Spider traps amphibian in northeastern Madagascar

Abstract Predation can take unexpected turns. For instance, various invertebrate species—most commonly spiders—may prey on vertebrates. Here, we report one observation of a spider (Sparassidae, Damastes sp.) feeding on an amphibian (Hyperoliidae, Heterixalus andrakata) inside a retreat in northeastern Madagascar. To our knowledge, this is the second report of vertebrate predation by spiders in Madagascar. Three additional observations of retreats built by the same spider species show that the spiders built similar retreats and were hiding at the rear end of the retreat. The retreats were built by weaving two green leaves together which were still attached to the tree. We speculate from the observations, that the retreat serves as a targeted trap that deceives frogs seeking shelter during daytime.

Here, we report on a predation by a spider of the genus Damastes sp. catching a small frog (Heterixalus andrakata, Glaw and Vences, 1991, Least Concern) in northeastern Madagascar. As we understand, this is the second report of spider predation on amphibians in Madagascar.

| Study area
We conducted field observations around Ambodiala (commune Farahalana, Sambava District) and Antsikory (commune Ampanefena, Vohemar District) in northeastern Madagascar ( Figure 1). The climate in this part of Madagascar is tropical-humid. The landscape was formerly covered with humid evergreen forests (Du Puy & Moat, 1996), but forests are nowadays fragmented (Vieilledent et al., 2018) and the landscape is dominated by smallholder agriculture.

| Incidental observations
We made four incidental observations during ecological surveys in the study area. DAM made the first incidental observation, at which the spider was feeding on the amphibian, in the morning on 25th October 2017 in a woody fallow in Ambodiala (14°24′47″ S, 50°5′17″ E) after a bird point count. The woody fallow is a former slash-and-burn (tavy) field on which rice was last cultivated in 2001. The shrubs and trees inside the woody fallow were around two to three meters high.

| Specimens
Two spider individuals were collected, euthanized, and fixed in 90% ethanol. We labeled voucher specimens with field numbers

| RE SULTS
We found four different spider retreats from Damastes sp. The retreats were open at the leave's base and the spiders were wellhidden at the rear end of the trap (i.e., the apex of the leaves) and not visible from the entrance.

| First incidental observation
After a bird point count in the morning (6:45 a.m.), we saw how a spider (Damastes sp.) was feeding on a frog (Heterixalus andrakata, Hyperoliidae) near the woven leaves of Tambourissa sp. The spider held on the head of the amphibian with the fangs. The amphibian posterior legs were above the back of the spider, while the head was down. The amphibian did not move anymore, so it seemed already killed (Figure 2c). When we approached the scene, the spider with prey went hiding into the retreat (Figure 2b). We took photographs and left the predation event. The tree leaves measured circa 26 cm in length and circa 9 cm in width at the widest point. The height of the leaves from the ground was around 120 cm.
In the afternoon (4:15 p.m. of the same day), we came back to the same place and the spider was still at the same place (hiding between the leaves). We collected the specimen (Figure 2a) but could not find the frog prey anymore. Around the tree, within a 2-m-radius, we found four other living individuals of Heterixalus andrakata. During the second through fourth incidental observations, we found the same spider species hiding between leaves of different tree species but we could not observe any predation events.

| Third incidental observation
The third observation resembled the second, but occurred in a different vanilla plantation within the same village, circa 300 m away from the second observation. The height of the woven leaves of Phyllarthron madagascariensis where the spider was hiding was around 170 cm above the ground.

| Fourth incidental observation
We found the spider hiding between leaves of Cedrela odorata ( Figure 2d). Before we took the picture, we found the spider at the far end of the retreat. When we took the picture, the spider was flushed out from the retreat. The length of the leaf was circa 8 cm with a width of circa 3 cm. We found the woven leaves around 50 cm from the ground.

| Predation event and retreat
We report one predation event of spider Damastes sp. eating a small frog Heterixalus andrakata. Additionally, we observed three individuals of the same species of spider sitting in retreats made by green leaves attached to the stem of the tree. The retreats generally show the same shape. Two leaves were woven by spider silk in the apex, edges and open in the base of the leaves seems enabling prey climbing up the stem of the tree to enter. The spiders do not seem to have a preference for a single tree species and the height from the ground also seems variable from those observations.
Spiders are the most cited invertebrate group preying on vertebrates (Barej et al., 2009). However, the majority of reports of amphibian predation by invertebrates stems from the Neotropics. Few predation events on Afrotropical anurans by invertebrates have been published (Babangenge et al., 2019). Reports from Africa are from Tanzania and Uganda, where fishing spiders prey on tadpoles (Vonesh, 2005), from South Africa, where crabs predate on amphibians, and from Cameroon, where wandering spiders prey on tree frogs (Barej et al., 2009).

Whether this geographic bias concerning amphibian predation by in-
vertebrates is indeed reflecting a difference in the frequency of such behavior or whether the bias is due to more research being conducted in the Neotropics (Martin et al., 2012;Meyer et al., 2015) remains, however, unclear. Our observation is, to our knowledge, the second report of spider predation on vertebrates from Madagascar after Glos (2003), which reports on spiders (Pisauridae) feeding on frogs (Heterixalus tricolor) on the reed grass stems within a pond in Kirindy dry forest.

| Characteristics of Damastes sp. spiders and Heterixalus sp. frogs
The genus Damastes is included in the Sparassidae family, a group that is called "huntsman spiders" (Rayor, 2018) and occurs around the world (Jäger, 2012). Most huntsman spiders do not build webs to capture their prey (Rayor, 2018). Instead, some of them are known for their running and hunting habit. But, the genus Damastes is an exception since members of the genus typically use a sit-and-wait approach (Soutinho et al., 2018). However, some Sparassidae have been found in their own silk nest which is fastened with debris, leaving leaves or stems that are completely surrounded by silk (Jackson, 1987). Furthermore, most species in the family Sparassidae are nocturnal (Rayor, 2018).
The genus Heterixalus is predominantly arboreal and typi- cally occurs in open areas within the human-dominated landscape (Blommers-Schlösser, 1982;Raharivololoniaina et al., 2003). The species H. andrakata is distributed in northern and northeastern Madagascar (Glaw & Vences, 2007). During our ecological survey, we found H. andrakata to be mostly active at night, but recorded some daytime activity in agroforests. However, the species is typically hiding away during daytime between leaves, possibly to avoid dehydration.

| Speculation on possible systematic trapping behavior
Previous reports of spiders preying on amphibians point to an opportunistic behavior and provide no evidence of specialization. Based on our report, we speculate that the spiders use targeted traps to prey on amphibians. We base this speculation on several strings of evidence. First, some reports describe spider retreats used by spiders as a protection from predators (Henschel & Jocqué, 1994;Nentwig & Heimer, 1987;Stradling, 1994;Thirunavukarasu et al., 1996).
However, these retreats may also be modified to serve as a trap (Nentwig & Heimer, 1987). Second, the behavior was observed independently in four spider individuals at four different sites suggesting that the retreat building is frequently performed by Damastes sp. in northeastern Madagascar. Third, a key factor facilitating the trapping behavior of Damastes sp. may be that Heterixalus andrakata and possibly also other arboreal frogs try to hide from sunlight during the day in order to avoid dehydration (Rodel & Braun, 1999).
When temperatures rise, the frogs look for shade and cover away from the ground, which the spiders provide in form of their retreat.
The frogs might favor the seemingly protected traps in an attempt to hide from other predators such as birds that scan the vegetation for prey. Based on these strings of evidence, we speculate that amphibians may not only be an opportunistic, indiscriminate, or accidental prey, but rather a targeted systematically exploited food source of Damastes sp. spiders.
Nonetheless, further research is required to confirm this, especially so, as we only report a single observation of the spider feeding on the frog. Additionally, large prey such as the frog is more likely to catch the attention of an observer, thereby posing the risk of being overinterpreted.

ACK N OWLED G M ENTS
We would like to thank the landowners in Ambodiala and Antsikory who let us survey their lands. We also thank our field assistants

CO N FLI C T O F I NTE R E S T
The corresponding author declares on behalf of authors that there is no conflict of interest to disclose.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article as no new data were created or analyzed in this study.