Herpetology Notes, volume 14: 157-161 (2021) (published online on 22 January 2021)

Egg-kicking behaviour by male gliding treefrogs (Agalychnis spurrelli Boulenger, 1913) does not dislodge competitors’ eggs

Elena K. Gomez1, Karen M. Warkentin1,2, and Brandon A. Güell1,*

The specific tactics anurans use for reproduction are how sexual selection acts on reproductive tactics in largely influenced by their temporal pattern of breeding explosive-breeding systems. and the subsequent operation of sexual selection on The gliding treefrog Agalychnis spurrelli Boulenger, reproductive characteristics (Wells, 1977, 2007). For 1913, is an explosive, arboreal-breeding species that instance, when breeding periods are prolonged, females ranges from Costa Rica to northwestern Ecuador often reproduce asynchronously. Males typically form (Duellman, 1968, 1970; Savage, 2002). Like other choruses or leks where females can choose their mates, explosive breeders, A. spurrelli mate during large and physical male-male competition for mates is low aggregations with highly male-skewed OSRs and males (Wells, 1977; Arak, 1983, 1988; Murphy, 1994; Sullivan engage in intense male-male scramble competition to et al., 1995; Llusia et al., 2013). In contrast, short find mates (Scott and Starrett, 1974; Güell and González, breeding periods (i.e., “explosive breeding”) reduce the 2019). In addition to classic scrambling behaviours, we number of opportunities for males to find a mate. Females often observe non-amplectant males position themselves breed more synchronously and are available for only a over recently laid eggs and perform alternating sets of short time, while operational sex ratios (OSR) at breeding kicking motions with their hind legs, sometimes for sites are often highly male-skewed and many males fail several minutes and even after amplectant pairs leave to mate. Thus, males of explosive breeders often engage (Fig. 1; video in Gomez et al., 2020). Scott and Starrett in intense scramble competition and female choice is (1974) described this behaviour as “egg-scraping” thought to be more limited (Wells, 1977, 2007; Howard, and hypothesized that it functions as an antagonistic 1980; Lee and Crump, 1981; Verrell and McCabe, 1986; strategy to remove and kill competitors’ offspring. If Sullivan et al., 1995). scraping male A. spurrelli sire eggs in other clutches, Although many anuran species have short breeding this egg-kicking behaviour could increase their relative periods (Wells, 1977; Sullivan et al., 1995), most of what reproductive success, making this behaviour adaptive. we know about the reproductive behaviour of explosive The egg-scraping hypothesis is speculative and based on breeders is based on aquatic-breeding temperate species just three direct observations during a single explosive- (Wells, 1977, 2007). Typical scrambling behaviours breeding event (Scott and Starrett, 1974); however, include gathering in dense aggregations and actively it is currently widely accepted (Savage, 2002; Wells, searching for mates. In addition, males of most species 2007). Here we test the egg-scraping hypothesis in a compete by kicking and pushing at competitors and large natural population of A. spurrelli on Costa Rica’s clasping amplectant pairs, in attempts to displace Osa Peninsula using video analyses of natural history amplectant males (Wells, 1977, 1979; Roberts, 1994; observations across multiple breeding events. Lee and Corrales, 2002). However, we know very little about if, or how, terrestrial egg-laying changes Materials and Methods We conducted our observations at Osa Conservation’s Shampoo Pond (8.4124°N, 83.3459°W) on Costa Rica’s Osa Peninsula (Güell et al., 2019). This site 1 Department of Biology, Boston University, 02215, Boston, serves as important permanent breeding habitat for at Massachusetts, USA. least ten locally abundant anuran species, including 2 Gamboa Laboratory, Smithsonian Tropical Research Institute, three explosive-breeding species (B. Güell, pers. obs.). 0843-03092, Panama, República de Panama. Explosive-breeding events of A. spurrelli at this site are * Corresponding author. E-mail: [email protected] typically concentrated in a ~20 m2 area on the northwest © 2021 by Herpetology Notes. Open Access by CC BY-NC-ND 4.0. edge of the pond and individuals aggregate on palm 158 Elena K. Gomez et al.

Figure 1. Series of images depicting male A. spurrelli egg-kicking behaviour during an explosive reproductive event on Costa Rica’s Osa Peninsula. Photos by Brandon A. Güell. fronds and leaves of Ardisia opegrapha that overhang the for A. spurrelli breeding aggregations. On days of pond surface and reach to 1–8 m in height (Thompson et explosive-breeding events, we opportunistically video al., 2016; Güell et al., 2019). We conducted daily surveys recorded adults performing egg-kicking behaviours. We of Shampoo Pond at dawn from May 14–August 23, recorded videos from within the middle of the pond or on 2018 and May 27–July 24, 2019 and visually checked its edge using a Canon EOS 5D Mark III camera and EF Egg-kicking Behaviour in Agalychnis spurrelli 159

70-200mm f/2.8L USM lens. Videos varied in length and kicking; this appears to be the optimal kicking position number of focal animals recorded. We recorded a total of (Gomez et al., 2020). If egg-kicking is an antagonistic 42 videos with males performing egg-kicking behaviour tactic directed at mated males, non-amplectant males for 17.2 minutes and quantified the number of kicks each would not benefit from competing against each other to male performed, the duration they spent egg-kicking, and dislodge mated males’ eggs. Finally, in addition to the the number of eggs dislodged per egg-kicking event. events analysed in this study we have observed dozens of hours of egg-kicking and, except for the single egg noted Results and Discussion above, we have never seen egg-kicking dislodge eggs. We propose the alternative hypothesis that egg- We observed multiple male A. spurrelli performing kicking by male A. spurrelli functions as an alternative egg-kicking behaviours (see Gomez et al., 2020) reproductive strategy, in which non-amplectant males during each of the reproductive events we surveyed. may fertilize some recently laid eggs. We hypothesize From the videos we recorded, we analysed a total that egg-kicking behaviours serve to spread the of 754 individual kicks by 57 different males at 47 kicking male’s sperm across multiple eggs within a unique egg masses across nine reproductive events and clutch, thereby increasing his chances of fertilizing any two breeding seasons (Table 1). On average, these 57 remaining unfertilized eggs, potentially in competition males kicked 13.2 ± 12.6 times (range = 1–61 times) with sperm already on the clutch. Theory suggests that for 18.2 ± 18.2 s. We found that egg-kicking almost the highly male-skewed OSRs and intense scramble never directly dislodged kicked eggs, except in one case competition of explosive breeders may lead to the where a single egg fell (Table 1). However, in contrast evolution of alternative reproductive tactics in males to typical A. spurrelli egg masses that are laid in thin (Wells, 2007; Oliveira et al., 2008). Indeed, explosive- monolayers directly on leaves (Gomez-Mestre and breeding males of many aquatic-breeding temperate Warkentin, 2007), this particular egg mass had several species are known to release sperm simultaneously with eggs laid on top of one another creating segments of paired males, often leading to sperm competition and multi-layered eggs that were loosely hanging by a few multiple paternity in clutches (Halliday and Verrell, 1984; eggs not directly attached to the leaf. Thus, we attribute Berger and Rybacki, 1992; Halliday, 1998; Roberts and this rare case of egg dislodgement to the fact that the egg Byrne, 2011). Male common frogs, Rana temporaria, that fell and the surrounding abnormally structured egg have also been documented engaging in clutch piracy, mass were loosely attached to the leaf before the male a post-mating tactic where non-amplectant males clasp began kicking (Gomez et al., 2020). and fertilize recently laid eggs in the water (Vieites et Our findings do not support the hypothesis proposed al., 2004). However, the role of non-typical scramble by Scott and Starrett (1974) that egg-kicking by male competition behaviours on reproductive success in A. spurrelli functions to dislodge competitors’ eggs. We explosive-breeding species have rarely been considered found no evidence that egg-kicking removes eggs from (Berger and Rybacki, 1992; Wells, 2007). Thus, it is their leaves by direct contact or by indirectly knocking plausible that the egg-kicking by male A. spurrelli is a them off, suggesting that egg-kicking serves another post-mating alternative reproductive tactic, somewhat function. There are several additional factors that further analogous to clutch piracy, that functions to directly support our findings. First, for this behaviour to be increase non-amplectant males’ reproductive success. adaptive, egg-kicking males would have to successfully Our current research aims to test this hypothesis. sire other eggs; without this, there would be no potential to increase their own (relative) reproductive success by Acknowledgements. This study was supported by the National killing competitors’ offspring (Scott and Starrett, 1974; Science Foundation (IOS-1354072 to KMW, DGE-1247312 to Vickery et al., 2003). However, OSRs during A. spurrelli BAG), and Boston University. We thank Osa Conservation for explosive-breeding events are extremely male-skewed their support and Jeanne Robertson for a pre-peer review. (Scott and Starrett, 1974; Güell et al., 2019) and it is likely that few egg-kicking males will find a mate; therefore, References most egg-kicking males would gain no benefit by Arak, A. (1983): Sexual selection by male–male competition in dislodging competitors’ eggs. Second, we have observed natterjack toad choruses. Nature 306: 261–262. several males kick at the same egg masses and compete to Arak, A. (1988): Female mate selection in the natterjack toad: position themselves at the top and centre part of the egg active choice or passive attraction? Behavioral Ecology and mass where they are able to contact the most eggs while Sociobiology 22: 317–327. 160 Elena K. Gomez et al.

Table 1. Date and number of individuals and kicks analysed of male Agalychnis spurrelli at Shampoo Pond on the Osa Peninsula of Costa Rica.

Breeding Number of Total number of Total number of Date Season males analysed kicks analysed eggs dislodged

27 May 15 199 1 6 June 1 5 0 15 June 7 51 0 2018 16 June 3 18 0 18 June 1 4 0 23 July 13 143 0

13 June 8 139 0 2019 15 July 8 162 0 24 July 1 33 0

Berger, L., Rybacki, M. (1992): Sperm competition in European Lee, J.C., Crump, M.L. (1981): Morphological correlates of male water frogs. Alytes 10: 113–116. mating success in Triprion petasatus and Hyla marmorata Duellman, W.E. (1968): The taxonomic status of some American (Anura: Hylidae). Oecologia 50: 153–157. hylid frogs. Herpetologica 24: 194–209. Llusia, D., Márquez, R., Beltrán, J.F., Moreira, C., Amaral, J.P. Duellman, W.E. (1970): The hylid frogs of Middle America. do (2013): Environmental and social determinants of anuran Monographs of the Museum of Natural History, University of lekking behavior: intraspecific variation in populations at Kansas 1–2: 1–753. thermal extremes. Behavioral Ecology and Sociobiology 67: Gomez, E.K., Güell, B.A., Warkentin, K.M. (2020): Gliding 493–511. treefrog reproductive behavior: “egg scraping” by males Murphy, C.G. (1994): Chorus tenure of male barking treefrogs, does not dislodge competitors’ eggs. Available at: https:// Hyla gratiosa. Animal Behaviour 48: 763–777. doi.org/10.6084/m9.figshare.12931448.v1. Accessed on 31 Oliveira, R.F., Taborsky, M., Brockmann, H.J. (2008): Alternative December 2020. Reproductive Tactics: an Integrative Approach. Cambridge, Gomez-Mestre, I., Warkentin, K.M. (2007): To hatch and hatch United Kingdom, Cambridge University Press. not: similar selective trade-offs but different responses to egg Roberts, J.D., Byrne, P.G. (2011): Polyandry, sperm competition, predators in two closely related, syntopic treefrogs. Oecologia and the evolution of anuran amphibians. In: Advances in the 153: 197–206. Study of Behavior, p. 1–53. Brockmann, H.J., Roper, T.J., Güell, B.A., González, K. (2019): Mating mayhem. Frontiers in Naguib, M., Mitani, J.C., Simmons, L.W., Eds., San Diego, Ecology and the Environment 17: 128. California, USA, Academic Press. Güell, B.A., González, K., Pedroso-Santos, F. (2019): Opportunistic Roberts, W.E. (1994): Explosive breeding aggregations and predation by two aquatic-feeding predators on an explosive- parachuting in a neotropical frog, Agalychnis saltator (Hylidae). breeding aggregation of arboreal gliding treefrogs (Agalychnis Journal of Herpetology 28: 193–199. spurrelli Boulenger, 1913; Anura: Phyllomedusidae) on the Osa Savage, J.M. (2002): The Amphibians and Reptiles of Costa Rica. Peninsula of Costa Rica. Herpetology Notes 12: 795–798. Chicago, Illinois, USA, University of Chicago Press. Halliday, T.R. (1998): Sperm competition in amphibians. In: Sperm Scott, N.J., Starrett, A. (1974): An unusual breeding aggregation Competition and Sexual Selection, p. 465–502. Birkhead, T.R., of frogs, with notes on the ecology of Agalychnis spurrelli Møller, A.P., Eds. San Diego, California, USA, Academic Press. (Anura: Hylidae). Bulletin of the Southern California Academy Halliday, T.R., Verrell, P.A. (1984): Sperm competition in of Sciences 73: 86–94. amphibians. In: Sperm Competition and the Evolution of Sullivan, B., Ryan, M.J., Verrell, P.A. (1995): Female choice and Animal Mating Systems, p. 487–508. Smith, R.L., Ed., San mating system structure. Amphibian Biology 2: 469–517. Diego, California USA, Academic Press. Thompson, M.E., Donnelly, M.A., Mendoza, M.S. (2016): Natural Howard, R.D. (1980): Mating behaviour and mating success in history notes. Agalychnis spurrelli (gliding leaf frog). Explosive woodfrogs Rana sylvatica. Animal Behaviour 28: 705–716. breeding aggregation. Herpetological Review 46: 434–435. Lee, J.C., Corrales, A.D. (2002): Sexual dimorphism in hind-limb Verrell, P.A., McCabe, N.R. (1986): Mating balls in the common muscle mass is associated with male reproductive success in toad, Bufo bufo. British Herpetological Society Bulletin 16: Bufo marinus. Journal of Herpetology 36: 502–505. 28–29. Egg-kicking Behaviour in Agalychnis spurrelli 161

Vickery, W., Brown, J., FitzGerald, G. (2003): Spite: altruism’s evil twin. Oikos 102: 413–416. Vieites, D.R., Nieto-Román, S., Barluenga, M., Palanca, A., Vences, M., Meyer, A. (2004): Post-mating clutch piracy in an amphibian. Nature 431: 305–308. Wells, K.D. (1977): The social behaviour of anuran amphibians. Animal Behaviour 25: 666–693. Wells, K.D. (1979): Reproductive behavior and male mating success in a neotropical toad, Bufo typhonius. Biotropica 11: 301–307. Wells, K.D. (2007): The Ecology and Behavior of Amphibians. Chicago, Illinois, USA, University of Chicago Press.

Accepted by Renato Nali