Frogs Alive!

Common Frog, Rana temporaria

I shouldn't have doubted. The tadpoles in the 'temporary pool' that I didn't think would survive..... have survived. Not only survived but metamorphosed into frogs. None were visible in the puddle that sheltered them as tadpoles, but I spotted a number of them in the undergrowth next to it. It seems that the wetter than usual summer has been a boon for them, maintaining water bodies that may otherwise have dried up. Individuals were not more than 2 cm in length, as can be seen from the picture below. The frogs are about 4 months old, based on average spawning dates for Rana temporaria in Ireland (1) and the size of the tadpoles when they were first spotted in May. However, I wouldn't try to put an exact age on them based on size.

Common Frog, Rana temporaria

Other than the obvious external changes, R. temporaria adults also have quite different stomachs to their tadpole stage. The gastric mucosa is removed and a new adult, definitive one develops, along with peripheral connective and muscular tissues (2).

References: 

1. Sterry, 2004. Collins Complete Guide to Irish Wildlife p. 94
   Rovira et al., 1995. Tissue Cell 27 pp. 13-22

A Puddle of Tadpoles

Rana temporaria tadpole

There is a forest, about 10 miles from Cork city. Its a commercially planted one, Sitka Spruce and Japanese Larch, with a border of Beech. The forest is bisected by a walkway, which acted as an access road when part of the forest was harvested some two years ago. The heavy machinery used in the harvest left depressions in the road which the recent rain has turned into dirty, shallow puddles – puddles that are home to a cloud of tadpoles.

A puddle of tadpoles

The puddles were at most 20 cm at their deepest parts, but the Common Frog (Rana temporaria) tadpoles were most visible at depths of 5 cm or less. It does seem odd that adult frogs would spawn in such shallow, temporary water source but not at all uncommon (as reported last year on this blog, for example) and can often lead to catastrophic losses (1). However the tadpoles are able to respond to this stress by speeding up their development, metamorphising faster when posed with the problem of pond (or in this case puddle) drying (1).

Rana temporaria tadpole

References:

1. Laurila and Kujasalo, 1999. Journal of Animal Ecology 68 pp. 1123–1132

Return of the Frog

Frog Spawn

Last March, this blog reported a batch of developing Common Frog tadpoles (Rana temporaria) that had succumbed to the intense cold of winter 2010-11. A combination of low rain fall and harsh frosts had left a large mass of c. 7 day old tadpoles stranded and frozen some distance from a marshy pool in which they were no doubt laid but which had since retreated. This year, re-examination of the site showed a new batch of eggs. This time they are well within the confines of the marsh pool - hopefully some of them will develop into adults.

Frog Spawn

Death of the Tadpole

Common Frog Tadpoles, Rana temporaria

It is the time of year for spawning by the Common Frog, Rana temporaria. Through out Ireland eggs are being laid that will become frogs in about 16 weeks time. However, making its way there is a tough journey for R. temporaria. Many are taken, as food by birds and large fish while some, like a large number of tadpoles I spotted recently, may not even make it that far. I discovered a large number of recently hatched (only about 7 days old) R. temporaria tadpoles in a large amount of mucus that must have been the remains of the egg casings alongside a marsh pond.

Dead Tadpole and Egg-Mass

The tadpoles were a good two feet away from the water and may have been laid during recent heavy rains. However there has been little such rain in the past few days that must have lead to water levels dropping, stranding the egg masses. When I discovered them, ice was present on their surface from the previous night, probably speeding their death.

Common Frog Tadpoles, Rana temporaria

Tadpoles of R. temporaria are quite resilient, so it was a shame to find so many of them ending up like this. Loman (1) has shown that stress on tadpoles, such as drying of habitats leads to an early metamorphosis. Such an adaptive response as this means the stresses on the tadpoles must have been severe.

Common Frog Tadpoles, Rana temporaria

In Ireland R. temporaria was considered a recent introduction, possible from Norman time c. 1000 years ago (2). This was due to the lack of archaeological evidence to support its arrival by natural colonisation. However, a phylogeographic study of R. temporaria in Europe published in 2009 (3) provided evidence that the Common Frog is native to Ireland, having survived the most recent glaciation periods to affect the country, possibly in a refuge in the South West of the country. Many of the frog haplotypes from Ireland were monophyletic groups that are unique to the country. Other groups were found that are more closely related to those found in the west of continental Europe and it is thought that these were introduced by human actions (or possibly across a land bridge with Britain). In fact Smith (4) states that in 1696 a Fellow of Trinity College Dublin moving common frogs from England to Dublin.

References:

1.   Loman 1999, Amphibia-Reptilia 20 pp. 421-430
2.   Sterry 2004, Collins Complete Guide to Irish Wildlife p. 94
3.   Teacher et al. 2009, Heredity 102 pp. 490–496
4.   Smith 1964, The British Amphibians and Reptiles 3rd edn.