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Extra-pair mate choice in the song sparrow (Melospiza melodia)

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Title: Extra-pair mate choice in the song sparrow (Melospiza melodia)
Author: Ames, Caroline Elizabeth
Degree Master of Science - MSc
Program Forestry
Copyright Date: 2009
Publicly Available in cIRcle 2009-11-10
Abstract: Extra-pair paternity (EPP) is common in birds yet its adaptive significance remains unclear. Since the strategy of EPP is thought to carry costs, females are predicted to obtain indirect genetic benefits (e.g. ‘good genes’) or direct material benefits (e.g. fertility insurance) from pursuing extra-pair copulations (EPCs). Breeding synchrony may also influence the costs and benefits of EPP to males and females. I examine ‘good genes’ benefits of EPP and the effect of breeding synchrony on EPP in a socially monogamous population of song sparrows wherein 29% of 751 offspring were sired by extra-pair males. The good genes hypothesis predicts that females mate with extra-pair males that have higher expected fitness than their social mate in order to improve the fitness of extra-pair young (EPY) compared to within-pair maternal half-siblings. Using traits closely linked to lifetime reproductive success, I found no evidence that EPY were fitter than their maternal half-siblings or that extra-pair males were fitter than cuckolded males. However, I found that middle-aged males on average were 3.1 —4.7 times more likely to sire EPY than first-year males and 1.3 —2.0 times more likely to sire EPY than very old males. This is consistent with similar, well-established patterns of age-related variation in annual reproductive success in song sparrows, suggesting that male success in siring EPY is influenced by experience and ability, rather than quality. I found a significant negative relationship between breeding synchrony among neighbors and the proportion of EPY within broods of focal males. This result supports the ‘mate Extra-pair paternity (EPP) is common in birds yet its adaptive significance remains unclear. Since the strategy of EPP is thought to carry costs, females are predicted to obtain indirect genetic benefits (e.g. ‘good genes’) or direct material benefits (e.g. fertility insurance) from pursuing extra-pair copulations (EPCs). Breeding synchrony may also influence the costs and benefits of EPP to males and females. I examine ‘good genes’ benefits of EPP and the effect ofbreeding synchrony on EPP in a socially monogamous population of song sparrows wherein 29% of 751 offspring were sired by extra-pair males. The good genes hypothesis predicts that females mate with extra-pair males that have higher expected fitness than their social mate in order to improve the fitness of extra-pair young (EPY) compared to within-pair maternal half-siblings. Using traits closely linked to lifetime reproductive success, I found no evidence that EPY were fitter than their maternal half-siblings or that extra-pair males were fitter than cuckolded males. However, I found that middle-aged males on average were 3.1 —4.7 times more likely to sire EPY than first-year males and 1.3 —2.0 times more likely to sire EPY than very old males. This is consistent with similar, well-established patterns of age-related variation in annual reproductive success in song sparrows, suggesting that male success in siring EPY is influenced by experience and ability, rather than quality. I found a significant negative relationship between breeding synchrony among neighbors and the proportion of EPY within broods of focal males. This result supports the ‘mate guarding constraint’ hypothesis predicting that EPP decreases as synchrony increases because a larger proportion of males allocate time toward guarding their fertile social mate, instead of toward pursuing EPCs. However, I found that paternity loss was similar for males that sired EPY outside their social mate’s fertile period (40.4% of 57 males lost paternity) and for males that sired EPY during their mate’s fertile period (37.8% of 37 males lost paternity). This result suggests that mate guarding did not constrain males in the pursuit of EPCs; however, the exact timing of EPCs was unknown and may have influenced this result.
URI: http://hdl.handle.net/2429/14734

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