Comparison Of Counter-adaptation In Two Sympatric Prinias To Defense Against Cuckoo Parasitism

Egg discrimination and rejection of the foreign parasitic egg is one of the most important evolved adaptations for hosts to counter brood parasitism during coevolution in parasite-host system. Correspondingly, egg rejection in different host species were extensively tested by plenty of empirical studies. The present study compared egg recognition and egg rejection in two small passerine hosts, the yellow bellied prinia Prinia flaviventris and the plain prinia Prinia inornata between mainland China and Taiwan island where have different parasitism pressure. Our result showed that hosts presented pretty egg recognition and egg rejection, and the egg-rejecting ability of prinias could be dramatically reduced when they are released from the selection pressure of cuckoo parasitism. However, the egg recognition ability can persist for an extended period of time (i.e.,2-3 million years) that has been demonstrated for the first time. Rejection motivation of a host should be a result of a trade-off between parasitism risk and rejection cost which affect the rejection decision. Our data provided the first evidence of a stepwise discrimination process in cuckoo hosts.Hosts can evolve anti-parasitic defenses to recognize and reject foreign eggs from their nests under selection pressure from cuckoo parasitism. Egg rejection mainly depends on egg appearance, and theory predicts that higher inter-clutch and lower intra-clutch variation in host egg appearance would facilitate the detection of the parasitic eggs. We compared egg appearance variation and egg rejection behavior of two prinia species between Guangxi, mainland China, and Taiwan island populations. The results revealed that parasitized prinias presented a larger inter-clutch and a lower intra-clutch variation in egg appearance than did non-parasitized hosts. Furthermore, the mainland host population evolved polymorphic eggs, whereas the island population laid monomorphic eggs. In addition, prinias under parasitism system with egg uniformity exhibited significantly higher egg rejection rate than population that have escaped from cuckoo parasitism.Characteristics of egg surface can serve as recognition cues that allow avian hosts to detect and reject foreign eggs when parasitized. Egg recognition cues include egg color, egg speckling or egg spot, UV light and blunt pole part, and so on.The blunt egg pole hypothesis suggests that the blunt egg pole serves as an essential signal in host egg recognition. In this study, eggs of yellow-bellied prinia were painted with black spots at either the blunt or sharp egg pole to determine and compare the responses of parasitic and non-parasitic populations to recognition cues at the blunt egg pole. The results showed that the parasitic population showed higher rate of rejection when eggs had spots at the blunt pole than when spots were present at the sharp pole. On the other hand, the non-parasitic population accepted all experimental eggs, which might be related to the decreased egg recognition resulting from the low exposure to cuckoo parasitism. Thus our findings supported the blunt egg pole hypothesis but also suggested that egg color is a more reliable recognition cue than egg spots in the yellow-bellied prinia. Furthermore, we confirm that UV light play an important role in the host egg recognition process.Cuckoo hosts may adjust their rejection decision to avoid negative effect from brood parasitism but this would depend on different parasitism risk, recognition costs and recognition mechanism. True recognition and discordancy are the two main egg recognition mechanism inhosts. We tested egg recognition of two prinia species under parasitism and non-parasitism system, and found that both two prinias could accurately distinguish and reject the alien egg from their nests without any self-referencing, and both mechanisms are used in their process of egg recognition and egg rejection. Furthermore, compared with population under low parasitism pressure, hosts have a significantly higher egg rejection rate and tend to evoke the egg recognition mechanism with a bias towards true recognition mechanism when suffering more intensive cuckoo parasitism, implying the plasticity in recognition mechanism.Parasites may, in the multi-parasite system, block the defenses of their hosts and thus thwart host recognition of parasites by frequency-dependent selection. Nest defenses as frontline may block or promote the subsequent stage of defenses such as egg and nestling recognition. We conducted comparative studies of the defensive strategies of a host of the Oriental cuckoo Cuculus optatus, the yellow-bellied prinia, and a host of the Common cuckoo Cuculus canorus in mainland China with multiple species of cuckoos and in Taiwan with a single cuckoo species. The aggressive defenses of hosts directed toward cuckoos were blocked in the presence of multiple cuckoo species, although cuckoo hosts such as yellow bellied prinia, were specifically able to distinguish its cuckoo parasite. Furthermore, our study supported the strategy facilitation hypothesis suggesting that the frontline of nest defense promotes the subsequent evolution of egg recognition. Our results significantly improved the knowledge of intra-specific variation in anti-parasitic behavior of hosts between single- and multi-cuckoo systems and provided a novel hypothesis for empirical testing.Hosts would suffer specific breeding costs from parasites, and life history theory predicts that, hosts of brood parasitism prefer to evolve small clutch size when they lose more eggs and nestling whereas there have no effect on adult survival rate. Such strategy would benefit from next successful breeding once they may escape from brood parasitism in the future. Clutch size of two prinias under parasitism and non-parasitism system was compared. Our data revealed that clutch size of the yellow bellied prinia in the parasitized population was much smaller than the non-parasitized population and the life history hypothesis was supported. However, in the plain prinia, smaller clutch size in the island population may also be due to other ecological factors such as the island syndrome.In addition, nest dismantling behaviour of yellow bellied prinia was observed and it was strongly related to cuckoo parasitism. Therefore, we suggested that nest dismantling behaviour is likely to evolve as a counter-adaptation of brood parasitism in the yellow bellied prinia.