Corresponding author:
dr Valentina Botto
Dipartimento di Biologia Animale e dell’Uomo, Università degli Studi di Torino Via Accademia Albertina, 13 – 10123, Torino (Italia)
Phone number: 011 6704582 Email: valentina.botto@unito.it
Date of submission: 02/09/2014 Number of pages (main text): 10 Figure legend: page 11
Spatial distribution of male Italian tree frogs (Hyla intermedia) at the breeding site
VALENTINA BOTTO1, CHIARA VITILLO1, MARTA DE FEO1 & SERGIO CASTELLANO1
1 Dipartimento di Biologia Animale e dell’Uomo, Università degli Studi di Torino
Riassunto. Il presente studio descrive l’uso dello spazio da parte dei maschi di una piccola popolazione di raganella italiana (Hyla intermedia). Per tutta la durata della stagione riproduttiva 2013, le posizioni dei maschi al sito riproduttivo (lek) sono state registrate su apposite schede di campo; in seguito i dati sono stati digitalizzati e analizzati. Per comprendere i meccanismi che stanno alla base del pattern di distribuzione osservato, le analisi sono state effettuate a 3 livelli (livello di popolazione, di singola serata e di singolo individuo).
A livello di popolazione, ne è emersa una distribuzione spaziale dei maschi non omogenea, con chiare preferenze per alcune zone del sito riproduttivo. Ad ulteriore conferma di tale pattern, un test di randomizzazione ha evidenziato che, a livello di singola serata, i maschi tendono ad aggregarsi in diversi centri di esibizione all’interno del sito. Infine, l’analisi grafica dell’utilizzo dello spazio a livello individuale ha evidenziato 3 diverse modalità di uso dello spazio: sito-fedeltà (6 maschi su 18), uso casuale dello spazio (8 maschi) e comportamento intermedio tra i due precedenti (4 maschi su 18).
Questi dati suggeriscono che l’utilizzo dello spazio da parte dei maschi di raganella italiana non sia sempre contesto-dipendente. In particolare, noi ipotizziamo che i maschi sito-fedeli scelgano dove posizionarsi indipendentemente dalla posizione degli altri maschi presenti al sito e che possano fungere da centri di attrazione per altri maschi, i quali cambierebbero la loro posizione opportunisticamente di notte in notte, per posizionarsi nelle zone con attività canora più intensa.
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Abstract. This paper describes the use of space by male Italian tree frogs (Hyla
intermedia) at a small lek. For the entire breeding season 2013, we recorded the
positions of lekking males on dedicated data-sheets; then, the collected data were digitized and analyzed. In order to provide insights into the mechanisms responsible for the observed patterns of distribution, we analyzed the data at three levels (population level, single night level and individual level).
The analysis at the population level showed a non-homogeneous distribution of males within the breeding site. Consistently, a randomization test showed that, at the single night level, males tended to aggregate in several exhibition centers within the site. Finally, the graphical analysis at the individual level showed 3 different patterns of use of space: site-fidelity (6 males out of 18), random use of space (8 males out of 18) and intermediate behavior between the two (4 males out of 18).
These data suggest that the use of space by male tree frogs is not always context-dependent. In particular, we hypothesize that site-faithful males choose their calling location independent of the social context and that they may act as attractors for those ‘site-unfaithful’ males that change their position opportunistically from night to night, in order to place themselves in the areas of highest calling activity.
Keywords. Hyla intermedia, spatial distribution, site-fidelity, lek 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
INTRODUCTION
In many Hylid species, during the breeding season males call from the vegetation around the ponds, maintaining a reciprocal distance of about 50-200 cm (Fellers, 1979), which seems to minimize reciprocal interferences and maximize female capability to locate males. In addition, the individual use of space is quite variable, with some males showing a strong site-fidelity and others often changing their calling position (Rosen and Lemon, 1974).
In the present study we analyze the spatial distribution of male Italian tree frogs (Hyla
intermedia) at a small lek. To provide insights into the mechanisms responsible for the
observed patterns of distribution, we conduct the analysis at three levels. At the population level, we consider all male locations recorded during the entire breeding season and test for the null hypothesis of uniform distribution. At the nightly level, we focus on the pattern of male aggregation: we measure the mean and the standard deviation of between-male distances and use a randomization test to verify whether males distribute either randomly, uniformly or aggregately at the breeding site. Finally, at the individual level, we analyze the use of space of each male throughout the breeding season, to verify his level of site-fidelity.
MATERIAL AND METHODS
Study site and field methods
The study took place in the spring-summer 2013 in Cascina Valletto (Roppolo, BI, NW Italy). The breeding site was a group of five ground pools measuring 100x100x30 cm, used for several years as a reproductive site by a small population of H. intermedia. 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
We monitored the breeding site for the whole breeding season, from April 22nd to June 27th. All the males visiting the site were collected and kept overnight in individual fauna-boxes. On the following morning, they were anesthetized (MS222 Sandoz) and weighted to the nearest 0.1 g (Pro-Weight MF 250); their SVL and WGRASP (distance between the elbows) were measured to the nearest 0.01 mm with a digital caliper (Mitutoyo Absolute, Model CD-15DCX). Finally, they were marked for individual recognition with subcutaneous alphanumerical tags (North Western Marine Technology Inc.) injected in the tarsal area on the left hind leg.
Every night we visited the breeding site regularly (every 45-60 minutes) and assessed the identity of males; we also recorded the exact position and behavior of each male on a dedicated data sheet.
Data analysis and statistics
The data sheets were digitalized and processed with the free software ImageJ. For each datasheet, we calculated the Cartesian coordinates of the positions of each male within the breeding site. When a male was captured more than once during the same night, we retained only the position of the first capture.
For the analysis at the population level, we divided each pool into four 50-cm squares, each indicated by a numerical code (from 1 to 20). For each square we counted the number of males and then we performed a Generalized Linear Model with Poisson error distribution and logarithm link function (dependent variable: number of captures per square; independent variables: male identity and square code). This approach allowed us to test possible differences in the use of space but also to control the effect of individual
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identity. The same analysis was performed using, as spatial units, the 5 pools instead of the 20 squares.
For every night of activity we calculated the average distance between individuals and the relative standard deviation. Then we calculated the seasonal average distance (2.03 m) and its standard deviation (0.91 m), that were used as reference values for the randomization test. Afterwards, we performed a randomization test where all positions recorded during the breeding season were randomly redistributed among the evenings, keeping the number of observations per night unchanged. Randomization was repeated 10,000 times; after each random assignment of positions, the test calculated nightly average distances. The probability of the null hypothesis of random distribution of males was then evaluated as the relative frequency of randomizations with mean and standard deviation lower than those actually observed.
Finally, we evaluated the use of space at the individual level, using a graphic approach rather than statistical analyses. We considered only males captured at least 4 times, and, for each of them, we plotted all the positions occupied during the breeding season on a single data sheet representing the breeding site.
RESULTS
Population level
During the breeding season, we recorded 191 male locations. The analysis showed that
males differ in the use of space both when we considered the 20 pools (χ2 = 126.595,
d.f. = 36, p < 0.001) and when we considered the 5 pools (χ2 = 126.076, d.f. = 36, p <
0.001). However, independent of male identity, the observed spatial distribution in the
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breeding site significantly differed from the one expected under the assumption of
uniform distribution both when we considered the 20 squares (χ2 = 49.584, d.f. = 19, p <
0.001) and when we considered the 5 pools (χ2 = 29.409; d.f. = 4; p < 0,001), with a
clear preference for the 2nd and the 5th pools of the breeding site (Fig. 1).
Single night level
After 10,000 permutations, the average distance between males at the breeding site was not significantly different from the one expected under the assumption of a random distribution (p = 0.0723). The observed values of standard deviation, instead, were significantly higher than those expected from a random distribution (p = 0.033). These results suggest that males tend to aggregate around multiple attraction centers within the breeding site.
Individual level
The use of space at the individual level was studied in 18 males captured at least 4 times. In confirmation of the individual differences in the use of space showed by the
population level analysis, we identified 3 different patterns (see Fig. 2). (i) Strong
site-fidelity: males tend to occupy close locations for several consecutive nights: for example, the specimen A13 (Fig. 2a) showed a strong site-fidelity to the 2nd pool at the beginning of season, while at the end of the season it was observed exclusively at the 5th pool. Overall, 6 out of the 18 males showed site-fidelity. (ii) Random use of space: for example, the specimen A19 (Fig. 2b) moved within the breeding site, without any clear pattern. In the population studied, 8 males showed a similar behavior. (iii) Intermediate behavior: for example, the specimen A50 (Fig. 2c) showed a moderate site-fidelity to 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129
the 2nd pool at the beginning of season and to the 5th pool at the end of the season, but it also performed from other areas of the breeding site. Overall, 4 males showed such behavior.
DISCUSSION
The population level study showed a non-uniform distribution of males at the site, with clear preferences for the 2nd and 5th pools. We exclude that this distribution is due to ecological conditions, since the ponds did not differ in either the amount or the type of aquatic vegetation, in the degree of shading or in water temperature. This kind of spatial distribution, with males aggregating into restricted areas of the lek, is common in some Avian species, such as Manacus manacus trinitatis (Snow, 1956) and Gallinago media (Höglund and Lundberg, 1987; Höglund and Robertson, 1990), in which males seem to prefer the central areas of the lek. In our study, however, the central pond was not the favorite one; on the contrary, it was the least used.
The nightly level analysis gave further insight into the mechanisms that underlie such spatial distribution. If male spatial choice was context independent (that is, if it was independent of the choice made by all the other males at the breeding site in a given night) then the randomization test should have provided no statistically significant result. Alternatively, if males tended to distance themselves from the others, then the observed between-male mean distances should have been higher and their standard deviation lower than those obtained by the random permutation of positions. Instead, if males tended to aggregate around a single attractor, then both the observed mean distances and their standard deviations should have been lower than those of the 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151
randomly permutated positions. We observed none of these three patterns. In fact, the permutation test showed that the observed mean distances were slightly (though not significantly) higher and their standard deviations significantly higher than those obtained by random permutations. We interpreted these results as evidence for a context-dependent spatial choice. The tendency of males to aggregate seems to prevail to the tendency to maintain reciprocal distance. However, aggregation is not around a single attractor, but around two or more attractors. We hypothesize that such attractors may be the most attractive (highly performing) males and that low-performing males may choose to move close to them in order to increase their probability of being perceived, assessed and eventually chosen by a female. In accordance with this hypothesis, the analysis at the individual level showed that whether some males are highly site-faithful, others are much less so and choose different locations in successive nights. According to our hypothesis, faithful males do not make context-dependent choice: they may be those acting as attractors for the low-performing males, who opportunistically change their positions from night to night.
ACKNOWLEDGEMENTS
We thank Alessandro Caputo, Simone Betto, Matteo Baldi and Marta Gea for their help during the field activity. The “Regione Piemonte – Direzione Ambiente – Settore Sostenibilità e recupero ambientale, bonifiche” provided the authorization for tree frog handling and capture (permits n° 152 and 153 issued on February 25th, 2013)
REFERENCES 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172
Höglund, J., Lundberg, A. (1987): Sexual selection in a monomorphic lek-breeding bird: Correlates of male mating success in the great snipe Gallinago media. Behav Ecol Sociobiol. 21: 211-216.
Höglund, J., Robertson, J. G. M (1990): Spacing of leks in relation to female home ranges, habitat requirements and male attractiveness in the great snipe (Gallinago media). Behav Ecol Sociobiol. 26: 173-180.
Fellers G. M. (1979): Aggression, territoriality and mating behavior in North American treefrogs. Anim Behav. 27: 107-119.
Rosen, M., Lemon, R. E. (1974): The vocal behavior of spring peepers, Hyla crucifer. Copeia. 1974: 940-950.
Snow, D. W. (1956): Courtship ritual: The dance of the manakins. Animal Kingdom, 59: 86-91. 173 174 175 176 177 178 179 180 181 182 183
FIGURE LEGEND
Fig. 1 – Spatial distribution of males throughout the breeding season, in each of the 20 squares of the breeding site. The shade of grey in each square indicates the concentration of males within the square (darker shades indicate a higher number of males)
Fig. 2 – Among male differences in the use of space, as exemplified by the individuals A13 (2a), A19 (2b) and A50 (2c). The grey tonality corresponds to different dates within the breeding season (light shades indicate the beginning of the breeding season, dark shades indicate the end of the breeding season).
