Written by Roger Downie, Froglife and University of Glasgow
Although we mostly think of amphibians mating in water, depositing their eggs there, and leaving them to develop unaided, the group actually includes a vast diversity of reproductive modes, 74 at the last comprehensive count (Nunes-de-Almeida et al., 2021), many of them including some kind of parental care. Now comes something new: Mailho-Fontana et al.(2024) report that in Siphonops annulatus, a widely-distributed South American caecilian, mothers produce and release a milk to feed their newly-hatched babies.
The 220+ species in the order Gymnophiona, commonly called caecilians, are acknowledged as the most poorly known amphibian group, a situation related to their tropical distribution and secretive burrowing habits. One feature that is known about the group is that a relatively high proportion of species (about 20%) is viviparous (giving birth to live young), compared to the tiny proportion of anurans and urodeles that reproduce in that way. In viviparous caecilians, the eggs are retained in the oviduct and hatch there, with the young nourished by oviductal secretions, growing substantially during a gestation period that can be as long as 11 months.
Egg-laying caecilians also show parental care, with mothers often looking after the eggs, and at least in some cases, feeding the hatchlings on modified, lipid-rich skin which the young browse on using specialised teeth. This behaviour has previously been reported in S. annulatus by Wilkinson et al. (2008).
Nunes-de-Almeida and colleagues were able to capture and observe for an extended period in captivity a sample of 16 adult female S. annulatus. After hatching, the young (4-13 per clutch) clustered around the coiled-up mother, keeping close to her vent. Periodically, the young touched the vent and emitted high-pitched sounds. These behaviours seemed to elicit the release of a transparent, viscous milk, which the young fed on. After feeding, the young became relatively inactive for a time. The young may compete for access to the milk, since data showed considerable variation in growth rate increase among a batch of young; on average, body mass increased by 130% during the week after hatching. Milk release was at least daily, and up to six times a day. As in Wilkinson et al.’s study, skin feeding also occurred, but less frequently, about once a week. Histological analysis of the female reproductive tract showed that milk is produced by oviductal glands which develop during the reproductive period. These are very similar to the glands which appear during gestation in viviparous species, but have not previously been reported in egg-layers. The lipid content of the milk is mainly composed of long-chain fatty acids, palmitic and stearic; the milk is also rich in proteins and carbohydrates. The period of maternal care, during which they do not feed, lasts about two months, during which maternal body mass declines by 30%, so this behaviour is very costly to the mothers.
Milk production is a diagnostic feature of mammalian reproduction, but it does occur occasionally elsewhere in the animal kingdom: in pigeons and flamingoes among the birds, for example, where the crop produces a nutritive secretion. It will be fascinating to discover if S. annulatus is an isolated case among the caecilians. Frost (2024) lists four species in the genus Siphonops, with five genera and 28 species in the family Siphonopidae, at least some of which are known to show skin-feeding. In my view, it would be surprising if this combination of hatchling behaviour and maternal physiology is restricted to a single species.
Acknowledgement I thank Malcolm Kennedy for drawing my attention to this fascinating discovery.
References
Frost (2024). Amphibian Species of the World. Accessed 15/3/24.
Nunes-de-Almeida et al. (2024). Milk provisioning in oviparous caecilian amphibians. Science 383, 1092-1095.
Wilkinson et al. (2008). One hundred million years of skin feeding? Extended parental care in a neotropical caecilian. Biology Letters 4, 358-361.
Wilkinson et al. (2013). A new species of skin-feeding caecilian, and the first report of the reproductive mode in Microcaecilia. PLOS ONE 8, e57756.
Potentially as early as January in the South East of England, and up to April in other areas of the UK, the common frog will emerge from their winter hideouts and head back to a pond. Male frogs will ‘piggy back’ on their chosen female using their nuptial pads to hold on tightly. This is a process known as ‘amplexus’. He will then fertilise her spawn as she lays it. Common frog eggs are laid in clumps, with up to 2,000 eggs laid in one single clump. Female frogs can lay up to 4,000 eggs over one breeding season.
Common toads will ritually return to the same pond for breeding each year where possible. They take advantage of warmer, damp spring evenings to leave their overwintering sites and return to their specific pond. This behaviour makes them notorious for crossing roads over night and putting themselves at risk of being run over. Our 
Once the warmer weather of spring hits, newts too will venture back to a suitable pond to breed and lay eggs. Males of each newt species will perform elaborate ‘dances’ to attract the attention of females, who will choose which male gets to fertilise their eggs. Newt eggs are laid in a very different way to toads and frogs. A female newt will lay each egg individually, she will lay the egg on the leaf of an aquatic plant and carefully fold the leaf around the egg to protect it, giving the leaf a very straight edge where it has been folded over. Newts lay less eggs than their toad and frog counterparts, but females will still lay hundreds over the season.
The natterjack toad has a distinctive call for multiple reasons. Their call is very raspy and higher in pitch than the common frog. The sound of their call carries for long distances, and can be heard from up to 2km away in the right conditions.