Embryo movement is more frequent in avian brood parasites than birds with parental reproductive strategies.

Stephanie C Mcclelland,Claire N Spottiswoode,Jess Lund,Matthew I M Louder,Silky Hamama,Steven J Portugal,Wolfgang Goymann,Miranda Reynolds,Ignas Safari,Luke A Mcclean,Marcel Honza,Molly Cordall,Tanmay Dixit,Mark E Hauber

doi:10.1098/rspb.2021.1137

Abstract

Movement of the embryo is essential for musculoskeletal development in vertebrates, yet little is known about whether, and why, species vary. Avian brood parasites exhibit feats of strength in early life as adaptations to exploit the hosts that rear them. We hypothesized that an increase in embryonic movement could allow brood parasites to develop the required musculature for these demands. We measured embryo movement across incubation for multiple brood-parasitic and non-parasitic bird species. Using a phylogenetically controlled analysis, we found that brood parasites exhibited significantly increased muscular movement during incubation compared to non-parasites. This suggests that increased embryo movement may facilitate the development of the stronger musculoskeletal system required for the demanding tasks undertaken by young brood parasites.

Highlights

Movement is essential for successful embryonic development across vertebrates [1,2]
Evidence of increased musculature in a virulent brood parasite has been observed in the chicks of common cuckoos (Cuculus canorus), which have a higher density of muscle fibres in their musculus complexus, the hatching muscle in their necks, compared to non-parasitic 2 birds [24]
The incubation period was divided into five stages to standardize embryonic development, and egg size was accounted for in the analyses

Summary

Introduction

Movement is essential for successful embryonic development across vertebrates [1,2]. Embryonic movement shapes the development of an animal’s musculoskeletal system and ranges from sporadic twitching of muscle tissue in the early stages of development, to coordinated motions akin to walking or flying closer to hatching [3,4]. Evidence of increased musculature in a virulent brood parasite has been observed in the chicks of common cuckoos (Cuculus canorus), which have a higher density of muscle fibres in their musculus complexus, the hatching muscle in their necks, compared to non-parasitic 2 birds [24] This is speculated to be an adaptation for hatching from significantly thicker eggshells, but may facilitate the eviction of host eggs and chicks. Given the evidence that muscle development is shaped by embryonic activity in birds, the increased embryonic movement provides a plausible mechanism by which denser and stronger muscles, including the musculus complexus, could be developed by young common cuckoos and other parasitic species This range of behaviours exhibited by parasitic chicks inspires predictions about differences in embryonic movement among parasite species. The muscular demands of less virulent parasitic species should be greater than those of non-parasitic species, since less virulent species must still outcompete host young, typically through heightened begging

Results

Discussion

Methods

66. Orme CDL et al 2006 Global patterns of geographic