No evidence for offspring sex-ratio adjustment to social or environmental conditions in cooperatively breeding purple-crowned fairy-wrens |
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Authors: | Sjouke A Kingma Michelle L Hall Anne Peters |
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Institution: | (1) Max Planck Institute for Ornithology, Vogelwarte Radolfzell, Schlossallee 2, 78315 Radolfzell, Germany;(2) Mornington Wildlife Sanctuary, Australian Wildlife Conservancy, PMB 925, Derby, WA, 6728, Australia;(3) Research School of Biology, Australian National University, Canberra, 0200, Australia;(4) School of Biological Sciences, Monash University, Clayton, VIC, 3800, Australia |
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Abstract: | When fitness returns or production costs vary between male and female offspring, selection is expected to favor females that
adjust offspring sex ratio accordingly. However, to what extent vertebrates can do so is the subject of ongoing debate. Here,
we explore primary sex ratios in 125 broods of cooperatively breeding purple-crowned fairy-wrens Malurus coronatus. We expected that females might adjust offspring sex ratio because this passerine species experiences considerable variation
in social and environmental conditions. (1) However, although helpers substantially increase parental fitness, females (particularly
in pairs and small groups) did not overproduce philopatric males (helper-repayment hypothesis). (2) Sex-ratio adjustment based on competition among individuals (helper-competition hypothesis) did not conceal helper-repayment effects or drive sex allocation on its own: while high-quality territories can accommodate
more birds, brood sex ratios were independent of territory quality, alone or in interaction with group size. (3) Additionally,
males are larger than females and are possibly more costly to produce (costly sex hypothesis), and (4) female offspring may benefit more from long-term effects of favorable conditions early in life (Trivers–Willard hypothesis). Nonetheless, large seasonal variation in food abundance was not associated with a consistent skew in primary sex ratios.
Thus, overall, our results did not support the main hypotheses of adaptive sex-ratio adjustment in M. coronatus. We discuss that long-term differential costs and benefits may be insufficient to drive evolution of primary sex-ratio manipulation
by M. coronatus females. More investigation is therefore needed to determine the general required sex differences in long-term fitness returns
for mechanisms of primary sex-ratio manipulation to evolve. |
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