Thread: Counting frogs
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Old 19-03-2003, 10:56 PM
Martin Sykes
 
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Default Counting frogs

"Kay Easton" wrote in message
...
Does anyone have any idea how to count the frogs in a pond?

Drop a grass snake in the pond. *all* the frogs will jump out. :-)

For example - I counted 36 frogs in the pond today - does that mean I
have 36 frogs? Or have I got nearer 50?

More seriously, apart from wondering why it's important, depending on how
long you spent looking you probably counted the same frog more than once.
Your presence there probably made them dive and pop up somewhere else. The
act of observation changes what you are observing.

As a rough guide, if you knew what percentage of its time a frog stayed on
the surface on average and how many frogs on average were visible at any one
time then the total number of frogs would be average visible/percentage of
time visible. So if on average there are 10 visible at any one time and they
spend only 25% of their time on the surface then you've got 40 frogs give or
take. The percentage time on the surface could be found if you've got one
easily identifiable frog you could time.

can one tell form how much spawn there is? Does each female lay one
batch or more than one batch - eg if I have 20 batches of spawn have I
got 20 females or could it have been laid by,say, 5 females?

I'm pretty sure they can lay more than one batch. This is only an indication
of female numbers though and is no guide to the number of males. However, I
think frogs can change sex from male to female depending on the temperature
so it's probabably possible to work out the likely ratio of female to male
at a given temperature.

How many females does each male mate with? I presume once he's
fertilised one batch of eggs, he won't just sit on the sidelines, he'll
see if he can find another female.

A male will try and mate with anything that moves. female frogs, fish, your
hand. Doesn't help you count them though.

Short of draining the pond and catching them all though, there's so much
uncertainty in any measurement that you'll probably be plus or minus 50% of
the true value. As soon as the spawn hatches (?) your number will be wrong
again anyway.

Martin

--
Kay Easton

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http://www.scarboro.demon.co.uk/garden/