Insulin-like growth factor 1 is related to the expression of plumage traits in a passerine species

Abstract

Avian plumage colors and ornaments are excellent models to study the endocrine mechanisms linking sexually selected traits and individual parameters of quality and condition. Insulin-like growth factor 1 (IGF-1) is an evolutionarily highly conserved peptide hormone. Its regulatory role in cell proliferation and differentiation and its high sensitivity to the nutritional state of individuals suggest it as an interesting candidate, possibly providing a link between body condition and individual capacity to grow elaborated ornamental features. We investigated whether IGF-1 levels during molting correlate with the expression of multiple ornaments in a sexually dichromatic passerine species, the bearded reedling (Panurus biarmicus). We collected blood samples of males and females shortly before the molting completed and measured the size and colors of ornamental traits. Our results indicate that in males, structural plumage colors, the size of the melanin-based ornament (beard), and tail length are independent traits. IGF-1 levels are associated with the length of the tail and the expression of male structural plumage components (UV coloration), but not the melanin-based ornament. In females, plumage color and tail length were independent traits, which were not related to IGF-1 levels. To the best of our knowledge, this study provides the first evidence that IGF-1 could play a role in the development of secondary sexual characters in a bird species. Significance statement IGF-1 is an evolutionarily highly conserved peptide hormone, which recently entered the center stage of research enquiry in evolutionary biology. It is considered as one of the key factors shaping individual life histories, but little is known about its effects on sexually selected traits. We investigated whether IGF-1 levels during molting predict the elaboration of multiple ornamental plumage traits in male and female bearded reedlings (Panurus biarmicus). Our results indicate that higher IGF-1 levels had positive effects on male structural plumage colors and tail feather length. This is the first study, bringing indication for a potential role of IGF-1 in the expression of plumage ornaments in a bird species. Our findings suggest that IGF-1 might serve as an ideal candidate to study the mechanisms linking condition and the capacity to develop sexually selected ornaments.

Effects of experimental increase in insulin-like growth factor 1 on feather growth rate, moult intensity and feather quality in a passerine bird

“…6). These findings support a recent study by Mahr et al (2020), showing that IGF-1 levels were positively correlated with structural plumage colouration in male bearded reedlings. Fault bars (also known as stress bars) are malformations of the feather, resulting from disruptions of feather growth that are often associated with stress or disease (Jovani and Rohwer, 2017).…”

“…3). While the treatment caused a pronounced elevation in IGF-1 levels on day 1, the peak hormone concentrations remained within the physiological range and were close to the highest concentrations found in naturally moulting individuals (Mahr et al, 2020). Our next sampling point was on day 4, by which time IGF-1 levels had returned to the baseline (and control) levels.…”

“…Based on previous research on the effects of IGF-1 on integument growth, renewal/replacement and development, we expected that IGF-1 supplementation would prolong and accelerate feather growth, resulting in longer and faster growing wing and tail feathers (Li et al, 2014;Weger and Schlake, 2005). We did not find this effect, even though natural variation in tail feather length has previously been shown to be associated with baseline IGF-1 levels in this species (Mahr et al, 2020). We consider the IGF-1 manipulation successful because a single injection of PLGAmicrospheres elevated IGF-1 levels significantly above control on day 1 post-treatment (Fig.…”

“…during the 4 days per treatment. Pujante et al, 2002) and natural variation in their length has been shown to be associated with baseline IGF-1 levels (Mahr et al, 2020). Sexually selected signals have been suggested to be related to insulin signalling as an assurance of their honesty (Warren et al, 2013).…”

“…Studies of how IGF-1 affects moult in other vertebrates are extremely scarce, but when available, seem consistent with its role in the regulation of moult. For instance, naturally shedding garter snakes were found to have higher circulating IGF-1 levels than non-shedding snakes (Sparkman et al, 2009), while a recently published study indicated a positive relationship between plumage quality, feather vane length and IGF-1 in a passerine species (Mahr et al, 2020). However, to date, there is only one experiment focusing on the relationship between moulting and IGF-1 in birds, revealing that stress-induced moult was associated with an increase in plasma IGF-1 levels in the broiler chicken (Mazzuco et al, 2005).…”