Why Are Kangaroos Born So Small? A Different Bet on Pregnancy

Gestation in marsupials is very short, running from around twelve days in some species to around a month in the largest, and the young is born at a stage of development that in a placental mammal would still be well inside the uterus. The newborn is blind, hairless, with hind limbs barely formed, and it weighs a fraction of a gram in most species. What is developed is exactly what is needed for the next task, being strong forelimbs with claws, a functioning nose and mouth and the nerves to coordinate them, and the animal climbs unaided through the mother's fur to reach a teat, attaches and does not let go for weeks. Development then continues attached to the teat, which is where most of the growing happens.

The contrast with placental reproduction is a set of trade-offs rather than a hierarchy:

• •The investment before birth is small, so a failed pregnancy costs the mother very little
• •A young that dies can be replaced quickly, which suits unpredictable conditions
• •The mother is not carrying a heavy foetus, so she remains mobile throughout
• •Milk composition changes continuously through the long lactation as the young develops
• •A female can carry an embryo in arrested development, a joey in the pouch and one at foot at once
• •The cost is a long lactation, which is where the energetic investment actually goes

The proximate reason concerns the immune system and the placenta. A developing embryo carries genes from the father and is therefore foreign tissue, which a mother's immune system would attack, and placental mammals have elaborate mechanisms suppressing that response locally for the duration of a long pregnancy. Marsupials have a simpler placenta and less developed suppression, and the pregnancy ends at about the point where the maternal immune system would begin to reject the young, which is one well-supported account of the timing. The strategy works because the pouch substitutes for the uterus, providing a protected environment in which development continues externally with the immune problem removed, and milk substitutes for the placenta as the supply route.

The distribution of marsupials is a geographical puzzle with a plate tectonic answer. They are concentrated in Australia and New Guinea, with a substantial group in South America and one species reaching North America, and that pattern was inexplicable until continental movement was accepted. Fossils show the group present in North America, then in South America, then across Antarctica, and finally in Australia, which traces a route across a southern supercontinent while those landmasses were joined and before Antarctica froze. Australia's isolation afterwards allowed marsupials to diversify into the roles placental mammals occupy elsewhere, producing marsupial equivalents of moles, mice, gliders, cats and wolves through convergent evolution, which is the standard illustration of similar pressures producing similar forms from different ancestry.

Several species add a further mechanism that makes the arrangement more capable than it appears. A female can mate again shortly after giving birth and the resulting embryo develops to a small cluster of cells and then stops, held in that state while a joey occupies the pouch, and resumes only when that pouch becomes free either through the joey leaving or through its death. That means a replacement is available almost immediately if conditions kill the current young, which is a substantial advantage in environments where drought and food failure are unpredictable. The mother can simultaneously produce two different milks from adjacent teats for young at different stages, which requires the mammary glands to operate independently and is a striking piece of physiology in its own right.

Gestation runs from twelve days to about a month, and the newborn weighs a fraction of a gram with only its forelimbs, nose and mouth developed enough to climb to the pouch. The pouch substitutes for the uterus and milk for the placenta, with the investment shifted into a long lactation. An embryo held in arrested development allows immediate replacement if the young is lost.