Tail Breakage and Predatory Pressure Upon Two Invasive Snakes (Serpentes: Colubridae) at Two Islands in the Western Mediterranean

Home , Horseshoe Whip Snake, Ladder snake

Canadian Journal of Zoology

Tail breakage and predatory pressure upon two invasive snakes (Serpentes: Colubridae) at two islands in the Western Mediterranean

Journal: Canadian Journal of Zoology

Manuscript ID cjz-2020-0261.R2

Manuscript Type: Article

Date Submitted by the 17-Jan-2021 Author:

Complete List of Authors: Febrer-Serra, Maria; University of the Balearic Islands Lassnig, Nil; University of the Balearic Islands Colomar, Victor; Consorci per a la Recuperació de la Fauna de les Illes Balears Draft Sureda Gomila, Antoni; University of the Balearic Islands; Carlos III Health Institute, CIBEROBC Pinya Fernández, Samuel; University of the Balearic Islands, Biology

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Zamenis scalaris, Hemorrhois hippocrepis, invasive snakes, predatory Keyword: pressure, Balearic Islands, frequency of tail breakage

1 Tail breakage and predatory pressure upon two invasive snakes (Serpentes:

2 Colubridae) at two islands in the Western Mediterranean

4 M. Febrer-Serra, N. Lassnig, V. Colomar, A. Sureda, S. Pinya*

6 M. Febrer-Serra. Interdisciplinary Ecology Group. University of the Balearic Islands,

7 Ctra. Valldemossa km 7.5, 07122 Palma, Balearic Islands, Spain. E-mail address:

8 [email protected].

9 N. Lassnig. Interdisciplinary Ecology Group. University of the Balearic Islands, Ctra.

10 Valldemossa km 7.5, 07122 Palma, Balearic Islands, Spain. E-mail address:

11 [email protected].

12 V. Colomar. Consortium for the RecoveryDraft of Fauna of the Balearic Islands (COFIB).

13 Government of the Balearic Islands, Spain. E-mail address: [email protected].

14 A. Sureda. Interdisciplinary Ecology Group / Research Group on Community Nutrition

15 and Oxidative Stress (NUCOX) and IdisBa / CIBEROBN (Physiopathology of Obesity

16 and Nutrition). University of the Balearic Islands, Ctra. Valldemossa km 7.5, 07122

17 Palma, Balearic Islands, Spain. E-mail address: [email protected]

18 S. Pinya. Interdisciplinary Ecology Group. University of the Balearic Islands, Ctra.

19 Valldemossa km 7.5, 07122 Palma, Balearic Islands, Spain. E-mail address:

20 [email protected]

21 * Corresponding author: S. Pinya. Interdisciplinary Ecology Group, Biology

22 Department, Guillem Colom Casasnovas Building. University of the Balearic Islands.

23 Phone. +34 971173177. Rd Palma-Valldemossa, Km 7,5. CP: 07122 (Palma de

24 Mallorca, Balearic Islands, Spain). E-mail address: [email protected]

1 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 2 of 34

26 ABSTRACT

27 Tail breakage is an important anti-predator mechanism in snake populations, which can

28 be used as a proxy for predation intensity as natural observations of predator-

29 prey interactions are scarce. Frequency of tail breakage was calculated for two

30 Iberian colubrids recently introduced in the Balearic Islands (Western

31 Mediterranean, Spain): Hemorrhois hippocrepis (Linnaeus, 1758) in Eivissa and

32 Zamenis scalaris (Schinz, 1812) in Formentera. The effect of sex, life-stage,

33 dorsal coloration pattern, body length and body condition on frequency of tail

34 breakage and on remaining subcaudal scale pairs (SBC) were analyzed and

35 compared between the native range and the invaded islands. An increase of the

36 frequency of tail breakage withDraft body size was found, supporting a size-related

37 effect, which also occurs in the native range. Frequency of tail breakage of H.

38 hippocrepis was lower in Eivissa when compared with the original area, while in

39 Formentera Z. scalaris showed a higher frequency, which could be related to the

40 different predator community on each island compared with the mainland. The

41 study of the main ecological aspects of these recent introduced species may

42 allow to assess their potential impact on insular ecosystems and their native

43 biodiversity as well as to promote future control actions in these areas previously

44 free of snakes.

46 Key words: ladder snake, Zamenis scalaris, horseshoe whip snake, Hemorrhois

47 hippocrepis, invasive snakes, frequency of tail breakage, predatory pressure, Balearic

48 Island

2 © The Author(s) or their Institution(s) Page 3 of 34 Canadian Journal of Zoology

49 INTRODUCTION

50 In many species, tail breakage is believed to provide an effective antipredator

51 mechanism (Savage and Slowinski 1996; Prudente et al. 2007) and it can be used as

52 evidence of predation rate (Turner et al. 1982; Arnold 1988). However, despite of tail

53 breakage being widely used as a proxy of predation it could be also caused by

54 intraspecific competition or other confounding factors, as it has been observed in lizards

55 (Jaksić and Busack 1984; Jaksić and Green 1984; Itescu et al. 2016). Tail breakage in

56 snakes, also known as pseudoautotomy (Savage and Slowinski 1996), is thought to

57 occur when snakes are grasped by the tail during a predation event and then, a non-

58 spontaneous intervertebral breakage is produced by the rotational movement of the

59 snake in its attempts to escape (Cooper and Alfieri 1993; Savage and Slowinski 1996).

60 The tail of snakes does not regenerateDraft after breaking (Slowinski and Savage 1995) thus

61 leaving a permanent scar from the predation episode. Although some studies suggest

62 that tail losses might indicate inefficiency of predation due to prey survival (Mushinsky

63 and Miller 1993), frequency of tail breakage is widely used as an indirect evidence of

64 the predation pressure on snake populations (Mendelson 1992; Slowinski and Savage

65 1995; Fitch 2003; Pleguezuelos et al. 2010; 2018; Santos et al. 2011). Furthermore,

66 length of the tail stump relative to snout-vent length (SVL) or remaining subcaudal

67 scale pairs (SBC) are commonly used as a measure of tail-loss severity in injured snakes

68 (Aubret et al. 2005).

69 Frequency of tail breakage regarding life-stage, sex and geographic distribution (Fitch

70 2003; Bowen 2004; Placyk and Burghardt 2005; Pleguezuelos et al. 2010, 2018; Costa

71 et al. 2014), as well as intraspecific differences in frequency of tail breakage between

72 populations subjected to different predatory risk have been studied (Santos et al. 2011).

73 Some authors suggest that differences in frequency of tail breakage between populations

3 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 4 of 34

74 could be related to different diversity of predators and predation pressure of each area,

75 variables which change geographically (Placyk and Burghardt 2005; Costa et al. 2014).

76 Pleguezuelos et al. (2010, 2018) suggested that the differences in frequency of tail

77 breakage among the Iberian colubrids Hemorrhois hippocrepis (Linnaeus, 1758) and

78 Zamenis scalaris (Schinz, 1822) in mainland Spain could be related to their foraging

79 habits, which determined their exposure to predators and hence, the predation risk

80 (Pleguezuelos et al. 2007). Moreover, intraspecific competition, including male-male

81 combats, should be considered, especially in large males (Madsen et al. 1993).

82 However, very few cases have been observed in H. hippocrepis in their natural habitat

83 (Jiménez-Cazalla and Lora 2016) and none has been reported in Z. scalaris.

84 Hemorrhois hippocrepis is a medium-large sized colubrid (Pleguezuelos and Feriche

85 2014) that is sexual dimorphic, withDraft males being longer than females with higher values

86 in both body length (SVL) and tail length (Feriche et al. 1993; Pleguezuelos and Feriche

87 2014). It is considered a long-tailed colubrid as its tail length represents 22.9 % of the

88 total length of the snake (range: 20.47 % - 25.21 %, Feriche 1989). In contrast, Z.

89 scalaris is a short-tailed colubrid (15.5 % of total length; Feriche et al. 1993) that lacks

90 sexual dimorphism in body length, but on average, males have longer tail than females

91 (Feriche et al. 1993). This species also has an ontogenetic and anti-predatory behavior

92 change in the dorsal coloration pattern, which shifts from a ladder pattern in juveniles to

93 a striped pattern in adults. Juveniles depend on their crypsis to avoid predation, while

94 adults rely on rapid escape and the confusion of predators because of their stripped

95 pattern (Pleguezuelos et al. 1990, 2007, 2010).

96 H. hippocrepis and Z. scalaris have been recently introduced in the Balearic Islands

97 (Western Mediterranean, Spain) from their native range, the Iberian Peninsula

98 (mainland Spain) (Pinya and Carretero 2011). In recent years, many invasive species

4 © The Author(s) or their Institution(s) Page 5 of 34 Canadian Journal of Zoology

99 have been introduced to the Balearic Islands because of increasing international trade

100 (Perrings et al. 2005). Among the large number of introduced species, reptile exotic

101 species (19) outnumber native ones (2) currently inhabiting the Balearic Islands (Pinya

102 and Carretero 2011). The first report of H. hippocrepis in the Balearic Islands was in

103 2003 in Eivissa, followed by Mallorca in 2004 and Formentera in 2011 (Pinya and

104 Carretero 2011). On the other hand, Z. scalaris was reported for the first time in 2003 in

105 Eivissa, followed by Mallorca in 2004 and Formentera in 2006 (Pinya and Carretero

106 2011). It should be noted that both Eivissa and Formentera islands had remained free of

107 snakes until the introduction of these colubrid species (Silva-Rocha et al. 2015).

108 Currently, any species of the Colubridae family inhabiting Eivissa and Formentera is

109 considered legally invasive, given its inclusion in the Spanish Catalogue of Invasive

110 Alien Species (RD 630/2013). Draft

111 Considering frequency of tail breakage as an indicator of frequency of predation (Fitch

112 2003; Pleguezuelos et al. 2010, 2018; Santos et al. 2011), the main objective of the

113 study was to assess the relationship between frequency of tail breakage and life-history

114 and morphological traits of these two introduced colubrids in the Balearic Islands. More

115 specifically, the aims of this study were: (a) to estimate the frequency of tail breakage in

116 these two exotic species in insular ecosystems (b) to analyze the effect of several

117 parameters such as sex, life-stage, body size (expressed by SVL), dorsal coloration

118 pattern (only in Z. scalaris) and body condition (BC, fat-body mass adjusted by body

119 mass) on the frequency of tail breakage and the remaining SBC of the insular

120 populations and, (c) to compare the potential predation pressure on the studied species

121 between the Balearic Islands and Iberian Peninsula, the mainland native distribution

122 area of the snakes. This study analyzes for the first time the frequency of tail breakage

123 of these two species in new invaded islands, so many of the results are highly

5 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 6 of 34

124 unforeseeable. We expect a positive relationship between body size and frequency of

125 tail breakage – i.e., larger, and older snakes with higher frequency of tail breakage, as

126 observed in previous studies (Mendelson 1992; Seligmann et al. 2008). Furthermore,

127 frequency of tail breakage is expected to be lower in the island ecosystems than in the

128 mainland because of the recent introduction of these two colubrids to island ecosystems.

129 Predation rates are expected to be lower in the new invaded areas, mainly due to the

130 differences in predator composition and abundance in the islands with respect to the

131 native range of the snakes, in accordance with the enemy release hypothesis (Roy et al.

132 2011). Taking advantage of the opportunity to study introductions in new territories and

133 compare them with their natural range (Huang et al. 2011; Lassnig et al. 2020) would be

134 an excellent way to better understand their potential impact.

135 MATERIALS AND METHODS Draft

136 Study area and sampling species

137 Snake sampling was conducted by Consortium for the Recovery of Fauna of the

138 Balearic Islands (COFIB) from the Government of the Balearic Islands, responsible for

139 control campaigns against alien species. Fieldwork was carried out by veterinarians and

140 field technicians of COFIB in Eivissa and Formentera (Balearic Islands, Spain, Western

141 Mediterranean) from March to October 2017 and 2018, months corresponding to the

142 main period of activity of the snakes. Snake specimens were captured by using traps

143 baited with live mice, which allowed the capture of both juvenile and adult specimens.

144 All traps were checked every nine days on average by the veterinarians. Captured

145 individuals were euthanized immediately by an injection of pentobarbital under

146 veterinary control and preserved frozen until further analyses at the University of the

147 Balearic Islands. After the study, all animal remains were incinerated by COFIB in

6 © The Author(s) or their Institution(s) Page 7 of 34 Canadian Journal of Zoology

148 accordance with national regulations on animal by-products non-intended for human

149 consumption (SANDACH).

150 Data collection

151 The following parameters were recorded in all the individuals: localization (island and

152 coordinates); body length (SVL), measured to the nearest mm with a flexible ruler; total

153 mass (to the nearest gram); fat body mass (to the nearest 0.01 g); and sex, determined by

154 introducing a sexing probe into the cloaca and by the examination of gonads during

155 necropsy. Dorsal color pattern of Z. scalaris was also recorded: a ladder pattern, with

156 transversal lines (found in juveniles); a striped pattern, with two longitudinal lines

157 (found in adults); and an intermediate pattern that was a combination of the scaled and

158 the striped colorations (found in the transition from juveniles to adults).

159 All the specimens were examined forDraft the integrity of the tail (broken or healed) and they

160 were classified as tail breakage or non-tail breakage specimens. Individuals with

161 missing tips (apical scale of the tail) were not considered tail breakage cases since the

162 apical scale of the tail can be removed by non-predator events as an incomplete skin

163 sloughing (Fitch 2003; Placyk and Burghardt 2005). The number of SBC in both injured

164 and non-injured individuals was also recorded as a measure of the tail length.

165 Life-stage was established according to the sexual maturity of specimens. Juveniles

166 were classified as sexually immature and adults as sexually mature individuals. It was

167 followed the classification criteria of sexual maturity proposed by Pleguezuelos and

168 Feriche (2006) for Z. scalaris (adult males ≥ 450 mm SVL, adult females ≥ 660 mm

169 SVL) and by Pleguezuelos and Feriche (1999) for H. hippocrepis (adult males ≥ 500

170 mm SVL, adult females ≥ 680 mm SVL).

171 Finally, historical data on the presence of potential predators of Z. scalaris and H.

172 hippocrepis in their native range as well as the occurrence of these predator species at

7 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 8 of 34

173 Eivissa and/or Formentera islands was provided, according to previous studies (López-

174 Jurado 2017 and references therein; Feriche 2017; Pleguezuelos 2019). The frequency

175 of predation of each predator on the snakes under study was also reported, according to

176 the literature provided in Table 2. Frequency of predation indicated what percentage of

177 the predator’s diet was represented by each snake prey.

178 Statistical analysis

179 Sex, life-stage and body size

180 Frequency of tail breakage was calculated for each sex and life-stage for both species

181 and in each island separately. Chi-square contingency tests were performed to compare

182 frequency of tail breakage between sex and life-stage of each species and to compare

183 frequency of tail breakage between the two studied species. The relationship between

184 SVL and frequency of tail breakageDraft was analyzed by logistic regression under R version

185 3.5.3 (R Core Team 2019). A Generalized Linear Model (GLM) assuming binomial

186 errors was performed to each species to model the effects of snakes’ SVL on the binary

187 response variable “presence or absence of tail breakage”. Sex of individuals was added

188 to the model as an additional covariate, to assess its potential effect combined with

189 SVL. Species were analyzed separately due to their different area of origin and their

190 different ecological traits. Residual analyses to prove error distribution and suitability of

191 the model as well as overdispersion checks were carried out. Finally, total deviance

192 explained by the model was calculated as follow: D2 (total deviance explained) = [(Null

193 deviance – Residual deviance)/Null deviance] · 100 (Cayuela 2009).

194 To analyze the relationship between the ontogenetic and behavioral shift in dorsal

195 coloration pattern of Z. scalaris and tail breakage, frequency of tail breakage was

196 calculated for each colour pattern. A chi-square contingency table, a Bonferroni post-

8 © The Author(s) or their Institution(s) Page 9 of 34 Canadian Journal of Zoology

197 hoc test and a mosaic plot were performed to compare the frequency of tail breakage

198 between snakes with ladder, intermediate and striped pattern.

199 Regarding SBC, mean (x), standard deviation (SD) and range (minimum - maximum)

200 were calculated for adult males and females in individuals with and without tail

201 breakage in both species. Statistical tests for normality (Shapiro Wilk normality test)

202 and homogeneity of variances (Bartlett test of homogeneity of variances) were

203 performed. Statistical significance was considered at p < 0.05. Student’s t-test (for

204 parametric data) and Wilcoxon-Mann-Whitney U test (for non-parametric data) were

205 carried out to assess statistically significant differences between sexes in number of

206 remaining SBC in individuals showing tail breakage and in individuals with intact tails.

207 To evaluate the multiple tail break hypothesis (MTBH), Pearson or Spearman

208 correlation tests analyzing remainingDraft SBC and SVL were performed, for parametric and

209 non-parametric data respectively. Through this analysis, the potential relationship

210 between the number of remaining SBC and body size we assessed. Tests were

211 performed on each species, both for combined and separate sexes.

212 Body condition

213 The potential effects of tail breakage and remaining SBC on BC of the adult individuals

214 were analyzed in individuals of 2018. To avoid the ontogenetic and sexual shift of fat-

215 body reserves (Pleguezuelos and Feriche 1999), juveniles were omitted from the

216 analysis and sexes were analyzed separately. Fat-body mass (g) was divided by the total

217 mass (g) of the specimens to calculate BC, which was considered a continuous variable.

218 Wilcoxon-Mann-Whitney U tests were carried out to assess statistically significant

219 differences in BC between injured and intact snakes, as data was non-parametric. In

220 stub-tailed adult snakes, potential correlations between fat body percentage and

9 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 10 of 34

221 remaining SBC were also analyzed through Pearson and Spearman correlation tests,

222 considering both males and females of H. hippocrepis and Z. scalaris.

223 RESULTS

224 Effects of biometric parameters on frequency of tail breakage and SBC

225 Sex, life-stage and body size

226 Up to 850 adult males, 258 adult females and 496 juveniles of H. hippocrepis from

227 Eivissa were analyzed (Table 1). Frequency of tail breakage of juveniles was

228 significantly lower than that of adults (χ2 test, df = 1, χ2 = 10.58, P < 0.005). However,

229 differences were not found between adult males and females (χ2 test, df = 1, χ2 = 0.10,

230 P = 0.75). Regarding Z. scalaris, up to 742 adult males, 316 adult females and 474

231 juveniles from Formentera were analyzed (Table 1). Juveniles had significantly a lower

232 frequency of tail breakage than adultsDraft (χ2 test, df = 1, χ2 = 54.72, P < 0.0001), and

233 among mature specimens, females had a higher frequency than males (χ2 test, df = 1, χ2

234 = 12.82, P < 0.0005).

235 Significant differences were found in the frequency of tail breakage between the two

236 species, with higher frequencies in Z. scalaris than in H. hippocrepis (χ2 test, df = 1, χ2

237 = 7.49, P < 0.01). Adult frequency of tail breakage of Z. scalaris was also higher than

238 adult H. hippocrepis (χ2 test, df = 1, χ2 = 14.44, P < 0.0005). Nevertheless, no

239 significant differences were found between juveniles of both species (χ2 test, df = 1, χ2

240 = 3.19, P = 0.074).

241 Logistic regressions (GLM) showed a significant effect of SVL for probability of tail

242 breakage in both H. hippocrepis (estimate = 0.036, std. error = 0.005, z = 7.429, P <

243 0.0001, n = 1596) and Z. scalaris (estimate = 0.088; std. error = 0.007; z = 13.073; P <

244 0.0001, n = 1529) (Fig. 1). However, no significant effect of sex was found for

245 probability of tail breakage in either of the two species (estimate = 0.162; std. error =

10 © The Author(s) or their Institution(s) Page 11 of 34 Canadian Journal of Zoology

246 0.180; z = 0.902; P = 0.367 for H. hippocrepis; estimate = 0.236; std. error = 0.170; z =

247 1.386; P = 0.166 for Z. scalaris), which meant that both sexes had the same likelihood

248 of tail breakage in each species. Therefore, the frequency of tail breakage increased with

249 body size in both sexes of the two analyzed species. Total deviance explained by the

250 model was 5.42 % in H. hippocrepis and 19.72 % in Z. scalaris.

251 Regarding dorsal coloration pattern of Z. scalaris, statistically significant differences

252 were found in the frequency of tail breakage between the three patterns analyzed (3 × 2

253 χ2 table, df = 2, χ2 = 150.50, P < 0.0001). A lower frequency was observed in

254 individuals with ladder pattern (small snakes) (3.96 %), followed by snakes with

255 intermediate pattern (intermediate snakes) (10.71 %) and snakes with striped pattern

256 (large snakes) (29,56 %) (Bonferroni post-hoc test: P < 0.001 for all pairwise

257 comparisons; Fig. 2). Draft

258 In terms of severity of tail-loss, no significant differences were found in remaining SBC

259 between sexes in injured individuals of H. hippocrepis (t-test, t = 1.30, P = 0.21).

260 Nevertheless, there was a sex difference in the mean number of SBC for individuals

261 with intact tails of the analyzed population, where males had more SBC than females

262 (M-W U test, W = 28178, P < 0.005). Regarding Z. scalaris, statistically significant

263 differences were not found in remaining SBC between males and females with broken

264 tails (t-test, t = 1.89, P = 0.061), despite the fact that differences were reported between

265 sexes for individuals with intact tails (M-W U test, W = 115000, P < 0.0001). In those

266 individuals, males also had higher number of SBC than females. Mean and range of

267 SBC values are presented in Table S1 and S2 (see supplementary material).

268 The number of remaining SBC did not differ according to SVL in H. hippocrepis

269 (Spearman correlation, rs = 0.16, n = 86, P = 0.13) when sexes were combined. When

270 sexes were analyzed separately, a positive correlation was found in H. hippocrepis

11 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 12 of 34

271 males (Spearman correlation, rs = 0.33, n = 57, P = 0.012) but it was not found in

272 females (Pearson correlation, r = -0.15, n = 28, P = 0.45). A very low positive

273 correlation was found in Z. scalaris (Spearman correlation, rs = 0.23, n = 186, P =

274 0.0018) when sexes were analyzed combined as well as in males when sexes were

275 considered separately (Spearman correlation, rs = 0.27, n = 104, P = 0.0055). However,

276 no correlation was found in Z. scalaris females (Spearman correlation, rs = 0.20, n = 81,

277 P = 0.080).

278 Body condition

279 BC did not differ between whole-tailed and stub-tailed H. hippocrepis males (M-W U

280 test, W = 556.5, P = 0.676, n = 114) and Z. scalaris females (M-W U test, W = 1585.5,

281 P = 0.425, n = 178). On the other hand, whole-tailed H. hippocrepis females (M-W U

282 test, W = 176, P < 0.05, n = 80) andDraft Z. scalaris males (M-W U test, W = 2344.5, P <

283 0.05, n = 194) had statistically significant higher BC than stub-tailed individuals. In

284 stub-tailed individuals no correlations were found between SBC and BC for H.

285 hippocrepis in both males (Pearson correlation: r = -0.33, n = 8, P = 0.43) and females

286 (Spearman correlation: rs = 0.39, n = 6, P = 0.44), as well as for Z. scalaris males

287 (Spearman correlation: rs = -0.13, n = 33, P = 0.47) and females (Spearman correlation:

288 rs = 0.03, n = 16, P = 0.92).

289

290 DISCUSSION

291 Effects of biometric parameters on frequency of tail breakage and SBC

292 Sex, life-stage and body size

293 In the present work, a higher frequency of tail breakage in Z. scalaris than in H.

294 hippocrepis was found (Table 1). Snake feeding patterns influence some life traits such

295 as risk of predation and, therefore, survival rate (Bonnet et al. 1999; Webb et al. 2003).

296 Both Z. scalaris and H. hippocrepis are considered active foragers (Pleguezuelos et al.

12 © The Author(s) or their Institution(s) Page 13 of 34 Canadian Journal of Zoology

297 2007; Feriche 2017) and they are expected to undergo a higher predation risk than those

298 sit-and-wait predators (Webb et al. 2003; Pleguezuelos et al. 2007). The highest

299 frequency of tail breakage among all Iberian colubrids was reported in Z. scalaris

300 meanwhile a lower frequency in H. hippocrepis compared to other snake species from

301 mainland Spain was found (Pleguezuelos et al. 2007, 2018). These differences could

302 derive from the capability of H. hippocrepis to inhabit sloped habitats and move faster

303 than Z. scalaris, which are favorable skills for escaping from predators (Pleguezuelos et

304 al. 2018). Furthermore, while H. hippocrepis is considered mainly diurnal, Z. scalaris

305 exhibits diurnal, crepuscular and nocturnal activity (Cheylan 1986; Pleguezuelos 1998).

306 Being active at night could expose Z. scalaris to additional predators with nocturnal

307 habits such as Felis silvestris catus Schreber, 1775 (Medina et al. 2008) thus increasing

308 its risk of predation attempts. Draft

309 According to some authors, long-tailed species such as H. hippocrepis usually show a

310 higher frequency of tail breakage than short-tailed snakes (Mendelson 1992;

311 Pleguezuelos et al. 2018). In H. hippocrepis tail length represents 22.9 % of the total

312 length of the snake (Feriche 1989) while it only represents 15.5 % of the total length of

313 the snake in Z. scalaris (from Feriche et al. 1993). For this reason, the frequency of tail

314 breakage of H. hippocrepis was expected to be higher than the frequency of Z. scalaris.

315 However, we observed the contrary, as also shown by Pleguezuelos et al. (2010) at the

316 original range of the species. These results could be explained by the activity pattern of

317 the species and its high exposure to predators when foraging (Pleguezuelos et al. 2007,

318 2010).

319 Regarding adults, no differences were found between sexes in H. hippocrepis, while Z.

320 scalaris females presented a higher frequency of tail breakage than males (Table 1).

321 Other authors did not found differences between sexes in both species (Pleguezuelos et

13 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 14 of 34

322 al. 2010, 2018) or in other species (White et al. 1982; Mendelson 1992; Bowen 2004;

323 Aubret et al. 2005; Dourado et al. 2013). Nevertheless, a higher frequency of tail

324 breakage in females has been associated with a greater probability to survive predator

325 attacks in the colubrid Thamnophis sirtalis (Linnaeus 1758) (Placyk and Burghardt

326 2005). It is still unknown if injuries at the base of the tail affecting the copulatory

327 organs of males could be fatal, so why this bias occurs has not been explained (Placyk

328 and Burghardt 2005). This hypothesis could be applied for the data obtained in the

329 present work, although some authors suggested that a firm existence of differences

330 between sexes in predatory pressure has yet to be demonstrated (Pleguezuelos et al.

331 2018).

332 In terms of life-stage, juveniles of both species differed significantly from adults in the

333 frequency of tail breakage (Table Draft 1). It was also observed in the GLM results, since

334 body size of H. hippocrepis and Z. scalaris was related to tail breakage (Fig. 1). A wide

335 number of authors supported a size-related hypothesis in their studies, both in H.

336 hippocrepis and Z. scalaris (Pleguezuelos et al. 2010, 2018) and also in other species

337 (White et al. 1982; Mendelson 1992; Dourado et al. 2013). According to this

338 hypothesis, the likelihood of being attacked increases with age (Pleguezuelos et al.

339 2010, 2018; Dourado et al. 2013), such that larger and, therefore, older snakes have a

340 higher frequency of tail breakage than younger snakes (Mendelson 1992; Seligmann et

341 al. 2008). Otherwise, juveniles have different anti-predatory strategies than adults, such

342 as remaining hidden or cryptic (Pleguezuelos et al. 2010). Small body sizes would not

343 be favorable to survive to predator attacks due to their weakness (Mendelson 1992,

344 Mushinsky and Miller 1993), so they would be removed from the population

345 (Mushinsky and Miller 1993).

14 © The Author(s) or their Institution(s) Page 15 of 34 Canadian Journal of Zoology

346 Gil and Pleguezuelos (2001) observed a prey-size selection of Circaetus gallicus

347 (Gmelin, 1788) upon both analyzed species, preferring prey of intermediate sizes

348 (between 700-1000 mm SVL) but neither very small (under 600 mm SVL) nor very

349 large prey (above 1200 mm SVL). These facts could fit with the results for Z. scalaris

350 since they showed a clearer sigmoid function compared with H. hippocrepis when

351 frequency of tail breakage was plotted against SVL (Fig. 1). It could suggest that the

352 probability of tail breakage increased sharply in the medium sizes, at the inflection point

353 of the curve (see Fig. S1 and Fig. S2 in Supplementary material). Although C. gallicus

354 has only been reported as a migrant species in the Balearic Islands (Table 2) these

355 results could fit with other potential predators found in the Balearic Archipelago (see

356 Table 2). This hypothesis would be supported by the ontogenetic change in dorsal

357 coloration pattern in Z. scalaris, whichDraft is thought to be an anti-predatory strategy and to

358 play a direct role in preventing predator attacks (Pleguezuelos et al. 2010). Small snakes

359 show a ladder pattern and depend on their crypsis to avoid predation, while large snakes

360 rely on their striped pattern to confuse the predator and then flee (Pleguezuelos et al.

361 2010).

362 No differences in remaining SBC were found between stub-tailed males and females for

363 both studied species, a pattern also observed in other Z. scalaris populations

364 (Pleguezuelos et al. 2010) and other snake species (Aubret et al. 2005). According to

365 some authors, stub-tailed males are prone to retain more SBC than females (Dourado et

366 al. 2013) due to the placement of hemipenes and the hemipenial retractor muscle at the

367 base of their tails (Mendelson 1992). In the present work, it should be considered that

368 the minimum value of SBC in H. hippocrepis males was 24 and the minimum in

369 females was 9 (Table S1 in Supplementary material) but an SBC range of 6 – 57 was

370 found in Z. scalaris males (Table S2 in Supplementary material). It could suggest a

15 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 16 of 34

371 harm in their copulatory organs, as it has been considered by Placyk and Burghardt

372 (2005) and Dourado et al. (2013).

373 Some authors found evidence of a correlation between remaining SBC and SVL in

374 long-tailed snakes and suggested several tail breaks over the life of the individuals

375 (Savage and Crother 1989; Slowinksi and Savage 1995). This hypothesis, named as the

376 multiple tail break hypothesis (MTBH) proposed by Mendelson (1992), has not been

377 proved in Z. scalaris and H. hippocrepis (Pleguezuelos et al. 2010, 2018). In the present

378 work, neither of the two species showed a clear correlation between size (SVL) and the

379 number of remaining SBC, supporting the studies carried out by Pleguezuelos et al.

380 (2010, 2018) in the native area of the two species.

381 Body condition

382 Partial tail loss does not seem to causeDraft a significant detrimental effect on locomotion,

383 energy reserves, or feeding efficiency in snakes (Aubret et al. 2005; Pleguezuelos et al.

384 2013, 2018). Several authors did not find a decrease in growth rate nor in feeding

385 success in stub-tailed individuals neither in mainland Spain populations of H.

386 hippocrepis and Z. scalaris (Pleguezuelos et al. 2013, 2018) nor in other snakes as

387 Notechis ater occidentalis (Glauert 1948) (Aubret et al. 2005). Furthermore, BC was

388 found to be unrelated to the severity of tail loss in Z. scalaris, measured by the

389 remaining SBC (Pleguezuelos et al. 2013). The results obtained are in accordance with

390 these facts, since no significant effects of tail breakage and severity of tail loss

391 (remaining SBC) on BC were found in the present work. Only whole-tailed H.

392 hippocrepis females and Z. scalaris males showed higher BC than stub-tailed snakes,

393 and some of these individuals had very short tail stubs (SBC range: 9 – 84; 6 – 57,

394 respectively). According to some authors, the loss of almost the entire tail could

395 decrease locomotion speed (Jayne and Bennet 1989) or damage copulatory organs in

16 © The Author(s) or their Institution(s) Page 17 of 34 Canadian Journal of Zoology

396 some snake species (Mendelson 1991; Dourado et al. 2013). Nevertheless, further

397 studies are needed to determine a possible relationship between tail breakage and

398 subsequent survival costs in the analyzed snakes.

399 Effect of potential predation pressure on tail breakage

400 A total of eight predator species for H. hippocrepis (three reptiles, four birds and one

401 mammal) and a total of 15 species for Z. scalaris have been documented in their native

402 range (two reptiles, eight birds and five mammals). In the islands currently studied, only

403 four potential predators of H. hippocrepis (two reptiles and two birds) are reported in

404 Eivissa, and six for Z. scalaris (one reptile and five birds) in Formentera (Table 2).

405 According to the frequency of predation, birds showed higher values than reptiles and

406 mammals (Table 2). The highest frequency of predation in birds was found in C.

407 gallicus, which is categorized as migrantDraft in Eivissa and Formentera for both colubrid

408 species. The highest percentage among mammals was found in Herpestes ichneumon

409 Linnaeus, 1758, which is not found in the Balearic Islands. However, F. s. catus was

410 also reported as a potential predator for both snake species and it is widespread in

411 Eivissa and Formentera.

412 In H. hippocrepis, adult frequency of tail breakage in mainland Spain was 13.1% (n =

413 328) (Pleguezuelos et al. 2018) while in Eivissa it was 11.64% (n = 1108, Table 1), so

414 the frequency in the new invaded area was slightly lower than in the native range of the

415 species. These results could be attributed to a greater diversity of predators on the

416 mainland than on the studied archipelago, as has also been previously suggested (Placyk

417 and Burghardt 2005). The two predatory reptiles found in Eivissa are alien introduced

418 species (RD1628/2011), consequently little is known about their population density and

419 their ecology in this new invaded island. Among birds, although C. gallicus is

420 considered the most snake specialized predator in mainland Spain (Gil and Pleguezuelos

17 © The Author(s) or their Institution(s) Canadian Journal of Zoology Page 18 of 34

421 2001) it has been only reported in Eivissa as migrant (López-Jurado 2017) (Table 2) and

422 to date there is no evidence that it acts as predator in the Island.

423 Regarding Z. scalaris, adult frequency of tail breakage in mainland Spain was 16.6 % (n

424 = 469) (Pleguezuelos et al. 2010) while in Formentera was 17.45 % (n = 1055, Table 1),

425 a slightly higher increase compared to the mainland. As it occurs in Eivissa, the predator

426 density in Formentera is lower than in the native range of Z. scalaris. Among birds, C.

427 gallicus, which has a predation frequency up to 39.5% on Z. scalaris in its native range

428 (n = 97) (Gil and Pleguezuelos 2001), is considered a migratory species (López-Jurado

429 2017). Nevertheless, such difference found between the two islands could be that, while

430 H. hippocrepis is widely spread in many regions of Eivissa, Z. scalaris is highly

431 condensed in a small area of Formentera named La Mola. Several potential predatory

432 birds breed in this area (Jaume andDraft Wijk 1996), such as Tyto alba (Scopoli, 1769) and

433 Asio otus (Linnaeus, 1758), nocturnal predators (Pavey et al. 2008) that have been

434 reported feeding on snakes (Gehlbach and Baldridge 1987; Yom-Tov and Wool 1997).

435 In addition, in Formentera stable populations of feral cats occur, which are known to

436 prey on snakes (Kelehear and Graham 2015). However, the slight differences found in

437 frequency of tail breakage in Eivissa and Formentera compared to the mainland could

438 also be caused by a wide range of variables, such as snake abundance, predator

439 abundance, sampling areas or years sampled, among others.

440 As no studies of these colubrid snakes have been conducted so far in insular

441 ecosystems, the results obtained reveal essential information about the ecological

442 aspects of these two introduced populations in the Balearic Islands. Results supported a

443 size-related hypothesis (White et al. 1982; Mendelson 1992; Pleguezuelos et al. 2010,

444 2018; Dourado et al. 2013) and injures mainly did not have a fitness cost for the snakes,

445 since no evidence of a decreased body condition was found. We used tail breakage as an

18 © The Author(s) or their Institution(s) Page 19 of 34 Canadian Journal of Zoology

446 indirect evidence of predation pressure and, apparently, it is related to potential

447 predators inhabiting the islands instead of, for example, intraspecific competition.

448 Furthermore, the enemy release hypothesis should also be considered in this

449 introduction event, so additional studies on the composition of the predators' diet would

450 be of great importance to interpret the obtained results. Although further research is

451 needed, this work shows for the first time the importance of assessing the traits of these

452 introduced populations in areas previously free of snakes. The knowledge of the main

453 ecological aspects of these populations may allow to promote future control actions and

454 to analyze the impact of these alien species on native biodiversity of islands ecosystems.

455

456 ACKNOWLEDGEMENTS

457 This research was done as part ofDraft a M.F.S. thesis at the University of the Balearic

458 Islands. The present work was carried out under an FPU contract with the Ministry of

459 Science, Innovation and Universities from the Spanish Government (FPU17/03484).

460 N.L. has a contract under the frame of Biodibal project (UIB-REE). This study has been

461 partially funded by the AAEE002/17 Acció Especial de Recerca del Govern de les Illes

462 Balears and by the Institute of Health Carlos III (CIBEROBN CB12/03/30038).

463

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635

636 LEGISLATION

637 Real Decreto 1628/2011, de 14 de noviembre, por el que se regula el listado y catálogo

638 español de especies exóticas invasoras. «BOE» núm. 298, de 12 de diciembre de

639 2011. Reference: BOE-A-2011-19398.

640 Real Decreto 630/2013, de 2 de agosto, por el que se regula el Catálogo español de

641 especies exóticas invasoras. «BOE» núm. 185, de 3 de agosto de 2013.

642 Reference: BOE-A-2013-8565.

Draft

644

645 Tables and figures

646 Table 1. Chi-square contingency table for comparison of the frequency of tail breakage

647 among adult males, adult females and juveniles of horseshoe whip snake Hemorrhois

648 hippocrepis and ladder snake Zamenis scalaris in Eivissa and Formentera islands,

649 respectively. Number of individuals with and without tail breakage (TB and non-TB,

650 respectively), total of individuals analyzed, and frequency of tail breakage (FTB) are

651 reported.

Species Sex Nº TB Nº non-TB Total FTB (%)

H. hippocrepis Juveniles 31 465 496 6.25

Adult males Draft97 753 850 11.41

Adult females 32 226 258 12.40

Total adults 129 979 1108 11.64

TOTAL 160 1444 1604 9.98

Z. scalaris Juveniles 16 458 474 3.38

Adult males 109 633 742 14.69

Adult females 76 240 316 24.05

Total adults 185 873 1058 17.45

TOTAL 201 1331 1532 13.12

652

654

655 Table 2. List of mainland potential predators of horseshoe whip snake Hemorrhois

656 hippocrepis and ladder snake Zamenis scalaris in the Iberian Peninsula (mainland

657 Spain). The presence of these predators at Eivissa and/or Formentera (Balearic Islands,

658 Spain) and their category are reported. Mainland potential snake preys, predation

659 frequency upon each snake species in their native range and references are also reported

660 (see references in Feriche, 2017 and Pleguezuelos, 2019). Predation frequency (%)

661 indicates what percentage of the predator’s diet is represented by each snake prey.

Mainland potential Occurrence at the Mainland potential References

predators Balearic Islands preys and frequency

(%)

DraftREPTILES

H. hippocrepis Eivissa (I) H. hippocrepis Márquez, 1987

M. monspesulanus Eivissa (I) Z. scalaris (0.5) Pleguezuelos, 1998;

H. hippocrepis Díaz-Paniagua, 1976

Timon lepidus Absent H. hippocrepis Pleguezuelos, 1997

BIRDS

Aquila adalberti Absent Z. scalaris (0.6) Deibes, 1978

H. hippocrepis

Aquila chrysaetos Absent Z. scalaris (5.2) Blanco et al., 2017;

H. hippocrepis

Bubo bubo Absent Z. scalaris (1.3) Antón et al., 2008

Buteo buteo Eivissa (H,R; M,S) Z. scalaris (0.7-1.8) Garzón, 1974; Zuberogoitia

Formentera (H,R; M,S) H. hippocrepis et al., 2006

Circaetus gallicus Eivissa (M,S) Z. scalaris (10.6-39.5) Amores and Franco, 1981;

Formentera (M,S) H. hippocrepis (8.7) Gil and Pleguezuelos,

2001.

Milvus migrans Eivissa (M,S) Z. scalaris (0.6) Fernández-Cruz, 1973

Formentera (M,R)

Milvus milvus Eivissa (M,S) Z. scalaris (0.13-0.3) Delibes and García, 1984

Formentera (M,S)

Neophron Eivissa (A) Z. scalaris (0.7) Donázar and Ceballos,

percnopterus 1988

MAMMALS Felis silvestris Eivissa and FormenteraDraft Z. scalaris (0.2) Moleón and Gil, 2003; (F.s.catus; I) H. hippocrepis Feriche, 2004

Herpestes Absent Z. scalaris (0.5-2.6) Palomares and Delibes,

ichneumon 1991a,b

Lynx pardina Absent Z. scalaris (0.1) Delibes, 1980

Martes foina Eivissa (extinct) Z. scalaris (0.5) Gil-Sánchez, 1996

Vulpes vulpes Absent Z. scalaris Díaz-Ruíz et al., 2013

662

663 Note: Categories for reptiles are I: introduced species, according to the Spanish

664 Catalogue of Invasive Exotic Species (RD1628/2011). Categories for birds are A:

665 accidental (far from its normal range); H: hibernator (only present during winter) and

666 M: migrant (only present in pre- and post-nuptial migrations). Population quantification

667 is expressed as R: rare (1-10 individuals) and S: scarce (11-100 individuals) according

668 to López-Jurado (2017, and references therein). Categories for mammals are I: invasive

669 species (Lowe et al., 2000).

Draft

671

672 Figure 1. Logistic regressions of probability of tail breakage in horseshoe whip snake

673 Hemorrhois hippocrepis (A) and ladder snake Zamenis scalaris (B) as a function of

674 SVL (cm) and sex.

675

676 Figure 2. Mosaic plot for comparison of the frequency of tail breakage among the three

677 dorsal coloration patterns defined for ladder snake Zamenis scalaris specimens. The

678 standardized (Pearson) residuals from independence are associated to colors and percent

679 values to sizes of the mosaic plot.

Draft

Figure 1. Logistic regressions of probability of tail breakage in horseshoe whip snake Hemorrhois hippocrepis (A) and ladder snake Zamenis scalaris (B) as a function of SVL (cm) and sex.

322x375mm (300 x 300 DPI)

Figure 2. Mosaic plot for comparison of the frequency of tail breakage among the three dorsal coloration patterns defined for ladder snake Zamenis scalaris specimens. The standardized (Pearson) residuals from independence are associated to colorsDraft and percent values to sizes of the mosaic plot.

677x381mm (300 x 300 DPI)