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Respiration Rate and Pattern in the Woollybear Caterpillar

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Respiration Rate and Pattern in the Woollybear Caterpillar The Journal of Experimental Biology 202, 47–53 (1999) 47 Printed in Great Britain © The Company of Biologists Limited 1998 JEB1717 METABOLIC OPPORTUNISTS: FEEDING AND TEMPERATURE INFLUENCE THE RATE AND PATTERN OF RESPIRATION IN THE HIGH ARCTIC WOOLLYBEAR CATERPILLAR GYNAEPHORA GROENLANDICA (LYMANTRIIDAE) VALERIE A. BENNETT1,*, OLGA KUKAL2 AND RICHARD E. LEE JR1 1Department of Zoology, Miami University, Oxford, OH 45056, USA and 2Department of Biology, University of Victoria, Victoria, British Columbia, Canada V8W 3P6 *e-mail: [email protected] Accepted 22 October; published on WWW 7 December 1998 Summary Arctic woollybear caterpillars, Gynaephora larvae. The seasonal capacity to adjust metabolic rate groenlandica, had the capacity to rapidly and dramatically rapidly in response to food consumption and temperature increase respiration rates up to fourfold within 12–24 h of (which can be elevated by basking) may promote the feeding and exhibited similar decreases in respiration of efficient acquisition of energy during the brief (1 month) 60–85 % in as little as 12 h of starvation. At the peak of summer growing and feeding season, while conserving their feeding season, the respiration rates of caterpillars energy by entering diapause when conditions are less also increased significantly with temperature from 0.5 to favorable. These adaptations, along with their long 15–20 22 °C for both fed and starved caterpillars (Q10=1–5). year life cycle and the retention of freeze tolerance year- Indicative of diapause, late season caterpillars had round, promote the survival of G. groenlandica in this depressed respiration rates which were less sensitive to harsh polar environment. temperature changes (Q10≈1.5), while respiration rates for caterpillars that had spun hibernacula were even lower. G. Key words: caterpillar, Gynaephora groenlandica, metabolic rate, groenlandica did not appear to demonstrate metabolic cold feeding, temperature, respiration, specific dynamic action, arctic adaptation compared with other temperate lepidopteran insects. Introduction Polar environments impose great challenges upon During the feeding period in June, the larvae spend most of organisms, the most obvious being extremely low temperatures their time thermoregulating by basking in the sunshine, raising and short summer growing seasons. Polar organisms, therefore, their body temperature by as much as 25 °C above the ambient must be masters at meeting these challenges: they endure long temperature (Kukal et al., 1988a). Curiously, before the main periods of darkness, cold, desiccation and starvation between peak of summer in July, the caterpillars cease feeding and spin short periods of relatively milder temperatures, limited hibernacula concealed within plant cushions and crevices moisture and patchy food sources. During these brief windows (Kukal, 1995). This diapause may provide some protection of opportunity, it is critical, particularly for ectotherms, that from the parasitoids Hyposoter pectinatus and Exorista sp. by they maximize energy input for growth and reproduction, behavioral avoidance (Kukal, 1993); however, it also further while minimizing energy expenditure during periods of less restricts an already narrow window of opportunity to exploit favorable conditions. another potential month of warm temperatures and abundant The arctic woollybear caterpillar [Gynaephora groenlandica food used by many other arctic insects. Within these (Wöcke), Lymantriidae] has one of the longest life cycles of hibernacula, the larvae remain inactive, at temperatures close all insects. It requires 14 years to complete its developmental to 0 °C, until they freeze in late summer at approximately −8 life cycle at Alexandra Fiord, Ellesmere Island (Kukal and to −10 °C. During this period of hypothermic conditioning Kevan, 1987), and probably closer to 20 years further north at before the onset of very low winter temperatures, the larvae Lake Hazen (O. Kukal, unpublished data). With the exception undergo mitochondrial degradation and demonstrate a lowered of 1 month during a single summer, their entire life span is oxidative metabolic rate (Kukal et al., 1989). This hypoxic spent in the larval stage. When acclimated to winter conditions, state occurs concomitantly with a build-up of cryoprotective the larvae are freeze-tolerant to at least −70 °C and, unlike most compounds, particularly glycerol, which may enhance their freeze-tolerant invertebrates, they retain the capacity to survive increased freeze tolerance during the winter. The mitochondria freezing to −15 °C even during the summer months (Kukal et are resynthesized relatively rapidly when the caterpillars al., 1988b). emerge from winter hibernacula the following spring. 48 V. A. BENNETT, O. KUKAL AND R. E. LEE Metabolic rate is an indirect indicator of the energy status 5 ml plastic syringes which were placed in a constant- of ectothermic animals, such as most insects (Heinrich, 1993; temperature bath (2L-M Isotemp Water Bath, Fisher Scientific, Mill, 1985). The overriding factor that dictates the diel and Pittsburgh, PA, USA). A disposable 20 µl micropipette was seasonal changes in behavior, and consequent changes in attached using hot glue to the tip of each syringe prior to metabolic rate, of these larvae appears to be temperature sealing the larva into the syringe. Once equilibrium had been (Kukal, 1991). However, our study indicated that the metabolic reached in the temperature bath, approximately 2.5 µl of 10 % rate is not only dependent on temperature but is also under the KOH (which was sufficient to absorb all the CO2 produced) influence of seasonal phenology and the feeding and was introduced into the micropipette tip to function as a physiological state of the caterpillars. manometer and to absorb CO2. An empty syringe with KOH This study examined how the respiration rate and pattern in its micropipette tip provided a control. When the larvae had reflect the energy status of Gynaephora groenlandica larvae. equilibrated for at least 15 min, the distance moved by the More specifically, the interaction between nutritional status KOH column as the larvae consumed O2 was read to the and temperature was examined in the following contexts: (1) nearest millimeter every 0.5–10 min, depending on bath by analyzing changes in larval respiration rate during temperature. New syringes and micropipettes were used for alternating periods of feeding and starvation; (2) by each new set of measurements. Once a steady rate of oxygen investigating the influence of body temperature on the rate of consumption had been established, a mean value was respiration and how this relationship changes in feeding versus calculated. Caterpillar live masses were the same before and starved larvae; (3) by comparing the respiration rate of larvae after respiration measurements (V. A. Bennett, O. Kukal and at the peak of their feeding season with that once they have R. E. Lee, unpublished data); therefore, there was no detectable stopped feeding and have spun hibernacula; (4) by comparing water loss during the course of measurements. This method the respiration rates of this polar species with those of other produces results that closely match those of other techniques Lepidoptera. (see Lee and Baust, 1982a,b). Influence of larval body temperature and feeding status on Materials and methods metabolic rate Study site, microhabitats and animals The relationship between body temperature and larval The experiments were conducted during June 1995 at Hazen metabolic rate was determined for larvae early in their feeding Camp, Ellesmere Island, North West Territories, Canada season by measuring rates of oxygen consumption in 11 larvae (81°49′N, 71°22′W). This site is one of the largest polar oases of similar body mass (instars IV and V) collected during the in the Canadian High Arctic Archipelago. It is characterized first week of June. Respiration rates were assessed at 0.5, 15, by higher temperatures and moisture levels compared with the 22 and 30 °C. surrounding polar desert (Parks Canada, 1994). The influence of starvation on the metabolic rate in larvae Two major microhabitats utilized by the larvae of G. collected during the second week of June (peak feeding season) groenlandica are found in the Lake Hazen area: patches of was determined. Nine larvae were held individually in plastic arctic willow Salix arctica, which are the primary site of 25 ml vials at 0 °C for 48 h in darkness, and then transferred feeding, and hummocks of Dryas integrifolia, where directly to 15 °C for repeated oxygen uptake measurements hibernacula are frequently found (Kukal, 1995). S. arctica over 12–24 h, during which time the larvae were starved. microhabitat temperatures were recorded during the feeding Subsequently, they were removed from respiration chambers season of the larvae in June using StowAway temperature and fed for 12 or 24 h with young leaves of S. arctica at 15 °C. loggers (Onset Computer Corp., Pocasset, MA, USA). The Oxygen uptake was again determined repeatedly at 15 °C over surface temperature probes were placed so that they were fully the next 12–24 h post-feeding. These cycles of feeding and exposed to the rays of the sun. Ambient temperature at ground starving with corresponding oxygen uptake measurements level was recorded using a shaded probe in the S. arctica were repeated three times over the course of 96 h. habitat. To determine whether this experimental protocol of alternate Most of the caterpillars were collected during the first
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