Written by Roger Downie, Froglife Trustee and University of Glasgow
In the UK, we are accustomed to amphibians breeding in the spring and depositing their eggs in freshwater bodies, usually ponds rather than streams or lakes. Frogs deposit their eggs as a clump of jelly; toads as strings; and newts wrap theirs individually in folded leaves. The embryos hatch as larvae and feed in the water until they are ready to metamorphose into juvenile versions of the adult form. The adults spend no time with their eggs after deposition. So far, so familiar. But, when we look beyond our UK species, we find a wide diversity of reproductive modes. How many, and what are they like?
The term ‘reproductive mode’ (RM) was coined by Breder and Rosen (1966) to help them make sense of reproductive diversity in fish. Later, Salthe and Duellman (1973), in the context of amphibians, defined RM as a set of characters including oviposition site, ovum and clutch characteristics, rate and duration of development, stage and size of hatchlings, and type of parental care, if any. Without using the term RM, Boulenger (1886) had identified 10 amphibian modes. A hundred years later, Duellman and Trueb’s (1986) textbook recognised 29 RMs in anurans, seven in urodeles and two in caecilians. Haddad and Prado (2005) extended this to 39 modes for all amphibians, and there have been a few additions since. However, Nunes-de-Almeida et al. (2021) have now published a new classification, identifying 74 RMs in amphibians, almost a doubling of the 2005 list. How and why?
Their method is to divide the reproductive process into a set of eleven characters where each species can be assigned to one of two (occasionally more) states. The characters are:
- Reproduction type: oviparity (egg-laying) or viviparity (eggs not laid: the female gives birth to larvae or juveniles). Viviparity is common in caecilians, but also occurs in a few frogs and salamanders.
- Oviposition macrohabitat: eggs are deposited into the environment or they develop in or on the body of either the female or the male parent.
- Spawning type: the distinction here is between cases where eggs are immersed in froth, or not. Froth is made from oviduct secretions in two ways: either a foam is generated by beating movements of the adults’ limbs; or bubbles are made by the female’s jumping movements.
- Oviposition substrate: either in water, or not in water: on the ground, or in vegetation, or attached to a parent.
- Medium surrounding the eggs: the main distinction here is between two kinds of aquatic habitat: lentic (still water, like a pond) or lotic (flowing waters, such as streams). The medium can also be air, as in eggs deposited on the ground, or attached to a parent’s body.
- Nest construction: a constructed nest is defined as a place to deposit eggs which the parents have made by digging, or cleaning, or building in some way. ‘Froth’ nests are excluded from this category (I’m not sure this exclusion is fully justified). Constructed nests can be burrows, or depressions, or cleared areas on the forest floor, or leaves folded around the eggs.
- Oviposition microhabitat: here, Nunes-de Almeida and colleagues find 15 variables: eggs on the surface of water, at the bottom of a pool, on the ground, on a leaf, on a rock, in a bromeliad tank etc.
The remaining characters distinguish different patterns of development:
- Embryonic development: can be indirect, with a larval stage, or direct – lacking a distinct larval form, and progressing directly from embryo to juvenile.
- Embryonic nutrition: all amphibians have yolky eggs, and the yolk provides the nutrients needed for embryonic development, but in some cases the mother provides additional nutrients. Where all nutrients derive from the yolk, development is termed lecithotrophic; where the mother provides extra, it is matrotrophic.
- Larval and newborn nutrition: when embryos hatch and become free-living, we consider them as larvae. Generally, this marks the stage when they begin to forage for food, although they still have some of the egg-yolk left. However, some species do not feed as larvae, but obtain their nutrition from their large remaining yolk reserves: these are termed endotrophic. Most larvae are exotrophic, obtaining most of their nutrition from external food sources. In a few cases, parents provide this nutrition. For example, so-called trophic eggs, unfertilised eggs deposited by females to feed their hatched larvae. Another example is the feeding of some caecilian young on their mother’s skin secretions.
- Place of larval development: mostly this occurs either in a pool (lentic) or a stream (lotic), but there are also cases of larval development on land, or attached to a parent’s body.
Overall, the authors reviewed RMs in 2171 species on which they could find adequate information: this is 26 % of all amphibians (8393 species, November 2021). Anurans showed 71 of the 74 RMs; urodeles 16 and caecilians seven. Most species showed a single RM, but some fitted up to four of the modes.
Nunes-de-Almeida and colleagues have made a valiant effort to classify the rich diversity of amphibian RMs, but it is not without some problematic aspects. One omitted feature is fertilisation mode: internal or external. This is a crucial feature in research on reproductive strategies relating to certainty of paternity and male competition. Another aspect largely omitted is parental care behaviour. Parental care can be defined as non-gametic investments in offspring that incur a cost to the parent, but which provide a benefit to the offspring. Parental care in amphibians is discussed in Croaking Science (date to come). The new RM classification explicitly excludes parental care on the grounds that parental care information is lacking for too many species. However, many kinds of parental care are actually included: for example, the provision of trophic eggs to larvae (character 10 above); while others such as larval transportation by adults are omitted. Another omitted feature which I find surprising is the differences in anuran spawn characteristics: single non-adhesive eggs, eggs in clumps, eggs in strings. It is likely that these differences are evolved characteristics important to reproductive success, so should be included in a classification of RMs. Another omission is the diversity of larval forms: there is huge diversity in tadpole form and behaviour, related to the habitats they live in: this may go beyond the usual definition of an RM, but is an important aspect of reproductive success. There are also occasional inconsistencies: phyllomedusine tree frogs wrap their egg clutches in leaves, and this is classed as a constructed nest (character 6 above); newts wrap their eggs individually in leaves, but this behaviour is not acknowledged as a kind of nest construction.
One excellent point made by the authors is about plasticity: i.e. individuals within a species may vary their RM, depending on circumstances. One example I’ve observed is the giant tree frog Boana boans. These frogs generally construct nests, as basins in gravel or sand (character 6 above), just beyond the edge of streams. However, where there is no suitable ‘beach’, the eggs are deposited at the water surface amongst emergent vegetation.
I’m sure that this new RM classification will stimulate discussion and research, and that later versions will include more species and modes. The authors hope that their work will stimulate the development of RM classifications for other taxa: how about reptiles?
Breder and Rosen (1966). Modes of Reproduction in Fishes. Natural History Press, New York.
Duellman and Trueb (1986). Biology of Amphibians. Johns Hopkins University Press, Maryland.
Haddad and Prado (2005). Reproductive modes in frogs and their unexpected diversity in the Atlantic forest of Brazil. Bioscience 55, 207-217.
Nunes-de-Almeida et al. (2021). A revised classification of the amphibian reproductive modes. Salamandra 57, 413-427.
Salthe and Duellman (1973). Quantitative constraints associated with reproductive modes in anurans. Pp 229-249 in: Vial (ed.) Evolutionary biology of the anurans. University of Missouri Press, Columbia.