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Sibling effects in adulthood
(i) Males
Overall, we found no indication of beneficial effects of siblings on male reproductive success in adulthood, but instead there was evidence of same-sex competition.
First, 65.7 per cent of males (n = 3201) who survived to adulthood reproduced in their lifetime, and each additional elder brother alive when the younger brother reached adulthood decreased this probability of ever reproducing (odds ratio, OR = 0.87, 95% CI = 0.80–0.96, Inline Formula p = 0.001), whereas there was no effect of elder sisters (Inline Formula p = 0.40; figure 2a; electronic supplementary material, table S2a). The model controlled for significant effect of family SES.
Second, the lifetime number of children (5.4 ± 0.07 s.e.) among males who reproduced at least once (n = 2104) was also negatively associated with their number of elder same-sex siblings alive at the onset of adulthood (β = −0.07 ± 0.01 s.e., Inline Formula p < 0.0001), whereas elder sisters had no significant effect on a male's number of children (Inline Formula p = 0.59; figure 2b; electronic supplementary material, table S3a). The model controlled for significant positive effects of the overall number of siblings.
When including age at first reproduction (on average 28.38 ± 0.13 s.e.) in the previous model, we found that it was, as expected, negatively associated with a male's lifetime number of children (Inline Formula p < 0.001). The negative effect of elder brothers was, however, still significant (Inline Formula p < 0.001), but its magnitude was decreased, suggesting that its effect on lifetime number of children was partly mediated by an effect on age at first reproduction (β = −0.05 ± 0.01 s.e., 29% decrease). This model controlled for significant effect of the sibship size (Inline Formula p = 0.014) and age at mother's death (Inline Formula p = 0.018).
(Ii) Females
Similar to males, we found no indication of beneficial effects of siblings on female reproductive success in adulthood, but there was again evidence of same-sex competition.
First, 74.7 per cent of females who survived to adulthood reproduced at least once (n = 3292), and this probability was negatively correlated with the number of elder same-sexed siblings (sisters) alive when the younger sister reached adulthood (OR = 0.90, 95% CI = 0.82–0.98, Inline Formula p = 0.02), but was not significantly associated with the number of elder brothers (Inline Formula p = 0.88; figure 2c; electronic supplementary material; table S2b).
Second, each elder sister alive at adulthood decreased their sisters' number of children (5.03 ± 0.06 s.e.) among those females who reproduced at least once (n = 2459, β = −0.04 ± 0.01, Inline Formula p = 0.001), whereas the number of elder brothers had no effect on lifetime fecundity (Inline Formula p = 0.32; figure 2d; electronic supplementary material, table S3b). The model controlled for significant effects of family SES.
Such effects might arise partly from females with elder sisters being less likely to ever marry (Inline Formula p = 0.005). Nevertheless, when adding age at first reproduction (25.8 ± 0.10 s.e.; Inline Formula p < 0.001) to the previous model on lifetime fecundity, the negative effect of elder sisters remained significant (Inline Formula p = 0.02; β = −0.03 ± 0.01 s.e., 25% decrease). The number of elder sisters was further negatively associated with the probability of marrying a landowning man (Inline Formula p < 0.001), whereas the number of elder brothers was not
(i) Males
Overall, we found no indication of beneficial effects of siblings on male reproductive success in adulthood, but instead there was evidence of same-sex competition.
First, 65.7 per cent of males (n = 3201) who survived to adulthood reproduced in their lifetime, and each additional elder brother alive when the younger brother reached adulthood decreased this probability of ever reproducing (odds ratio, OR = 0.87, 95% CI = 0.80–0.96, Inline Formula p = 0.001), whereas there was no effect of elder sisters (Inline Formula p = 0.40; figure 2a; electronic supplementary material, table S2a). The model controlled for significant effect of family SES.
Second, the lifetime number of children (5.4 ± 0.07 s.e.) among males who reproduced at least once (n = 2104) was also negatively associated with their number of elder same-sex siblings alive at the onset of adulthood (β = −0.07 ± 0.01 s.e., Inline Formula p < 0.0001), whereas elder sisters had no significant effect on a male's number of children (Inline Formula p = 0.59; figure 2b; electronic supplementary material, table S3a). The model controlled for significant positive effects of the overall number of siblings.
When including age at first reproduction (on average 28.38 ± 0.13 s.e.) in the previous model, we found that it was, as expected, negatively associated with a male's lifetime number of children (Inline Formula p < 0.001). The negative effect of elder brothers was, however, still significant (Inline Formula p < 0.001), but its magnitude was decreased, suggesting that its effect on lifetime number of children was partly mediated by an effect on age at first reproduction (β = −0.05 ± 0.01 s.e., 29% decrease). This model controlled for significant effect of the sibship size (Inline Formula p = 0.014) and age at mother's death (Inline Formula p = 0.018).
(Ii) Females
Similar to males, we found no indication of beneficial effects of siblings on female reproductive success in adulthood, but there was again evidence of same-sex competition.
First, 74.7 per cent of females who survived to adulthood reproduced at least once (n = 3292), and this probability was negatively correlated with the number of elder same-sexed siblings (sisters) alive when the younger sister reached adulthood (OR = 0.90, 95% CI = 0.82–0.98, Inline Formula p = 0.02), but was not significantly associated with the number of elder brothers (Inline Formula p = 0.88; figure 2c; electronic supplementary material; table S2b).
Second, each elder sister alive at adulthood decreased their sisters' number of children (5.03 ± 0.06 s.e.) among those females who reproduced at least once (n = 2459, β = −0.04 ± 0.01, Inline Formula p = 0.001), whereas the number of elder brothers had no effect on lifetime fecundity (Inline Formula p = 0.32; figure 2d; electronic supplementary material, table S3b). The model controlled for significant effects of family SES.
Such effects might arise partly from females with elder sisters being less likely to ever marry (Inline Formula p = 0.005). Nevertheless, when adding age at first reproduction (25.8 ± 0.10 s.e.; Inline Formula p < 0.001) to the previous model on lifetime fecundity, the negative effect of elder sisters remained significant (Inline Formula p = 0.02; β = −0.03 ± 0.01 s.e., 25% decrease). The number of elder sisters was further negatively associated with the probability of marrying a landowning man (Inline Formula p < 0.001), whereas the number of elder brothers was not
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