Oviposition site selection, or nest site selection, is one of the key factors that determine reproductive success. A superior oviposition site can lead to increases in offspring fitness and survivorship (Howard 1978; Kolbe and Janzen 2002). Selection of oviposition sites is dependent on a number of biotic and abiotic factors, such as temperature, vegetation, predation, and presence of conspecifics (Davis 2005; Rudolf and Rödel 2005; von May et al. 2009; Pike et al. 2011; Kern et al. 2013). Though the process of selecting of a particular site can be quite complex, studies have generally focused on oviposition site selection of either males or females only (but see Howard 1978). Even though we know that animals of different sex can respond differentially in several biological issues, such as disease resistance (Bernal and Pinto 2016), signal sensory pathway (Hoke et al. 2010), local adaptation (Svensson et al. 2018) and offspring discriminating ability (Insley et al. 2003; Ringler et al. 2016), we still have limited knowledge in the sex-specific preference of oviposition site. Similarly, while oviposition sites preference can be affected by multiple factors (Rudolf and Rödel 2005), few studies have examined the effects of and interaction between factors. A better understanding of oviposition site selection allows us to gain insight into how site suitability is determined and how characteristics that are preferred by males and females help maximize their reproductive output.
In anurans, choice of oviposition sites can depend on temperature, water volume, vegetation, competition, and predation (Howard 1978; Rudolf and Rödel 2005; von May et al. 2009). These characteristics not only affect offspring development and survival during the egg stage (Rudolf and Rödel 2005), but also have more long-lasting effects on subsequent life stages in cases where larval environment is dependent on the location of the nests. This is particularly notable in cases where larval environment is determined by nest location. In anurans, oviposition sites is often assumed to be selected or determined by females. This is because, the majority of cases, females invest more into gamete production and will often move around once amplexus is formed before settling at an oviposition site. Therefore, the significance of male preference in oviposition site selection is often overlooked. However, in resource-defense mating systems, where males occupy and defend territories (e.g. Wells 1977; Chuang et al. 2013), potential oviposition sites could be pre-selected by males before they advertise for females. The effects of male choice in oviposition site may be even more pronounced in species with higher male investment, such as those with paternal care. These factor may shift the relative weights of male and females in determining the final oviposition site. Despite the potential interplay between gamete production, resource-defense, and parental care, few studies have examined how oviposition site select differs between the sexes.
To examine the role females and males play in determining oviposition sites in a resource-defense mating system, we focused on Kurixalus eiffingeri (Family: Rhacophoridae), which is distributed in Taiwan and the Ryukyu islands (Kuramoto 1973; Ueda 1986). An arboreal species, K. eiffingeri breeds in tree holes or bamboo stumps where water has collected within the internodes (Kam et al. 1997; Kam et al. 1998a). Potential breeding sites are generally spread out within the forest. These sites are occupied by males, which defend their territories and call to attract females (Kam et al. 1996, Fig. 1). Once a female amplexus with a calling male, eggs are deposited and attached to the inner wall of the treehole or bamboo stump occupied by the male (Fig. 1). Both males and females exhibit parental care, though the timing and type of care do not overlap between the sexes. Males care for their offspring by attending the eggs (Kam et al. 1996; Chen et al. 2007; Chuang et al. 2019), while females return periodically to feed their tadpoles by laying unfertilized eggs until tadpoles reach metamorphosis (Ueda 1986; Kam et al. 1996).
Previous observations in K. eiffingeri indicate that only around 40% of potential breeding sites are occupied by males (Lin and Kam 2008). Since oviposition sites are not a limiting factor, males should select a given site based on the benefits it provides in increased reproductive success, via increases in mating opportunities or offspring survivorship. Similarly, since calling males are not a limiting factor, females should select mates based on the benefits of having a preferable male or a preferable oviposition site. Given the male territoriality, female mate choice, paternal egg attendance, as well as maternal tadpole attendance, K. eiffingeri provides a unique case to explore factors that affect oviposition site selection between males and females. One of the main factors that affect offspring survival is egg desiccation (Lin and Kam 2008). For terrestrial-breeding amphibians, laying eggs outside of aquatic environments comes at a distinct cost, since their amniotic eggs are faced with a higher rate of evaporation in the harsh terrestrial environment. Therefore, it is not surprising that K. eiffingeri egg clutches are more likely to be found at oviposition sites with more water present and more potential space to accumulate water in the future (Kam et al. 1996; Lin and Kam 2008). However, what remains unknown is if males are choosing oviposition sites based on site characteristics, if females have a preference for the same site characteristics, and if the site characteristics chosen by males or females are those that correlate with offspring survivorship.
Herein, we examine how oviposition sites are selected in K. eiffingeri and how it affects offspring survivorship. Using natural field observations, we determine (1) if males are selecting oviposition sites based on the physical characteristics of the site (2) if females are selecting oviposition sites based on the same site characteristics and if they are selecting mates based on male characteristics. We hypothesize that males will show a preference for oviposition site characteristics that directly affect egg stage survivorship (i.e. more water and larger tree hole size). Moreover, we hypothesize that females will narrow the selection further based on the same oviposition site characteristics. Finally, we hypothesize that females will prefer larger males with better body conditions, since it may serve as an indicator of the quality of care that males are able to provide during egg attendance.