How 'roos got pouches, Hot eggs on high

Q:  I have always wondered how kangaroos got their pouches.  (Mary, Toronto, Canada)

Eastern grey kangaroo. Courtesy of Wikipedia.

A:  During the Mesozoic period, about 125 million years ago, in what is now China, early mammals split into four branches; three still exist:  placental mammals (more formally called eutherian mammals) like us, monotremes like a platypus and marsupials like a kangaroo.   Early marsupials were small shrew-like creatures that lacked a pouch.  A mother gave birth to a tiny baby, which locked onto and hung from a teat as it completed its development.  This worked fine for minute babies, but not for the bigger ones produced by larger animals. 

"The young would be too heavy to carry this way," says zoology professor Geoff Shaw of the University of Melbourne.  Today, all larger marsupials have a pouch, but many smaller species either lack pouches or have only a rudimentary one. 

The pouch is a simple structure — a fold of skin around the area with the mammary glands and teats.  The ancestral marsupial species had to develop a pouch to grow larger than rat size.  The pouch supports the baby, and lessens the burden on the teat.  That's how the kangaroo got her pouch.

A female kangaroo (called a doe, a flyer or a jill) has more control over her reproduction system than placental mammals do; this capability helps her survive bad times.  Normally, she gives birth to a honeybee-size baby.  The tiny thing finds its way from birth canal to safety in its mother's pouch in about three minutes.

The baby is a "curious mixture":  some things like forelimbs (needed to get to the pouch) are highly developed, says Shaw.  Hind limbs (not needed yet) are rudimentary. The mouth and tongue (needed to attach to the teat) are well developed.  The reproductive organs are not yet differentiated; it is an "it".  For comparison, human fetuses become either male or female in the first trimester.

Somewhat surprisingly, the baby climbs, unaided by its mother, from the birth canal to the pouch, where it changes direction, claws down and attaches to a teat.  We don't know how it finds its way, but gravity and smell may provide cues.  Once attached, the baby will not release the teat for six months, and will not be fully weaned for a year. 

Newborn kangaroo fetus sucking on a teat in the pouch. Courtesy of Geoff Shaw with the University of Melbourne http://kangaroo.genome.org.au and Wikipedia, copyright, used with permission.

The mother's milk, though, can dry up, if a drought sears the land or some other catastrophe that deprives her of food.  The young kangaroo in the pouch dies, but she saves energy and manages to survive.  Moreover, she usually has another embryo dormant in the uterus because, after giving birth, she immediately mates again.  When no baby suckles at her teats, the dormant embryo activates.  It will be born in four weeks.

In summary, the kangaroo got her pouch to support a large baby while it suckles and matures to an independent life.  This development allowed larger marsupials to gradually evolve from the original smaller species. 

By the way, marsupial animals also have effective placentas and the smaller species use them, exclusively.  The larger species use pouches only to complete their baby's development, but use their placentas to help exchange gas and nutrients before the baby is born.  Marsupials are named from the Greek marsupios, meaning purse.

Further Reading:

Life cycle of the tammar wallaby by Marilyn Renfree and Geoff Shaw, University of Melbourne

Lactation in mammals, University of Illinois, Urbana

Kangaroo, Wikipedia

History of Australian marsupials by Amber McKenney, University of California, Los Angeles

Q:  How long does it take to boil an egg on Mt Everest?  (Matt, Broken Hill, New South Wales, Australia)

Mount Everest as seen from the nearby mountain, Kalapatthar.  Courtesy of Pavel Novak and Wikipedia.

A:  Suppose we're on top of Mt. Everest.  A 50 mph (80 km/h) gale blasts the mountaintop this fine June day. We've pitched a tent, got the stove going and put a pot of water on to boil.  Gradually the tent warms above freezing.  Finally, the water's boiling, and I drop an egg in.  How long does it take for it to boil?  About 20 minutes for a soft-boiled egg.

How did I arrive at the answer?  First, I looked up the temperature at which water boils at Mt. Everest's altitude: 29,035 feet (8850 m).  At that altitude, water boils at 156° F (69° C).  The Mt. Everest boiling water is much cooler than boiling water at sea level (212° F, 100° C) because the mountaintop air pressure is only about a third of that at sea level.  

By the way, if you don't have tables handy, a good rule of thumb is that the temperature of boiling water decreases by 3.33° C for each kilometer of altitude.  Or, in English units:  1.83° F for each 1000 feet of altitude.  Knowing this, "people who live in Denver or who backpack in mountains now have an easy way for determining the boiling point at their elevation," says meteorologist Craig Bohren, author of What Light through Yonder Window Breaks? 

Furthermore, I assumed that the initial temperature of the egg is as cold as if I had removed it from a refrigerator:  4° C.  Then, I turned to Google to find the soft-boil temperature.  According to the FDA code, an egg is safe for immediate consumption if I raise the egg's temperature to about 63° C for 15 seconds.  This was reassuring since the temperature (69° C) at which water boils on top of Mt. Everest is greater than the necessary cooking temperature (63° C).  Therefore, I can actually soft boil the egg. 

Now, how long must I cook it?  Bohren came up with 24 minutes, based on chemical-reaction speeds.   The rate of chemical reactions doubles with every increase in temperature of 10° C — another useful rule of thumb.  Boiling an egg involves chemical reactions as the egg protein cooks. 

Thus, for every drop in temperature of 10° C, the speed halves and the time it takes doubles.  The boiling temperature (69° C) on Mt. Everest is about 30° C lower than at sea level (100° C).  Therefore, the time doubles (for the first 10° C), doubles again (four times, for the second 10° C) and doubles again (eight times, for the last 10° increment).  Thus the time for boiling increases by a factor of eight. 

A soft-boiled egg takes about 3 minutes to cook at sea level.  Thus, Bohren predicts the soft-boiled egg will take about 24 minutes on top of Mt. Everest. 

Finally, I estimated 17 minutes based on a heat-transfer formula derived by physicist Charles D.H. Williams of Exeter University in England. Click here for the heat-transfer details. 

But is either estimate right?  Like any good scientist, I checked the predictions with an experiment.  First pressing a helper into service, I gathered my apparatus:  a pot for boiling water, a candy thermometer, a plastic stirring spoon, ice cubes and a couple of 57-g eggs. 

Kitchen-science apparatus. Photo courtesy of Lanney Atchley.

We cooled the initially boiling water (93° C temperature, at my altitude) to the temperature at which water boils on Mt. Everest (69° C), put the egg in the hot water, maintained the 69° C temperature for 17 minutes and stirred the water continually to simulate the mixing associated with boiling. 

A 20-minute soft-boiled egg on "Mt. Everest." Photo courtesy of Lanney Atchley.

When the timer dinged, we removed the egg from the hot water, and plunged it into ice water to stop the egg cooking.  Then we sampled the egg.  Good, but the white part of the egg was a tad runny.  We repeated the experiment on a second egg, increasing the time to 20 minutes.  The results:  a perfect soft-boiled egg.  See figure. 

Both Bohren's and William's predictions are consistent with our experiment.

Further Reading:

Does lightning strike the top of Mount Everest? Weatherquesting

What Light through Yonder Window Breaks by Craig Bohren. New York, John Wiley & Sons, Inc: 1991.

The science of boiling an egg by Charles D.H. Williams, Exeter University

Egg food safety, Egg Nutrition Center

Panoramas: Turn slowly around and see the 360-degree top-of-the-world view from Mount Everest’s summit. Photos by Roderick Mackenzie

(Answered June 13, 2006)