The Relationship Between the Presence of Satellite Males and Nest-Holders’ Mating Success in the Azorean Rock-Pool Blenny Parablennius sanguinolentus parvicornis Rui F. Oliveira*, Natacha Carvalho , Jason Miranda à, Emanuel J. Gonc¸alves*, Matthew Groberà & Ricardo Serra˜o Santos  *Unidade de Investigac ¸a ˜o em Eco-Etologia, Instituto Superior de Psicologia Aplicada, Lisbon,  Departamento de Oceanografia e Pescas, Universidade dos Ac ¸ ores, Horta, Ac ¸ ores, and àLife Sciences, Arizona State University West, Phoenix Abstract In the Azorean rock-pool blenny, sexually active males may adopt alternative reproductive tactics. In the present paper the relationship between the presence of satellite males and the reproductive success of nest-holders was investigated by comparing nests with and without an associated satellite male. Males with an associated satellite male suffered more conspecific intrusions but they did not display a higher frequency of attacks towards conspecifics. Nest-holder males were more aggressive towards other conspecifics than towards satellites and the tolerance of nest-holders towards satellites was inversely correlated with the time spent by the satellites in the breeding territory, which suggests control by the nest- holder male of the satellite investment in shared territorial defence. Nest-holders with an associated satellite male had higher condition factors and received more female visits and more spawnings. These results bear two possible interpretations. (1) Nest-holders benefit from the presence of a satellite male by increased attractiveness of their nests to females; satellite males are mutualists helping to defend the nest-owner’s territory and to attract females, which is why they are tolerated. (2) Satellite males associate preferentially with more successful nest- holder males which have higher condition factors, and by doing so have more opportunities to achieve parasitic fertilizations. Only experiments will allow these two hypotheses to be distinguished. Corresponding author: Rui F. Oliveira, Unidade de Investigac¸a˜o em Eco- Etologia, Instituto Superior de Psicologia Aplicada, Rua Jardim do Tabaco 34, 1149-041 Lisboa, Portugal. E-mail: ruiol@clix.pt Ethology 108, 223—235 (2002) Ó 2002 Blackwell Wissenschafts-Verlag, Berlin ISSN 0179–1613 U. S. Copyright Clearance Center Code Statement: 0179-1613/2002/1083–0223$15.00/0 www.blackwell.de/synergy