Contribution of Individual Honeybees to Social Thermoregulation

Research project P 13916 Contribution of Individual Honeybees to Social Thermoregulation Anton STABENTHEINER 28.06.1999 Despite their small size honeybees are able to regulate body temperature at a similarly high level as mammals or birds during long periods of time. They use this ability not only as a preparation for proper flight or to achieve high motility but also to guarantee fast development of their brood by regulating the brood nest temperature at a high level (34 C). The mechanisms that coordinate heat production of the several thousand individuals of a honeybee colony for the purpose of a constant nest climate are only partly understood. The investigations concentrated mainly on the general description of the microclimate within a colony and the effect of varying ambient conditions on it. It is known that it is mainly the bees and not the larvae themselves which take care for an optimal brood nest temperature. However, there is a great lack of knowledge about which bees produce the necessary heat for brood rearing. The present project, therefore, will focus on the question of which age classes of the bees are responsible for colonial heat production. Heat production of individual bees will be determined by indirect calorimetry through measurement of O2 consumption and/or CO2 production. However, it is known that the bees` body temperature and thus their heat production may change drastically and in an unpredictable manner by transferring them from the social context in the colony to a gas measurement system. Therefore, heat production during natural behavior will be determined in a way that the body temperature inside the intact colony is measured by infrared thermography - without contact. From these temperatures - for the first time heat production can be calculated via calibration curves determined from simultaneous measurements Of O2 consumption (or CO2 production) and body temperature of individual bees. Bees will be kept in temperature controlled observation hives which allow them to fly out and to forage without restriction. Body temperature and heat production of marked bees will be compared between standard conditions and conditions of "cold stress" (during cooling of the hive), related to the type of nursed brood and whereabouts (larvae or pupae, drone brood or queen cells, honey or pollen stores), and compared between daytime when the bees fly out of the hive and forage, and nighttime when no foraging occurs and the colony life slows down. The contribution of different age classes to heat production will be corrected according to the age distribution of the hive bees and compared with the energy turnover of the whole colony. Freshly hatched bees are unable to activate their flight muscles for the purpose of heat production. Therefore, to allow proper interpretation whether the bees of an age class decide to stay cool though they would have the ability of endothermic heat production or whether they are not able to heat their thorax up, we will investigate at what age bees become able to contribute significantly to thermogenesis. The method of infrared thermography and its combination with O2 consumption (CO2 production) measurements will allow a completely new approach to the raised questions. The unique approach of the project will provide the international scientific community with further missing links which will allow a better understanding of the mechanisms of colonial thermoregulation. 6

Honeybees are, unlike other insects, capable of intense heat production with their flight muscles without wing movement. They use this ability to heat their brood nest to temperatures of 34-36 C. We investigated the contribution of individual honeybees to social thermoregulation. In particular, we investigated the question of whether there exists an age-dependent division of labour concerning active (endogenous) heat production. Infrared thermography enabled us to measure the bees` body temperature during different tasks without disturbance of their social interactions. The experiments showed that 2 to 40 days old bees contribute equally to colonial heat production, i.e. that there is no age-dependent division of labour as can be found for other social tasks. Only bees younger than two days contribute less to heat production, because they are not yet able to activate their flight muscles for this purpose after emergence from the brood cells, and reach full endothermic capacity only at an age of 5-7 days. If it is to determine an individual bee`s heat production during the normal colony life, however, direct methods - by transferring them from the social context in the colony to a respiration measurement system - are not applicable, because in this case their heat production may change drastically (up to 500-fold) and in an unpredictable manner. As an indirect approach we measured their respiration (which can be used to determine heat production) and body temperature simultaneously, and used the correlation of both to estimate their heat production during undisturbed colony life from body temperature measurements. Many bees of a colony, however, are not engaged in active heat production. They remain at air temperature. Their contribution to heat production is passive and smaller than that of the bees with activated flight muscles. Their heat production depends on their position in the variable thermal environment of the colony. The same holds for the larvae. We found that the respiratory metabolism of honeybee larvae increases with temperature much more than in inactive worker bees. Our experiments suggest that the strong temperature dependency of larval metabolism allows the bees to speed up brood development particularly strong by regulating its temperature at a high level (34-36 C). On the other hand, this temperature dependency forces them to regulate the brood temperature within narrow limits in order to avoid delayed brood development during cold weather.