Egg colour in the spotless starling: signalling potential and genetic architecture
- Autores: Stefanía D´Arpa
- Directores de la Tesis: Diego Gil Pérez (dir. tes.), Lorenzo Pérez Rodríguez (dir. tes.)
- Lectura: En la Universidad Autónoma de Madrid ( España ) en 2023
- Idioma: inglés
- Número de páginas: 11
- Títulos paralelos:
- El color del huevo en el estornino negro: potencial de señalización y arquitectura genética
- Tribunal Calificador de la Tesis: Juan Carlos Senar (presid.), Nuria Polo Cavia (secret.), Judith Morales Fernaz (voc.)
- Programa de doctorado: Programa de Doctorado en Ecología por la Universidad Autónoma de Madrid y la Universidad Complutense de Madrid
- Materias:
- Enlaces
- Tesis en acceso abierto en: Biblos-e Archivo
- Resumen
In the early 2000s it was proposed that, in birds, the colour of the eggs could play a role in mate choice by males, as a post-mating sexual signal (the Sexually Selected Eggshell Colouration Hypothesis, SSECH). Eggshell colouration, in this perspective, would be a condition-dependent trait signalling female quality to her mate. The aim of this thesis is to test the predictions and assumptions of this hypothesis using the spotless starling (Sturnus unicolor) as a study model.
We have used experimental manipulations and correlative analyses on a large database built over eight years of data collection in the field. We tested for a relationship between female body condition before laying and eggshell colouration using correlative analyses (Chapter 1) and a flight handicapping experimental manipulation (Chapter 2). We investigated if the early life condition could affect adult eggshell colouration, considering the body conditions of the females as nestlings (Chapter 1) and in following the effects of an in-ovo injection of androgens in a cohort of starlings during the development of their life (Chapter 3). We then examined longitudinally the variations in eggshell colouration of females that repeatedly reproduced in our population to analyse the change in this trait with age (Chapter 1). As sexual traits are expected to have a genetic basis, we described the genetic architecture of eggshell colouration in our population and estimated its heritability through an animal model (Chapter 4). Lastly, since the deposition of pigments in the eggshell is considered a female reproductive investment, we considered egg size as an alternative index of reproductive investment in Chapters 3 and 4.
We found that eggshell coloration is not related to body condition of the female, either in the adult or in the early life stages. The former result was further confirmed by the lack of effect of our experimental manipulation of female condition on eggshell coloration. Moreover, we found a decrease in eggshell colouration with age. Our animal model revealed that eggshell colouration in our population has a moderate to high heritability, egg size had a moderate heritability, but both of them have a low evolutive potential. Lastly, for egg size, we found that the hormonal treatment acted in interaction with the laying order, causing a steeper decrease in volume from first to las egg in treated clutches.
Our results suggest that, in our study species, eggshell colouration is not a sexually selected trait. Several pieces of evidence reinforce this interpretation. Firstly, variations (natural or experimentally induced) in the condition of adult females did not induce a change in the colouration of their eggs, as it would be expected for a condition-dependent sexual signal. Secondly, egg colouration also does not appear to respond to postnatal developmental difficulties due to stress factors in the nest, as commonly found for sexual ornaments in adults. Similarly, egg colour does not mirror physiological impairments caused during early development by increased exposure to prenatal androgen levels in females. Moreover, the decrease in eggshell colouration with age does not reflect the expected pattern of sexual traits, whose expression should increase instead, at least in the first years of age. Lastly, the low evolutive potential for egg colouration and size we found suggests that it is unlikely that these traits would respond to selection.
Throughout this thesis, we have considered female condition thorough body condition indices that are sensible to several internal factors of the organism. Their suboptimal function may lead to a mass loss that is mirrored by the body condition indices that we used. Although previous authors have reported positive and significative associations between these indices and eggshell colouration, our results suggest that in the spotless starling eggshell colouration is not informative of female body condition at laying, or the harshness of the conditions experienced during her early life. If the predicted association between female quality and eggshell colour exists, it should be addressed by focusing on oxidative stress, as specifically stated by the SSECH.
