Do stingrays lay eggs?

I got a phone call from Sofi today with a very specific question: Do stingrays lay eggs? It seems that she was playing in the pool, pretending she was swimming with stingrays who were about to have babies. But she needed to stop the story short because she was not sure if stingrays lay eggs or give birth to live young. What a great question!

While researching to find her answer, I came across conflicting information to what I thought should be a pretty simple answer. It turns out it is all about semantics. There are approximately 500 species of rays, and though stingrays are a type of ray, not all rays are "sting" rays. Some stingray species, such as devil rays, don't posses stinging spines. There are also other ray species, including as guitarfish, sawfish, electric rays, and skates, which belong to different phylogenetic families.

Even though stingrays, skates and sharks are a closely-related group of fish (called elasmobranchs), there are key differences between them. Namely, the biggest difference between a stingray and a skate is how they give birth. All stingrays give birth to live young, while skates lay eggs and attach them to the reef in hard pouches sometimes called "mermaid's purses".

So, Sofi, true stingrays give birth to fully-developed live young (called pups) which look just like miniature versions of the adult.

Click this link to watch a video about a family of stingrays at the Houston Zoo:
http://www.youtube.com/watch?v=jpBo4mO-_dU&eurl=http%3A%2F%2Fwww%2Ezooborns%2Ecom%2Fzooborns%2F2009%2F01%2Fstingray%2Dbabies%2Dare%2Dcute%2Dtoo%2Ehtml&feature=player_embedded

Why do koalas sleep so much?

The San Diego Zoo has an amazing koala exhibit. It's always the first place I head to, and I could watch those fuzzy little guys all day long. If you're lucky, some of the koalas are awake (most likely eating). If you're REALLY lucky, one or two may even be walking around (in the direction of the food). But most of the time, the koalas are sleeping.

When kids at the exhibit ask why they sleep so much, I usually hear parents say something along the lines of "because they're tired" or "because that's what koalas do", or that the koalas are "high" on eucalyptus oil. Well, those aren't much of an answer (and being drugged on eucalyptus is only a myth), so I thought I'd post the real story.

It isn't surprising that the koalas at the zoo are usually sleeping. Koalas naturally sleep between 18 and 22 hours a day! They have a very slow metabolic rate and need to sleep so much in order to save energy. Koala's main food source is eucalyptus. Eucalyptus leaves are low in nutrition and very high in fiber, which makes them extremely difficult to digest. The koala's slow metabolism allows the animal to retain the leaves in their digestive system for long periods of time in order to extract all the energy they can from them. And since that isn't much in the first place, sleeping in the trees is a method of conserving the energy they do have.

Disclaimer: Don't try this at home! Eucalyptus leaves are extremely poisonous. A koala's digestive system has adapted to host a specific strain of bacteria which detoxifies the chemicals in the eucalyptus leaves that would harm almost all other animals.

Koalas have also evolved a highly adapted skeletal structure that allows them to sit in the tree branches for long periods of time. Their spines are curved to allow them to sit comfortably, and they do not have a tail. Even the koala's rump is covered in very dense fur which provides an extra cushion for sitting.

So ... Do ducks have teeth?

No, ducks don't have teeth. Ducks have bills made of keratin, the same protein that makes up our fingernails and hair, but no bird today has actual teeth. Prehistoric goose-like birds called Darsornis had sharp, bony, tooth-like structures on their bills, but this feature was lost as the birds evolved, likely due to dietary changes and to reduce their body weight and make flying easier.

Some ducks may look like they have small teeth because their bills are serrated. The serration allows the ducks to strain away the water from aquatic vegetation they eat. Some ducks may also use the ridges on their bills to help them catch and hold on to small fish.

Ducks swallow their food whole. Instead of using teeth to breakdown the plants, seeds, insects or fish that they eat, ducks (and birds in general, some reptiles, crocodiles, fish, and even worms) possess an organ called a gizzard. The gizzard is part of a bird's digestive system, located between the bird's true stomach and intestines. The gizzard is a strong muscular sack which performs the mechanical work of grinding the food so that it is easier for the animal to digest. Ducks swallow grit (small pieces of gravel or stones) which helps to breakdown the food while being ground inside the gizzard.

For Sofi

The other day at the San Diego Zoo's Wild Animal Park we stopped to feed the ducks at one of the ponds. We dug through our pockets for quarters to buy handfuls of mash from the machine, and as we did the ducks began to gather in anticipation. Instead of tossing the little pellets into the pond, I chose to feed the two mallards quacking at my feet. I opened my hand and they gently took the food, one pellet at a time.

This drew a little crowd of parents and kids, and after a few minutes a boy started to ask (loudly and repeatedly) "Dad, do ducks have teeth? Do ducks have teeth? Do ducks have teeth? Do ducks have teeth?" It was all I could do not to give that kid (and his dad) a short lecture, but as I walked away I thought that Sofi, my favorite 4-year old friend and budding scientific genius, could give him a great answer. And I wonder if that boy's question got answered.

I've always had a passion for science and teaching, and try to share my enthusiasm however I can to whoever will listen. Science is cool, fun, interesting, important, and it's all around us. There is so much to learn about the world, and I hope to use this blog to make Sofi proud.