I found this picture of my first daughter and I the other day. She might have been around 9 months old and we'd spent the day in Banff. The Ergo fit my body perfectly and I was able to use it for all of my babies, though the twins opened my eyes to many more tandem babywearing options.
Biologically, mammals tend to fall into four categories: "cache", "nest", "follow", or "carry".
The "cache" type have extremely high protein in their mother's milk and can extract their mother's milk quickly allowing their mother to leave their baby for longer periods of time, such as up to 12 hours. The "cache" babies remain silent so as to not attract predators and therefore do not cry. They also have the ability to regulate their body temperature. Examples are deer and rabbits.
"Nest" babies are often born in litters, immature (blind and unable to move), their mother's milk is lower in fat and protein than "cache" mammals so they need to be fed every 2-4 hours. They rely on their nest for warmth. Examples are dogs and cats.
"Follow" babies are born quite mature, so they can easily follow their mom. Their mother's milk is lower in fat and protein than "cache" and "nest" mammals therefore they require feeding more frequently. Examples include horses, cows and giraffes.
"Carry" babies, on the other hand, maintain continuous contact with their mom, feed often, suckle slowly, cry and show signs of distress away from their parent, use the parent to help keep them warm and have comparatively low protein and fat content in their mother's milk, making feedings frequent around the clock and carrying necessary. Examples include kangaroos and apes.
Humans have amongst the lowest levels of fat and protein in their breastmilk, meaning that, whether we choose to or not, we've been biologically designed to feed frequently and find carrying our babies quite convenient.
Personally, I would have loved to carry my twins more, but my twin pregnancy was hard on my back.
How about you? Did you enjoy carrying your baby? Did you have a favorite type of carrier?