Polyandry in frogs and toads: evidence of mixed paternity?

Written by Dr Laurence Jarvis (external contributor)

Polyandry, where a female mates with multiple males, is widespread across the animal kingdom. It has been documented from several taxa, particularly fish, insects and birds.

Polyandry has been observed in many species of amphibians (Roberts & Bryne, 2011) and may be more common than previously thought (Halliday, 1998). There are two main types of polyandry in frogs and toads: simultaneous and sequential (Roberts and Bryne, 2011). Simultaneous polyandry typically occurs where the sperm from several males reaches a female’s eggs at the same time. This is most likely in explosively breeding species such as the European Common Frog (Rana temporaria) or Common Toad (Bufo bufo) where several males may be present around one female.

Each male may attempt to fertilise a female’s eggs as she releases them into the water. In these instances, a female may be forced to obtain sperm from several males due to continued harassment. In sequential polyandry, a female may seek out additional males. This occurs more frequently in species where males defend territories and there are opportunities for the female to mate multiple times, perhaps over hours or days.

This may confer significant benefits for the female such as enhanced protection or parental care of young, as well as obtaining mixed genes for her offspring (Roberts and Bryne, 2011). However, despite the fact that polyandry may be relatively common in frogs and toads, actually proving mixed paternity can be difficult (Wang et al., 2017). Genetic studies using molecular markers from DNA sequences are required to fully understand the paternal origins of a given female’s clutch. Over recent decades several studies have examined the potential for polyandry in a range of frog and toad species.

The Omei treefrog (Zhangixalus omeimontis) lives in subtropical and mountainous forests of China. It is an arboreal species, laying its eggs on leaves overhanging water (IUCN SSC Amphibian Specialist Group, 2020). Wang et al. (2017) provide a useful summary of the breeding behaviour this species. During the breeding season males will congregate in standing water, forming leks, each calling to attract a female.

On arrival, a female will be grabbed by a male and she will ascend a tree to find a suitable leaf on which she will lay her eggs. Several additional males will follow and attempt to fertilise her eggs as she releases them. In some cases, up to eight males may attempt to form amplexus with a female (Wang et al., 2017). The male provides no parental care and once a female has laid her eggs she will produce mucus to envelope them within a foam. Once hatched, the tadpoles will fall into the water below and continue their development (Wang et al., 2017).

Wang et al. (2017) examined the degree of multiple paternity in the Omei treefrog using genetic markers. Perhaps unsurprisingly, in nearly all cases, females had genes from multiple males. Out of the 30 females studied, only one had genetic material from a single male. The rest had genes from between two and four fathers. Interestingly, the greater the number of males present around a given female, the less the chances of obtaining paternity. For example, when only one or two males were present, each male had 100% chance of paternity. However, with seven or eight males, for a given male, there was only a 45% chance of passing on his genes. Therefore, in this species, it is more advantageous for a male to join a mating crowd with fewer competitors (Wang et al., 2017). Although it appears that a female is at the mercy of advancing males, there may be benefits for her. By receiving sperm from several males, she is more likely to receive multiple genes, some of which may be ‘good genes’ or those with compatible genotypes (Wang et al., 2017).

It is perhaps not surprising that simultaneous polyandry occurs in frog species with large breeding aggregations, since many males are present at one time and there are many opportunities for males to release their sperm onto the eggs of different females. However, does this occurrence actually provide genetic benefits?

The quaking frog (Crinia georgiana) has been a model species for behavioural studies for many decades. It is a common and widespread frog across the coastal plains of southwestern Australia. Males congregate in large numbers at water bodies during the spring. Females are attracted to aggregations of males and upon arrival are immediately engaged in amplexus by a male.

Previous research has demonstrated that approximately 50% of matings result in polyandry (Roberts et al., 1999). However, it is uncertain whether there are any genetic benefits for this occurrence. For example, do egg clutches fertilized from more than one male experience greater viability, especially in an unpredictable breeding environment? Bryne and Roberts (2000) tested this hypothesis by carrying out laboratory experiments on the quaking frog. Interestingly, clutches fertilised by more than one male did not experience any increase in survival or growth (Bryne and Roberts, 2000). This suggests that multiple paternity in this species does not provide increased genetic benefits such as more robust larvae in an unpredictable environment.

This was unexpected, since studies on other taxa (e.g. bumblebees and field crickets) have demonstrated distinct advantages of multiple paternity such as improved hatching success or lower parasite load. Therefore, multiple paternity may not necessarily result in genetic benefits to the offspring and may have evolved due to other selective pressures.

There is increasing evidence to demonstrate the existence of sequential polyandry in amphibians. This is more subtle and involves deliberate attempts by a female to seek additional males with which to mate.

Pickersgill’s Banana Frog (Afrixalus delicatus) is a widespread species inhabiting grasslands of south and east Africa. In this species, the male and female pair and the female lays a clutch of eggs on a leaf just above, or on top of, the surface of water (AmphibaWeb, 2026). She carefully folds this over and leaves them to develop. However, she may obtain another male with which to mate and repeats the process several times. Therefore, the female fertilises portions of her whole clutch from different males.

This sequential laying of different portions of the clutch may occur in other species. The green and golden bell frog (Litoria aurea) from Australia has been documented to partake in simultaneous polyandry (Beranek et al., 2021). Males congregate in large aggregations at breeding ponds during the spring and there are opportunities for many males to congregate around one female. However, there is unconfirmed evidence that a female may withhold releasing her whole clutch at once (Christy, 2001 in Beranek et al., 2021). This provides the female with the opportunity to move to another area of the pond to allow the rest of her clutch to be fertilised by a different male.

Terrestrial toadlets of the genus Pseudophryne inhabit grasslands and forests of Australia. The brown toadlet (P. bibronii) has an unusual breeding strategy. The male constructs a terrestrial nest, usually out of dead leaves or soil. Here he will call to attract a female. Mating occurs within the nest and the male will carry out all parental care duties, attending the eggs until they are flooded by rainwater and hatch (O’Brien et al., 2018). Females will move to the territories of several males and potentially mate multiple times. In this species Bryne and Keogh (2009) found that females mated with up to eight different males, laying clutches in the nests of each.

Similar sequential polyandry has been observed in other Pseudophryne frogs. However, this behavioural trait is not necessarily consistent across similar species. More recently, O’Brien et al. (2018) found that in the red-backed toadlet (P. coriacea) 92.6% of females mated just once with only 3.7% taking part in sequential polyandry. The reasons for these stark differences between species are not clear, but may be linked to the evolutionary status of different species (O’Brien et al., 2018).

Overall, polyandry is widespread amongst many frog and toad families. Several studies have demonstrated that females may mate with multiple males, either incidentally, or actively seeking additional matings. Genetic studies have been crucial in demonstrating the degree of multiple paternity in different species. The evidence for the genetic benefits of polyandry appears inconclusive and experimental studies are required to determine the relative costs and benefits of polyandry in frogs and toads.

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