Why do some mammals have short pregnancies while others take years? Wallaby study sparks new insights

Researcher
Dr Jessica Dudley, Dr Oliver Griffith
Writer
Fran Molloy
Date
16 December 2024
Faculty
Faculty of Science and Engineering
Macquarie University scientists compared placental mammals with wallabies, opossums and dunnarts to show how mammals evolved to manage inflammation during pregnancy, allowing them to extend their gestation from days to months.

Why do some mammals have long pregnancies, while others give birth much more quickly? The answer may lie in how their bodies handle inflammation during pregnancy, according to new research from Macquarie University.

Three of a kind:  The research team studied tammar wallabies (pictured), opossums and fat-tailed dunnarts to discover more about why pregnancy duration differs in mammals. Image: Oliver Griffith

The research, led by Dr Oliver Griffith, Lecturer in Evolutionary Genomics in the School of Natural Sciences, has been published in Proceedings of the National Academy of Sciences (PNAS).

The team's paper reveals how controlling inflammation was key to extending pregnancy times.

“Our data suggest that controlling inflammation when the embryo attaches to the uterus allows for longer pregnancies in mammals,” says lead author of the paper Dr Jessica Dudley, who is a Postdoctoral Fellow in Evolutionary Biology.

Marsupial Mamas

The research team compared three marsupials: tammar wallabies, opossums and fat-tailed dunnarts, and examined how genes related to inflammation behaved in each animal when the embryo attached to the mother’s uterus. Their findings showed striking differences between species.

In the field: Dr Oliver Griffith, pictured, says the team's discovery not only helps us understand how mammals evolved different pregnancy lengths, but also provides insights into the link between inflammation and successful pregnancy.

In short-gestating opossums, which complete pregnancy in just 12-13 days, and fat-tailed dunnarts with their 13-14 day pregnancy, almost all inflammation-related genes became active during embryo attachment. However, tammar wallabies, whose pregnancy lasts about 28 days, showed a different pattern.

“The tammar wallaby shows us a different evolutionary approach to extending pregnancy - instead of switching off inflammation after it starts, like humans do, wallabies prevent it from happening in the first place,” says Dr Griffith.

The ability to control inflammation during pregnancy appears to be a key factor in allowing a developing baby to stay in the womb longer.

The study reveals that the earliest mammals probably had short pregnancies due to inflammation when embryos attached to the uterus, similar to what occurs in opossum and dunnart pregnancy. Over millions of years, two groups evolved different solutions to this problem: “placental mammals” (including humans) and macropodids (kangaroos and wallabies).

“This research changes how we view inflammation in pregnancy, suggesting that the relationship between mother and developing baby depends on carefully controlled inflammation over an extended period,” says Dr Dudley.

Maternity leave

The length of pregnancy varies dramatically across different mammal species.

Lead author: Dr Jessica Dudley (pictured), Postdoctoral Fellow in Evolutionary Biology, says the new data suggests controlling inflammation when an embryo attaches to a uterus allows for longer pregnancies in mammals.

European hamsters give birth after 15-18 days, house mice carry their young for 19-21 days, and rabbits for about a month. Cats and dogs have similar gestation periods of around two months, while pigs carry their young for approximately 114 days.

In contrast, human pregnancies last 280 days - about nine months. Even longer pregnancies are seen in giraffes, which carry their calves for 430-460 days (approximately 15 months), while African elephants hold the record for the longest known mammalian pregnancy at 640-660 days, nearly two years.

“We discovered that while opossums and dunnarts activated almost all their inflammatory genes during embryo attachment, wallabies only used a few specific inflammatory genes at certain times during pregnancy,” says Dr Griffith.

Tammar wallabies were an interesting subject because they represent a unique middle ground in pregnancy length. The wallabies have a set breeding season, so all babies in a group are born around the same time. Their pregnancy lasts about 28 days, but development continues after birth, with the joey growing in the mother's pouch for another 8-9 months.

Pregnant Pause

The scientists analysed gene expression in uterine tissue samples collected from gravid (pregnant) and non-gravid (non-pregnant) wallabies, opossums and dunnarts at different stages of pregnancy.

By comparing these tissue samples using RNA sequencing, they could see which inflammatory genes were active or inactive during embryo attachment and throughout pregnancy.

“The ability to control inflammation during pregnancy appears to be a key factor in allowing a developing baby to stay in the womb longer,” says Dr Griffith.

This helps explain the vast difference between the brief 12-day gestation of an opossum, the month-long pregnancy of a tammar wallaby, and the 22-month pregnancy of an African elephant.

In humans, pregnancy involves three stages of inflammation: an initial inflammatory response when the embryo implants, followed by a long anti-inflammatory period allowing the baby to develop, and finally, inflammation returns during labour.

Wallabies take a different approach, maintaining low inflammation throughout pregnancy and only activating specific inflammatory genes when needed.

“This discovery not only helps us understand how mammals evolved different pregnancy lengths - but also provides insights into the complex relationship between inflammation and successful pregnancy,” says Dr Griffith.

Dr Oliver Griffith is a lecturer in Evolutionary Genomics in the School of Natural Sciences, Faculty of Science and Engineering, Macquarie University.

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