SWAMP WALLABIES
Lucy Cooke on the extraordinary reproductive feat of one marsupial
IMAGINE BEING PREGNANT FOR YOUR entire adult life. That’s the reality for the swamp wallaby – a stocky member of the kangaroo family with a unique talent for maximising her reproductive capacity.
Life in the Australian outback is tough. Temperatures can be lethal and rainfall sporadic, making food sources unpredictable for vegetarians such as wallabies and kangaroos. Evolution has equipped these marsupials with some fiendish adaptations to cope with the stress of daily life.
Hopping for a living doesn’t just look like fun, it’s also the most energetically effective means of travelling long distances in search of dinner. The secret is an oversized tendon in their hind legs that acts like a spring, storing elastic energy from the downward force of gravity and body mass, which then propels the animal forward for free. An Australian zoologist monitored red kangaroos on treadmills, attached to heart monitors and oxygen. This surreal experiment revealed that when hopping along at the speed of a world-class marathon runner, the red kangaroo uses half as much energy as a human. With a top speed of about 65kmph, these 2m pogo sticks can also travel three times faster.
Female wallabies and kangaroos are equally efficient in their approach to motherhood. By having not one, but two uteri, the shortest pregnancies on record – at about 28 days – and a pouch to develop their undercooked baby in that provides two different formulas of milk to cater for different life stages, they are able to run a veritable production line of baby-making. Mothers can have an embryo in one uterus; an early-stage pouch joey suckling a special immuneboosting milk; and a semi-dependent youth at foot nursing a more lipid-rich milk from a separate nipple.
Mothers can also pause the development of their embryo in the uterus, so it’s only born during favourable times. Once the newborn joey begins to suck milk, the freshly fertilised embryo enters a long period of developmental arrest that can last up to 11 months depending on environmental conditions. When the sucking stimulus from the young in the pouch declines, the dormant embryo starts growing again and the cycle begins anew.
The swamp wallaby has taken this to an extreme. She comes into oestrus, mates and forms a new embryo in her spare uterus one to two days before the end of her existing pregnancy. While most mammals require a break between pregnancies, the swamp wallaby is the only one that can claim the feat of being permanently pregnant throughout her adult life. Surfing a constant rollercoaster of hormones, contractions and nursing sounds exhausting, but it has enabled this wallaby to conquer all the eastern states, from the tip of Queensland to the bottom of southern Australia.
Catch up with Political Animals, Lucy’s new three-part series.
Lucy is a broadcaster, zoologist and author of Bitch: A Revolutionary Guide to Sex, Evolution and the Female Animal.