Full Description

"Male signals are frequently studied in a single behavioral context, but in some cases they may assist multiple functions, namely for both male– male competition and female mate choice. Boatwhistles are known as the mate attraction calls of toadfishes typically produced during the breeding season. However, recent observations with the Lusitanian toadfish Halobatrachus didactylus (Batrachoididae) indicate that the emission of boatwhistles is not restricted to this period, which suggests a function in other behavioral contexts such as agonistic territorial interactions."

"Resident males defended their shelters producing sounds, mostly boatwhistles, towards intruders. Parental males revealed higher aggression levels, exhibiting additional threatening and attack behaviors. Boatwhistles registered during agonistic events were compared with the mate advertising boatwhistles recorded from small aggregations of nesting males in a natural breeding intertidal area. Agonistic boatwhistles were produced in lower and variable calling rates comparing with the advertising ones that were typically emitted in long series of calls. Agonistic boatwhistles were similar in duration and frequency harmonic structure (with a middle tonal phase) to the advertising calls, but presented less amplitude modulation, and lower dominant and fundamental frequencies. These acoustic differences were probably related to differences in calling rates and broadcast demands associated to the distance to the intended receiver. We provide first evidence that, apart from attracting mates, the toadfish boatwhistles also function as active ‘keep-out’ signals during territorial defense."

"Congruently, the Lusitanian toadfish Halobatrachus didactylus (Bloch and Schneider 1801) produces these distinct calls, boatwhistle and grunt train, most likely associated with mating activities and agonistic interactions (dos Santos et al. 2000; Amorim et al. 2006; Vasconcelos & Ladich 2008). Recent observations, however, indicate that boatwhistling is not restricted to the mating season in this species, since it has been detected all year round when water temperature remained higher than 19C (Amorim et al. unpublished data). This suggests that, besides attracting gravid females to the male’s nest, the boatwhistle may have other functions such as territorial defense."

"Only the type I males establish nests under rocks in shallow waters during the breeding season and are able to emit the advertisement tonal call (boatwhistle) to attract females at distance (Amorim & Vasconcelos 2008). The Lusitanian toadfish exhibits an unusual large acoustic repertoire composed of four commonly produced sounds: boatwhistle, grunt train, long grunt train and double croak, and other less frequent sound emissions such as croak and mixed croak–grunt calls (dos Santos et al. 2000; Amorim et al. 2008)."

"During territorial intrusions, only resident males produced sounds typically inside or at the entrance of their nests."

"The resident males always responded towards the intruders by producing sounds (93% of the trials) and ⁄ or exhibiting visual displays (53%) (see Table 1). Vocal activity, only detected in residents, varied between 1 and 47 sounds per specimen and mainly included boatwhistles (92% of the calls), but also grunt trains (7%) and other less frequent vocalizations (1%). A total of 301 sounds were recorded from all males. Some of the boatwhistles (10 from a total of 278 sounds, detected in four out of the 15 trials) were followed by a grunt train resulting in a mixed call."

"The boatwhistles produced by territorial males during confrontation with intruders (AB) were similar in duration (U test: U = 53, nAB = 12, nRB = 13, p > 0.05) and presented a harmonic structure with a middle tonal phase identical to those emitted by males to attract females to their nests (RB) (Figs 2 and 3a). These sounds differed considerably from the other vocalizations of the species repertoire. However, we found significant differences between ABs and RBs, recorded at the same distance from the nest entrance (see Section ‘Methods’), in several other acoustical parameters, namely ABs revealed less amplitude modulation, that is, amp P1 ⁄ amp P2 1 (U test: U = 6, nAB = 12, nRB = 13, p < 0.001, Fig. 3b). The dominant (DF) and fundamental (FF) frequencies were significantly lower in the AB (U test: DF, U = 11, nAB = 12, nRB = 13, p < 0.001; FF, U = 31, nAB = 12, nRB = 13, p = 0.010, Fig. 3c, d). In the ABs, the spectral energy was almost evenly distributed within the first three harmonics but coincided predominantly with the fundamental frequency; whereas, in the RB the energy peaked mostly at the second harmonic (Fig. 2). In six of 12 fish (50%), the dominant frequency of ABs varied within the same individual between the first and the second harmonics. On the contrary, the dominant frequency of RBs was detected in the first, second or fourth harmonics in seven of 13 fish (54%). Most of ABs were emitted singly, but infrequently we observed ABs produced in series of up to 15 boatwhistles. A series of 15 ABs was detected only once."

"Agonistic boatwhistles presented less amplitude modulation and generally had lower dominant and fundamental frequencies. This last parameter indicates that the agonistic calls are produced at slower muscle contraction rate (Fine et al. 2001). In particular, the spectral energy of agonistic boatwhistles was almost evenly distributed within the first three harmonics but the dominant frequency usually corresponded to the fundamental frequency, whereas in reproductive boatwhistles most of the sound energy generally appeared in the second harmonic (see Figs 2 and 3)."

"Toadfish advertising sounds are typically produced at relatively constant (McKibben & Bass 1998; Fine & Thorson 2008) and higher rates for longer periods of time up to several hours (personal observations in H. didactylus)."

Observation Environment Quotes

"Prior to the onset of the breeding season we placed 60 artificial concrete nests (internal dimensions: 50- cm long, 30-cm wide and 20-cm height) with a hemicylinder shape and closed at one end, approx. 1.5 m apart, in three rows, along an intertidal area of the Tagus River estuary (Military Air Force Base, Montijo, Portugal; 3842’N; 858’W). Fish spontaneously occupied these shelters and we were able to access the animals at low tides during the whole period between May and July. We used 42 of these specimens (total length, TL = 38–52 cm; body mass = 985–2452 g) for testing behavioral responses during territorial intrusions and 16 males for recording mate advertising sounds (TL = 35–48 cm; body mass = 963–1819 g). In the territorial intrusion experiments, the fish used within each trial were similar in size with residents and intruders differing in 0.25–3.0 cm TL, with the exception of one trial where the size differed in 7.8 cm TL. All animals used were type I males, which were easily identified on the basis of size (Modesto & Cana´rio 2003a). Type II males and females caught in the study area were typically much smaller (generally TL < 30 cm, body mass < 500 g, personal observations) than the tested type I males and gravid females additionally differ by their larger abdomens. Moreover, the morphotypes’ identity could be confirmed by gently pressing the males’ abdomen since type I males have larger accessory glands (Modesto & Cana´rio 2003a) and release a dark-brown seminal fluid (personal observations). We maintained males to use in the territorial intrusion experiments in round stock tanks (plastic swimming-pools, 2-m diameter and water depth c. 0.5 m) near the intertidal toadfish nesting area where subjects were collected. "

"We created an aggregation of shelters, readily occupied by type I males during the breeding season, similar to the natural aggregations where toadfish males emit advertising boatwhistles in choruses to attract mates (Amorim & Vasconcelos 2008). We used a group of eight concrete nests placed in the intertidal study area of the Tagus estuary to confine males that spontaneously occupied these shelters and record their vocal activity (see experimental setup in Fig. 1b). Nest’s entrances were closed with a plastic net to prevent fish from escaping and to ensure male identity throughout the recordings."

Behaviour Description Quotes

"Male signals are frequently studied in a single behavioral context, but in some cases they may assist multiple functions, namely for both male– male competition and female mate choice. Boatwhistles are known as the mate attraction calls of toadfishes typically produced during the breeding season. However, recent observations with the Lusitanian toadfish Halobatrachus didactylus (Batrachoididae) indicate that the emission of boatwhistles is not restricted to this period, which suggests a function in other behavioral contexts such as agonistic territorial interactions."

"Resident males defended their shelters producing sounds, mostly boatwhistles, towards intruders. Parental males revealed higher aggression levels, exhibiting additional threatening and attack behaviors. Boatwhistles registered during agonistic events were compared with the mate advertising boatwhistles recorded from small aggregations of nesting males in a natural breeding intertidal area. Agonistic boatwhistles were produced in lower and variable calling rates comparing with the advertising ones that were typically emitted in long series of calls. Agonistic boatwhistles were similar in duration and frequency harmonic structure (with a middle tonal phase) to the advertising calls, but presented less amplitude modulation, and lower dominant and fundamental frequencies. These acoustic differences were probably related to differences in calling rates and broadcast demands associated to the distance to the intended receiver. We provide first evidence that, apart from attracting mates, the toadfish boatwhistles also function as active ‘keep-out’ signals during territorial defense."

". However, the emission rate was considerably lower compared with the boatwhistles, which indicates that the grunt trains of the Lusitanian toadfish are probably more used during highly distress events such as when fish are being handheld (Vasconcelos & Ladich 2008) like in other batrachoidids (e.g. Opsanus tau, Cohen & Winn 1967)."

Sound Name Quotes

"Male signals are frequently studied in a single behavioral context, but in some cases they may assist multiple functions, namely for both male– male competition and female mate choice. Boatwhistles are known as the mate attraction calls of toadfishes typically produced during the breeding season. However, recent observations with the Lusitanian toadfish Halobatrachus didactylus (Batrachoididae) indicate that the emission of boatwhistles is not restricted to this period, which suggests a function in other behavioral contexts such as agonistic territorial interactions."

"Resident males defended their shelters producing sounds, mostly boatwhistles, towards intruders. Parental males revealed higher aggression levels, exhibiting additional threatening and attack behaviors. Boatwhistles registered during agonistic events were compared with the mate advertising boatwhistles recorded from small aggregations of nesting males in a natural breeding intertidal area. Agonistic boatwhistles were produced in lower and variable calling rates comparing with the advertising ones that were typically emitted in long series of calls. Agonistic boatwhistles were similar in duration and frequency harmonic structure (with a middle tonal phase) to the advertising calls, but presented less amplitude modulation, and lower dominant and fundamental frequencies. These acoustic differences were probably related to differences in calling rates and broadcast demands associated to the distance to the intended receiver. We provide first evidence that, apart from attracting mates, the toadfish boatwhistles also function as active ‘keep-out’ signals during territorial defense."

"Congruently, the Lusitanian toadfish Halobatrachus didactylus (Bloch and Schneider 1801) produces these distinct calls, boatwhistle and grunt train, most likely associated with mating activities and agonistic interactions (dos Santos et al. 2000; Amorim et al. 2006; Vasconcelos & Ladich 2008). Recent observations, however, indicate that boatwhistling is not restricted to the mating season in this species, since it has been detected all year round when water temperature remained higher than 19C (Amorim et al. unpublished data). This suggests that, besides attracting gravid females to the male’s nest, the boatwhistle may have other functions such as territorial defense."

"Only the type I males establish nests under rocks in shallow waters during the breeding season and are able to emit the advertisement tonal call (boatwhistle) to attract females at distance (Amorim & Vasconcelos 2008). The Lusitanian toadfish exhibits an unusual large acoustic repertoire composed of four commonly produced sounds: boatwhistle, grunt train, long grunt train and double croak, and other less frequent sound emissions such as croak and mixed croak–grunt calls (dos Santos et al. 2000; Amorim et al. 2008). "

"During territorial intrusion trials, 22 specimens produced sounds (TL = 40.5–52.0 cm; body mass = 985–2452 g) which were classified as boatwhistles, grunts and ‘other sounds’ that occurred less frequently."

"Only boatwhistles emitted in series with more than 15 sounds and ⁄ or produced constantly during 1 h (namely c. 4–10 calls per min on average), have been selected and classified as reproductive boatwhistles. These criteria were based on previous observations of the Lusitanian toadfish breeding chorus behavior."