Going Underground: Iberian Brown Frog Is First Known to Reproduce in Caves

Another frog post - success! Today's blog is about the Iberian brown frog, and thankfully some good frog news at last. This article will be up on my London Student blog at the end of this week.

Iberian brown frog (Rana iberica) by Goncalo M Rosa

Quite a few amphibians are known to spend time underground, but it is mostly salamanders that actually live there. Frogs, while often taking shelter in caves, for example, do not tend to have any significant part of their life cycle occur there, and there are no frogs known that have to live in subterranean habitats. Indeed the first tadpoles ever found underground in the Palearctic region, were only discovered in 1996, in Italy. New research in the journal Subterranean Biology, however, suggests that the Iberian brown frog, Rana iberica, not only lives in caves on the Iberian Peninsula but reproduces in them too.

The frog was discovered in artificial drainage galleries during a survey for the gold-striped salamander, which is known to live in these caves. Following the initial observation of a group of frogs in May 2010, the researchers Gonçalo Rosa and Andreia Penado made several further trips, and from December 2011 to December 2012 visited every month to try to understand what the frogs used this subterranean habitat for. They observed secondary sexual characters in males, such as nuptial pads (used to grip onto the female during mating), breeding behaviour and then larvae.

Iberian frog (Rana iberica) tadpole by Goncalo M Rosa

Presence of larvae was recorded and then the developmental stages were studied in order to improve the previously limited knowledge of the biology of this species. Hibernation has been observed in other populations at similar elevations during the coldest months, but is not seen in the cave-dwelling frogs, suggesting that the subterranean conditions are more stable than those outside. The underground population is active year-round and has a breeding season lasting from December to April. Many observations by the researchers may also be relevant to the species as a whole. First-time observations for the species include tadpole oophagy, where tadpoles cannibalise unhatched eggs, and predation on tadpoles by salamander (Salamandra salamandra gallaica) larvae.

The discovery of the cave refugia may be very significant for the survival of the species in the region. The area has been subject to dramatic climate change recently, with increased aridity presenting a particular threat to amphibians. Living in caves may protect the frogs from changes in temperature and aridity and provide important refugia.