Functional Ecology 2004 18, 925–930 © 2004 British Ecological Society 925 Blackwell Publishing, Ltd. Respiration and thermogenesis by cones of the Australian cycad Macrozamia machinii R. S. SEYMOUR,*† I. TERRY‡ and R. B. ROEMER§ *Department of Environmental Biology, University of Adelaide, Adelaide 5005, Australia, ‡Department of Biology, and §Department of Mechanical Engineering, University of Utah, Salt Lake City, USA Summary 1. While cycads are often considered to be wind-pollinated, it is now clear that insects are pollen vectors in many species. This study addresses the role of thermogenesis in pollination biology of the dioecious cycad Macrozamia machinii P.I. Forster & D.L. Jones. 2. The patterns of thermogenesis in intact male and female cones were assessed with thermometry and respirometry throughout the pollination period in the field. 3. Thermogenic episodes in male cones occurred from about 17.00–00.00 h on succes- sive evenings, in association with dehiscence of sporangia and presence of their pollinating weevils (Tranes sp.). 4. Temperatures of the 167 g male cones rose ≈6 °C above ambient, and mean rate of oxygen consumption peaked at 7·7 μmol s -1 (3·6 W). Regulation of male cone temper- ature was not evident, and thermogenesis of female cones was insignificant. 5. Male cones probably heat to augment scent production and enhance weevil activity, including mating and egg-laying, but female cones may benefit from reduced visitation and freedom from damage by weevil larvae. Male cones may be sacrificial in providing the reward to the pollinators while the female cones are safeguarded. Key-words: cone, Cycadaceae, heat production, pollination, respiration, temperature Functional Ecology (2004) 18, 925–930 Introduction Pollination biology of cycads is characterized by thermogenic cones and insect pollen vectors (Norstog & Stevenson 1986; Tang 1987a, 1987b; Donaldson 1997; Terry et al . 2004). The pattern in these gymno- sperms is similar in some respects to that in several groups of basal angiosperms, which have thermogenic flowers or inflorescences coupled with insect pollina- tion (Gottsberger 1990; Endress 1994; Seymour & Schultze-Motel 1997; Thien, Azuma & Kawano 2000). Cycads are the older group (Soltis & Soltis 2003) and may represent the origin of insect pollination, although extinct seed ferns and gymnosperms (Cayto- niales and Bennettitales) may also have been insect pollinated (Labandeira 2002; Pellmyr 2002). Insects, primarily beetles and thrips, are attracted to the cones, where they find rewards in the form of food, mating sites and brood sites for larvae (Tang 1987a; Donaldson 1997). Thermogenesis probably promotes scent production to attract insects (Terry et al . 2004) and may have other functions important to the plant, possibly including the timing of pollen development and release. The warm environment in the plant has also been hypothesized to provide a reward for insect visitors by enhancing their activities, for example feeding, digesting, mating and egg-laying (Seymour & Schultze-Motel 1997), and this has been demonstrated quantitatively for beetles visiting thermogenic arum lilies (Seymour, White & Gibernau 2003b). To extend our knowledge of the physiological correlates of the pollination biology of cycads, we studied thermogenesis in intact male and female cones of the endemic Australian cycad Macrozamia machinii throughout the pollination sequence in the field, and in relation to observations of insect activities. Thermogenesis by cycad cones has been assessed from temperature elevations in intact cones (Tang 1987a, 1987b; Tang, Sternberg & Price 1987; Terry 2001; Terry et al . 2004), and metabolic rate in isolated sporophylls, measured either as carbon dioxide pro- duction (Tang et al . 1987) or direct heat production in a calorimeter (Skubatz, Tang & Meeuse 1993). These studies have clearly associated thermogenesis with pollination biology, and have provided the first meas- urements of metabolic intensity during thermogenesis. However, because temperature elevations are influenced by mechanisms of heat loss, principally evaporation, convection and radiation, they may not be accurate †Author to whom correspondence should be addressed. E-mail: roger.seymour@adelaide.edu.au