The Physiological And Biochemical Mechanisms Of Short Incubation Period In Brood Parasitism Of Birds

Brood parasite is a special reproductive strategy in birds and insects.Parasites lay eggs in host's nest and hosts will take the charge of egg incubation.Hatching earlier than their hosts is critical for birds leading a brood parasite living and its evolutionary significance have been widely appreciated.How brood parasitic birds shorten the incubation period,however,is largely unknown.According to the hypotheses of shortening incubation period in egg-laying vertebrates,an embryo could enhance physiological function and enzyme activity to accelerate the development rate and therefore shorten incubation period.We proposed three underlining mechanisms that make parasitic birds to shorten their incubation period.First,eggs may contain more yolk content to supply more energy for faster developmental rate;Secondly,an embryo may improve heart rate during incubation,increase heart mass,enhance cardiac output,to enhance metabolic level and developmental rate;Third,an embryo may up-regulated mitochondrial function and enzyme activity,increase liver mass to enhance metabolic level and developmental rate.To exploring the proximate mechanisms by which the eggs of parasitic birds hatch faster than their hosts,we compared the differences in physiological and biochemical basis associated with metabolism between common cuckoo(Cuculus canorus)and their host oriental reed warblers(Acrocephalus orientalis)in zhalong national nature reserve,China.First,we founded that the eggs of common cuckoos hatched one day earlier than those of oriental reed warblers.Secondly,based on the results of embryonic physiology and biochemistry,we found that:(1)eggs of common cuckoos had more yolk energy than those of their hosts and embryos consumed more yolk content during incubation than their hosts;(2)embryos of common cuckoos had lower heart rates(fH),but heavier heart mass and greater cardiac output(heart mass*heart rate)than their hosts;(3)the activities of metabolic enzymes,including Succinate dehydrogenase,lactate dehydrogenase,and adenosine triphosphatase did not differ between the two species.The activities of cytochrome coxidase and mitochondrial respiration were higher in the host than common cuckoo.After liver mass and protein concentration were taking into account,common cuckoos had higher metabolic levels than their hosts.Our results suggest that,cuckoo embryo have more yolk content to support high metabolic level,which lead faster development rate and earlier-hatching.