Telomere length (TL) is known to covary with a suite of physiological traits involved in energy balances that may influence breeding success. Thus, TL may be an indicator of individual quality, influencing mate choice and breeding strategies. In the Brown Booby (Sula leucogaster), a long-lived seabird, we investigated in a total of 72 pairs whether TL may indicate the individual physiological phenotype by examining how it relates to mitochondrial DNA copy number (mtDNAcn) and corticosterone (CORT) levels. Then, we tested if TL is associated with mating patterns by looking for covariation with skin coloration (a condition-dependent sexual trait), whether there is assortative mating by TL, and if the degree of TL similarity between partners influences an individual's likelihood of changing mates between reproductive seasons. Because environmental conditions can have important effects on the condition and reproductive strategies of individuals, the study was performed in 2 colonies with contrasting breeding conditions. We found that TL was positively related to mtDNAcn (but not CORT levels), suggesting that individuals with shorter telomeres may have a reduced mitochondrial density. TL was reflected in sexual coloration, but only in the colony with good breeding conditions. There was positive assortative mating by TL; however, the degree of assortative mating was unrelated to the probability of mate change, which greatly varied between the 2 colonies. Our results suggest that TL is associated with the physiological phenotype of individuals and their breeding patterns. Importantly, contrasting environmental conditions influenced the link between TL and sexual coloration and the propensity of individuals to change mate, highlighting the importance of environmental variation on mating patterns. center dot Telomere length (TL) may act as an integrative indicator of individual quality, which influences mating preferences and breeding success. If TL indicates individual quality, it would be expected that TL would be related to other key physiological traits linked to the individual energy balances.center dot We found in the Brown Booby that TL is related to mitochondrial DNA copy number (mtDNAcn), a parameter that in birds has been related to mitochondrial density and (dys)function, and to skin color, a condition-dependent sexually selected trait, but was unrelated to the individual corticosterone levels.center dot The probability of mate change varied greatly between 2 colonies with contrasting breeding conditions, and in both colonies, there was positive assortative mating by TL. However, the degree of assortative mating was unrelated to the probability of mate change.center dot Our findings suggest that TL might be an indicator of the individual quality and may influence mating patterns. Interestingly, our results highlight the importance of environmental variation on mating patterns. Se sabe que la longitud de los telomeros (TL) covaria con un conjunto de rasgos fisiologicos involucrados en los balances energeticos que pueden influir en el exito reproductivo. Por lo tanto, TL podria ser un indicador de la calidad del individuo, influyendo en la eleccion de pareja y las estrategias reproductivas. En el bobo cafe (Sula leucogaster), un ave marina de larga vida, investigamos en un total de 72 parejas, si la TL puede indicar el fenotipo fisiologico individual examinando como se relaciona con el numero de copias de DNA mitocondrial (mtDNAcn) y los niveles de corticosterona (CORT). Tambien, probamos si la TL esta asociada con los patrones de apareamiento evaluando la covariacion entre la TL y la coloracion de la gular y las patas (un rasgo sexual dependiente de la condicion), si hay apareamiento concordante por TL, y si el grado de similitud en TL entre las parejas influye en la probabilidad de que un individuo cambie de pareja entre temporadas reproductivas. Debido a que las condiciones ambientales tienen efectos importantes, el estudio se realizo en dos colonias con condiciones de reproduccion contrastantes. Encontramos que la TL se relaciono positivamente con el mtDNAcn (pero no con los niveles de CORT), lo que sugiere que los individuos con telomeros mas cortos pueden tener una densidad mitocondrial reducida. TL se reflejo en la coloracion sexual, pero solo en la colonia con buenas condiciones para la reproduccion. Encontramos apareamiento concordante positivo por TL, sin embargo, el grado de apareamiento concordante no se relaciono con la probabilidad de cambio de pareja, que vario considerablemente entre las dos colonias. Nuestros resultados sugieren que la TL se relaciona con el fenotipo fisiologico de los individuos y los patrones de reproduccion. Notablemente, las condiciones ambientales contrastantes influyeron en el vinculo entre TL y la coloracion sexual y la propension de los individuos a cambiar de pareja, destacando la importancia de la variacion ambiental en los patrones de reproduccion.
