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Optimal clutch size and male incubation investment in the male-only incubating emu (Dromaius novaehollandiae)

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Abstract

The incubation capacity hypothesis states that clutch size is limited by the number of eggs an adult can successfully incubate due to the individual’s morphology and physiology. Clutch size may be reduced in females because of the costs of egg production or in both parents due to the need to provide post-hatching care of young, complicating the testing of an individual’s incubation capacity. We tested the incubation capacity hypothesis by studying a species with male-only parental care and precocial young, the emu Dromaius novaehollandiae, in which males can theoretically maximise care during incubation. Clutch size in emus varied considerably from 3 to 32 eggs, with males with larger natural, unmanipulated clutch sizes investing most in incubation in terms of the time they spent attending the nest. However, we found that when manipulating clutch size (to 4, 11 and 20 eggs), males responded to both clutch size and the magnitude of clutch size change during early incubation. Males with a median-sized clutch hatched the greatest number of eggs with moderate investment in incubation, while large nests elicited greater investment but a lower probability of hatching and fewer eggs to hatch; though when clutch sizes were manipulated, the increase in incubation investment between median and large clutches was not significantly different. Median clutches likely represent the male’s incubation capacity, which moderates male fitness and may drive selection for median-sized clutches in stable environments. However, the large variation in emu clutch sizes suggests that optimal clutch size may be driven by where females choose to lay or that optimal clutch size might be seasonally variable, with changing environmental and predation pressures.

Significance statement

Avian reproductive success is restricted by the number of eggs parents can successfully incubate. The incubation capacity of a species is often difficult to test, however, as care is often divided between the parents and between incubation and the substantial post-hatching care of young. Using the male-only incubating emu, who invests heavily in incubation but little in post-hatching care, we test the effect of clutch size on male behaviour and hatching success. Males with a median-sized clutch (~ 11 eggs) hatched the greatest number of eggs with moderate investment in incubation, while large nests elicited similar or greater investment but lower hatching success. Our findings suggest that for species with uniparental care and precocial young, the incubation capacity is a key driver of reproductive success.

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Data availability

The data that support the findings of this study are openly available in figshare at http://doi.org/10.6084/m9.figshare.16987912.

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Acknowledgements

Thank you to past emu researchers who contributed to the discussion in the project formation: Professor Stephen Davis, Dr Dominique Blache, Professor Graeme Martin, Associate Professor David Groth, Neil Hamilton and Dr Maria Calviño-Cancela. Thank you to Alex Barley (AB), Faelan Mourmourakis (FM), Kate Murphy, Stephen Mahony and the Haire family. For assistance in statistical analysis, thank you to Desi Quintans. We thank two anonymous reviewers and Associate Editor Dr Dustin Rubenstein whose comments greatly improved the manuscript.

Funding

This project was funded by the New South Wales Department of Planning, Industry and Environment and the Hawkesbury Institute for the Environment.

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  1. Hawkesbury Institute for the Environment and the School of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW, 2751, Australia

    Julia Ryeland, Clarissa M. House, Kate D. L. Umbers & Ricky-John Spencer

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  1. Julia Ryeland
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  2. Clarissa M. House
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All authors contributed to the project concept design and revision of the manuscript. JR collected and analysed all data and drafted the manuscript.

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Correspondence to Julia Ryeland.

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Ethics approval

This project was conducted under the following Western Sydney University Animal Ethics licences: A11897, A11898 and A12475. All work was approved by the Office of Environment and Heritage (scientific licence #SL101821), and the use of animals adhered to all guidelines set forth by the Australian National Health and Medical Research Council (NHMRC).

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The authors declare no competing interests.

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Communicated by D. Rubenstein

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Ryeland, J., House, C.M., Umbers, K.D.L. et al. Optimal clutch size and male incubation investment in the male-only incubating emu (Dromaius novaehollandiae). Behav Ecol Sociobiol 75, 168 (2021). https://doi.org/10.1007/s00265-021-03110-4

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