Parturition and perfect prematurity: birth in marsupials

Abstract

Marsupials are distinguished from eutherian mammals in their mode of reproduction. They give birth to a highly altricial young, which completes its development whilst attached to a teat, usually within a pouch. The marsupial neonate has relatively well-developed digestive, respiratory and circulatory systems but retains its fetal excretory system with a fully functional mesonephric kidney and undifferentiated gonads and genitalia. We have investigated birth in the tammar wallaby (Macropus eugenii) and shown that the tiny (400 mg) fetus determines the time of its own delivery. Although plasma progesterone falls, and oestradiol associated with the postpartum oestrus typically rises, around the time of parturition, neither hormone is essential for the timing of birth. However relaxin may loosen the connective tissue of the cervix and vaginae for birth. Labour starts suddenly and is completed within minutes. Both prostaglandins and mesotocin are essential for the contractions that deliver the young. Prostaglandins from the reproductive tract act via the brain to control parturient behaviour. In the last 2 days of gestation fetal adrenal glucocorticoid production increases, promoting lung maturation and surfactant production and ultimately triggering labour. The accessibility of the altricial neonatal marsupial provides a unique opportunity for experimental manipulation of organ development and maturation.

A pronounced uterine pro-inflammatory response at parturition is an ancient feature in mammals

“…It is possible that this is analogous to a state of labour in the opossum. However in other marsupials labour is brief; it is only a matter of minutes in macropods [56].…”

Embryo-endometrial interactions during early development after embryonic diapause in the marsupial tammar wallaby

“…In late gestation the maturing fetus produces increasing amounts of cortisol that may provide the fetal signal for parturition (Ingram et al, 1999. At term, progesterone falls rapidly concomitantly with short pulses of prostaglandin F2a and mesotocin (Parry et al, 1996) that trigger uterine contractions and parturition (Shaw andRenfree, 2001, Shaw andRenfree, 2006). Mating occurs about 1 h post-partum (Rudd, 1994), and ovulation and fertilisation occur about 40 hrs post-partum (Renfree, 1994, Renfree andLewis, 1996).…”

Reduced phenotypic covariation in marsupial limbs and the implications for mammalian evolution

“…There are significant reasons to suppose that marsupials should exhibit different levels of phenotypic covariation between their fore‐ and hindlimbs, relative to eutherians. In contrast to eutherians, marsupials give birth after very short gestations to highly altricial young that complete their development while attached and feeding at the teat (Muller, 1969; Sharman, 1973; Tyndale‐Biscoe, 1973; Lillegraven, 1975; Tyndale‐Biscoe & Renfree, 1987; Hughes & Hall, 1988; Renfree, 1993; Shaw & Renfree, 2006). Most marsupial young complete the journey from the birth canal to the teat through a swim‐like crawl, using their robust forelimbs to anchor their premature bodies while their hindlimbs hang passively (Gemmell, 2002).…”