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): 9 Figure legend: page 10
Number of figures: 2 (pages 11-12) 1 2 3 4 5 6 7 8 9 10 11 12
Preliminary observations on male reproductive behavior in a population of Italian tree frog (Hyla intermedia) in central Piedmont
VALENTINA BOTTO1 & SERGIO CASTELLANO1
1 Dipartimento di Biologia Animale e dell’Uomo, Università degli Studi di Torino
Via Accademia Albertina, 13 – 10123, Torino (Italia) 13 14 15 16 17 18 19
Riassunto. In molte specie con sistema riproduttivo a lek, il successo riproduttivo dipende sia dalla qualità degli ornamenti, sia dal tempo trascorso al sito riproduttivo (attendance): i maschi che trascorrono più notti al sito si accoppiano più spesso. Il presente studio è stato svolto su una piccola popolazione di raganella italiana (Hyla intermedia), un anfibio anuro con sistema riproduttivo a lek, e ha analizzato a) i fattori biotici e abiotici che influiscono sul numero di maschi presenti al lek in una determinata sera, e b) il successo riproduttivo maschile e i fattori che lo influenzano.
Durante la stagione riproduttiva 2013, i maschi sono stati marcati individualmente e registrati. Per ogni sera è stato effettuato un conteggio dei maschi presenti al sito riproduttivo e sono stati misurati diversi parametri ambientali (temperatura di aria e acqua, umidità atmosferica, velocità del vento e quantità di precipitazioni cumulative giornaliere). Inoltre, i maschi sono stati pesati e misurati e per ognuno è stato calcolato l’indice di condizione, il numero di notti spese al sito riproduttivo e il numero di accoppiamenti ottenuti.
Il numero di maschi presenti al sito riproduttivo è risultato variabile nel corso della stagione, con 4 picchi di attività. Un GLM ha evidenziato che il numero di maschi presenti al sito riproduttivo in una data serata è influenzato sia da fattori abiotici (velocità del vento, precipitazioni) che da fattori biotici (numero di maschi presenti al sito nel corso della serata precedente).
Il successo riproduttivo maschile non è risultato casuale e le analisi statistiche hanno dimostrato che esso è influenzato dal peso e dall’attendance. I risultati del presente lavoro confermano quindi che in Hyla intermedia (come in molti altri Ilidi) l’endurance rivalry è un’importante forma di competizione tra maschi e ha un ruolo importante nel determinare il successo riproduttivo maschile.
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
Abstract. In many lek-breeding species, male reproductive success depends both on the quality of the ornaments and on the time spent at the breeding site (attendance): males who spend more nights at the site mate more often. This study focuses on a small population of Italian tree frog (Hyla intermedia), a lek breeding Anuran, and investigates: i) the biotic and abiotic factors that affect the number of males at the lek in a given night, and ii) male mating success and the factors that influence it.
During the 2013 breeding season, male tree frogs have been individually marked and recorded. For each evening we counted the males at the site and measured several environmental parameters (water temperature, air temperature, relative humidity, wind velocity and daily total rainfall). In addition, we weighed and measured males, calculated their condition index and counted the number of nights spent at the breeding site and the number of matings obtained.
The number of males at the breeding site showed four peaks during the breeding season and was influenced by both abiotic (wind velocity, rainfall) and biotic factors (number of males at the site during the previous evening).
Male reproductive success was not random and was positively influenced by body weight and attendance. The results of this study thus confirm that in Hyla intermedia (as in many other Hylids) endurance rivalry is an important form of competition between males and plays an important role in determining male reproductive success.
Keywords. Hyla intermedia; attendance; reproductive success; sexual selection. 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64
INTRODUCTION
Lek-breeding species represent an ideal model for the study of inter-sexual selection. In these species, male reproductive success is highly variable: a few individuals monopolize the majority of the matings, while many males never mate. In lekking species, males often bear conspicuous ornaments that may play a role in female choice; however, in some cases male reproductive success may also be influenced by factors other than phenotypic appearance, such as attendance or behavior at the lek (Andersson, 1994).
In many species of lekking anurans, male competition does not involve physical aggression, but rather consists in the emission of vocalizations (advertisement calls); in these cases, male reproductive success will depend on the acoustic parameters of the calls. Another common form of competition among lekking amphibians is endurance rivalry: in many species, indeed, the reproductive success of a male is directly correlated with the number of nights spent at the breeding site.
The Italian tree frog (Hyla intermedia) represents an ideal model for the study of these kinds of competition. During the breeding season, male tree frogs aggregate into restricted areas and perform their advertisement calls, forming noisy choruses that attract females, which in turn choose their partner mainly on the basis of their call properties. The breeding season lasts more than one month, and during this time males spend several days at the breeding site. Male attendance is likely to be influenced by both meteorological conditions and by individual energy reserves. Leks are therefore a perfect situation to study endurance rivalry.
This paper describes the environmental factors that influence the presence of males in a small breeding site in central Piedmont; in addition, it presents preliminary results on the distribution of male mating success and on the factors that affect it. The work represents a preliminary contribution of an ongoing project that aims at investigating how abiotic and 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88
biotic factors interact with each other in affecting both the type and the strength of male competition and the reliability of males’ sexual signals.
MATERIAL AND METHODS Study area and field activities
The study took place in the spring-summer 2013 in Cascina Valletto, in the municipality of Roppolo (province of Biella, Piedmont). The study site was represented by a group of five ground pools measuring 100x100x30 cm, used for several years as a reproductive site by a small population of H. intermedia. Male tree frogs were recorded every night from April 22nd
2013 to June 27th 2013 (67 nights) in order to study how temperature affects the expression
and reliability of advertisement calls (data not shown in the present paper). At the beginning
and end of each recording we measured air temperature, atmospheric humidity and water temperature.
During the recording sessions, the site was visited regularly and the position and behavior of all tree frogs were recorded. The internal (cloacal) temperature of each calling male was measured with an Appa 51 thermometer. Individuals of both sexes were also checked for their individual marks (see below), in order to assess their identity.
Unmarked tree frogs 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 with subcutaneous alphanumerical tags (North Western Marine Technology Inc.) injected in the tarsal area on the left hind leg. The third finger of the right hind leg was clipped in order to perform a skeletochronological study (data not shown in the present report).
89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112
Statistical analyses
To investigate which factors affected male breeding activity, we built a Generalized Linear Model with Poisson error distribution and logarithm link function, in which the number of males at the breeding site (dependent variable) was regressed against five meteorological variables and an autocorrelation term. The meteorological variables were: (i) air temperature, (ii) water temperature and (iii) relative humidity, which were measured at the beginning of each recording session, (iv) daily mean wind velocity and (v) daily total rainfall, which were downloaded from the database of ARPA Piemonte (weather station of Piverone, about 5 km from the study site). Since we observed a highly significant first-order temporal autocorrelation (Durbin-Watson Test: auto-correlation=0.354, D-W statistics=1.267, p=0.004), we included in the model also the number of males at the site in the previous night. A model selection procedure was then used to select the most parsimonious model.
To investigate the factors affecting male mating success, first we compared the observed distribution of mating success with that expected under the assumption of random mating (Poisson distribution), then we carried out a Generalized Linear Model with Poisson error distribution and logarithm link function. The full model included as independent variables SVL, body-condition index (the residuals of the logarithm of weight regressed against the logarithm of SVL), weight, and attendance (number of nights spent at the chorus).
All statistical analyzes were performed with R 2.15.0.
RESULTS Number of males at the breeding site
The breeding season lasted 67 nights (from April 22nd to June 27th); during this period 37
males were observed at the chorus. On 42 nights (62.7%), at least one male attended the lek. The number of males at the breeding site was irregular throughout the season, and exhibited 4 peaks of activity (Fig. 1). The GLM analysis showed that the number of males was 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137
significantly affected by wind velocity (B = 0.333, t = 2.528, p = 0.0115), by rainfall (B = 0.0388, t = 7.134, p < 0.001) and by the number of males at the breeding site on the previous night (B = 0.087, t = 5,954 , p < 0.001), whereas water and air temperature did not show any significant effect.
Male reproductive success
During the breeding season, we observed 32 matings. Only 17 males mated; 7 of them mated once, 6 mated twice, and only 4 were mated 3 times or more (Fig. 2). Twenty males (54%) never mated. The Chi-square test indicates that male reproductive success was not randomly distributed among males (χ2 = 17.162, df = 3, p < 0.001). The GLM showed a significant
effect of both males’ attendance (B = 0.120, t = 3.602, p < 0.001) and weight (B = 0.548, t = 3.254 , p < 0.005) on male reproductive success.
DISCUSSION
The data show that male reproductive activity is influenced by both abiotic factors (wind velocity and rainfall) and biotic factors (number of males attending the site on the previous night). Similar results were found in another population of H. intermedia in central Piedmont (Cannarsa, 2010). The present study also confirms that male reproductive success depends on attendance. The relationship between reproductive success and attendance is common in many species with a lek breeding system, and has been highlighted in several species of Hylids, including H. arborea (Friedl and Klump, 2005), H. chrysoscelis (Godwin and Roble, 1983) and H. versicolor (Bertram et al., 1996).
Despite the importance of attendance in determining reproductive success, many males remain at the breeding site for a limited number of nights. In Anurans, calling activity is metabolically very expensive (Godwin and Roble, 1983; Murphy, 1994; Grafe and Thein, 2001) and such costs can seriously limit the number of nights that a male can spend calling. 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161
Therefore, males that spend more nights at the breeding site are likely to have better ability to recover energy, and therefore to be superior to their competitors.
In the present work, we do not consider the role of calling on male mating success. However, a previous study on an Italian tree frog population showed that, independent of the number of nights spent at the breeding site, males calling at higher call rates were more likely to mate than those calling at lower call rates (Castellano et al., 2009). The same study has also showed a positive correlation between call rate and lek attendance, providing no evidence for a population tradeoff between these two important determinants of male mating success.
ACKNOWLEDGEMENTS
We thank Alessandro Caputo, Simone Betto, Matteo Baldi, Marta Gea and Chiara Vitillo 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)
162 163 164 165 166 167 168 169 170 171 172 173 174
REFERENCES
Andersson M. (1994): Sexual selection. Princeton University Press, Princeton, New Jersey.
Bertram, S., Berrill, M., Nol, E. (1996): Male mating success and variation in chorus attendance
within and among breeding seasons in the gray treefrog (Hyla versicolor). Copeia, 729-734.
Cannarsa E. (2010): Comportamento e successo riproduttivo nelle femmine di raganella italiana (Hyla
intermedia). Unpublished master’s thesis Dissertation, University of Turin, Italy.
Castellano, S., Zanollo, V., Marconi, V., Berto, G. (2009): The mechanisms of sexual selection in a
lek-breeding anuran, Hyla intermedia. Anim Behav. 77: 213-224.
Friedl, T.W.P., Klump, G.M. (2005): Sexual selection in the lek-breeding European treefrog: body
size, chorus attendance, random mating and good genes. Anim Behav. 70: 1141-1154.
Godwin, G.J., Roble S.M. (1983): Mating success in male treefrogs, Hyla chrysoscelis (Anura:
Hylidae). Herpetologica. 39: 141-146.
Grafe, T.U., Thein, J. (2001): Energetics of calling and metabolic substrate use during prolonged
exercise in the European treefrog Hyla arborea. J Comp Physiol B. 171: 69-76.
Murphy, C.G. (1994): Chorus tenure of male barking treefrogs, Hyla gratiosa. Anim Behav. 48:
763-777. 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190
FIGURE LEGEND
Fig. 1 - Number of males attending the chorus on each day of the breeding season. Arrows show the four main peaks of activity.
Fig. 2 - Distribution of mating success in the males of the study population. The majority of males did not mate (n=20), while only a minority of them mated 3 or more times (n=4).
191 192 193 194 195
FIG. 1: 196 197
FIG. 2: 199 200
