12 Jun 2018
Environmental influences on the expression and selection of melanin-based traits in the Common Kestrel (Falco tinnunculus)

BRANTA — David López-Idiáquez

Environmental influences on the expression and selection of melanin-based traits in the Common Kestrel (Falco tinnunculus)

Institution: Universidad Autónoma de Madrid (UAM), Spain

Supervisors: Jesús Martínez-Padilla

Details: PhD, 2017

Department of Evolutionary Ecology, Natural History Museum of Madrid. Calle Jose Gutierrez Abascal, 2. 28006. Madrid. Spain.

Subject Keywords: Melanin-based colouration, signalling, animal communication, environmental effects, senescence, age-dependent patterns, status signalling, female colouration, personality
Species Keywords: Common Kestrel, Falco tinnunculus


The animal kingdom is a constant source of conspicuous structures and behaviours that have drawn the attention of the scientific community. These traits play a crucial role in animals’ life, since they are the pillars where communication systems lay on and help individuals to resolve many conflicts, both within and between species. Animal communication can occur by very different means, but always in a bidirectional fashion that involves a sender, modulating the behaviour of the receiver. When the communication process occurs among individuals of a given species, signalling takes place in different contexts, depending on the interests of signallers and receivers. For example, signallers and receivers can share interests, as fitness benefits of one part may depend upon the fitness benefits of the other. This scenario can be found when individuals involved in the communication process are genetically related, like for instance, between parents and offspring. Alternatively, the interests of signallers and receivers can collide, when individuals compete for limited resources for example. Finally, the interests of signallers and receivers can diverge. For instance, during mate choice, males signal their quality aiming to obtain as many females as they can, while females may use these signals to find a single male of the highest quality. However, an essential requirement to obtain stable communication systems, is that the signals has to be a reliable proxy of individual condition. Signal honesty is based on the differential cost of production or maintenance paid by low and high quality individuals. Form an evolutionary perspective, the presence of stable communication systems is indicative that, at least in average, the signals used in them are reliable. If they were not reliable, Natural Selection will favour individuals that not react to those dishonest signals and, in the end, the communication system vanishes.

It is easy to realise that all individuals within a single species do not exhibit the same traits with equal intensity, as can be seen for example, in the antlers of red deer. The most obvious difference is that in this species, males and not females show conspicuous antlers. In addition, among males there is also a variance in the size of their antlers depending on different individual characteristics. This example perfectly illustrates that the expression of a certain trait, secondary sexual in this case, is determined by several factors, that can be inherent to the individual, like sex or age, or be associated with environmental heterogeneity, like food availability. Besides, these factors do not act in isolation, and they can interact between them. The study of the factors that modulate signal expression is crucial to understand both, the message that is transmitted and the evolutionary pressures behind the expression of the trait.

The use of colour-based traits is one of the most typical ways of signalling. Among terrestrial animals, birds are of the most coloured groups, exhibiting traits varying in multiple colours and shapes. These colourations can be grouped in two different categories. First, pigmentary colouration, whose physiological origin occurs by the deposition of pigments like melanin or carotenoids. Second, structural colouration, in which colours are generated by the interaction of the microscopic structure of the feather and light, as for example the iridescent gorgets present in many hummingbirds.

Melanin-based traits are one of the most common in the animal kingdom. This pigment is responsible of many colours, however, melanins are, usually, associated with colours like black, grey, brown and reddish-brown. Melanins are generated from the amino acid tyrosine, in a process known as melanogenesis. In this process, two different types of melanin can be produced, eu-melanin and pheo-melanin, that are responsible of blackish and brownish colorations respectively. Melanin-based traits have a crucial role within animal communication. It has been, for example, shown that the presence of certain melanin-based traits can be used by females to choose their mates during mate choice, reflect individual age or reliably show individual quality to name some. Still, the main role of these traits has been pointed towards a signalling function of the individual status, showing the competitive capabilities of the bearers.

In this thesis, I describe the different mechanisms and functions associated with the expression melanin-based traits, using the common kestrel (Falco tinnunculus) as study species. Kestrels are medium-sized diurnal raptors, exhibiting a reversed sexual dimorphism, where males are 20% smaller than females. In addition, they also are dimorphic in coloration, males being more conspicuous than females. Specifically, males have grey heads, rump and tails, and brown bodies with a variable number of black spots covering it. Females exhibit duller plumages, mostly brown with black bars in the body, except in the rump and tail, where they can present a variable proportion of grey colouration. Kestrels have a homogeneous distribution in the Iberian Peninsula, and our study areas are located in central Spain. First, Campo Azálvaro region, located between Segovia and Ávila, there are 62 nest-boxes that have been subjected to an individual-based monitoring since 1994. Second, with the aim of increasing the number of observations and the environmental heterogeneity, we also studied a kestrel population located in Villalar de los Comuneros (Valladolid) during the experiment performed in Chapter IV.

The pivotal point of this thesis is to study the factors that modulate the expression of different melanin-based traits and their function in different environments. These aspects are developed in 5 different chapters, in addition to a general introduction (Chapter I). The first two are focused on exploring the factors that modulate the expression of melanin-based traits:

- In Chapter II, I explore how age and environment modulate the expression of the number and size of the spots shown in the plumages of adult males and females.

- In Chapter III, I study the influence of the above-mentioned factors in a different trait, rump colouration, only in adult females, as is the sex where there is variation in the expression of this trait.

In the remaining three chapters, we explored the function that melanin-based traits have during different stages of kestrels’ life-cycles.

- In Chapter IV, I focus on the role of female rump colouration as a signal of status within an intra-sexual competition context during the pre-laying period.
- In Chapter V, I explore the association between the duration of the post-fledgling dependence period, offspring rump colouration and survival rates to the first winter. In addition, I study the influence that this period has on future reproduction of parents.
- In Chapter VI, I explore the role of plumage and rump colouration as personality index during the nestling growth.

Finally, in Chapter VII, I present and discuss in an integrative manner the main results obtained in this thesis.

Published Papers

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López-Idiáquez, D., Vergara, P., Fargallo, J.A., Martínez-Padilla, J. 2016. Old males reduce melanin-pigmented traits and increase reproductive outcome under worse environmental conditions in common kestrels. Ecology and Evolution. DOI: 10.1002/ece3.1910.
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López-Idiáquez, D., Vergara, P., Fargallo, J.A., Martínez-Padilla, J. (2016). Female plumage coloration signals status to conspecifics. Animal Behaviour. 121(101-106). DOI: 10.1016/j.anbehav.2016.08.020
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