Martin Nichelmann

Ontogeny of thermoregulation in precocial birds.

The aim of this paper is to summarise the results of earlier experiments on thermoregulation and heat balance in birds, to present new results concerning thermoregulation during the perinatal period in precocial embryos and to develop a model of the ontogeny of thermoregulation over the whole lifespan of birds. The ontogeny of thermoregulation in precocial birds is characterised by three phases with different efficiency of the system. In the prenatal phase, all control elements of the thermoregulatory system can function, but the efficiency of the system is low. It is postulated that endothermic reactions during the prenatal period do not have a proximate (immediate), but rather an ultimate influence on the efficiency of thermoregulation. They may support adaptivity to expected environmental conditions and may be involved in epigenetic adaptation processes. During the early postnatal phase, the thermoregulatory system develops and matures. Summit metabolism and resting metabolic rate and their thermoregulatory set points increase. Preferred temperature is significantly different during different behavioural activities. The phase of full-blown homeothermy starts at approximately the 10th day of life. It is characterised by an activation order of thermoregulatory control elements and by secondary chemical thermoregulation. The influence of thermal and non-thermal climatic factors on heat production and heat loss may be described by mathematical models.

Development of heart rate irregularities in chick embryos

Heart rate (HR) irregularities in chick embryos were defined as large fluctuations (>10 beats/min) comprising irregular, brief deceleration and/or acceleration of instantaneous HR (IHR). IHR was determined directly from the arterial blood pressure while adequate gas exchange was maintained through an eggshell and chorioallantoic membrane. Five embryos were examined on each day from day 11 to day 19 of incubation. Baseline HR was stable until day 12-13, and on around day 13-14 transient, rapid deceleration of HR (termed V pattern) began to appear, with a subsequent increase in its frequency and magnitude. The acceleration patterns (lambda, avian omega, and periodic patterns) appeared later, and the IHR became increasingly irregular, with additional, spontaneous deceleration and acceleration patterns toward hatching. Additional experiments with intravenous administration of autonomic drugs clearly showed that rapid deceleration of HR was mediated by parasympathetic nervous function but did not always show clear relations of sympathomimetic and sympathetic blocking agents to the acceleration patterns.Heart rate (HR) irregularities in chick embryos were defined as large fluctuations (>10 beats/min) comprising irregular, brief deceleration and/or acceleration of instantaneous HR (IHR). IHR was determined directly from the arterial blood pressure while adequate gas exchange was maintained through an eggshell and chorioallantoic membrane. Five embryos were examined on each day from day 11 to day 19 of incubation. Baseline HR was stable until day 12-13, and on around day 13-14 transient, rapid deceleration of HR (termed V pattern) began to appear, with a subsequent increase in its frequency and magnitude. The acceleration patterns (lambda, avian omega, and periodic patterns) appeared later, and the IHR became increasingly irregular, with additional, spontaneous deceleration and acceleration patterns toward hatching. Additional experiments with intravenous administration of autonomic drugs clearly showed that rapid deceleration of HR was mediated by parasympathetic nervous function but did not always show clear relations of sympathomimetic and sympathetic blocking agents to the acceleration patterns.

Some characteristics of the biological optimum temperature

Abstract 1. 1.|The concept of the biological optimum temperature (BOT) is developed. 2. 2.|BOT constitutes the ambient temperature ( T a ) at which the organism is expsed to minimum thermal stress. It can be derived from the relationship between ( T a ) and rectal temperature as well as between ( T a ) and the level of energy metabolism. 3. 3.|BOT is not constant but varies with age, breed, time of day, acclimatization, floor conditions, group size, air humidity, air velocity and amount of food intake.

Biological rhythms in birds--development, insights and perspectives.

The aim of this review is to show that probably the internal clock of precocial birds is imprinted in the prenatal period by exogenous factors (zeitgeber). The activity of organ functions occurs early during embryonic development, before this function is ultimately necessary to ensure the survival of the embryo. Prenatal activation of some functional systems may have a training effect on the postnatal efficiency. The development of physiological control systems is influenced by endogenous and exogenous factors during the late prenatal and early postnatal period: epigenetic adaptation processes play an important role in the development of animals; they have acquired characteristics which are innated but not genetically fixed. As a rule, the actual value during the determination period has a very strong influence on the set-point of the system. This will be explained using the example of thermoregulation. It is shown in detail that it seems to be possible to imprint the prenatal development of circadian rhythms by periodic changes of the light-dark cycle but not by rhythmic influence of acoustic signals. Altogether, there are more questions open than solved concerning the perinatal genesis of circadian rhythms in birds. Topics are given for the future research.

Influence of Prenatal and Postnatal Acclimation on Nervous and Peripheral Thermoregulation

The aim of the present study was to investigate whether prenatal and postnatal adaptation to different ambient temperatures affects the autonomic (heat production, heat loss, rectal or colonic temperature), behavioral (preferred ambient temperature) and nervous mechanisms (neuronal thermosensitivity of the preoptical area of the anterior hypothalamus) of thermoregulation. The experiments were carried out in postnatal, differently acclimated adult rabbits (60 days at 6-7, 20 and 30 degrees C) and adult rats (3 to 6 weeks at 5 and 21 degrees C) and in differently incubated 1- to 10-day-old Muscovy ducklings and turkeys (last week of incubation at 34.5, 37.5 and 38.5 degrees C). The results of the experiments are summarized as follows: (1) Postnatal acclimation changes the threshold ambient temperature of heat loss and heat production. For example, cold-acclimated rabbits have a lower threshold temperature for evaporative heat loss and thermoregulatory heat production than heat-acclimated ones. (2) Prenatal acclimation changes postnatal thermoregulatory behavior as well as autonomic thermoregulatory mechanisms. Birds incubated at higher (38.5 degrees C) or lower (34.5 degrees C) temperatures than the usual 37.5 degrees C for the last week of embryonic development have higher or lower preferred ambient temperatures during the first 10 days post hatching. Besides this, cold-incubated birds have a higher heat/ production and clonic temperature in the first days post hatching than normally/ incubated or heat-incubated ones. (3) Extracellular recordings from hypothalamic neurons in brain slices from differently acclimated rats have shown that adaptation to different ambient temperatures changes firstly the temperature sensitivity of the hypothalamic neurons and secondly the modulatory action of the neuropeptides bombesin and thyrotropin releasing hormone.

Avian Thermoregulation during the Perinatal Period

Three different types of processes occur during the perinatal development of temperature regulation in avian species: (a) development of endothermy, (b) epigenetic determination of the thermoregulatory system, (c) thermoregulatory learning processes in the early postnatal phase.

A method for noninvasive, long-term recording of the avian embryo heart rate

A technique for obtaining a noninvasive electrocardiogram (ECG) from avian embryos is described. Accounts of electronic signal processing and computer-aided long-term recording are included. Furthermore, an algorithm is demonstrated for extracting the relevant information on heart rate from noisy data samples. The method provides the basis for monitoring the heart rate of the avian embryo on different time scales and under different influences.

Efficiency of Thermoregulatory Control Elements in Precocial Poultry Embryos

Toexplainallaspectsof theefficiencyof thethermoregulatorysysteminpoultryembryos,theactivityandthedynamicsofdifferent control elements have to be investigated systematically during the prenatal period.The relevant methodology and results, citedinthispaperaremainly fromourowngroupbutarecomparedwithrelatedexperimentsdescribedinthescientificliterature. They may be summarised: Thermoregulatory behaviour is developed early in poultry embryos. Temperature of allantoic fluid (Taf) represents the internal body temperature in the last third of incubation because Taf and Tc are much the same at normal (37.5C) and low incubation temperatures (34.5C) after internal pipping. Fowl embryos show endothermic reactions from day14 and Muscovy duck embryos from day 22. In precocial avian embryos, body core temperature increases in accordance with heat production. In contrast to heat production the efficiency of heat loss is high in precocial avian embryos. Development of physiologicalcontrol systems starts withnon-co-ordinatedandproximate (immediate) non-adaptive reactions. In summary, endothermic reactions occur very early during embryonic development but their efficiency is limited. Due to the van’t Hoff rule, low temperatures decrease the net heat production but the trajectories of endothermy are stimulated and related epigenetic adaptation mechanisms are activated. Using behavioural mechanisms the embryo is protected against super-cooling during the natural incubation process. The heat loss mechanisms are most efficient against heat stress, which may occur for only very short time during incubation, protect the embryo against disturbances caused by hyperthermia.

Temperature guardian neurons in the preoptic area of the hypothalamus

Applying the slice method extracellular recordings of 218 hypothalamic neurons in Muscovy ducks during sinusoidal temperature changes were investigated. Seven neurons reacted in a hitherto unknown manner to temperatures very near the physiological limits. Four were exclusively sensitive to temperatures around 36.1 degrees C and three to temperatures around 42.3 degrees C. We recommend to call this kind of neurons temperature guardian neurons. The presented results suggest that the current neuronal model of temperature regulation of vertebrates should be extended by aspects of the two-tier theory of Bligh [J. Bligh, The thermosensitivity of the hypothalamus and thermoregulation in mammals, Biol. Rev. 41 (1966) 317-367].

Effects of ambient temperature, age and wind speed on the thermal balance of layer-strain fowls.

1. In White Leghorn laying hybrids aged 14 to 399 d the effects were measured of ambient temperature (Ta) and wind speed (WS) on heat production (HP), evaporative heat loss (EHL), nonevaporative heat loss with or without considering heat storage (NEHL1 or NEHL2), conductance (c), total insulation (I) and colonic temperature (Tc) as well as on biological optimum temperature (BOT), thermoneutral temperature (TNT) and threshold temperature for evaporative heat loss (Te). 2. All thermal balance variables were affected by age, Ta and WS. 3. The relationships between Ta and HP or NEHL2 were best expressed in terms of a polynomial function at low WS, and in terms of a linear function at medium and high WS; the relationship between Ta and NEHL1 at all WS studied was best described in terms of linear functions, the slope of which increased with decreasing Ta and rising WS; those between Ta, EHL and c were best described in terms of exponential functions and between Ta and I in terms of quadratic functions, the rate of increase of which was higher at low Ta and medium and high WS. 4. The description of relationships between the age of birds and their HP, EHL, NEHL1 and NEHL2 was performed after logarithmic transformation and yielded a linearly decreasing function with advancing age. 5. Threshold temperatures increased as WS increased from 0.2 to 1.2 m/s; for EHL (Te) by up to 7.5 K, for Tc (BOT) and for HP (TNT) by 10 K. 6. The effects of age, Ta and WS on HP and both forms of NEHL were summarised by multiple regression equations.