Abstract
In a few species, males invest more than females in parental care while the females invest in mating competition and producing multiple broods for several mates. Species in the family Jacanidae are commonly used for studying this type of breeding system (called sex-role reversal), and previous studies found discrepancies and variation between species in the expected characteristics of reversed sex roles. Yet, a better understanding of sex role differences in breeding behavior in such species is crucial for disentangling possible evolutionary mechanisms leading to this peculiar breeding system. Sex-role reversal in the pheasant-tailed jacana Hydrophasianus chirurgus has been documented long time ago. Since the very early observation of this species, however, there was no attempt to provide a comprehensive and quantitative description of their breeding. This study aims to fill these knowledge gaps by investigating the sex role differences in the breeding behavior of pheasant-tailed jacanas, by observing and monitoring a breeding population in Taiwan. We focused on three main characteristics of sex-role reversal: (1) competition between females for access to males, such as agonistic and courtship behaviors, (2) polyandrous mating, and (3) male-only care. As expected, we found that females provide most of the territory defense toward conspecifics. Males also participated in agonistic behaviors, although less frequently than females. Furthermore, contrary to what was expected, we found that males spent more time than females on courtship behavior. Polyandrous females performed mating and laying sequentially with different mates but maintained the pair bonds simultaneously with multiple males. For the first time for the species, we could estimate that the average number of mates per female (i.e., degree of polyandry) was 2.4 and that at least 81.8% of the females in the population were polyandrous. Finally, our observations corroborated that brood care is predominantly provided by males, nevertheless females were also participating to some degree in brood attendance but never in direct care (i.e., brooding). This study highlights that some aspects of polyandrous breeding might deviate from stereotyped view on sex-role reversal, and stress the importance of further within species and comparative studies in order to fully understand the mechanisms leading to sex-role reversal.
Highlights
In species providing parental care the sex difference in parental investment, as defined by Trivers (1972), varies widely
We investigate possible sex differences in the courtship behavior, where we would expect females investing more in courtship when starting a new clutch, since the benefits of producing additional offspring should be higher for the females than for the males with already existing brood
We highlight some patterns that are important to take in account: (1) males participated in territorial defense especially toward males that were not paired with the same female and males invested more in courtship behaviors than females
Summary
In species providing parental care the sex difference in parental investment, as defined by Trivers (1972), varies widely. In most of the species, on average, females invest more in parental care (Clutton-Brock, 1991; Cockburn, 2006; Székely et al, 2013), which is often termed as conventional sex-roles. In a few species, we observe the opposite: males invest more than females in parental care while the females invest in mating competition and breeding with several mates This is the case in species that have male-only parental care with a polyandrous type of mating system, called sex-role reversal (Trivers, 1985; Andersson, 1994). In a sex-role reversed species, the females are expected to take upon territoriality and compete with other females for access to mates through aggressiveness. The dominance of females on males in sex-role reversed species is confirmed by their reversed sexual size dimorphism with females being bigger than males and expected to be more aggressive (Székely et al, 2007)
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