Organización social y señalización del estatus de dominancia: el lúgano como especie modelo

Abstract

La tesis describe la organización social del lúgano (Carduelis spinus), especie nómada y social, mediante estudios en libertad y en cautividad mostrando que el tamaño medio de los grupos estables de lúganos es de 5,2 individuos y que el tiempo necesario para alcanzar la estabilidad social es de 15 - 20 dias. El lúgano macho presenta una mancha de color negro de melanina en forma de babero, visible sobre el amarillo verdoso del pecho, esta mancha presenta una gran variabilidad en el tamaño (0 – 66 mm2) y es utilizada como señal de estatus social. Existe una correlación significativa entre el tamaño del babero y la dominancia, de manera que los lúganos de babero negro grande son dominantes sobre los de babero pequeño, esta correlación es independiente de la edad. La distribución de frecuencias de tamaño de babero es bimodal, esto indica que sobre este carácter actúa una selección diversificadota o disruptiva, en la que los fenotipos extremos de la distribución tienen más ventajas que los intermedios. Mediante un diseño experimental de elección entre individuos de babero grande y pequeño, con manipulación del tamaño del babero, se demuestra que el babero negro en el lúgano no solo esta correlacionado con la dominancia, sino que funciona como una verdadera señal estatus social, porque los lúganos evitan comer junto a individuos de babero grande (dominantes), tanto si estos son naturales o si son agrandados artificialmente, pero no los evitan cuando se les reduce el tamaño del babero a pesar de mostrar comportamiento de dominante. Esto demuestra que los lúganos prestan mas atención al color del plumaje (señal) que al comportamiento, y que son capaces de reconocer la dominancia relativa de los otros individuos cuando se encuentran por primera vez sin necesidad de realizar ningún comportamiento agonístico, y así pueden evitar estar al lado de los dominantes. Pero muestran una preferencia por comer junto a los compañeros subordinados del grupo, esta preferencia puede ser debida a la mayor eficiencia de los grupos en buscar y encontrar alimento. Tanto los animales dominantes como los subordinados obtienen ventajas de la señalización de estatus social: los dominantes porque al anunciar su estatus, no necesitan recurrir a enfrentamientos agonísticos para mantener su estatus y la preferencia de acceso a los recursos, para ellos es importante tener subordinados cerca, ya que así aumenta la tasa de vigilancia del grupo, y esto les permite tener una tasa metabólica baja (menor coste energético). Pero reciben mas agresiones. Los subordinados, por su parte están pendientes de la presencia de los dominantes, no tienen acceso preferencial a los recursos y esto se traduce en una mayor coste energético (tasa metabólica mayor), pero al señalizar su bajo estatus social, reciben menos agresiones de los dominantes (la mayoría de las interacciones agonísticas se producen entre dominantes), y esto les permite beneficiarse de la vida en grupo (mayor tasa de alimentación, protección frente a los depredadores). Los costes y beneficios tanto para dominantes como para subordinados explican la distribución bimodal de les frecuencias en el tamaño del babero, demostrando que ser dominante o subordinado son dos estrategias igualmente exitosas. Los lúganos de babero medio son los que tienen más costes sociales, tienen los mismos costes que los dominantes, reciben muchas agresiones, pero no tienen ninguna ventaja, ya que difícilmente pueden vencer a los subordinados. El babero negro del lúgano no funciona como ornamento sexual; las hembras escogen como pareja a los machos con una mayor franja amarilla del ala, no escogen a los machos en función de su estatus de dominancia. El tamaño de la franja amarilla alar no esta correlacionada con el tamaño del babero negro en el lúgano. Ambos caracteres son señales y las señales múltiples dentro de un mismo individuo pueden coexistir cuando cada una es seleccionada por un receptor distinto, parece que se han desarrollado por presiones selectivas diferentes: la franja amarilla del ala por selección sexual y el tamaño del babero negro pos selección social. El lúgano es una especie modelo para realizar estudios sobre señalización de estatus social porque: es un pájaro social, en el que sus hábitos nómadas hacen que interaccione con muchos individuos desconocidos (este hecho da sentido a la señalización de estatus), presenta variabilidad en el tamaño del babero negro, dimorfismo sexual (solo los machos tienen babero negro), la dominancia no esta correlacionada con la edad y es una especie que se adapta bien a la vida en cautividad.

Rohwer proposed in 1975 that the variation and extent of color patches in the plumage of wintering birds could work as badges of social status. The major advantage of these signals would be that individuals of unequal status competing for limited resources would not need to risk accidental injury or waste energy assessing the relative fighting ability of potential opponents. The status-signalling hypothesis has been tested in several species with variable plumage. These studies have, however, produced contradictory results. The Eurasian Siskin (Carduelis spinus) shows great variability in the extent of blackish plumage in the bib. This chin patch, is present only in males, which are dominant over females. Dominance, however, is unaffected by age. This lack of a relationship between age and dominance, and the absence of the badge in females simplifies analysis of the significance of the chin patch in status signalling. The aim of this Thesis is to describe variation in the size of the chin patch in male Eurasian Siskins and test (both in captivity and in the field) whether this variation is related to dominance status, and which may be the costs and benefits of this signalling system. The first chapter of the Thesis analyses the social organization of the Siskin. It is shown that the average group size for the species is 5.2 individuals. Behavioural integration associated with the fusion of two flocks is analyzed in captive siskins (Curduelrs sptncts) by quantifying changes in social behaviour with time since joining. In general there was an increase in the incidence of tolerant behaviour, supplanting attacks and hopping withdrawals with time since fusion of the flocks. However, the number of displays and flights showed the opposite, negative, trend. Taking dominance status into account, the greatest change in behaviour with time since joining is an increase in tolerance by dominants of new flock companions. Factorial analysis of correspondences (CA) was used to study how different birds changed their behaviour with time since joining a flock. This analysis showed that the introduction of new birds did not disrupt relationships with familiar birds, and that residents are dominant in interactions with the incoming new flock companions. The analysis also demonstrated that relationships within the new flock had stabilized 20 days after the flocks had joined. In the second chapter I analysed the variation and function of the black bib of the Siskin. Badge size in male siskins ranged between 0.0 and 66.4 mm2, with a mean of 20.04 ± SE of 0.43 mm 2 (n = 949). Adults had on average larger badges than yearlings (adult X = 23.43 ± 0.76 mm2, n = 311; yearling X = 18.39 ± 0.50 mm2, n = 638; Mann-Whitney test, U = 78,449.5, P < 0.001). We recorded 39 agonistic interactions in the field in which both contestants were males and one of the individuals clearly had a larger badge than the other. Larger badged birds won 77% of these interactions, significantly more than expected by chance (binomial test, two-tailed Z =0.0014), and so were appreciably dominant over smaller-badged birds. The large-badged individuals also initiated most of the interactions (79%), a trait typical of dominant siskins (Senar 1985). The correlation in a captive flock between badge size and social status (i.e. dominance score from CA) was 0.71 (F =10.0, df =1,10, P < 0.01; Fig. 2). Multiple regression analysis of social status on badge size, age, mass, wing and third-primary length showed that the badge size was the only variable that explained the observed variation in social status when all the other variables were held constant. The third chapter explores the functionality of the black bib as a signal of dominance. Experimental enlargement or removal of the siskin black bib, which has been shown in the previous chapter to be a reliable predictor of dominance, strongly suggests that dominance discrimination is based on the use of badges of status. Results show that siskins discriminate between dominant and subordinate individuals on the first encounter without requiring cues such as overt aggression, and prefer to join subordinate individuals. In the fourth chapter I explore the possibility of status signalling reducing the metabolic rate of dominant individuals. The higher metabolic rate of dominant individuals, found in different species, has been interpreted as the cost that prevents subordinates from cheating by adopting large badges of status. However, an alternative prediction for status-signalling species, in which subordinates may recognize dominants, is that subordinates have the higher metabolic rate because of the greater stress of locating and actively avoiding aggressive interactions with them. In this chapter, the size of the black bib of the siskin was negatively correlated with metabolic rate in daylight, even when controlling for the bird’s activity level in the respirometer chamber and its body mass. The size of the black bib, however, was not correlated with metabolic rate in darkness. This suggests that the difference between dominance classes is not related to intrinsic physiological differences, but that subordinates are more susceptible to stressful conditions. When controlling for metabolic rate, a positive correlation appeared between dominance status and body mass. This stresses the importance of knowing the effects of social status on energy requirements for understanding the relationship between body mass and dominance. It is concluded that an advantage of status signalling to dominant birds is to help them to reduce their metabolic expenditure. In chapter five, the advantage of status signalling was explored from the point of view of subodinate birds. Analyses of interaction rates between birds of different bib size classes showed that dominant individuals mostly attacked to other individuals with large badges of status, in a like-vs-like aggression. This allowed subordinate (small badged) individuals to receive few agonistic attacks. This is suggested as the advantage to subordinates of the status signalling system. In chapter six it is provided evidence that in siskins, wing patches rather than the size of the black bib, function in mate choice. Mate-choice experiments showed that females were attracted by the size of the yellow wing stripe of the male, but not by the size of its black bib, body size, general plumage brightness or age. Experiments on birds with manipulated yellow wing stripes showed that females were sensitive to the size of this colour patch, irrespective of other male qualities. The preference of female siskins for males with larger wing patches when searching for a mate may be explained by the relationship of this trait to foraging ability, which would ensure females good parental investment from the chosen male. This stresses that the black bib is not selected by sexual selection but by processes related to social selection. Overall, the Thesis shows that the Siskin is a very good model species for the study of status signalling. The nomadic habits of the species causes many different unfamiliar individuals to interact to each other along a winter. Under this circunstance, status signalling may be highly advantegeous. The work shows that advantages appear both for dominant and subordinate birds, which ensures the stability of the system. This system is maintained by social selection processes.