Why do Red-Bellied Black Snakes have smaller heads today than in 1935? Here is some evidence.

Evidence 1

Evidence 2

Evidence 3

Evidence 4

Evidence 5

Evidence 6

Evidence 1Red-Bellied Black Snakes live in Australia. They eat many small animals, including frogs and toads. Snakes eat their prey by opening their mouths wide and swallowing the prey whole, like the picture below.When Red-Bellied Black Snakes eat cane toads, the snakes often die, because the toads are poisonous.

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Evidence 2In 1935, a poisonous toad (the cane toad) was introduced to Australia. From 1935 until now, the toad has slowly spread from the east coast of Australia into the areas farther from the coast.

1935-1974HOME NEXT

1975-1980BACK NEXT

1981-1986BACK NEXT

1986-2001BACK NEXT

Evidence 3

 Snakes that live in areas with no cane toads 

 Snakes that live in areas with many cane toads 

Number of Red-Bellied Black Snakes examined

75 75

Color of snakes All were black with red bellies.

All were black with red bellies.

Percent of snakes with heads big enough to eat cane toads

76% 22%

The scientists compared 150 Red-Bellied Black Snakes in parts of Australia, and they examined the percent of the snakes with heads big enough to eat the toads. They also examined the color of the snakes. This is what they found:

With your group, discuss what to conclude from this evidence.BACK HOME

• The snakes whose heads are too small to fit the toads cannot eat them.

Evidence 3

Evidence 4

Sydney Zoo NewsletterMarch 13, 2013

Babies Born This Weekat the Zoo

We have newborn Red-Bellied Black Snakes ! We recently acquired a female Red-Bellied Black Snakes carrying fertilized eggs, and the eggs have now hatched. We have six hatchlings. You can see several of the eggs in the top photo, and in the bottom photo you can see a fine baby male poking his head out and saying hello to the world. Such a cute, small head.

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Evidence 5

Scientists at the University of Sydney carefully studied 250 Red-Bellied Black Snakes and their offspring using modern DNA evaluation techniques.

They determined that head size in Red-Bellied Black Snakes is a genetically inherited trait.

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Evidence 6

Scientists put radio tags on 90 Red-Bellied Black Snakes in a part of Australia that has many poisonous toads and 90 Red-Bellied Black Snakes in a part of Australia that does not have any poisonous toads. They recaptured the snakes that were still alive 80 days later. The table in the next few slides shows the number of snakes they recaptured and the number that had a smaller head size.

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Evidence 6

In groups, make a prediction about the following two questions before move onto the next slide.

1. Will the head size of the snakes placed in areas with poisonous toads increase, decrease, or remain the same?

2. Will the head size of the snakes placed in areas without poisonous toads increase, decrease, or remain the same?

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Evidence 6Here are the results of the study:

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Number of Snakes Captured

Number of snakes with a decrease in head size 

Number of snakes whose head size 

remained the same size

Snakes placed in areas with

poisonous toads85 0 85

Snakes places in areas without

poisonous toads80 0 80