Why do we have baby teeth?

Baby teeth, also known as milk teeth or deciduous teeth, start forming in the jaws of a baby in utero with the front teeth almost fully formed (apart from the roots) by the time of birth. Baby teeth erupt from about 6 months starting with the front teeth and are usually all present by the age of two and a half years. The first permanent grinding tooth (molar) erupts just behind the last baby molar. Then the front baby teeth get slowly replaced with permanent teeth and by about 12 years of age all the permanent teeth are erupted in the mouth and by adulthood most people have their 3rd molars (“wisdom teeth”).

The jaws of infants and children are far too small to accommodate the larger permanent teeth. Baby teeth are essential for the development of the mouth. They maintain the jaw length, and provide guides for the eruption pathway and therefore proper placement of permanent teeth.

Humans aren’t the only species who have two sets of teeth, but not all animals who have teeth have two sets. Some animals, such as hamsters and moles, only have one set of teeth in their lifetime. Most other vertebrates such as reptiles and fishes have the ability to replace their teeth over and over again. The tooth sizes are very similar and non-specialsed.

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So, really, the question is why do we only have one set of baby teeth and permanent teeth!

Mammals have very specialised sets of teeth that need to fit together properly to work well. Each tooth has a specific function and they need to work together as a unit, which makes chewing much more efficient for the purposes of getting nutrients from food. If they are constantly being shed and replaced throughout life the precision matching of shape and size of neighbouring teeth that enable that efficient chewing is lost.

But there is a trade off with developing such specialised teeth – it takes more energy to make them, so we are left with one set of teeth during development and one throughout our whole adulthood.

 

First sighting of a wild newborn chimpanzee taken from mother and cannibalised.

The first ever witnessed case of a newborn wild chimpanzee being taken from the mother and subsequently eaten was published by researchers from Japan this week in the American Journal of Anthropology. The authors explain that this most likely occurred because the pregnant chimpanzee did not go on “maternity leave”, a period near the time of birth that the mother chimpanzee hides and gives birth alone.

Just a matter of seconds after the mother, Devota, gave birth and before she could touch her newborn, an adult male chimpanzee, Darwin, picked up the infant and took it into the bush. The researchers therefore couldn’t confirm if it was a live or still birth or the sex of the infant. Darwin and other members of the group were subsequently seen eating the newborn. At the time of the incident in 2014, there were about 60 members in the group, with 20 adult females and 10 adult males.

Screen Shot 2018-02-03 at 11.55.38 PM.pngPhoto of Darwin with the infant that was subsequently cannibalised (Nishie and Nakamura 2018).

There have been 45 sightings of infanticide in wild chimpanzee groups in six different populations, but never around the time of birth. This incident occurred in group “M” from the Mahale Mountains National Park in Tanzania, which have been studied since 1968. The authors found clear evidence for maternity leave from data from their daily attendance records between 1990 and 2010 with a total of 94 births to 36 females.

Infanticide perpetrated by males has been observed in many primate species and is usually explained as being a male reproductive strategy, whereby males increase their chance of fathering an infant by killing unrelated infants, and results in the mothers becoming fertile again sooner after birth.

The authors propose explanations for why Devota gave birth without “maternity leave”: 1) the baby was preterm and was therefore “not expected”; 2) it was her first pregnancy and had not learned the behaviour of “maternity leave”; and 3) she felt safe in the group as she may have copulated with a number of males around the time of conception of the infant.