Facultative and non-facultative sex ratio adjustments in a dimorphic bird species

Simone Santoro*, Andy J. Green, John R. Speakman, Jordi Figuerola

*Corresponding author for this work

Research output: Contribution to journalArticle

4 Citations (Scopus)

Abstract

If parental allocation to each offspring sex has the same cost/benefit ratio, Fisher's hypothesis predicts a sex ratio biased towards the cheaper sex. However, in dimorphic birds there is little evidence for this, especially at hatching. We investigated the pre-fledgling 1) sex ratio, 2) body condition and 3) sex-differential mortality in a population of the glossy ibis Plegadis falcinellus, in southern Spain between 2001 and 2011. We defined two age groups for the period between hatching and fledging. We also compared pre-fledgling with the autumn sex ratio. Metabolic rates were estimated by the doubly labeled water (DLW) technique to establish that sons (the bigger sex) were 18% more energy demanding than daughters, and to compute the predicted Fisher's sex ratio (0.465). As population size increased between years, body condition decreased in both sexes, and mortality increased more for daughters than sons prior to fledging. At the same time, the proportion of males among chicks close to fledging increased (average sex ratio: 0.606) while the proportion close to hatching decreased (average sex ratio: 0.434, in line with Fisher's prediction). Furthermore, the proportions of males at fledging and the following autumn were negatively correlated across years. We suggest that, as population density increased and conditions worsened the larger sex had relatively higher survival. These differences in survival produce a shift from a facultative female-biased sex ratio at hatching into a non-facultative male-biased sex ratio of fledglings. Additionally, the excess of males at fledging was counterbalanced by sex-related dispersal during the autumn. Overall, glossy ibis sex ratio is a product of a combination of facultative and non-facultative adjustments triggered by environmental conditions, driven by rapid population growth, and mediated by highly interrelated life-history traits such as body condition, mortality, and dispersal.

Original languageEnglish
Pages (from-to)1215-1224
Number of pages10
JournalOikos
Volume124
Issue number9
Early online date6 Feb 2015
DOIs
Publication statusPublished - Sep 2015

Keywords

  • doubly-labeled water
  • body condition
  • reproductive success
  • energy-expenditure
  • american kestrels
  • passerine bird
  • metabolic-rate
  • CO2 production
  • silver spoon
  • population

Cite this

Facultative and non-facultative sex ratio adjustments in a dimorphic bird species. / Santoro, Simone; Green, Andy J.; Speakman, John R.; Figuerola, Jordi.

In: Oikos, Vol. 124, No. 9, 09.2015, p. 1215-1224.

Research output: Contribution to journalArticle

Santoro, Simone ; Green, Andy J. ; Speakman, John R. ; Figuerola, Jordi. / Facultative and non-facultative sex ratio adjustments in a dimorphic bird species. In: Oikos. 2015 ; Vol. 124, No. 9. pp. 1215-1224.
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abstract = "If parental allocation to each offspring sex has the same cost/benefit ratio, Fisher's hypothesis predicts a sex ratio biased towards the cheaper sex. However, in dimorphic birds there is little evidence for this, especially at hatching. We investigated the pre-fledgling 1) sex ratio, 2) body condition and 3) sex-differential mortality in a population of the glossy ibis Plegadis falcinellus, in southern Spain between 2001 and 2011. We defined two age groups for the period between hatching and fledging. We also compared pre-fledgling with the autumn sex ratio. Metabolic rates were estimated by the doubly labeled water (DLW) technique to establish that sons (the bigger sex) were 18{\%} more energy demanding than daughters, and to compute the predicted Fisher's sex ratio (0.465). As population size increased between years, body condition decreased in both sexes, and mortality increased more for daughters than sons prior to fledging. At the same time, the proportion of males among chicks close to fledging increased (average sex ratio: 0.606) while the proportion close to hatching decreased (average sex ratio: 0.434, in line with Fisher's prediction). Furthermore, the proportions of males at fledging and the following autumn were negatively correlated across years. We suggest that, as population density increased and conditions worsened the larger sex had relatively higher survival. These differences in survival produce a shift from a facultative female-biased sex ratio at hatching into a non-facultative male-biased sex ratio of fledglings. Additionally, the excess of males at fledging was counterbalanced by sex-related dispersal during the autumn. Overall, glossy ibis sex ratio is a product of a combination of facultative and non-facultative adjustments triggered by environmental conditions, driven by rapid population growth, and mediated by highly interrelated life-history traits such as body condition, mortality, and dispersal.",
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T1 - Facultative and non-facultative sex ratio adjustments in a dimorphic bird species

AU - Santoro, Simone

AU - Green, Andy J.

AU - Speakman, John R.

AU - Figuerola, Jordi

N1 - Funded by the Ministerio de Economia y Competitividad Junta de Andalucía

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N2 - If parental allocation to each offspring sex has the same cost/benefit ratio, Fisher's hypothesis predicts a sex ratio biased towards the cheaper sex. However, in dimorphic birds there is little evidence for this, especially at hatching. We investigated the pre-fledgling 1) sex ratio, 2) body condition and 3) sex-differential mortality in a population of the glossy ibis Plegadis falcinellus, in southern Spain between 2001 and 2011. We defined two age groups for the period between hatching and fledging. We also compared pre-fledgling with the autumn sex ratio. Metabolic rates were estimated by the doubly labeled water (DLW) technique to establish that sons (the bigger sex) were 18% more energy demanding than daughters, and to compute the predicted Fisher's sex ratio (0.465). As population size increased between years, body condition decreased in both sexes, and mortality increased more for daughters than sons prior to fledging. At the same time, the proportion of males among chicks close to fledging increased (average sex ratio: 0.606) while the proportion close to hatching decreased (average sex ratio: 0.434, in line with Fisher's prediction). Furthermore, the proportions of males at fledging and the following autumn were negatively correlated across years. We suggest that, as population density increased and conditions worsened the larger sex had relatively higher survival. These differences in survival produce a shift from a facultative female-biased sex ratio at hatching into a non-facultative male-biased sex ratio of fledglings. Additionally, the excess of males at fledging was counterbalanced by sex-related dispersal during the autumn. Overall, glossy ibis sex ratio is a product of a combination of facultative and non-facultative adjustments triggered by environmental conditions, driven by rapid population growth, and mediated by highly interrelated life-history traits such as body condition, mortality, and dispersal.

AB - If parental allocation to each offspring sex has the same cost/benefit ratio, Fisher's hypothesis predicts a sex ratio biased towards the cheaper sex. However, in dimorphic birds there is little evidence for this, especially at hatching. We investigated the pre-fledgling 1) sex ratio, 2) body condition and 3) sex-differential mortality in a population of the glossy ibis Plegadis falcinellus, in southern Spain between 2001 and 2011. We defined two age groups for the period between hatching and fledging. We also compared pre-fledgling with the autumn sex ratio. Metabolic rates were estimated by the doubly labeled water (DLW) technique to establish that sons (the bigger sex) were 18% more energy demanding than daughters, and to compute the predicted Fisher's sex ratio (0.465). As population size increased between years, body condition decreased in both sexes, and mortality increased more for daughters than sons prior to fledging. At the same time, the proportion of males among chicks close to fledging increased (average sex ratio: 0.606) while the proportion close to hatching decreased (average sex ratio: 0.434, in line with Fisher's prediction). Furthermore, the proportions of males at fledging and the following autumn were negatively correlated across years. We suggest that, as population density increased and conditions worsened the larger sex had relatively higher survival. These differences in survival produce a shift from a facultative female-biased sex ratio at hatching into a non-facultative male-biased sex ratio of fledglings. Additionally, the excess of males at fledging was counterbalanced by sex-related dispersal during the autumn. Overall, glossy ibis sex ratio is a product of a combination of facultative and non-facultative adjustments triggered by environmental conditions, driven by rapid population growth, and mediated by highly interrelated life-history traits such as body condition, mortality, and dispersal.

KW - doubly-labeled water

KW - body condition

KW - reproductive success

KW - energy-expenditure

KW - american kestrels

KW - passerine bird

KW - metabolic-rate

KW - CO2 production

KW - silver spoon

KW - population

U2 - 10.1111/oik.01889

DO - 10.1111/oik.01889

M3 - Article

VL - 124

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JO - Oikos

JF - Oikos

SN - 0030-1299

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ER -