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Description: Intense sexual selection usually leads to strong male-male competition over reproduction. In extremely competitive environments, males can increase their reproductive success by employing alternative reproductive tactics or/and engage in cooperation with other males (Taborsky et al. 2008; Díaz-Muñoz 2014). Mathematical models showed that cooperation can have different costs and benefits for the participants based on their ranks (van Schaik et al. 2004). Lower ranking males might establish coalitions against the dominant male(s) either to deprive them from their rank and occupy their niche in the hierarchy (all-up, rank changing coalitions), or take matings away from them and thus, increase their own reproductive success while flattening the payoff distributions (non-rank changing, leveling coalitions). On the other hand, dominant individuals are predicted to team up with low ranking partners if they can help a lower ranking relative increase its rank. Alternatively, they might team up with other higher ranking individuals to prevent lower ranking ones to team up against them, thus they can stabilize their ranks (rank stabilizing coalitions). These models were developed and tested mostly in multi-male societies with coalitions (e.g. primates or lions), in which the cooperation mainly aims to physically prevent or exclude other male individuals from matings. Another wide-spread male reproductive cooperation is co-display between two or more males. Here, usually the cooperation forms between one or more lower ranking and non-breeding individuals and a dominant breeding male. In these systems the benefits of the non-breeding lower ranking individuals were thoroughly studied and considered mostly as indirect. However, the benefits of the dominant and the breeding partner are still often unclear, as studies failed to show direct benefits of co-display in terms of increased female attraction or reproductive success (Jones 2017). Here, we study co-display in male Ruffs, Philomachus pugnax. Ruffs represent an unusual system where cooperative interactions between lekking males have elements of not only co-display but also coalitions. This provides a unique opportunity to study the costs and benefits of cooperation for individuals with different ranks, and at the same time to better understand the function of co-display. Ruffs feature three genetically determined morphs: Independents, Satellites and Faeders (Küpper et al. 2016). In males the three morphs represent three different mating strategies (Widemo 1998). Residents are territorial Independents that gather on leks during the breeding season and aggressively compete with each other to attract females to their own courts. Among Residents there is an expressed hierarchy with a strong mating skew, in which the alpha male often monopolizes half of the total matings over the breeding season. The second morph are Satellites. These non-aggressive and non-territorial males do not display on leks alone but join the court of a Resident for co-display. Co-display seems beneficial for the dyad (van Rhijn 1991; Hugie and Lank 1997) however the formation and function of co-display was never systematically studied. In contrast to most other systems with male co-display, Satellite Ruff males also directly compete with their co-displaying dominant partner for matings. As a consequence, the Resident risks losing matings as the cooperation turns into competition when females arrive to his court. Therefore, the benefit of cooperation for the Residents is less clear and may vary according to rank, lek size and competitiveness (Tolliver et al. in prep). Nevertheless, Residents of different ranks are often observed to co-display with Satellites both in captivity and on natural leks (van Rhijn 1991; own observations). Therefore, Ruffs provide an excellent opportunity to study benefits of cooperation across males of different ranks in an extremely competitive environment, in which co-display may have the character of a coalition, i.e. prevent other lekking males to gain matings. We aim to investigate the benefits of co-displaying for different ranking Residents by analysing the cooperative and competitive interactions, and female choice between co-displaying and single displaying Resident males in a controlled experimental setting. We aim to answer the following questions. Question 1: Does co-display increase attractiveness? At the lek level: 1.1. Does a lek with the same set of Resident males attract more females when having a Satellite than without having one (within lek comparison)? 1.2. Do leks that have a Satellite attract more females than the same sized leks with no Satellite (between leks comparison)? At the individual level: 1.3. Do Residents attract more females to their courts when paired with a Satellite than when unpaired (within Resident male comparison)? 1.4. Is the benefit of co-display for Residents dependent on their rank (between Resident males comparison)? Question 2: Does co-display change the courting strategy of Residents? 2.1. Do Resident males alter their investment into aggressive and courtship behaviour when paired with a Satellite in comparison to when they are unpaired (within Resident male comparison)? 2.2. How does the rank of the Resident alter the investment into aggressive and courtship behaviour during co-display (between Resident males comparison)? 2.3. Are paired high ranking males more aggressive to their Satellite partner than paired lower ranking males?