Baby Answers

by Deb on November 18, 2011

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Do you know about other babies?

1.     Precocial and altricial describe the two opposite types of babies. Precocial babies are large and capable, often they are single or have very small litters. A classic precocial baby is a horse that can get up and run with the herd within hours of birth. They generally require long pregnancies or incubations. Altricial babies are the ones that are born like wiggly pink things and require weeks or months of care.

I have good examples of both precocial and altricial strategies in my birds – chickens and budgies. The chicks hatch together after 21 days, are born with down and stagger around straight away. Within a few days they have feathers and are flying around and out of their enclosure. I have hatched two clutches in an incubator and they are happily feeding and caring for themselves, we just had to get one or two eating and drinking and the rest look after themselves.

The budgies hatch over time and are at all stages of development, they are only in the eggs for 11 days. When they hatch they really are like floppy little embryos and their parents fly back and forth for the next month while their eyes open, they grow down then feathers and generally look more like birds. After a month they are flying clumsily out of the nest.

Humans are very interesting, because our babies are ‘secondarily altricial.’ I made that up myself but it expresses the situation well. If you look at our closest relatives they are precocial but spend a long time teaching their babies. Our minimal litters seem to be pointing towards a precocial strategy, but our pregnancies are far too short and our babies far too helpless. So our large brains and upright walking, which drove early births, mean that our babies are much more like the pink wriggly things than their cousins. Aren’t you glad you don’t have a litter of 8 to look after?

2.     Monotremes are the egg laying mammals – the echidna and platypus. They are not some primitive type of mammal, it’s just that live birth isn’t as cut and dried as ‘we do it and everyone else doesn’t’. There are snakes and fish that give birth to live young and there are even stomach brooding frogs. They swallow their eggs then stop eating and their stomach modifies to become more like a uterus where the tadpoles develop until little froglets are born through their mouths. Live birth is something that is useful in some circumstances and has developed many times.

Monotremes retain their eggs inside for a while and actively transfer nutrients to them, then still spend a lot of time caring for their young in nests. They are distant cousins within the mammal family, like the grandchildren of your Grandmother’s sister – they share recognisable traits and you know they are related to you, but they don’t have other things that are distinctively your family’s and they have some extras that you don’t have. Monotremes have the fur, milk, bones and teeth of mammals, but they have eggs and slightly different brains and they have developed electroreception and poisonous spurs which we don’t have.

Look below for video on a cute baby echidna puggle.

3.     The sex of baby crocodiles is determined by the temperature but it isn’t just high and low. Male crocodiles form in the middle, between 31-32 degrees, and both higher and lower temperatures will produce females. Crocodiles lay their eggs in large nest mounds that they scrape up, which includes a lot of plants from the river bank. This makes compost which heats the mound, but it will vary in temperature in different places so they generally have a mix of males and females in a clutch. There is some concern the narrow range necessary to produce males will make them sensitive to global warming.

4.     Ostriches and pipefish  are both animals where males look after or hatch the babies. It is common in the ratites with ostriches, emus, cassowaries and kiwis all having males on the nest. Sea horses and leafy sea dragons are relatives of the pipefish and they do it too. In fact they don’t just carry the eggs around in their brood pouch, it has definite hormonal and cellular changes and is basically a functioning uterus. For a fun take on it, check out this post by Bec Crew, a prize winning Australian science blogger.

5.     The picture is a Port Jackson Shark egg. Sharks are an ancient and extremely broad group, they use a wide range of breeding strategies ranging from live birth to eggs and also in between. All sharks have internal fertilisation rather then the ‘spread them and let them meet up’ that most fish use.

Most sharks practice ovoviviparity which means they produce eggs where the embryos partially develop, then hatch still inside the mother. They continue developing supported by their yolk and secretions from the mother’s uterus but they aren’t attached the way a placental mammal baby is. Some take it a step further and the first embryo to emerge will eat the other eggs to further support it before it is born. Some types of sharks even have pups that fight and eat each other in the uterus, only the strongest one or two are born.

Some sharks skip the eggs altogether and have a placental connection with their young, their reproduction is more like placental mammals than other fish.

And some sharks lay eggs. But these are not the brief eggs of amphibians, reptiles and birds, Port Jackson shark embryos spend 10-11 months developing. This means they need to be very tough to last that long and most don’t survive. The shells are thick and leathery and the mothers screw them into rock crevices, hence the screw-like thread around them.

And for some extreme cuteness, have a look at this echidna puggle.

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