13Telomeres are non-coding genetic repeats protecting the ends of linear chromosomes. Long 14 telomeres are often associated with high individual survival, and inter-individual variation in 15 telomere length has recently been proposed as a proxy for individual quality. Therefore, one 16 might expect individuals of either sex with long telomeres to be of higher intrinsic quality and 17 to be preferred in the context of mate choice. Thus, in sexually monomorphic species where 18 individuals discriminate mates on the basis of signals of intrinsic quality, mate choice should 19 lead to assortative pairing by telomere length, and it should be associated with breeding 20 performance. We tested these two predictions in the king penguin (Aptenodytes patagonicus, 21 Miller, 1778), a sexually monomorphic seabird. Over 3 years of study and 73 penguin pairs 22 under contrasting environmental conditions, we found strong assortative pairing by telomere 23 length. Interestingly, only female telomere length was positively associated to chick survival 24 up to fledging, and this relationship was only apparent when foraging conditions at sea were 25 average. The positive link between telomere length and breeding success confirmed that 26 telomere length is somehow related to individual biological state at a given time. The 27 proximate mechanisms by which birds assess individual state related to telomere length 28 remains to be discovered. 29 30