Mammal Reproduction

The approximately 4700 mammals on the planet come in all shapes and sizes. But they all have similarities that make them mammals.

The approximately 4700 mammals on the planet come in all shapes and sizes. But they all have similarities that make them mammals.

  • They all possess hair or something similar
  • They can all regulate their body temperature, (as do the birds)
  • They can all produce milk to feed offspring.

Mammals occupy a huge part of the Earth’s surface, from pole to pole and everywhere in between, including the oceans. They are an astoundingly successful group! There are many species, and who can say that we’ve found all of the mammals living in our world?  (Of course, we mammals don’t even begin by comparison to the insects - with an estimated 10 million species.)  We also regard the mammals as among the most sophisticated animals.

Fossil evidence shows that ‘mammals’ have been around on the planet for around 200 million years. Their legs were straighter and they could chew their food. They wandered around, trying to avoid the attention of large reptiles. After around 100 million years later, they began to evolve into a more diverse group. Some – the platypus and echidna from the Monotremes – still have some ‘reptilian’ features. They both have mammalian hair, they produce milk and are warm blooded, but ….. they lay eggs like reptiles. The hatchlings still have an ‘egg-tooth’ to help them escape from their eggs. But they are not ‘primitive’.

The marsupials have moved a bit further on to being ‘placental’ mammals. The embryo develops for a short time in the uterus, and then transfers to a pouch, where development continues. The new-born are very tiny. An adult koala weighs in at around 8kg. A new-born koala weighs just about 0.5g.  To see how tiny the baby is, we can work out the percentage of its weight by comparison to the adult.

8kg. = 8000g.  So 0.5g as a percentage of 8000g is (0.5/8000) X 100%.  Therefore, a 0.5g baby is around 0.00625% of its mother’s weight. That’d just over 6 thousandths the weight!

Badger weight depends upon season and food availability. Let’s say that the sow weighs in at 9kg – 9000g. A baby badger will be born with a weight around 100g. (75g to 130g).   

So (100/9000) X 100% = 1.111rec.  So a baby badger, whilst still small, weighs 1.1% of its mother’s weight.  The reason? Badgers are true placental mammals, and their young are developed for much longer in the womb before birth. Much more development has to take place in the pouch of the marsupial.

In all mammals, a pregnancy begins as a blastocyst becomes embedded in the wall of the uterus. However, in the badger, it is not quite straight-forward.  Badgers use a system called ‘delayed implantation’ or ‘embryonic diapause’. They can mate at any time of the year, ( commonly between February and May, and between July and September) but this system ensures that the babies are born at the best time of year in terms of temperature and food availability. After mating, the sow retains the eggs in her uterus in a state of suspended animation. They draw just enough nutrient and oxygen they need to survive. They are implanted into the wall of the uterus around December.

The tissues of the womb and that of the embryo remain in close contact, so that nutrients, waste materials, carbon dioxide and oxygen can pass between them. However, the bloods of mother and baby don’t fully mix, so that the mother doesn’t reject the embryo. But transfer between the blood systems is so great that the needs of the growing embryo can be delivered for quite a while.  The benefit from this is that the offspring of placental mammals can be born at a quite mature stage. This system is key to the biological success of the mammals. (Delayed implantation is also used by other mammals, such as stoats and seals.)

After a gestation period of around 8 weeks, the babies are born in the sett during February. The sow will bring the baby badgers to the surface at around 10 weeks old, in April or May. They can then begin to feed during the maximum months if the year, before winter sets in again.  They become independent by the autumn and are mature by the age of two.

Ernest Neal thought that mating throughout the year might help to strengthen clan relationships. Hans Kruuk pointed out that multiple matings with different males might result in multiple embryos, which could mean that the siblings could all have different fathers!

Of course, one of the most important parts of mammalian reproduction is milk production or ‘lactation’.  Milk is a very nutritious food. It contains proteins, carbohydrates and fats. The large molecules are built up from the much more basic chemicals such as amino acids, from the mother’s diet and reserves. The proteins in the milk are important to the continued strong growth of the cub. Antibodies help give the young animals the power to fight infection.

One of the most important advantages of milk production is that the young animals do not have to find their own food at a very vulnerable time. It also allows the cub to conserve its energy and use it for growth.

Weaning begins around 3 months after birth. The sow will then introduce them to some solid food, such as worms. This is their opportunity to learn from mother what is good and what is to be avoided!

Newly born badger cubs have a covering of fine grey hair, and the facial stripes can be seen.  They are often reared in a special chamber within the sett, where ventilation, etc., is good. Badgers are very clean animals, and mother will remove their waste products until they are able to use dung pits around the territory.

Graham Temby  2011

(Figures from OU source.)