Temperature, size and developmental plasticity in birds

Abstract

As temperatures increase, there is growing evidence that species across much of the tree of life are getting smaller. These climate change-driven size reductions are often interpreted as a temporal analogue of the observation that individuals within a species tend to be smaller in the warmer parts of the species' range. For ectotherms, there has been a broad effort to understand the role of developmental plasticity in temperature–size relationships, but in endotherms, this mechanism has received relatively little attention in favour of selection-based explanations. We review the evidence for a role of developmental plasticity in warming-driven size reductions in birds and highlight insulin-like growth factors as a potential mechanism underlying plastic responses to temperature in endotherms. We find that, as with ectotherms, changes in temperature during development can result in shifts in body size in birds, with size reductions associated with warmer temperatures being the most frequent association. This suggests developmental plasticity may be an important, but largely overlooked, mechanism underlying warming-driven size reductions in endotherms. Plasticity and natural selection have very different constraining forces, thus understanding the mechanism linking temperature and body size in endotherms has broad implications for predicting future impacts of climate change on biodiversity.

Endocrine flexibility can facilitate or constrain the ability to cope with global change

“…Disentangling these sources of variation in the context of climate change is particularly challenging, because changing temperature regimes can simultaneously result in altered selection pressures leading to evolution and in systematic differences in developmental environments that trigger plasticity [9]. This important distinction has recently been explored for climate-induced changes in phenology [6,67] and body size [3,9,10], but has not been systematically explored in endocrine flexibility. Better understanding and characterizing variation in endocrine flexibility is a first step in determining when it acts as a facilitator or constraint on coping with climate for both individuals and species.…”

“…A growing number of studies have documented the effects of changing temperature regimes on morphology [3,4], behaviour [5,6], species ranges [7] and population stability [8]. These population-wide responses occur through a combination of evolutionary responses to selection and developmental plasticity that are induced, in part, by experiencing a warmer and more variable climate [3,4,9,10]. Understanding these patterns is crucial to predicting the effects of climate change, because evidence has mounted that in many cases populations that are unable to keep pace with climate change are more vulnerable [11] and that particular behaviours, life history strategies, or morphologies predispose some species or populations to increased or decreased capacity to cope with a warming world [12].…”

Climate change and its effects on body size and shape: the role of endocrine mechanisms

“…For example, in addition to nutrient availability, IGF-1 levels are also very sensitive to temperature. A meta-analysis involving experimentally warmed bird eggs and nestlings revealed that 61% of studies detected a decrease in size related to higher temperatures during development, and suggested changes in IGF-1 signalling resulting from developmental plasticity as a potential mechanism [108]. Interestingly, the effects of temperature can vary depending on both the intensity and duration of exposure.…”

“…Interestingly, the effects of temperature can vary depending on both the intensity and duration of exposure. For example, acute exposure to elevated temperatures tends to increase IGF-1 levels and growth and/or feeding rates [16,109], and chronic exposure to elevated temperatures can have opposing effects [108]. Because changes in temperature can also modify metabolic rates and/or resource availability, it will be important to separate the direct effects of temperature acting on the endocrine mechanisms from indirect effects mediated through changes in resource availability [110].…”

A long‐term study of size variation in Northern Goshawk Accipiter gentilis across Scandinavia, with a focus on Norway

“…A number of bird species have undergone body size changes during the 20th century. These changes are varied with some species increasing (Kaňuščák et al., 2004; Moreno‐Rueda & Rivas, 2007) but the majority showing a decrease in overall body size (Jakober & Stauber, 2000; Riddell et al., 2019; Weeks et al., 2020, 2022; Yom‐Tov & Yom‐Tov, 2006). This overall decrease, seen not only in birds, has led to some suggesting it is a possible global response to climate change (Daufresne et al., 2009; Gardner et al., 2011; Ryding et al., 2021; Sheridan & Bickford, 2011).…”