Endocrinology of thermoregulation in birds in a changing climate

Abstract

The ability to maintain a (relatively) stable body temperature in a wide range of thermal environments is a unique feature of endotherms such as birds. Endothermy is acquired and regulated via various endocrine and molecular pathways, and ultimately allows wide aerial, aquatic, and terrestrial distribution in variable environments. However, due to our changing climate, birds are faced with potential new challenges for thermoregulation, such as more frequent extreme weather events, lower predictability of climate, and increasing mean temperature. We provide a compact overview on thermoregulation in birds and its endocrine and molecular mechanisms, pinpointing gaps in current knowledge and recent developments, focusing especially on non-model species to understand the generality of, and variation in, mechanisms. We highlight plasticity in thermoregulation and underlying endocrine regulation, because thorough understanding of plasticity is key to predicting responses to changing environmental conditions. To this end, we discuss how changing climate is likely to affect avian thermoregulation and associated endocrine traits, and how the interplay between these physiological processes may play a role in facilitating or constraining adaptation to a changing climate. We conclude that while the general patterns of endocrine regulation of thermogenesis are quite well understood, at least in poultry, the molecular and endocrine mechanisms that regulate e.g. mitochondria function and plasticity of thermoregulation over different time scales (from transgenerational to daily variation) need to be unveiled. Plasticity may ameliorate climate change effects on thermoregulation to some extent, but the increased frequency of extreme weather events, and associated in resource availability, may be beyond the scope and/or speed for plastic responses. This could lead to selection for more tolerant phenotypes, if the underlying physiological traits harbour genetic and individual variation for selection to act on – a key question for future research.

Great tits differ in glucocorticoid plasticity in response to spring temperature

“…Importantly, we found that this negative relationship was explained by within-individual plasticity. A likely mechanism is increased energetic demands associated with thermoregulation that can lead to within-individual increases in corticosterone at lower temperatures (summaries: [23,62]). In our study, temperatures at capture varied between approximately 4-27°C (electronic supplementary material, figures S1 and S2).…”

Testing for context-dependent effects of maternal thyroid hormones on offspring survival and physiology: an experimental approach manipulating temperature in a wild bird species

“…Maternal effects may translate environmental cues from the mother to the offspring for example via maternally transferred hormones (hereafter 'maternal hormones'), potentially increasing offspring survival in the predicted conditions (adaptive maternal effects, 1,[2][3][4]. Maternal hormone-mediated effects have been recently highlighted as a potential mechanism and source of phenotypic plasticity to respond to changing climate (5)(6)(7), yet empirical evidence is scarce. Experimental studies on hormone-mediated maternal effects have revealed increasingly contradictory results, for example, elevated maternal androgens both increasing, decreasing or having no effect on offspring growth (3).…”

“…We experimentally manipulated both egg TH levels and post-hatching nest-box temperature in a wild population of pied flycatchers. We chose to study interactions between THs and temperature variation, because both factors are involved in thermoregulation (7,20), which is crucial for early-life altricial nestlings (21).…”

Lifelong Effects of Thermal Challenges During Development in Birds and Mammals

“…The effects of embryonic temperature on postnatal thermoregulation have been studied particularly in poultry since temperature fluctuations inside rearing facilities have consequences for welfare and economic return (Naga Raja Kumari and Narendra Nath, 2018). Thermal sensitivity is the greatest when the hypothalamus-thyroid-pituitary-adrenal (HTPA) axis forms (Loyau et al, 2015), in line with the modulatory role of thyroid hormones in avian thermoregulation (Ruuskanen et al, 2019). In the chicken, this commences during the middle third of embryogenesis, when even brief (2-5 h) exposure to hypo-or hyperthermic incubation alters thyroid and glucocorticoid hormone secretion in response to a thermal challenge after hatching, and results in phenotypic changes that improve chicks' capacity to deal with cold or heat at least until market age of ca.…”