70 S.-Afr. Tydskr. Dierk. 1992,27(2) Effects of temperature on chirp rates of tree crickets (: Oecanthidae)

A.B. Toms Department of General Entomology, Transvaal Museum, P.O. Box 413, Pretoria, 0001 Republic of South Africa

Received 2 October 1991; accepted 3 February 1992

The relationship between temperature and chirp rate is described for three African tree crickets, capensis, 0. karschi and O. sycomorus. Knowledge of the variation in the chirp rates of 0. karschi and 0. capensis makes it possible to discriminate between the songs of these species with a stopwatch. Their songs can also be used to estimate air temperature. When signals like songs change with temperature, the response may also be affected. Some problems associated with temperature dependent communication systems are discussed.

Die verhouding tussen temperatuur en t,jirptempo word beskryf vir drie Afrika boomkrieke, Oecanthus capensis, 0. karschi en O. sycomorus. Kennis van die verskille in die tjirptempo's van O. karschi en O. capensis maak dit moontlik om tussen die sang van die twee spesies te onderskei met 'n stophoriosie. Hulle sang kan oak gebruik word om lugtemperatuur te meet. Wanneer seine soos kriekklanke verander met temperatuur, kan die reaksies ook be"invloed word. Sommige probleme geassosieer met temperatuur afhanklike kommunikasie-stelses, word bespreek.

Surprisingly little is known about the effects of temperature Several studies have been made on the relationship on rates within poikilothenn nervous systems (Walker between temperature and chirp rate of the American snowy 1975). Since poikilothenn metabolic rates generally increase (e.g. Brooks 1882; Dolbear with temperature, pulse rates and chirp rates of the calling 1897; Block 1966; Walker 1969). Although the calls of most songs of crickets generally increase with temperature. Since southern African species have now been described (roms & songs that are utilized during the process of mate location One 1988), and O. karschi, incorrectly referred to as O. can change with temperature, it may also be necessary for burmeisteri, is known for its baffle-making behaviour

.

) the response to change with temperature (e.g. Walker 1957). (Prozesky-Schulze, Prozesky, Anderson, and van der Merwe 0

1 Since cricket songs play a crucial role in the location of 1975; Bennet-Clark 1~89) few data on chirp rates have been 0 2 conspecifics of the opposite sex, it is also probable that we published for any African species of the Oecanthus. d e

t will have to understand evolutionary changes in the Most male crickets are capable of producing sounds by a

d neurophysiology of sound production much better, before rubbing their modified forewings (tegmina) together. During (

r we can hope to understand speciation. stridulation a scraper on one wing comes into contact with a e h Of all the changes in songs which are correlated with file on the other. Two factors affect the frequency of the s i l changes in temperature, by far the most conspicuous, and song, the rate at which the scraper makes contact with the b u the easiest to monitor, are changes in chirp rate. In some file teeth, and the physical attributes of the resonating P

e species this phenomenon is so distinct that it can be used to membrane (Sismondo 1979). File tooth impact rate depends h t

calculate air temperature reasonably accurately. Although it on the distance between successive teeth and tegminal y b

has been incorrectly suggested that almost any cricket can closure rate. In tree crickets, tooth impact rate is more d e be used to calculate temperature with one fonnula (e.g. important than the characteristics of the vibrating membrane t n Skaife 1953), most crickets have unique songs and are in frequency determination, so frequency also changes with a r

g totally unsuitable as 'thennometer' crickets for a variety of temperature. Each tooth impact causes one vibration of the e

c reasons. Since most crickets sing with a pulse rate (= wing tegminal membranes, so the frequency of the sound in Hz is n

e closure rate) that is too fast to count without instruments, a directly related to the tooth impact rate. One sound pulse is c i l

good thennometer cricket would need to have its song produced during each tegminal closure, while one chirp r e broken into chirps. The chirp rate should be regular, slow consists of a group of pulses produced at more or less d n enough to count, and loud enough to hear clearly. Ideally, regular intervals (Figure I; Toms 1984; Bennet-Clark 1989). u

y the cricket should have a large surface area to volume ratio, a so that its body temperature quickly adjusts to changes in air Methods w e t temperature. The more pronounced the change in chirp rate Data collected at several localities and various ambient a

G with temperature, the more accurate the estimates will be. temperatures (roms 1988; Toms & Otte 1988) were used to t e Finally, the should sing from an elevated position, compare changes in chirp rates of African Oecanthidae with n i

b such as a tree, shrub or grass, and not from a concealed published data for Oecanthus fultoni. In the species which a

S place, such as a burrow or crack, so that it is quickly call slowly enough for chirp rate to be counted without

y exposed to changes in air temperature. Few crickets possess b special techniques (capensis and karschi), tape recordings of

d all of these characteristics, apart from the chirping members each individual were analysed by timing the duration of 10 e c

u of the genus Oecanthus, generally known as tree crickets. or more chirps and interchirps with a stopwatch and then d o r p e R S. Afr. I. Zool. 1992.27(2) 71

tun

,,, .. , ,un

o I TIME (seconds)

Figure 1 Audiospectrogtams showing the constant chirp rate of the song of OeCanllui.s capensis at 21.8°C and the variable chirp rate of O. karschi at 22,4°C. The song of O. sycomorus has been published elsewhere (Toms & Otte 1988).

14 ...... ,I"

12

° 10

.' .

) .'

0 .' 1

...... " 0 .' 2 .' d e t a 4 d ( r e .. ' ~,.J?"'"t:c. h s i l ...... ,..... "!" ,...... :11'.' ... " •.•.....•..•.••••.••.•... ;. •..•::.-..:::::::: ...... ' •.••••..•... ••..•..•.•....•.•••. , •.••• l.~.,~ .....,.."~.::: .. :.~. _4_ 4. --- .. , .... ~=_~ b u , /l~~rt t-t f- -i- - ~-;«; 000 ° P

ceo __ j-/-' -Ii.. ,_ .... --i-r, ° ° e ... ~ ...... h t . .. , ...... ~.; ,," ' o ,...... y b •. Q - sycomorus d "0'" capensis e t ... - fultoni n -0- karschi a r 10 15 20 25 39 g

e AIR TE~RATURE DC c n e c i FIgure 2 Change in chirp rate with temperature in Oecanlhus capensis, O. ful'oni, O. karschi and O. sycomorus, with extrapolations to l

r show how the regression lines of the African species converge where Y = 0 on the X-intercept. Data obtained from audiospectrographs e d were used for O. capensis and O. sycomorus, while the data for the other species were obtained with a stopwatch (see Table 1). n u y a

w ca1culating the chirp rate. These results were compared with the air at least 1 min before any temperatures were recorded. e t chirp rates obtained by measurement, using dial callipers, of

a Least-squares linear regression equations were ca1culated G

the duration of a single chirp and inter-chirp period (10 with a microcomputer. t e chirps were measured and averaged in O. sycomorus), from n i b an audiospectrogram produced with a Kay 7029A spectrum Results a

S analyser. Special care was exercized in the collection of

y ambient-temperature data after each recording, at or as near As is the case with O. fulloni. the relationship between chirp b

d to the calling site as possible, but always < 2 m from the rate and temperature of all three African species appears to e c calling site. A 50°C ± O.I°C thermometer was exposed to be linear (Figure 2, Table 1). u d o r p e R 72 S.-Afr. Tydskr. Dierk. 1992,27(2)

The regression lines for O. karschi and O. fultoni are Table 3 Differences between predicted and almost parallel but the slope of the regression line for O. actual temperatures using the formulae in capensis is greater. In O. sycomorus, the rate of change of Table 2 chirp rate is even more pronounced than in O. capensis (Figure 2, Table 1). Standard Standard Species Range °C deviation error 11

Discussion julto1li -1,2 to 2,4 0,79 0,09 n 'The songs of three chirping African Oecanthidae change capensis -2,0 to 1,4 0,68 0,11 35 karschi -2,4 to 4,4 1,75 0,24 51 significantly with temperature in a predictable way. The chirp rates of O. karschi and O. capensis are slow enough to count in the field, so they can be used to identify the species below about IO"C, while this may be possible for some concerned. If the chirp rate is greater than three chirps per American species and O. fultoni has been tape recorded second, it is probably capensis. Below 20°C a chirp rate below WOC (Figure 2). The fact that O. capensis has not exceeding two chirps per second is probably produced by been tape recorded below 17°C, or above 23°C, is probably capensis. It is also possible to estimate temperature from the due to limited sampling. How and why different species chirp rates of these species, in a similar way to O. fultoni should have different rates of change and why their rates (Table 2). Although the chirp rate of O. sycomorus changes should converge at the X-intercept is not known. It is inter­ rapidly with temperature, it sings at a rate which is too fast esting to note that the regression lines for O. fultoni and O. to count without the use of instruments. karschi are almost parallel, which suggests that something Walker (1975) showed that a linear relationship generally about the mechanism of changes in metabolic rates with provided more accurate predictions than an exponential one, temperature is similar in these two species. and that the extrapolated regression lines of most species In O. capensis, the r 2 value is higher when chirp rates tended to converge where Y = 0 on the X-intercept. The were calculated from audiospectrograms than with a stop­ chirp rates of the species in this study converge at about watch, while the opposite is true for O. karschi (Table I). ~C, providing further evidence for this phenomenon. In The measurement of a single chirp in O. karschi is not contrast, the X-intercept for fultoni and rileyi, another accurate because the chirp rate is not always constant American chirping species, is about 4°C (Walker 1962b). (Figure 1). In contrast, the chirp rate of O. capensis is constant, but rates calculated with a stopwatch depend on The fact that the regression lines for African and American

. the response time of the operator. Higher chirp rates of O. ) species converge at different temperatures, suggests that 0 capensis (four or more chirps per second above 20"C), are 1 there may be a basic physiological difference between Afri­ 0

2 also more difficult to count. It is interesting to note that the can and American chirping Oecanthus. The African species d r 2 value for O. fultoni is higher than those of O. capensis e t studied here would not be capable of singing at temperatures

a and O. karschi (Table I). Differences in correlation coeffici­ d

( ents can arise from differences in sample size, sample range, r e Table 1 Regression equations of chirp rate plotted sampling bias and other factors. Some of the difference in h s i against temperature for Oecanthus fultoni, 0. capensis, correlation coefficients may also have arisen as an effect of l b O. karschi and O. sycomorus the method of measurement Data for O.fultoni were collec­ u

P ted specifically for analysis of effects of temperature, while

e Species Method Regression equation r 2 11 h the data used here were collected primarily for taxonomic t

y fultOili stopwatch y chirps / min =-21,9 + 6,8b 0,91 77 purposes. When Block collected his data the chirp rate was b fultOili stopwatch y chirps / s =--{),36 + O,I1.x counted for 1 min in the field. Any breaks in the song would d e t capellSis stopwatch y chirps / s =-1,95 + O,26.x 0,72 32 have made it necessary to start again. Tape recordings col­ n 0,78 a capellSis speClrograph y chirps / s =-2,57 + O,29.x 35 lected for taxonomic purposes may have several breaks, r g karschi stopwatch y chirps / s =-1,10 + 0,12.% 0,73 51 since only 2 s of singing are generally needed for a spectro­ e c IrIlrschi speClrograph y chirps / s =-1,13 + 0,12.% 0,54 48 graph. The fact that the chirp rate of O. capensis changes n e rycomorus spectrograph y chirps / s =-6,44 + O,74.x 0,90 21 faster with temperature than that of O.fultoni or O. karschi c i l

means that it is a better thennometer cricket in certain r e respects. Calculation of temperature from chirp rate is d n Table 2 Simplified regression equa­ accurate within the limits of ± 2°C for O. capensis, while u

y tions for the calculation of tempera­ in O. karschi this drops to ± 4°C (Table 3). This also a ture from chirp rate. The first equa­ w means that two O. karschi males at the same locality, and e t tion for O. fultoni was published by the same temperature, could produce songs with different a

G Walker (1962a) chirp rates. Convergence of regression lines at the X-inter­ t e cept makes it possible to make relatively reliable predictions n Species Regression equation i

b about variation in songs even if only one tape recording is a julto1li .x"F = No. chirps in 13 s + 40

S available. However, the X-intercepts for crickets of different

y fulto1li .x°C = No. chirps in 8 s + 4 faunas may be different, so predictions of this nature will be b capellSis .x°C = No. chirps in 3 s + 11 d more reliable for new members of a fairly well-known fauna e karschi .x°C No. chirps in 6 s + 12 c = than for completely unknown groups. u d o r p e R S. Afr. I. Zoo!. 1992,27(2) 73

Problems with mate recognition References Signals which change with temperature pose special prob­ BENNET-CLARK, H.C. 1989. Songs and the physics of sound lems for the receiver. A wide band receiver, capable of re­ production. In: Cricket Behavior and Neurobiology. (Eds) ceiving the entire range of variation in signals, would also Huber, E, Moore, T.E. & Loher, W. 8: 227-261. be capable of receiving excessive noise, including sounds BLOCK, B.C. 1966. The relation of temperature to the chirp-rate produced by other species. For example, O. capensis and O. of male snowy tree crickets, Oecantluu fuitoni (Orthoptera: karschi both call at about 2,2 chirps per second, but at ). Ann. ent. soc. Am. 59: 56-59. different temperatures (Figure 2). When additional data for BROOKS, M.W. 1882. Influence of temperature on the chirp of O. capensis are available, I suspect that it will be found that the cricket. Pop. Sci. Mon. 20: 268. they can continue to sing at temperatures down to at least DOLBEAR, A.E. 1897. The cricket as a thermometer. Am. Nat. 13°C, at rates at least as low as one chirp per second. This 1897: 971. would mean that chirp rates between 1 and 2,2 could be pro­ MA YR, E. & DOBZHANSKY, T. 1945. Experiments on sexual duced by either species. With regard to pulse rates, a similar isolation in Drosophila IV. Modification of the degree of problem exists. O. capensis and O. karschi call at similar isolation between D. pseudoobsclUa and D. persimilis and of pulse rates and have similar ranges of pulse rates with some sexual preferences in D. prosaltans. Proc. Natl Acad. Sci. 31: overlap in the available sample (foms & Otte 1988: Figure 75-82. 20). In trilling species O. neosimilis, O. ruJopictus and O. ga/pini are all capable of singing at 30 pulses per second at PROZESKY-SCHULZE, L., PROZESKY, O.P.M., ANDERSON, E and VAN DER MERWE, G.IJ. 1975. Use of a self-made different temperatures (foms & Otte 1988: Figure 26a). Although the extent to which cricket songs change with baffle by a tree cricket. NatlUe 255: 142-143. temperature can be considerable, Walker (1957) showed that SKAIFE, S.H. 1953. African Life. Longmans, Cape Town. the female response may also be affected by temperature. In 387 pp. other words, the female auditory system may only be capa­ SIS MONDO, E. 1979. Stridulation and tegminal resonance in the ble of perceiving a limited range of the possible variation in tree cricket . J. Compo Physiol. 129: male songs as appropriate signals at any given temperature. 269-279. The fact that the regression lines of different species of TOMS, R.B. 1984. Directional calls and effects of turning crickets converge at the X-intercept means that the possibili­ behaviour in crickets. J. ent. Soc. sth. Afr. 47: 309-312. ty for confusion may increase at low temperatures in certain TOMS, R.B. 1988. A phylogenetic approach to the evolution of species. In Drosophila almost no hybridization occurs acoustic communication with special reference to crickets.

. Unpublished Ph.D. thesis, University of the Witwatersrand, ) between Drosophila pseudoobscura and D. persimilis at 0

1 24,soC but at 16,5°C heterogametic and homogametic ma­ Iohannesburg. 0

2 TOMS, R.B. & OITE, D. 1988. New genera, species and records tings occur at almost the same frequency, when D. persimi­ d of East and Southern African tree crickets (Orthoptera: e lis males are used (Mayr & Dobzhansky 1945). Temperature t a dependent changes in receptivity could occur as a result of Gryllidae: Oecanthinae). Ann. Tvl. Mus. 34: 469-521. d ( different rates of change in metabolic rates of different WALKER, T J. 1957. Specificity in the response of female tree r e crickets to the calling songs of the males. AM. Ent. Soc. Am. h species, and the convergence of regression lines at low tem­ s i l peratures. In order to understand these changes, it would be 50: 626-636. b

u necessary to understand how changes in temperature affect WALKER, T J. 1962a. The and calling songs of P emitters and receivers. Although little is known of the United States tree crickets (Orthoptera: Gryllidae: e h t mechanisms of thermal variation in metabolic rates affecting Oecanthinae). I. The genus Neoxabea and the niveus and y transmitters, even less is known about thermal variation in varicornis groups of the genus Oecantluu. AM. Ent. Soc. Am. b

d receivers. 55: 303-322. e t WALKER, T J. 1962b. Factors responsible for intraspecific n a r Acknowledgements variation in the calling song of crickets. Evolution 16: g

e L.E.O. Braack, I. Garland, D. Otte, N.I. Passmore, H.E.H. 407-428. c n Paterson, I. Ie Roux, E.H. Taylor, L.R. Taylor and the WALKER, T J. 1969. Acoustic synchrony: two mechanisms in e c i the snowy tree cricket. Science 166: 891-894. l National Parks Board are thanked for their help in various

r WALKER, TJ. 1975. Effects of temperature on rates in e ways. Special thanks are due to two anonymous referees for d poikilotherm nervous systems: evidence from the songs of n their suggestions. Financial support by the Foundation for u Research Development and the Transvaal Museum is grate­ meadow katydids (Orthoptera: Tettigoniidae: Orchelimum). J. y a fully acknowledged. compo Physiol. 101: 57-69. w e t a G t e n i b a S y b d e c u d o r p e R