Louise Quesnel, Allison MacKay, David M. Forsyth, Kevin R. Nicholas, and Marco Festa-Bianchet
Maternal care in mammals has long interested biologists. Evolutionary ecologists hope to better understand the adaptive mechanisms underlying lactation, including what causes differences in milk provisioning, including offspring characteristics such as sex.
Maternal care and maternal effects have been extensively documented in large terrestrial mammals, and many of these studies have looked at ungulates or seals. That’s where we come in: we looked at milk composition in a large mammal that we expected to show a lot of individual variability, because mothers are expected to have complete control over lactation. We thought that because of this characteristic, marsupials would have great potential to advance our understanding of factors driving intraspecific variability in maternal care and therefore in female reproductive strategies.
Kangaroos are extraordinarily peculiar mammals. They share many ecological traits with other large herbivores: they are polygynous and highly sexually dimorphic. Their reproductive cycle, however, is very different. Females give birth to an underdeveloped young after a gestation of only 36 days, followed by an extremely long lactation that last up to 22 months. This is roughly four times longer than for eutherian mammals of the same size, for example deer. Females can also give birth at any time of the year, implying that they face different energetic challenges depending on the timing of their reproduction, and we expected to see evidence of these seasonal effects on milk composition. Also, for young kangaroos, milk composition follows stage-specific changes that are set to signal tissue development and growth, and mothers of sons may differentially allocate certain nutrients in the milk compared to mothers of daughters.
Our paper published in the Journal of Zoology sought to answer the questions: what are the ecological determinants of milk composition in a free-ranging marsupial? Do sex of young or environment variability influence the maternal care? We measured protein and lipid concentrations in 103 samples of eastern grey kangaroo (Macropus giganteus) milk collected over two years from marked females in beautiful (but windy) south-eastern Australia. Now, collecting milk from a sleeping kangaroo comes with some technical challenges, but what we found was quite interesting, so it was worth it.
First, our study was conducted during two years of highly contrasting conditions. Year 2 was very dry, resulting in very low resource availability, with about half as much forage production compared to year 1. In year 2, females lost body condition and few attempted to reproduce. It is safe to say that our study females were under a lot of metabolic stress during year 2, and that stress affected the composition of their milk. Milk produced during the low forage year was much lower in protein and lipids than milk produced the previous year, when more forage was available. Moreover, milk content was different between females that were sampled at the same stage of lactation, but at different periods of the year. Kangaroos that gave birth earlier and were sampled in late winter produced milk that was higher in lipids, compared to females that gave birth later and reached the same stage of lactation in late spring-early summer. The asynchronous lactation of kangaroos allowed us to identify a very resource-driven and conservative maternal care strategy, as females face the costs of lactation in a seasonal but also rather unpredictable environment from year to year.
We also found differences in milk provided to sons and daughters. Supporting the hypothesis of condition-dependent and sex-biased maternal care, mothers in good condition that nursed a son produced milk that was higher in protein that mothers in good condition that had a daughter. This result suggests that females that can provide additional care to their offspring should direct that care to the sex that is more likely to benefit from it, and is likely to provide higher fitness returns on the additional care. Male kangaroos should benefit more than females from a large size, as it should increase their mating success which is mostly determined by their ability at fighting other males. Obtaining milk rich in proteins early in their life may favour rapid growth and be an adaptive mechanism for mothers to obtain higher lifetime fitness.
Our two-year study provided further evidence that kangaroos are an attractive study species for research in evolutionary ecology. Their lactation strategy integrates the environmental conditions they live under, and the sex of their offspring, to modulate resource allocation to reproduction. Our research is part of a larger long-term monitoring program of marked kangaroos. Future research on this population could investigate inter-individual variation in milk composition, looking at the repeatability of milk composition within females and the contribution of individual quality in the provisioning of care.