Avian and Poultry Biology Reviews 17 (1), 2006
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Manipulation
of Primary Sex-Ratio: an updated review
Carlos
Alonso-Alvarez
Unidad
de EcologıŽa, Instituto de InvestigacioŽn en Recursos CinegeŽticos
(IREC-CSIC-UCLM- JCCM). Ronda de Toledo, s/n.
13005
ABSTRACT
Some
bird species would be able to manipulate primary sex ratio, i.e.
the proportion of male and female offspring at the laying time. Such trait
could be considered being mostly under maternal control. Nevertheless, the
exact mechanism involved remains undetermined. Knowledge of the mechanism would
contribute to any assessment of its cost in terms of resources and/or time, which would be important to formulate
predictions of those scenarios where it could have evolved. In fact, small
costs of sex ratio control could overcome the adaptive value of adjusting the
proportion of sexes in the progeny. Pike and Petrie (2003) published an
excellent review on the potential mechanism involved. However, many new
experimental and correlational evidences have appeared from that date,
providing new and interesting perspectives. Thus, hormonal control is obtaining
strong support from recent findings. The present review updates the current
knowledge on this subject with emphasis on the question of whether sex ratio
manipulation is a widespread trait among avian taxa. Finally, future directions
for research, including not only those related to the mechanism per
se, but also those linked to conditions necessary for the evolution of
this trait, are presented. Thus, the necessity of determining fitness functions
of male and female offspring and both parents, as well as the problem of if
this trait is the result of a evolutionary constraint
or strategy, are also addressed.
Keywords:
birds, primary sex-ratio, proximate mechanisms, segregation distortion, sex
allocation, sex-specific
embryo mortality
Reproductive
biology and sperm competition in Australian fairy-wrens
Melissah Rowe* and
Stephen Pruett-Jones
Department
of Ecology and Evolution, University of Chicago, 1101 East 57th Street, Chicago,
Illinois 60637, USA
ABSTRACT
In
birds, the outcome of sperm competition is determined by the relative timing
and order of matings, sperm numbers, and the differential fertilizing capacity
of male ejaculates. The intensity of sperm competition varies across species
according to mating system, specifically the copulation behavior of females.
The Australian Maluridae (fairy-wrens, grasswrens, and emu-wrens) exhibit a
diversity of reproductive behaviors and adaptations thought to be associated
with sperm competition. Australian malurids live in sedentary social groups and
reside on temporally stable territories. The species are socially monogamous,
with males and females forming multi-year pair bonds,
and exhibit facultative cooperative breeding. In contrast to the relative
conservatism of social systems, the malurid genera vary widely in reproductive
behaviors. Fairy-wrens (Malurus) exhibit some of the highest known rates of
reproductive promiscuity for socially monogamous species, and males possess many
anatomical and behavioral adaptations associated with sperm competition
including relatively large testes and sperm stores. In contrast, male
grasswrens (Amytornis) and emu-wrens (Stipiturus) show relatively small testes
and sperm
stores. These differences are likely a consequence of
relatively lower rates of reproductive promiscuity in the grasswrens and
emu-wrens. Fairy-wrens and the other malurids illustrate the complex
interactions between social and reproductive behaviors and the intensity of
sperm competition and sexual selection in birds.
Keywords:
Maluridae, fairy-wren, Malurus, sperm competition,
extra-pair paternity, promiscuity