Odonatologica 31(2): 129-138 June 1, 2002 Activity patterns and thermoregulation in a tropical dragonflyassemblage P. De Marco+Jr. * and D.C. Resende Laboratory ofQuantitative Ecology, Department ofGeneral Biology, Federal University of Viçosa, BR-36571-000 Viçosa, MG, Brazil. Received February 6, 2001 / Revised andAccepted October 2, 2001 is Solar exposure a key factor determining odonate activity, particularly in tropical areas. Small sized perchers, classified as thermal conformers, can begin their activity when air temperature is sufficiently high, and larger species become active when direct the is In the in exposure to sun possible. this study, activity patterns a neotropical dragonfly assemblage present on the Federal University of Vigosa, SE Brazil, have been described and tested: decrease in followingpredictions about their thermoregulatorybehaviour (a) a the in the activity of percher dragonflies warmest periods is expected due to highthoracic conformers will be controlled temperatures; (b) species by temperature, not luminosity, whereas in heliothermic species, the initiation and termination of their activity is only In the low air constrained by luminosity. dry season, temperatures represent a limiting factor the in to beginning and the end of activity, resulting a shorter total activity time. Orthemis discolor and Micrathyria hesperis showed a decrease in activity in the middle of the in the Perithemis that had day rainy season. mooma was the only sp. a higher abundance near midday. As this sp. had a light-colouredthorax compared to the others, it is suggested that it could minimize the effect of the high temperatures. There is a clear also differences in the effect of seasonon activity time, and large intensity ofthis effect When direct among species. clouds precluded exposure to sun, variations only in the did affect M. temperature not the activity of Erythrodiplaxfusea. hesperis and O. discolor, the small I but the activity of sized P. mooma remained dependenton temperature. These results that the minimum size heliotherm could be highlighted body to be a a complex function ofbehavioural andmorphologicalcharacteristics, includingbody colour,preferred substrate and perch posture. INTRODUCTION to MAY the odonates could be classified into three with According (1976), groups respect to their ability to deal with variations in ambient temperature: (a) thermal * Correspondentauthor 130 P. De Marco Jr & D.C. Resende conformers; (b) heliotherms or (c) endotherms. It is mainly the relationship between heat exchange and body size that determine this classification. The of heat with processes exchange the environment are convection heat exchange andirradiation (CHE) heat exchange (IHE). CHE is dependent on the surface to volume but IHE is the surface ratio, only dependent on area directly exposed to the sun. From smallto there is decrease in the surface volume with large species, a to ratio, a consequent decrease in CHE, and an increase in the importance ofIHE for thermoregulation. Thermalconformers show high conductanceand their body temperature varies with environmental temperature, mainly due to convection heat exchange. These species be of small size. The heliotherms are expected to a have a larger size and consequently lower conductance. Their activity must be primarily determinedby solar irradiation. The endothermic from species escape the previous scenario by producing endogenous heat and controlling their haemolymph circulation to enable them to thermoregulate (HEINRICH & CASEY. 1978;MAY, 1991;HEINRICH, 1993). MAY (1976) showed that, in heliotherms, the conductance coefficientand thermoregulatory ability decrease with size. This within the body suggests that, even same physiological group, a relationship is expected between thermoregulation and body size. Odonate often classified species are behaviourally as perchers and fliers (CORBET, 1962,1999;MAY, 1976,1991;DEMARCO, 1998). This behaviouralclassification is directly relatedto thermoregulation. Percher species are usually thermal conformers or heliotherms. Small sized perchers, classified as thermal conformers, can begin their when the air is activity temperature sufficiently high. Larger species can be active as soon as direct exposure to the sun occur. Otherwise, it is possible that at dawn only thermal conformers could maintain their activity with lower luminosity but with air the minimum critical temperature exceeding temperature for flight. odonates Very larger may rely on endothermy for body temperature to exceed the minimumcritical for since convection temperature flight, heating or irradiationheating will be this the which a time-consuming process. Following argument, fliers, are recognised as endotherms, must be the larger species. Males ofpercher species spend most of their time in active territorial disputes that include fast flights and intense thoracic muscle activity, increasing the thoracic temperatures (MAY, 1977; MARDEN, 1989). Other flight activities such as guarding females and also could increase thoracic mating temperatures (ISHIZAWA, 1998). Thoracic in odonates is elevated much 40-45°C temperatures some to as as (POLCYN, 1994; MARDEN et al„ 1996), near to the lethal temperature for some species: 48,5°C (MARDEN, 1995). Due to the general relationship between convection heat exchange, irradiation heat and exchange body size, some general predictions were made concerning the activity ofthe odonate in percher species a given assemblage: (a) a decrease in activity of the percher dragonflies in the warmest moments was expected due to high thorax temperatures (HEINRICH & CASEY, 1978; MAY, 1979); (b) thermal conformers species, usually with small size, will be controlled by temperature, not luminosity. in 131 Activity patterns a tropical dragonflyassemblage whereas larger percher species, usually heliothermic, initiateand terminatetheir activity only as constrained by luminosity. Very large species are fliers and principally endothermic. this In study we aimed to describe the general activity patterns in a neotropical dragonfly assemblage and relate these data to the predictions about their thermoregulatory abilities. METHODS Dragonfly activity patterns were studied in two adjacent ponds on the Federal University of Vi$osa, Vi?osa, Minas Gerais, Brazil (20°45’S, 42°5TW). The climate ofthe region is wet sub-tropical, with the dry season from May to September. (GOLFARI, 1975). Mean annual rainfall is between 1500 and 2000 mm, with relative humidity about 80%, and the mean annual temperature ranged from I4.0°C to 26.1°C (VALVERDE, 1958). observations made and each month from Quantitative were during August 1994 on at least one day 1996 1997. 1800h. 15min February to January These observations began at 0600h and finished at Every a in walk was made at a constant velocity past the two ponds and every individual was counted oneminute weather recorded each walk. intervals. Air temperature, measured in shadow, and conditions were during Data to and and also were groupedaccording dry (March to September) wet(October to February) season grouped in intervals ofone hour duringthe day. To test the predictions about the effects of luminosity onactivity of these dragonflies the relationship between the of active individuals and presence temperature was investigated using data from the rainy and for and A season, carrying out a separateanalysis sunny cloudysamples. Logistic Regression approach 2 was used to analyse this dependency, using x to test for significant results according to HOSMER & LEMESHOW We called Time the difference between the final time the (1989). Activity that species was observed to be active to the initial time that it was active. For analyses that concerned body size, the hind wing length was measured for at least five males in each species. GENERAL BEHAVIOURAL OBSERVATIONS The most abundantpercher dragonfly species found in the ponds were: Perithemis mooma Kirby, Orthemis discolor (Burnt.), Erythrodiplax fusca (Ramb.), E. media Borror and Micrathyria hesperis Ris. P. mooma is an amber-winged dragonfly that usually perched on emergent macrophytes and at low heights (10 to 20 cm). They were aggressive against is that conspecifics. M. hesperis a clear-winged dragonfly usually perched on emergent and 20 This floating vegetation (10 to cm). species was not very aggressive and were observed usually interacting with P. mooma. Both E. mediaandE.fusca usually perched on the marginal vegetation and on the soil around the lakes. When perched inside the lakes, they chose perches similar to P. mooma and M. hesperis. O. discolor selected perches often higher than 50 cm. They were very aggressive against conspecifics. A single male appeared to defend the whole pond and they rarely interacted with the other percher species. 132 P. De Marco Jr & D.C. Resende ACTIVITY PATTERNS In all species the females were rarely observed at the ponds. They usually appeared 10:00 for at h, except O. discolor, which had an additional peak density at 14:00 h in the rainy season (Fig. 1 ). In all species, femalesarrived at the pond and usually copulated for a few minutes. It is possible that some females stayed so briefly at the pond that they could notbe detected duringthe observationsbut, with respect to thermoregulation, these events were ofminor importance. Fig. 1. Daily variation of air temperature (A); also male (squares) and female (circles) abundance of O. discolor E. P. M. in the (B), E. fusca (C), media (D), mooma (E), hesperis (F), rainy season(open symbols) and dry season(filled symbols) in twoadjacent ponds in MG. Bars represent standard deviation. in Activity patterns a tropical dragonfly assemblage 133 Table I Seasonal variation in total activity time and air temperatures in the initial (T ) and final (T ) activity of in fi in MG dragonflies a pondat Vigosa, Hind wing Dry season Wet season Species Size (mm) T(°C)T, (°C) T (°C) Activity T(°C)T (°C) T (°C) Activity Species fin fln n n timetime (h) time (h) P. mooma 20.620.6 26.5 22.0 5.00 24.024.0 27.5 8.75 M. hesperishesperis 23.723.7 22.5 24.0 5.50 24.024.0 30.5 8.75 E. media 25.025.0 20.0 26.0 5.50 — — — E.E.fuscafusca 25.7 18.018.0 28.0 5.25 25.025.0 31.0 7.50 O.