Emerging sex allocation research in mammals: marsupials and the pouch advantage

Abstract

1. Adaptive adjustments in offspring sex ratios in mammals have long been reported, but the conditions and mechanisms that prompt shifts in the proportion of sons and daughters born are still unclear. 2. Empirical evidence indicates that offspring sex in mammals can be related to a diversity of environmental and maternal traits. However, the underlying assumptions regarding offspring and maternal fitness are rarely tested. 3. Physiological mechanisms of maternal selection of offspring sex may occur at many stages during the prolonged maternal investment stage, and a pluralistic approach to studying mechanisms might prove fruitful. 4. This review highlights the apparent frequency, in marsupial mammals, of sex ratio bias, which has largely been recorded as conforming to one of a few hypotheses. 5. Marsupials are ideally suited to experiments involving cross-fostering of offspring, which can allow rigorous tests of the fitness consequences of rearing one sex vs. the other. The reproductive biology of marsupials lends the group to detailed studies of the timing and physiological correlates of offspring sex biases. 6. Many components of metatherian biology may prove advantageous in experimental studies of sex allocation in mammals, and together may provide a prosperous avenue for examining adaptive and mechanistic hypotheses in mammalian sex allocation.

Offspring sex ratio in mammals and the Trivers‐Willard hypothesis: In pursuit of unambiguous evidence

“…From theory to practice The use of "condition" in context of the TWH refers to capacity to provide maternal care [10] (see also [11,16,18,20]). From theory to practice The use of "condition" in context of the TWH refers to capacity to provide maternal care [10] (see also [11,16,18,20]).…”

“…The TWH has been extensively tested in a wide range of species, especially mammals [14][15][16][17][18]. Despite its intuitive appeal, however, this hypothesis has produced equivocal results at best [14].…”

Milk Composition during Lactation Suggests a Mechanism for Male Biased Allocation of Maternal Resources in the Tammar Wallaby (Macropus eugenii)

“…Unbalanced sex ratios frequently occur in marsupials [17], [19] and male biased sex ratios are increasingly problematical in captive breeding programs that play an important role in conservation and management [20]. It is therefore essential that a greater understanding of facultative sex allocation in marsupials be developed.…”

Sexual conflict in action: An antagonistic relationship between maternal and paternal sex allocation in the tammar wallaby, Notamacropus eugenii

“…The difference between the 66.6% male offspring in the younger cohort and the 20% male offspring in the older cohort suggests the possibility of maternal adaptive control between birth and weaning. Marsupials have the "pouch advantage" (Robert & Schwanz, 2011), whereby offspring are born at a relatively early stage of development, with females yet to invest large amounts of energy into offspring (Hayssen, Lacy, & Parker, 1985;Renfree, 1983;Tyndale-Biscoe, 2005). Eutherian mothers would require signals of offspring sex at an early stage to selectively abort after the peri-conceptual stage (Hardy, 1997); it is, therefore, remarkably easier for a marsupial mother to abort or abandon the current offspring (Robert & Schwanz, 2011).…”

“…Marsupials have the "pouch advantage" (Robert & Schwanz, 2011), whereby offspring are born at a relatively early stage of development, with females yet to invest large amounts of energy into offspring (Hayssen, Lacy, & Parker, 1985;Renfree, 1983;Tyndale-Biscoe, 2005). Eutherian mothers would require signals of offspring sex at an early stage to selectively abort after the peri-conceptual stage (Hardy, 1997); it is, therefore, remarkably easier for a marsupial mother to abort or abandon the current offspring (Robert & Schwanz, 2011). It is possible that the trend toward male offspring in the younger cohort, the suggested "birth offspring SR," was driven by sperm SR and is not in line with maternal interests, resulting in an antagonistic relationship between parents.…”