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

Sex ratio biases are often inconsistent, both among and within species and populations. While some of these inconsistencies may be due to experimental design, much of the variation remains inexplicable. Recent research suggests that an exclusive focus on mothers may account for some of the inconsistency, with an increasing number of studies showing variation in sperm sex ratios and seminal fluids. Using fluorescent in-situ hybridization we show a significant population level Y-chromosome bias in the spermatozoa of wild tammar wallabies, but with significant intra-individual variation between males. We also show a population-level birth sex ratio trend in the same direction towards male offspring, but a weaning sex ratio that is significantly female biased, indicating that males are disproportionately lost during lactation. We hypothesise that sexual conflict between parents may cause mothers to adjust offspring sex ratios after birth, through abandonment of male pouch young and reactiva...