Why do hosts with obvious egg polymorphism suffer low parasitism rates under avian brood parasitism? A theoretical consideration

摘要

Polymorphism is a general phenomenon in the natural world evolving as a result of frequency-dependent selection. Such polymorphism has recently received increasing attention in parasitic cuckoo-host systems, in which egg polymorphism in hosts is regarded as a specific adaptation against cuckoo parasitism. Here, we provided a theoretical analysis to investigate the effect of host egg polymorphism on the rate of successful parasitism (SPR) by cuckoos to help better understand the role of egg polymorphism in cuckoo-host systems. Five key conclusions can be drawn from these calculations: (1) The contrast of egg phenotypes in hosts significantly reduces cuckoo fitness by reducing the SPR of cuckoos; (2) hosts can minimize the SPR of cuckoos by laying polymorphic eggs with equal proportion of the two morphs if cuckoos also evolve egg dimorphism as a counter-adaptation; (3) the escalation of egg polymorphism in hosts further reduces the SPR in cuckoos, although the efficiency of such an effect decreases with the number of host egg morphs; (4) cuckoos can maximize their SPR by laying a ratio of egg morphs equal to the ratio of host egg morphs; and (5) an increase in the number of cuckoo egg morph does not increase the total SPR of cuckoos for parasitizing hosts with polymorphic eggs. Egg polymorphism in hosts is regarded as a specific adaptation against cuckoo parasitism. Here, we provided a theoretical analysis to show that the disruptive evolution of egg phenotypes in hosts significantly reduces cuckoo fitness by reducing the rate of successful parasitism by cuckoos (SPR). Hosts with obvious egg polymorphism suffer low parasitism rates and they can minimize the SPR of cuckoos by laying polymorphic eggs with equal proportions of the two morphs if cuckoos also evolve egg dimorphism as a counteradaptation, whilst cuckoos can maximize their SPR by laying a ratio of egg morphs equal to the ratio of host egg morphs.