Male‐specific mortality biases secondary sex ratio in Eurasian tree sparrows Passer montanus

Abstract

Sex allocation theory predicts that parents bias the offspring sex ratio strategically. In avian species, the offspring sex ratio can be biased at multiple growth stages, although the mechanisms are not well known. It is crucial to reveal a cause and timing of biased offspring sex ratio. We investigated (i) offspring sex ratio at multiple growth stages, from laying to fledging; and (ii) the stage at which offspring sex ratio became biased; and (iii) the cause of biased offspring sex ratio in Eurasian tree sparrows Passer montanus. Sex determination of 218 offspring, including hatchlings and unhatched eggs from 41 clutches, suggested that the offspring sex ratio was not biased at the egg‐laying stage but was significantly female‐biased after the laying stage due to higher mortality of male embryos. Half of the unhatched eggs showed no sign of embryo development (37/74, 50.00%), and most undeveloped eggs were male (36/37, 97.30%). Additional experiments using an incubator suggested that the cause of embryo developmental failure was a lack of developmental ability within the egg, rather than a failure of incubation. This study highlights the importance of clarifying offspring sex ratio at multiple stages and suggests that offspring sex ratio is adjusted after fertilization.

Sex-specific signalling of individual personality by a mutual plumage ornament in a passerine

“…random sample). Evidence from another population (Kato et al 2017) shows that in tree sparrows, the mortality of male embryos during incubation can cause a similar femalebiased adult sex ratio, as observed in our population. Therefore, we are reasonably confident that our sample is random, but we are aware that a sampling bias caused by different trapping probability of males and females cannot be excluded completely.…”

Sex-Biased Mortality and Sex Reversal Shape Wild Frog Sex Ratios

“…Although there is evidence of sex-biased mortality in juvenile mammals, birds, and even some fish (Kruuk et al, 1999;Cichon et al, 2005;Svensson et al, 2007;Orzack et al, 2015;Moran et al, 2016;Kato et al, 2017;Firman, 2019), to our knowledge, this is the first record of sex-biased mortality in larval amphibians in the wild. Importantly, we find the same pattern of male-biased mortality across two independent frog populations which inhabit contrasting environments, suggesting sex-biased mortality may be a relatively common phenomenon.…”

“…An alternate, and non-exclusive, hypothesis to sex reversal is that amphibian sex ratio variation is driven by sexbiased mortality (Figure 1). Sex-biased mortality prior to birth or hatching has been increasingly observed in wild animal populations, particularly in mammals and birds, but also occasionally in fish (Kruuk et al, 1999;Cichon et al, 2005;Svensson et al, 2007;Orzack et al, 2015;Moran et al, 2016;Kato et al, 2017;Firman, 2019). Multiple factors, including temperature stress and parental condition, have been identified as factors underlying sex-biased mortality, suggesting that sex-biased mortality may not be an adaptive process, as has been suggested for sex allocation, but a maladaptive response to stressful rearing conditions (Kruuk et al, 1999;Goth and Booth, 2005;Eiby et al, 2008;DuRant et al, 2016;Firman, 2019).…”

Sex determination and optimal development in the Moorish gecko, Tarentola mauritanica

“…This lack of studies/ evidence could indicate that this is a rather rare mechanism in herps, but it cannot be discarded as a possible explanation for the obtained results, only because there are few studies demonstrating this phenomenon. On the contrary, this seems to be a common mechanism well described in mammals, birds and some fishes (Firman, 2020;Kato et al, 2017;Morán et al, 2016). In all these examples, sex bias is correlated with a decrease in fecundity resulting from the sex-biased mortality of embryos or eggs.…”