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Predation on parasites is an important ecological process, but few experimental studies have examined the long-term impacts on the prey. Cleaner fish prey upon large numbers and selectively feed on the larger individuals of the ectoparasitic stage of gnathiid isopods. Removal of cleaner fish Labroides dimidiatus for 1.5–12.5 years negatively affects coral reef fishes, but the mechanism is unclear. A reduction in local parasite populations or the size of individual parasites would benefit all susceptible fishes. We tested whether cleaner presence reduces local gnathiid populations using 18 patch-reefs distributed between two sites (both at Lizard Island, Great Barrier Reef) which were maintained cleaner-free or undisturbed for 12 years. Using emergence traps (1 m²), free-living gnathiid stages were sampled before and after cleaner fish were removed during the day and night, up to 11 times over the course of the experiment. There were effects of the removal in the predicted direction, driven largely by the response at one site over the other involving 200% more gnathiids, but manifested only in the daytime sampling after 4 months. There was also a main effect (36%) for the shared sample dates at both sites after 12 years. Gnathiid size occasionally differed with cleaner presence, but in no consistent way over time. Contrary to our predictions, changes in free-living gnathiid population numbers and their size structure rarely reflected the changes in fish populations and individuals observed on cleaner-free reefs. Therefore, evidence that this predator alone regulates gnathiids remains limited, suggesting other contributing processes are involved.

Mutualisms are pivotal in shaping ecological communities. Iconic images of cleaner fish entering the mouths of predatory fish clients to remove ectoparasites epitomize their mutual benefit. Experimental manipulations of cleaner wrasse reveal declines in fish size and growth, and population abundance and diversity of client fishes in the absence of cleaner wrasse. Fishes grow more slowly and are less abundant and diverse on reefs without cleaner wrasse, both for larger species that are regularly cleaned and have high ectoparasite loads (“attractive species”), and for those smaller species that are rarely cleaned and are rarely infested with parasites (“unattractive species”). We therefore considered whether these previously observed declines in individual and population parameters on reefs without cleaners were related to increased ectoparasite infestation using an attractive species (Hemigymnus melapterus, Labridae) and an unattractive species (Pomacentrus amboinensis, Pomacentridae). Traps with these fish as a form of bait were deployed to sample blood-sucking gnathiid ectoparasites (Gnathiidae: Isopoda) on reefs from which cleaners (Labroides dimidiatus, Labridae) have been removed for 13 yr. Cleaner fish could not enter traps to access the clients/hosts, but gnathiids could enter the traps to infest hosts; thus, this method sampled the indirect effect of cleaners on gnathiid infestation of fish. Infestation was higher on reefs without cleaners than on those with them. The effect was only detected during the daytime when cleaners are active and only on the attractive species (H. melapterus). Thus, cleaner presence indirectly reduced fish exposure to parasites in a species that is highly susceptible to parasites, but not in one that is rarely infested with parasites. This suggests that cleaner presence indirectly reduces exposure of a common fish species to harmful parasites, which may explain some observed benefits in fishes at this location.

Joint group membership is of major importance for cooperation in humans, and close ties or familiarity with a partner are also thought to promote cooperation in other animals. Here, we present the opposite pattern: female cleaner fish, Labroides dimidiatus, behave more cooperatively (by feeding more against their preference) when paired with an unfamiliar male rather than with their social partner. We propose that cooperation based on asymmetric punishment causes this reversed pattern. Males are larger than and dominant to female partners and are more aggressive to unfamiliar than to familiar female partners. In response, females behave more cooperatively with unfamiliar male partners. Our data suggest that in asymmetric interactions, weaker players might behave more cooperatively with out-group members than with in-group members to avoid harsher punishment.

The cleaner wrasse Labroides dimidiatus often touches 'client' reef fish dorsal fin areas with its pelvic and pectoral fins. The relative spatial positions of cleaner and client remain constant and the cleaner's head points away from the client's body. Therefore, this behaviour is not compatible with foraging and the removal of client ectoparasites. As clients seek such 'tactile stimulation', it can be classified as an interspecific socio-positive behaviour. Our field observations on 12 cleaners (observation time of 112h) suggest that cleaners use tactile stimulation in order to successfully (i) alter client decisions over how long to stay for an inspection, and (ii) stop clients from fleeing or aggressive chasing of the cleaner in response to a cleaner fish bite that made them jolt. Finally, predatory clients receive tactile stimulation more often than non-predatory clients, which might be interpreted as an extra service that cleaners give to specific partners as pre-conflict management, as these partners would be particularly dangerous if they started a conflict. We therefore propose that cleaner fish use interspecific social strategies, which have so far been reported only from mammals, particularly primates.

Social learning has potential advantages over individual learning but one challenge is to identify valuable information. One possibility is to not randomly learn from any social partner but mainly from specific role models like for example the mother or high ranking group members. A potential mechanism for such directed social learning could be that individuals observe the actions of role models more often than of other group members. Field experiments showed that in vervet monkeys - a species with female philopatry - dominant females are more closely watched than dominant males in an artificial fruit-type social learning task. Here, we quantified social attention to males and females under natural conditions in six groups of vervet monkeys (Chlorocebus aethiops) at Loskop Dam Nature Reserve, South Africa. Using the focal sampling method, we quantified the frequencies with which all adult individuals were observed by other group members of known age class, rank, sex and degree of relatedness during foraging bouts and grooming interactions. We found that group members generally paid more attention to females than to males. This effect remained when we excluded matriline members from the analyses. Furthermore, we found that an individual's rank did not affect the attention it received from other group members. These results suggest that philopatry may promote social attention independently of an individual's rank and across situations.

It has been suggested that individuals may prevent partners from cheating by building up relationships slowly, giving very little in the beginning and raising the stakes in subsequent moves if partners reciprocate. I tested this idea with field experiments on the cleaner-fish Labroides dimidiatus and its \"client\" reef fish. Clients visit cleaners at their small territories, so-called cleaning stations, to have parasites removed. Cleaners were first observed and then caught and either put back on their original territory or moved to a new site. I noted a variety of cleaner and client behaviour to evaluate how, if at all, relationships are built up. Cleaners and resident clients indeed build up relationships, but with heavy initial investment. There was no evidence that cleaners build up relationships with client species that have access to several cleaners. Finally, it appeared that cleaners constantly invest in relationships with predatory clients, possibly to reduce the risk that predators try to catch them. I propose that asymmetries between partners with respect to either payoff values or strategic options are the major reason why the results do not fit the so-called raising-the-stakes strategy.

It is generally assumed that most primates live in mono-specific or poly-specific groups because group-living provides protection against predation, but hard evidence is scarce. We tested the anti-predation hypothesis with observational and experimental data on mixed-species groups of red colobus (Procolobus badius) and diana monkeys (Cercopithecus diana) in the Taï National Park, Ivory Coast. Red colobus, but not diana monkeys, are frequently killed by cooperatively hunting chimpanzees. Association rates peaked during the chimpanzees' hunting season, due to changes in the behaviour of the red colobus. In addition, playbacks of recordings of chimpanzee sounds induced the formation of new associations and extended the duration of existing associations. No such effects were observed in reaction to control experiments and playbacks of leopard recordings.

1. In marine cleaning symbioses, small fish or shrimps remove ectoparasites and other material from cooperating 'client' fish. Until now, there has been conflicting evidence with respect to the importance of cleaning for client health and in particular with respect to the impact of cleaning organisms on local fish diversity. 2. Data are provided on the impact of the cleaner wrasse Labroides dimidiatus on reef fish distribution at Ras Mohammed National Park, Egypt. During four field seasons, I regularly sampled fish species composition in 46 reef patches. Natural fluctuations in cleaner fish presence or absence at these reef patches allowed me to assess how this variable correlates with reef fish diversity. In addition, cleaner fish were removed experimentally from some reef patches and added at others to test whether the presence or absence of cleaner fish causes changes in fish diversity. 3. Both cleaner fish disappearance and cleaner fish removal had no immediate effects on fish abundance, i.e. within the first few weeks. A significant decline in fish diversity was only detectable after 4-20 months. 4. In contrast, cleaner immigration or experimental addition led to a significant increase in fish diversity within the first few weeks. 5. Generally, effects were most pronounced for client species that visit reef patches and soon move off again, and less pronounced (but still significant) for resident clients that do not move between reef patches. Surprisingly, the presence or absence of cleaner fish also affected the distribution of resident fish that hardly ever interact with them. 6. This study demonstrates that cleaner fish are a key organism for local reef fish diversity.

The feeding and defecation behaviour of the surgeonfish Ctenochaetus striatus was investigated at Ras Mohammed National Park (South Sinai, Red Sea). The fish feed on coral rock mainly by sweeping loose sediment with their flexible broom like teeth into their mouths. Feeding occurred exclusively on coral rock, but defecation took place only outside the grazing area above sand in small, precisely defined areas.

Humans are more likely to help those who they have observed helping others previously. Individuals may thus benefit from being altruistic without direct reciprocity of recipients but due to gains in 'image' and associated indirect reciprocity. I suggest, however, that image-scoring individuals may be exploitable by cheaters if pay-offs vary between interactions. I illustrate this point with data on cleaner-client reef fish interactions. I show the following: (i) there is strong variation between cleaners with respect to cheating of clients (i.e. feeding on client tissue instead of parasites); (ii) clients approach cleaners, that they observe cooperating with their current client and avoid cleaners that they observe cheating; (iii) cleaners that cheat frequently are avoided more frequently than more cooperative cleaners; (iv) cleaners that cheat frequently behave altruistically towards their smallest client species; (v) altruistic acts are followed by exploitative interactions. Thus, it appears that cleaners indeed have an image score, which selects for cooperative cleaners. However, cheating cleaners use altruism in potentially low-pay-off interactions to deceive and attract image-scoring clients that will be exploited.