not provide parental care (Wells, 1981), Martof (1953b) also found that adult R. clami- they are free to respond to changes in site qual- tans moved little and restricted their movements ity without affecting offspring survival. Al- to small areas during the nonbreeding season. though temperatures and predator densities Movement outside the breeding season is usu- can change abruptly, oviposition site quality ally related to foraging (Lamoureux et al., should be relatively constant over short time in- 2002), and because R. clamitans are best consid- tervals such as the 3–6 days required for R. clam- ered sit-and-wait predators, extended periods of itans eggs to hatch. Because male movements little movement should be expected. during the breeding season were small over Although males in the nonbreeding season short time intervals (Ͻ 1 week) and I observed exhibited high site tenacity, they cannot be con- aggressive behavior on two occasions, a limited sidered territorial because they exhibited no ag- degree of territoriality may have been present. SHEPARD—GREEN FROG AGGRESSION AND SITE TENACITY 163

However, without knowing whether movements for financial assistance. This work was conduct- were associated with changes in resource quali- ed under permit NH99.0355 issued by the Illi- ty, it is not possible to ascertain whether the be- nois Department of Natural Resources and Illi- havior of male R. clamitans in my study was truly nois State University Animal Care and Use Com- territoriality. mittee protocol 1-2000-02. An alternative hypothesis to territoriality is that males instead actively maintain an individ- LITERATURE CITED ual distance (Wilson, 1975). Distinguishing territoriality from the maintenance of an individ- BACKWELL,P.R.Y.,AND N. I. PASSMORE. 1990. Aggres- ual distance is difficult. In the latter case, indi- sive interactions and intermale spacing in choruses viduals would not defend a specific resource but of the Leaf-Folding Frog, Afrixalus delicatus. S. Afr. rather a personal space that would move with J. Zool. 25:133–137. the animal. Aggressive behavior could result BOICE, R., AND R. C. WILLIAMS. 1971. Competitive when conspecifics approach one another closer feeding behavior of Rana pipiens and Rana clami- than some threshold distance. The lower den- tans. Anim. Behav. 19:548–551. BOLIVAR-G,W.,T.GRANT, AND L. A. OSORIO. 1999. sity and greater intermale distances during the Combat behavior in Centrolene buckleyi and other nonbreeding season (Shepard, 2002) may ex- centrolenid frogs. Alytes 16:77–83. plain the absence or rarity of male-male BRODE, W. E. 1959. Territoriality in Rana clamitans. encounters during this time. An individual Herpetologica 15:140. distance may be maintained throughout the ac- CONANT, R., AND J. T. COLLINS. 1991. A field guide to tivity season, but the function of maintaining reptiles and amphibians: eastern and central North spacing may vary temporally. Maintenance of America. 3d ed. Houghton-Mifflin, Boston, MA. spacing during the breeding season may reduce CRAWFORD, J. A. 2000. Investigation of aggressive and interference and enable females to localize and satellite behavior in the Cricket Frog Acris crepitans. Unpubl. master’s thesis, Illinois State Univ., Nor- choose individual males more easily (Pierce and mal. Ralin, 1972; Whitney and Krebs, 1975). DUELLMAN,W.E.,AND A. H. SAVITZKY. 1976. Aggres- Selection of the oviposition site may be a pri- sive behavior in a centrolenid frog with comments mary means by which parents can directly influ- on territoriality in anurans. Herpetologica 32:401– ence their offspring’s probability of survival in 404. some species. In R. catesbeiana, there is evidence DYSON,M.L.,AND N. I. PASSMORE. 1992. Inter-male of lower embryonic mortality at sites defended spacing and aggression in African Painted Reed by larger males (Howard, 1978b). However, the Frogs, Hyperolius marmoratus. Ethology 91:237–247. cues males use to select calling sites and females FELLERS, G. M. 1979. Aggression, territoriality, and use to select oviposition sites are unknown. Ar- mating behavior in North American treefrogs. Anim. Behav. 27:107–119. eas of seemingly equal quality often are not HOWARD, H. E. 1920. Territory in bird life. Murray, used for calling or oviposition by anurans London. (Whitney and Krebs, 1975; Howard, 1981). Un- HOWARD, R. D. 1978a. The evolution of mating strat- til temporal variation in site quality can be as- egies in bullfrogs, Rana catesbeiana. Evolution 32: sessed, resource limitation and site tenacity can- 850–871. not be adequately evaluated, and it will be dif- ———. 1978b. The influence of male-defended ovi- ficult to discern between territoriality and indi- position sites on early embryo mortality in bull- vidual distance. frogs. Ecology 59:789–798. ———. 1981. Male age-size distribution and male mating success in bullfrogs, p. 61–77. In: Natural ACKNOWLEDGMENTS selection and social behavior, R. D. Alexander, and D. W. Tinkle (eds.). Chiron Press, New York. I thank L. Brown for valuable guidance as my KAUFMANN, J. H. 1983. On definitions and functions major professor. I also thank A. Kuhns, H. Bur- of dominance and territoriality. Biol. Rev. 58:1–20. dett, J. Crawford, and C. Gutmann for field as- KLUGE, A. G. 1981. The life history, social organiza- sistance; S. Juliano and K. Eckerle for statistical tion, and parental behavior of Hyla rosenbergi Bou- advice; S. Juliano, E. Mockford, and D. Whit- lenger, a nest-building gladiator frog. Misc. Publ. man for helpful suggestions and discussions re- Univ. Mich. Mus. Zool. 160:1–170. garding this project; L. Brown, J. Caldwell, J. LAMOUREUX, V. S., J. C. MAERZ, AND D. M. MADISON. Crawford, M. Dreslik, A. Kuhns, and C. Leary 2002. Premigratory autumn foraging forays in the Green Frog, Rana clamitans. J. Herpetol. 36:245– for critically reviewing earlier versions of the 254. manuscript; J. Cavenaugh and the Lockport LUTZ, B. 1960. Fighting and incipient notion of ter- Duck Club for access to their property; and the ritory in male tree frogs. Copeia 1960:61–63. Illinois State University Department of Biologi- MAHER,C.R.,AND D. F. LOTT. 1995. Definitions of cal Sciences and Graduate Student Association territoriality used in the study of variation in ver- 164 COPEIA, 2004, NO. 1

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