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

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. [ABSTRACT FROM AUTHOR]