nightfly's picture

Dead female frog.

Observed: 3rd March 2013 By: nightflynightfly’s reputation in Amphibians and Reptilesnightfly’s reputation in Amphibians and Reptilesnightfly’s reputation in Amphibians and Reptiles
3 March 13 (606)
3rd March 13 Egg pics in plate 22.50 (8)
3rd March 13 Egg pics in plate 22.50 (30)
3rd March 13 Egg pics in plate 22.50 (34)
3 Mar 13, egg size at 11pm (30)
3 Mar 13, egg size at 23 (7)
3 Mar 13 eggs at 15 past midnight (3)
4 March 13, eggs at 1730 (25)
4 March 13, eggs at 1730 (33)
Description:

The coating on the freshly exposed eggs from within the ovisacs can only be described as glue. It began to take on water immediately and the glue was on its way to becoming the familiar jelly capsule around the ova. The purpose of the adhesive quality of the coating would appear to be to keep the spawn together- it stuck to anything it came into contact with as can be seen.
Having had this on my fingers I can understand why this might put some predators off consuming these ova. It is a highly adhesive glue before it takes on its full capacity of water. This might make it difficult to eat and would explain why some predators choose not to eat it.
I had been expecting to produce a mass of jelly as in the cases of star jelly by placing the frog and all its innards in water overnight but the last pic shows the results, a frog, a batch of frogspawn and an oviduct.
Editing- My theory about the unpalatability of the ova wouldnt explain why a predator couldnt just swallow the frog whole and therefore avoid contact with the sticky ova but it would appear from other observations that some predator(s) are avoiding not only the ova but the oviduct also when they consume a frog. Maybe the potential for the spawn to significantly increase in mass is another reason that a predator might not wish to have it inside its alimentary canal?
Editing again- What I have called ovaries above may be better termed ovisacs(altered accordingly). Egg storage not egg production. Big difference, my error.

Identifications
Species interactions

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Comments

nightfly's picture

I just openned this frog up,

I just opened this frog up, it is so fresh it must only be dead a short time, I think it met its maker today. I am starting to wonder about the heron I saw leave the lake as I arrived, it had a slightly guilty look as it wandered away out of sight- this frog has a broken back, the vertebrae are snapped cleanly apart just behind the shoulders. It is an egg laden female, well developed ova within and an oviduct half full of black ova- the oviduct is quite long. Pics to follow.

Cathal.

Masked Marvel's picture

Hello Cathal Would be

Hello Cathal

Would be interesting to see if you can find the organ which produces the jelly, to see what it looks like in a 100% confirmed frog predation case.

nightfly's picture

Hi Masked Marvel, That was my

Hi Masked Marvel,

That was my intention last night when I opened the frog. It is a female, full of ova. The ova in the ovaries swole up when put into water, there doesnt seem to be an alternative source for the jelly. It is already on the ova even in the ovaries long before passing through the oviduct. The only thing it requires to become frogspawn as we know it is contact with water.

I'll post illustratory images later. What I havent fully figured out yet is the occurances of masses of ova-less jelly but without any doubt whatsoever the ova have a 'just add water' film of expanding stuff on each and every one of them even within the ovaries. On lunch break, later.....

Cathal.

Masked Marvel's picture

It's interesting that the

It's interesting that the eggs appear to already have their gelatinous coating in the ovisac. According the Malcolm Smith's New Naturalist book they receive the coating as they pass down the oviducts.

nightfly's picture

Hi Masked Marvel,This begs

Hi Masked Marvel,

This begs the question, do the ova pass through the oviduct before or after storage in the ovisac?

If they are given their coating in the oviduct(which would make sense in many ways) which is also what I thought the oviduct was for then they must go through the oviduct prior to storage in the ovisac? This would then mean they are not spawned through the oviduct, just produced by it?

The more I look the less I seem to know- one point which might be consistent with the above postulation is that when the frog was opened, the majority of the oviduct was found to be anterior to the ovisac and not posterior where I expected it to be.

Cathal.

nightfly's picture

When I approached the small

When I approached the small lake on Sunday evening, I saw a heron fly from close to where I approached on the western side to the NE corner and land there. It was there for a minute or so and then it flew off out of sight to the east. No other birds or animals were seen and this is a small lake on open tree-less moorland, if any other birds had been there I'd have seen them, eg hooded crows. It took me about1.5 hours to get round the southern end of the lake and to work my way right up around the eastern side (photographing and fishing as I went) to the NE corner.

It was exactly there at a popular spawning site in the NE corner where the heron had landed that I found this recently deceased frog. It had a tiny cut on its back giving rise to the trickle of blood on its belly and when opened later at home it was found to have a broken back. I think the heron has killed this frog but I cant be sure why it hasnt eaten it. Maybe it didnt have time to deal with it and remove its load of sticky ova due to my approaching. The frog was so very fresh as can be seen from the pics.

A male and female couple was found a minute previous. This female may have been one half of another couple and the heron killed both but just made off with the male due to my approaching. Whatever the case I think the heron is no. 1 prime suspect.

Cathal.

DavidHowdon's picture

Was it a mature heron?

I've seen immature ones struggle with food stuff before - learning how to eat all the different things obviously takes a bit of time.

nightfly's picture

David, It crtainly looked big

David,

It crtainly looked big enough to be a mature heron. It was definitely a fully grown heron but I couldnt suggest if it was in its first, second or third year from the view I got. All I can say is it wasnt small.

Cathal.

dejayM's picture

Stuff..

This is pretty spectacular stuff and you may need an outsider (of the inner biological ring) to say that - I am one.
It's this sort of post, the comment, the consideration and sheer determination to get to the bottom, that carries the tradition of iSpot (only recently discovered because I have been browsing iSpot posting history lately).
Cathal. Excellent PictureSeq; did you put the results of your anatomization in the nearby ditch or pond - the eggs look quite viable?
Good stuff - thanks
Derek

nightfly's picture

Hi Derek, Just thought I

Hi Derek,

Just thought I would share the details as it is quite intriguing why the heron would kill but not eat the frog. Im not sure how a heron swallows a frog but I'd be very surprised if it couldnt do it 'in one go' as it would eat a fish.

I havent put the spawn anywhere yet and I doubt if there is any point as it wont be fertilized. The male fertilizes the ova externally as they come out of the female, if I'm not mistaken.

I have to say again Derek, the nature of the glue-like coating on the ova in the frog has to be experienced to be believed. It has a rubbery elastic quality and it is very adhesive. It would surely be very troublesome to eat, especially if you got it stuck to your big long beak!! Cheers.

Cathal.

dejayM's picture

external

..fertilizing yes, of course.
Years ago, I came across loads - I mean dozens - of dead frogs in a leat. Something had simply speared and left them. High in the hills I doubt it would have been a human act.

My frogs have about finished. Interestingly one batch looks quite unviable. The yolks (can I call them that?) are dull sepia coloured. I shall remove the whole clump in case they carry disease.
Heron no longer breed here but I see them on the shore most days. They have been caught here with Danish, Swedish and Norwegian rings attached.
My pond is too overhung for them to bother.
Thanks for your insight.
Derek