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[Herpetology • 2016] A Unique Mating Strategy without Physical Contact During Fertilization in Bombay Night Frogs Nyctibatrachus humayuni with the Description of A New Form of Amplexus and Female Call

Breeding Behaviour in Nyctibatrachus humayuni

Willaert, Suyesh, Garg, Giri, Bee & Biju, 2016

DOI: 10.7717/peerj.2117

Abstract

Anurans show the highest diversity in reproductive modes of all vertebrate taxa, with a variety of associated breeding behaviours. One striking feature of anuran reproduction is amplexus. During this process, in which the male clasps the female, both individuals’ cloacae are juxtaposed to ensure successful external fertilization. Several types of amplexus have evolved with the diversification of anurans, and secondary loss of amplexus has been reported in a few distantly related taxa. Within Nyctibatrachus, a genus endemic to the Western Ghats of India, normal axillary amplexus, a complete loss of amplexus, and intermediate forms of amplexus have all been suggested to occur, but many species remain unstudied. Here, we describe the reproductive behaviour of N. humayuni, including a new type of amplexus. The dorsal straddle, here defined as a loose form of contact in which the male sits on the dorsum of the female prior to oviposition but without clasping her, is previously unreported for anurans. When compared to known amplexus types, it most closely resembles the form of amplexus observed in Mantellinae. Furthermore, we prove that, opposed to the situation in most anurans, male semen release happens before egg deposition. We hypothesize that the male ejaculates on the female’s dorsum and that sperm subsequently runs from her back and hind legs before fertilizing the eggs. A second feature characterizing anuran breeding is the advertisement call, mostly produced solely by males. Despite recent descriptions of several new Nyctibatrachus species, few studies have explored their vocal repertoire. We describe both the male advertisement call and a female call for N. humayuni. The presence of a female call has not been reported within Nyctibatrachidae, and has been reported in less than 0.5% of anuran species. Altogether, our results highlight a striking diversity and several unique aspects of Nyctibatrachus breeding behaviour.

Figure 4: A comparison of known amplexus positions found in anuran amphibians with the new amplexus mode in Nyctibatrachus humayuni.
(A–F) Known amplexus positions. (A) Inguinal. (B) Axillary. (C) Cephalic. (D) Head straddle. (E) Glued. (F) Independent (adapted from Duellman & Trueb, 1986: 69). (G–I) Dorsal straddle, with the male’s hands on sides of the female’s head but not clasping the female. (G–H) Dorsal views. (I) Side view. (J–K) Dorsal straddle, with the male’s hands holding a twig but not clasping the female. (J) Side view. (K) Close-up of the side view.

Arrows indicate the male grasping position. The male is drawn grey and the female white. DOI: 10.7717/peerj.2117

Figure 1: Schematic sequence of reproductive behaviour in Nyctibatrachus humayuni.
(A) A female approaches a calling male. (B) The female sits in front of the male and creeps backwards, until her feet touch the male’s head. (C) The male mounts the female and forms a dorsal straddle, and most likely sperm is released on her back during this moment. (D) When the female is about to deposit the eggs, the male dismounts. (E) Immediately after the male dismounts, the female deposits the eggs and remains motionless with her hind legs stretched around the eggs. (F) After oviposition, the female leaves the oviposition site, and the male sits on or near the eggs and continues to call.

The male is drawn white and the female grey. DOI: 10.7717/peerj.2117

Figure 2: (A–E) Sequence of breeding behaviour in Nyctibatrachus humayuni; (F–G) Egg development.
(A) Female approaches a calling male. (B) Female touches male just before the dorsal straddle (arrow indicates the position of female’s leg on male’s head). (C) Male mounts the female in a dorsal straddle, and most likely sperm is released on her back during this moment (arrow indicates the male’s hand positioned on the leaf, but not clasping the female). (D) Female deposits eggs and remains motionless with her hind legs stretched around the eggs. The male is mostly seen sitting close-by without any physical contact with the female. (E) After the female leaves the oviposition site, the male sits on or near the eggs and continues to call. (F) Freshly laid eggs, pigmented (egg diameter 3.5 ± 0.2 mm, n = 20). (G) Developing embryos on the 19th day, just before hatching out of the eggs.

DOI: 10.7717/peerj.2117

The Bombay Night frogs Nyctibatrachus humayuni in Dorsal straddle:
A new amplexus mode in frogs. photo: SD Biju

Conclusion

The breeding behaviour of Nyctibatrachus humayuni has several unique elements: a new type of amplexus, the release of semen before oviposition and the presence of a female call. These findings further highlight the tremendous variation present in the reproductive behaviour of anuran amphibians. Nyctibatrachus frogs are one of several unique taxa in the Western Ghats biodiversity hotspot, which is heavily threatened by anthropogenic activities (Myers et al., 2000; Bossuyt et al., 2004; Van Bocxlaer et al., 2012). A good understanding of each species’ ecology, including reproduction, is of major importance for planning and successfully implementing conservation strategies. Additional studies further exploring the unique and diverse behaviour in Nyctibatrachus frogs are, therefore, badly needed. Special attention should be paid to describing the amplexus type, determining the moment of fertilization and assessing the presence and function of female calling behaviour.

Eggs of the Bombay Night frog Nyctibatrachus humayuni being eaten by a snake.

photo: SD Biju

Bert Willaert, Robin Suyesh, Sonali Garg, Varad B. Giri, Mark A. Bee and S.D. Biju. 2016. A Unique Mating Strategy without Physical Contact During Fertilization in Bombay Night Frogs (Nyctibatrachus humayuni) with the Description of A New Form of Amplexus and Female Call. PeerJ. 4:e2117. DOI: 10.7717/peerj.2117

ResearchGate.net/publication/303945595_A_unique_mating_strategy_without_physical_contact_during_fertilization_in_Bombay_Night_Frogs_Nyctibatrachus_humayuni_with_the_description_of_a_new_form_of_amplexus_and_female_call

Discovery of a new mating position (the 7th to be discovered) in frogs.
http://bit.ly/25KopKM via @ThePeerJ @EurekAlertAAAS

Narahari Gramapurohit, Sachin Gosavi and Samadhan Phuge. 2011. Unique courtship and spawning behaviour in wrinkled frog, Nyctibatrachus humayuni. Amphibia-Reptilia.

ResearchGate.net/publication/215634691_Unique_courtship_and_spawning_behaviour_in_wrinkled_frog_Nyctibatrachus_humayuni

Krushnamegh Kunte. 2004. Natural History and Reproductive Behavior of Nyctibatrachus cf. humayuni (Anura: Ranidae). Herpetological Review. 35(2), 137–140.
http://www.biodiversitylab.org/sites/default/files/images/website/Kunte04_NyctiHerpRev.pdf