Hypoxia during embryonic development increases energy metabolism in normoxic juvenile chicks.

Environmental changes during perinatal development can affect the postnatal life. In this sense, chicken embryos that experience low levels of O2 over a specific phase of incubation can have their tissue growth reduced and the ventilatory response to hypoxia blunted, at least until hatching. Additionally, exposure to low level of O2 after birth reduces the thermogenesis as well. In the present study, we tested the hypothesis that hypoxia over the third week of incubation affects the thermoregulation of juvenile chicks at an age when thermogenesis is already expected to be well-developed. To this end, we measured body temperature (Tb) and oxygen consumption (V̇02) under acute hypoxia or different ambient temperatures (Ta) of 1 and 10day-old chicks that have been exposed to 21% O2 for entire incubation (Nx) or to 15% O2 in the last week of incubation (Hx). We also assessed the thermal preference under normoxia or acute hypoxia of the older chicks from both incubation groups in a thermocline. Hypoxia over incubation reduced growth but did not affect the cold-induced thermogenesis in hatchlings. Regarding the juvenile Hx, present data indicate a catch up growth with higher resting V̇02, a thermal preference for warmer Tas and a possible higher thermal conductance. In conclusion, our results show that hypoxia over the third week of incubation can affect the thermoregulation at least until 10days after hatch in chickens.

Chicken hatchlings prefer ambient temperatures lower than their thermoneutral zone.

We investigated whether or not the preferred ambient temperature (Tapref) of the 1-day old chicken hatchling, a precocial neonate with excellent locomotory capacity, clearly identifiable thermogenesis and independence from maternal care, coincides with the lower critical temperature (LCT) of thermoneutrality and minimal oxygen consumption (V̇(O(2))). Tapref of single chicks measured in a thermocline (N=16) averaged 33.5±0.3 °C (mode, 33.3±0.4 °C). The same value was obtained in hatchlings studied in pairs. LCT was computed from the ambient temperature (Ta)-V̇(O(2)) relationship, constructed by slowly decreasing the Ta of a respirometer from 38 to 29 °C over 2.5h, while continuously measuring V̇(O(2)) by an open-flow methodology; LCT averaged 36.4 °C±0.3 or 36.8 °C±0.4, depending on the method of computation. In all hatchlings Tapref was lower than LCT (P<0.001), by a magnitude that depended on the method of computation of the two variables, 2.8 °C±0.3 (P<0.001) or 3.9 °C±0.5. The Tapref-LCT difference implied that, at Tapref, V̇(O(2)) was higher than at thermoneutrality. We conclude that in the chicken hatchling thermal preference does not coincide with thermoneutrality, probably because during development what seems optimal from a thermoregulatory viewpoint may not necessarily be so for other regulatory functions.

Thermogenesis, vocalization, and temperature preference of 1-day-old chicken hatchlings after cold-exposure in late embryogenesis.

In a thermal gradient the preferred ambient temperature (T(a) pref) of chicken hatchlings is a few degrees lower than thermoneutrality. To investigate whether a correlation may exist between T(a) pref and the autonomic thermogenic capacity or not we studied a group of hatchlings (N = 15) exposed to cold at end-incubation, a procedure known to increase their postnatal thermogenesis. Chicken embryos were exposed to cold (34.5 °C instead of 38 °C) at days 18-20 of incubation. By comparison to Controls (N = 15), they hatched a few hours later, with similar body weight, body temperature, vocalization (number of sounds produced per unit time), and oxygen consumption (VO2, measured in a respirometer by an open-flow methodology). When exposed to slow cooling these hatchlings had a higher lower critical temperature (LCT) of thermoneutrality and higher VO2, and slightly higher vocalization than Controls. In a thermal gradient, T(a) pref averaged 34.3 ± 0.3 °C, or 1 °C higher than in Controls (33.4 ± 0.3 °C; P < 0.05), in proportion with their higher LCT (38 ± 0.1 °C instead of 36.7 ± 0.3 °C; P < 0.001), so that the T(a) pref - LCT difference (-3.6 ± 0.3 °C) was similar to Controls (-3.3 ± 0.3 °C). In conclusion, in chicken hatchlings T(a) pref was lower than LCT irrespective of the magnitude of their thermogenic response. It was estimated that, at T(a) pref, VO2 was ~20 % higher than at thermoneutrality. Such metabolic increase could carry some physiological advantage and the choice of T a pref may reflect the hatchling's needs to maintain VO2 slightly elevated.

Breathing pattern and ventilatory chemosensitivity of the 1-day old Muscovy duck (Cairina moschata) in relation to its metabolic demands.

Adult birds have a ventilatory equivalent (pulmonary ventilation-oxygen consumption ratio, V˙ E/ [Formula: see text] ) lower than mammals because of the superior gas exchange efficiency of their respiratory apparatus. In particular, adult Muscovy ducks (Cairina moschata) have been reported to have an extraordinary low ventilatory equivalent (~14mL STPD·mL BTPS(-1)). We asked if similar high efficiency was already apparent in duck hatchlings. Breathing pattern and V˙E were measured by the barometric technique and [Formula: see text] by an open-flow methodology in 1-day old Muscovy duck hatchlings (N=21); same measurements were performed on chicken hatchlings (N=21) for purpose of comparison. During air breathing V˙E/ [Formula: see text] was slightly, yet significantly, lower in ducklings (20.8) than in chicks (25.3), mostly because of a lower breathing frequency (f). The hatchlings of both species (N=14 per group) responded to inspired hypoxia (15 or 10% O2) or hypercapnia (2 or 4% CO2) with a clear hyperventilation; however, in ducklings the hypercapnic hyperventilation was smaller than in chicks because of a smaller increase in tidal volume and lower f. We conclude that duck and chicken hatchlings just a few hours old have the high ventilatory efficiency typical of birds, although possibly not as high as their adults. The low f and blunted V˙E response to hypercapnia of the newborn duck could be related to the aquatic habitat of the species. In such a case, it would mean that these characteristics are genetic traits, the phenotypic expression of which does not require diving experience.

Ventilatory response to hypoxia of the 1-day old chicken hatchling after prenatal cold-induced hypometabolism.

Sustained prenatal hypoxia decreases the growth and metabolic rate of the embryo and causes a blunted hypoxic ventilatory response (HVR) in the newborn. The most likely interpretation is that the sustained hypoxic stimulation may interfere with the normal prenatal development of the chemoreceptors. However, we wanted to consider the possibility that the prolonged hypoxic hypometabolism may be a contributing factor. Chicken embryos were incubated at 35°C (Cold group, N=14), which is known to lower the embryonic oxygen consumption (VO2) by ≈ 30% throughout incubation, or at 37.5°C (Controls, N=16). Cold incubation delayed hatching by ≈ 2 days. The 1-day old hatchlings had normal pulmonary ventilation (VE), measured by the barometric technique, and oxygen consumption (VO2), simultaneously measured by an open flow methodology. During acute hypoxia (≈ 15% or ≈ 11% O2) the hyperventilation (increase in VO2), the hyperpnea and the hypometabolism were almost identical between the two groups of hatchlings. We conclude that a sustained decrease in metabolic rate during the embryonic period by itself does not carry obvious consequences on the newborn's resting VE and HVR.