Body temperature maintenance acclimates in a winter-tenacious songbird

ABSTRACTFlexibility in heat generation and dissipation mechanisms provides endotherms the ability to match their thermoregulatory strategy with external demands. However, the degree to which these two mechanisms account for seasonal changes in body temperature regulation is unexplored. Here we present novel data on the regulation of avian body temperature to investigate how birds alter mechanisms of heat production and heat conservation to deal with variation in ambient conditions. We subjected Dark-eyed Juncos (Junco hyemalis) to chronic cold acclimations of varying duration and subsequently quantified their metabolic rates, thermal conductance, and ability to maintain normothermia. Cold-acclimated birds adjusted traits related to both heat generation (increased summit metabolic rate) and heat conservation (decreased conductance) to improve their body temperature regulation. Increases in summit metabolic rate occurred rapidly, but plateaued after one week of cold exposure. In contrast, changes to conductance occurred only after nine weeks of cold exposure. Thus, the ability to maintain body temperature continued to improve throughout the experiment, but the mechanisms underlying this improvement changed through time. Our results demonstrate the ability of birds to adjust thermoregulatory strategies in response to thermal cues and reveal that birds may combine multiple responses to meet the specific demands of their environments.

Seasonal Acclimatization in the American Goldfinch Revisited: To What Extent do Metabolic Rates Vary Seasonally?

We evaluated seasonal changes in cold tolerance, basal metabolic rate (BMR), and summit metabolic rate (Msum) for American Goldfinches (Carduelis tristis) from southeastern South Dakota to determine if goldfinches differ in pattern of metabolic acclimatization from other species of small birds. Goldfinches were captured in winter (January–February), spring (April), and summer (June–August) and tested on the day of capture. Cold exposure tests involved subjecting individual birds to a decreasing series of temperatures in an atmosphere of 79% helium to 21% oxygen (helox) concurrent with open-circuit respirometry. The helox temperature eliciting hypothermia was designated the cold limit (Tcl). Whole-animal metabolic rates were analyzed. Winter goldfinches demonstrated significantly higher BMR (46%) and Msum (31%) and significantly lower Tcl (−9.5°C vs. 1.3°C) than their summer counterparts. Spring goldfinches also showed significantly higher Msum (21%) and significantly lower Tcl (−5.3°C) than summer birds. Winter birds had higher BMR (23%) and Msum (8%) than spring birds. In winter birds, Tcl was also significantly lower than in spring birds. These data support the view that prominent winter increases in Msum and BMR are components of winter acclimatization in American Goldfinches from South Dakota and that seasonal changes in metabolism in goldfinches are similar to those for other small temperate-wintering birds. La Aclimatación Estacional en Carduelis tristis Revisitada: ¿En qué Grado Varían Estacionalmente las Tasas Metabólicas? Resumen. Evaluamos los cambios estacionales en la tolerancia al frío, la tasa metabólica basal (TMB) y la tasa metabólica pico (Mpico) en individuos de Carduelis tristis del sudeste de South Dakota para determinar si esta especie difiere de otras aves pequeñas en el patrón de aclimatación metabólica. Las aves fueron capturadas en invierno (enero–febrero), primavera (abril) y verano (junio–agosto) y sometidas a exámenes el día de captura. Los exámenes de tolerancia al frío consistieron en someter a las aves a una serie decreciente de temperaturas en una atmósfera de 79% helio y 21% oxígeno (helox) al mismo tiempo que se practicaba respirometría de circuito abierto. La temperatura que provocó hipotermia se designó como el límite de tolerancia al frío (Tfr). Se analizaron tasas metabólicas de animales completos. Las aves capturadas en el invierno presentaron TMB y Mpico significativamente mayores (46% y 31%, respectivamente) y Tfr significativamente menor (−9.5°C vs. 1.3°C) que las capturadas en el verano. Las aves de primavera también presentaron Mpico significativamente mayor (21%) y Tfr significativamente menor (−5.3°C) que las aves de verano. Las aves de invierno tuvieron mayores TMB (23%) y Mpico (8%) que las de primavera. En aves de invierno, Tfr también fue significativamente menor que en las aves de primavera. Estos datos apoyan la idea de que los incrementos invernales prominentes en Mpico y TMB son componentes de la aclimatación de invierno de C. tristis de South Dakota y que los cambios estacionales del metabolismo en esta especie son similares a los de otras aves que inviernan en la zona templada.

Translocation from flowering to daughter tillers in perennial ryegrass (Lolium perenne L.)

Recent New Zealand cultivars of Lolium perenne often have a high rate of tiller death and replacement in late spring–early summer. A majority of the new tillers are daughter tillers of flowering tillers. Previous research has led to a hypothesis that defoliation may influence the amount of assimilate exported by flowering tillers, and hence the rate of daughter tiller formation. To test this hypothesis, flowering tillers of of nutrition that enabled the dam to gain 100 g/day of conceptus-free liveweight in addition to expected fetal growth) during the mid pregnancy (post-shearing) period (from Day 70 of pregnancy (P70) until P101). At the end of this period ewes were offered either maintenance or a high level of nutrition during late pregnancy (P102–P140). Pregnancy shearing was found to increase lamb birthweight (average birthweight 5.4±0.1 v. 5.1± 0.1 kg) without differentially affecting the birthweight of singles or twins. Dam feeding level post-shearing did not affect the birthweight response to mid pregnancy shearing. Pregnancy shearing treatment had no effect on the summit metabolic rate of twin lambs. However, when results of this study are compiled with those of previous mid to late pregnancy shearing studies under similar conditions, it appears that the birth weight response from pregnancy shearing is greatest where lightweight lambs are born to unshorn (control) ewes.

Temperature regulation in the new-born lamb. V. Summit metabolism

"Summit metabolism" was estimated by measuring respiratory exchange during a 20 min period of falling rectal temperature. The rate of fall was controlled at about 1°C per 20 min, by varying the wind velocity while the lamb was exposed to conditions of high heat loss. At body temperatures near normal, summit metabolism was not predictable from rectal temperature. Below 36°C the metabolic rate was proportional to rectal temperature. When expressed as kilocalories per kilogram per hour, summit metabolism in young lambs was approximately constant at all body weights, and hence summit, metabolism per unit of surface area increased with increasing body weight. Heavy lambs are therefore able to maintain homeothermy under conditions of higher heat loss than light lambs. Summit metabolism was usually established at about 17 kcal kg-1 hr-1 within half an hour of birth, i.e. heat production increased rapidly to 15 times foetal levels or five times "basal" levels. There was no increase after ingestion of milk, and the summit metabolic rate appeared to decline slowly with advancing age. It also declined during prolonged exposure to cold and during prolonged fasting, particularly in very young lambs. Blood analyses indicated a great mobilization of fat and carbohydrate during exposure to conditions which evoked a summit response. The practical significance of these results is discussed.