Male common loons signal greater aggressive motivation by lengthening territorial yodels.
Abstract: We examined two critical predictions of the hypothesis that male Common Loons (Gavia immer) communicate greater aggressive motivation by increasing the number of repeat syllables within their territorial yodels. We observed (from >3,500 hrs of field observations of 58 males) the probability that territorial interactions escalated from territorial flyovers by intruders to stereotyped 'social gatherings' to escalated fights between residents and intruders was positively correlated to the number of repeat syllables given by individually-banded males. Males yodeling during these escalated contests often assumed the upright 'vulture' posture rather than the usual 'crouch' posture, reflecting an escalated aggressive motivational state. Territorial pairs responded sooner and with more threat and alarm vocalizations to playback yodels that contained more repeat phrases. This reflected a greater willingness to attack by residents to perceived intrusions by males of higher aggressive motivational state. Our study demonstrates the ability of loons to communicate greater aggressive motivation by lengthening acoustic territorial threat signals, which not only may be important for conveying imminent attack, but may also reflect important tactics for individuals of poorer fighting ability to deter territorial evictions. Our results also raise questions regarding what receiver-dependent and receiver-independent selective factors are responsible for maintaining signal honesty in this non-oscine bird.
Article Type: Report
Subject: Aggressive behavior in animals (Research)
Territoriality (Zoology) (Research)
Authors: Mager, John N., III
Walcott, Charles
Piper, Walter H.
Pub Date: 03/01/2012
Publication: Name: The Wilson Journal of Ornithology Publisher: Wilson Ornithological Society Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2012 Wilson Ornithological Society ISSN: 1559-4491
Issue: Date: March, 2012 Source Volume: 124 Source Issue: 1
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 285207244
Full Text: Most threat signals communicate information regarding an individual's inherent, or condition-dependent fighting ability (Parker 1974) and/or willingness to attack (or aggressive motivation) (Maynard Smith 1982, Bradbury and Vehrencamp 1998, Hurd and Enquist 2001). Features that communicate fighting ability often reflect stable physical attributes, like physical size, that predict success in aggressive encounters (Parker 1974, Archer 1988). An animal's willingness to attack is often influenced by ephemeral factors such as health and motivational state of both the animal and its competitors (Maynard Smith 1982, Hurd and Enquist 2001). Many birds benefit from communicating varying levels of aggressive motivation within territorial signals to avoid conflicts that consume energy and can cause serious injury. Birds, especially oscines, often communicate heightened motivation to attack by modulating frequencies of their songs (Morton 1977, 1982). Others do so by matching or overlapping songs of their competitors (e.g., Dabelsteen et al. 1997, Vehrencamp 2001). Recent studies indicate some birds, particularly non-oscines, add or remove phases from their vocalizations to communicate greater aggressive motivation (e.g., Capp and Searcy 1991, Langemann et al. 2000).

Male Common Loons (Gavia immer) defend allpurpose territories on freshwater lakes by aggressive threat vocalizations called yodels (Sjolander and Argen 1972; Rummel and Goetzinger 1975, 1978). Yodels are given only by male Common Loons, and are considered to be territorial threat signals because males aggressively approach and yodel at conspecific territorial intruders (Vogel 1995; J. N. Mager, unpubl, data). Structurally, yodels consist of a 3-4 note introductory phrase followed by a strophe of two-syllable repeat phrases (Fig. 1). Most frequency elements of a yodel exhibit low intra-individual variability (Barklow 1979; Vogel 1995; Walcott et al. 1999; Mager et al. 2007a, b) and each male can lengthen yodels by increasing the number of repeat phrases. Barklow (1979) observed males added more repeat phrases to their yodels when intruders wandered deeply into breeding territories, and suggested that because intrusions pose a greater threat to resident males, longer yodels might communicate a greater willingness to attack. This hypothesis has been generally accepted, but has been supported by few, and only anecdotal, data. For example, males add repeat phrases to yodels when territory quality (and resource value) is enhanced (Mager et al. 2007b), which may indicate they defend higher quality territories more aggressively. There has been no published study to date that has examined critical predictions of this long-standing hypothesis.

We specifically tested Barklow's hypothesis that yodels containing more repeat phrases reflect a greater motivational state by examining two critical predictions: (1) males should give longer yodels during social situations in which the probability of attack is greatest, and (2) territorial pairs should respond more aggressively to yodels that vary only in number of repeat phrases.

[FIGURE 1 OMITTED]

METHODS

Study Site and Resident Population.--We conducted this study from 2002 to 2007 on ~100 study lakes in Oneida County, Wisconsin, USA, northwest of Rhinelander (45[degrees] 42' N, 89[degrees] 37' W). Loons are genetically monogamous and exhibit high territory and mate fidelity (individuals return to the same territories for 3-7 years, on average; Piper et al. 1997a). Territorial intrusions occur frequently between April and July (Mager and Walcott 2007) and occur most often between 0430 and 1100 hrs each day (Mager 1995).

Assessing the Effect of Social Context on Yodel Duration.--We assessed the impact of varying number of repeat phrases on behavior of signalers and receivers by considering whether longer yodels were given during five stages of the intrusion process in which the probability of attack (i.e., the proportion of situations that resulted in an attack) increases (following Le Prell et al. 2002). These were: (1) when conspecifics flew over the territory, (2) when conspecifics landed on the territory but remained >20 m from the resident male, (3) when intruders approached to within 20 m of the resident male but did not engage in 'social gatherings' that consist of stereotyped circling on the water's surface in head-to-tail orientation that may include 'splash dives' (Sjolander and Argen 1972) with pair members, (4) when participants engaged in social gatherings, and (5) when intruders attacked or fought a resident. Data collected between 2002 and 2004 verified that residents were more likely to attack an intruder (repeated-measures ANOVA [F.sub.3,246] = 1.654, P < 0.0001; Fig. 2), when intrusions transitioned from flyovers to actual landings (paired t = 8.093, df = 155, P < 0.0001), from landings to approaches (paired t = 2.178, df = 155, P = 0.0073), and from approaches to social gatherings (paired t = 6.969, df = 155, P < 0.0001; Fig. 2).

Observations of Natural Yodels.--We observed 58 pairs of individually-banded loons between 15 April and 31 July 2002-2007 between 0430 and 1430 hrs (CDT). We conducted daily 1-hr behavioral-time samples (Altmann 1974) of 4-6 territorial pairs, using 'all-occurrences' recording methods (Martin and Bateson 1993) to count flyovers, the extent of intrusions (e.g., did pairs approach within 20 m of the intruder, did the residents and intruders engage in a social gathering), aggressive behavior (e.g., surface chases involving the swimming and flapping of wings of one individual toward another, or actual fights in which individuals actually grab one another by either the bill or neck and beat one another with their wings, following descriptions in McIntyre (1988) and Piper et al. (2008), and yodels. We recorded the time of day, the yodeler's physical posture (i.e., 'crouch' or 'vulture' after Rummel and Goetzinger 1978), the estimated distance between yodeler and perceived receiver, and the number of repeat phrases given when each male yodeled.

[FIGURE 2 OMITTED]

Playback Experiment.--We assessed how loons responded to yodels having fewer/more repeat syllables by recording vocal responses of focal pairs to broadcast yodels that simulated an unfamiliar male's intrusion into the territory. We used a Portadat MDP 500 acoustic recorder (HHB, London, UK) equipped with a Sennheiser MHK-70 shotgun microphone (Old Lyme, CT, USA) to broadcast yodels from males recorded earlier that season at a distance between 15 and 30 m from males in the study area whose territories were >8 km from a focal pair's territory (to reduce social recognition) (Waas 1991, Mager et al. 2010), and did not play the same yodel to more than one pair (to prevent pseudoreplication) (Kroodsma 1989). We created undistorted yodels containing one, four, and seven repeat phrases by adding identical repeat phrases to the same yodel using Canary acoustic software (Version 1.5, Cornell University Bioacoustics Research Program, Ithaca, NY, USA). Thus, each yodel was identical in all acoustic parameters except for the number of repeat phrases. We began playbacks between 2130 and 0230 hrs on three successive nights. We randomized the order yodels were played on the three nights (e.g., 1-repeat phrase the first night, 7-repeats the second, and 4-repeats the third night to one pair; 4, 7, 1 repeat phrases on successive nights to another pair, etc.). Each night, we broadcast yodels through a RadioShack (Fort Worth, TX, USA) 20-Watt amplifier and Super PowerHorn model 40-1,445 speaker at 90 dB (measured 10 m from the speaker) and rebroadcast the same yodel (having the same number of repeat phrases) at 5 and 10 rain after the first. We recorded the time and type of all vocal responses during the 15-min period following the first playback using the same recorder and microphone from which we recorded the playback yodels.

We quantified the number of tremolo, wail, and yodel responses from focal pairs in addition to the latency before first vocalization. We interpreted the number of tremolos to reflect the extent pairs were threatened by the yodel, and the number of wail responses to reflect the pair's level of alarm and interest to contact following Barklow (1979) and McIntyre (1988). We used repeated-measures ANOVA and associated post-hoc tests to investigate differential responses by pairs to the three classes of playback. We used the PROC MIXED procedure (SAS after Singer 1998, Johnson 2002) to construct linear growth models to examine whether responses given could be attributed to more repeat phrases while controlling for variability among pairs. We accepted significance at a Bonferroni-corrected Qt of 0.05 for all statistical tests.

RESULTS

We recorded conspecific intrusions on 57 territories and the yodeling responses of 58 banded males defending those territories ([bar.x] [+ or -] SE number of seasons an individual was observed = 3.7 [+ or -] 0.2, median = 3 seasons) during 3,900 hrs of observation covered by our 6-year study. The number of prospecting loons flying over territories varied significantly with each territory ([F.sub.56,151] = 1.718, P = 0.0051), but the frequency of distant intrusions ([F.sub.56,151] = 1.505, P = 0.0268), approaches ([F.sub.56,151] - 0.862, P = 0.7349), social gatherings ([F.sub.56,151] = 1.316, P = 0.0975), and aggressive chases/fights ([F.sub.56,151] = 1.202, P = 0.1914) did not vary with territory. Intrusions that involved the close approach of the intruder without a social gathering occurred rarely; consequently, we report mean yodeling responses by males to these types of intrusions, but omit them from repeated-measures analyses of yodeling rates and number of repeat phrases given with stage of territorial intrusion. None of the contests resulted in eviction of territorial males.

[FIGURE 3 OMITTED]

We observed 1,495 yodels from 58 banded males (mean [+ or -] SD; 13 [+ or -] 1 yodels recorded/individual/ year, range = 1-51). Forty-eight of the 58 males yodeled in at least two of the five social contexts between 2002 and 2007; few gave yodels in all five. Pooling all yodels from all males, males yodeled primarily at conspecifics (87.5% of all yodels), but also yodeled at humans (10.8%), and Bald Eagles (Haliacetus leucocephalus) (1.7%). We observed the four contexts (omitting infrequent close intrusions that did not lead to social gatherings) of territorial intrusion on 48 of the 56 territories (some territories had more than 1 male loon over successive years). The frequency (# yodels/occurrence) males yodeled at conspecifics was related to the extent of intrusion (Friedman's ANOVA [chi square] = 25.394, n = 48 males, df = 3, P < 0.0001; Fig. 3). Males gave significantly more yodels when intruders landed on the territory than when intruders flew over the territory (Wilcoxon Z = 2.99, P = 0.0028), and gave progressively fewer yodels per social gathering (Wilcoxon Z = 2.92, P = 0.0035). However, male yodeling rate did not decrease as social gatherings escalated into chases or fights (Wilcoxon Z = 0.62, P = 0.54). Close intrusions that did not lead to social gatherings rarely occurred, but the likelihood that a resident male would yodel was lowest (14.3% of all such occurrences) during this situation, implying some signal, or assessment of lower threat, associated with the intruding loon.

Males gave most calls as 'crouch' yodels (94% of all given), but only gave crouch yodels at flyovers, distant intruders, and close intruders that did not escalate into social gatherings. Males gave more 'vulture-posture' yodels when territorial contests escalated: 10.1% of all calls given during social gatherings were crouch yodels, but 67.8% of all yodels given during physical chases, and/or fights were vulture-posture yodels (Fig. 3). Males (n = 25) observed yodeling in both the crouch and vulture postures during a given year gave yodels with more repeat phrases when in the vultureposture ([bar.x] [+ or -] SE = 4.95 [+ or -] 0.40) than in the crouch-posture (3.72 [+ or -] 0.26; paired t = 3.10, df = 24, P = 0.0049). Intruders rarely yodeled on territories of residents (130 yodels in the 6 years of study), and tended to give proportionately more vulture yodels (33/130 or 25.4% of yodels) than residents (84/1,495, or 5.6% of yodels).

[FIGUR 4 OMITTED]

Males had 3.6 [+ or -] 0.1 repeat phrases/yodel (range = 0.0-20.0), although the number of repeat phrases each male produced was a function of the number of yodels heard (y = 3.320 log x + 1.064; [r.sup.2] = 0.613; P < 0.0001). Most yodels (86.1%) contained between two and five repeat phrases (mode = 3). There was individual variation in the number of repeat phrases given by males per crouch posture yodel (ANOVA [F.sub.sub.57,129] = 3.270, P < 0.0001), but not per vulture posture (ANOVA [F.sub.sub.25,12] = 1.431, P = 0.26) yodel, not controlling for the different contexts of territorial intrusion.

Males had 42.9% more repeat phrases per yodel as intrusions escalated from flyovers and landings to social gatherings and physical attacks (Fig. 4). Males had longer yodels when flyovers escalated to landings (paired t = 3.158, df= 39, P = 0.0031); did not give significantly longer yodels when intruders came within 20 m of the male (paired t = 0.025, df = 14, P = 0.98); and gave longer yodels when close intrusions transitioned into social gatherings (paired t = 3.168, df = 10, P = 0.01), but not when social gatherings transitioned into chases and/or fights (paired t = 0.09, df = 17, P = 0.93).

There was a clear difference in how pairs responded vocally to broadcast yodels that differed in the number of repeat phrases. Pairs vocalized over twice as quickly, and gave almost four times as many tremolos to longer yodels. Resident males gave four times as many yodels to seven-repeat yodels versus one-repeat yodels (Table 1).

DISCUSSION

Individuals that experience frequent territorial contests often produce graded threat displays to communicate a heightened state of aggressive motivation (van Rhijn 1980, Bradbury and Vehrencamp 1998). Populations experience frequent conspecific intrusions that escalate into potentially lethal confrontations (Piper et al. 1997b, 2000), and it is not surprising that male Common Loons, as Barklow (1979) proposed, lengthen territorial yodels to communicate greater aggressive motivation.

Territorial interactions between residents and intruders among loons proceed through a sequence of actions that provide more reliable information about fighting ability but entail greater costs consistent with sequential-assessment models of contests (Enquist and Leimar 1983, 1987). Flyovers provide intruders information about territory quality (Piper et al. 2006), but if a resident yodels, it also provides the prospecting loons information about territory occupancy, the resident male's identity (Walcott et al. 1999, 2006; Mager et al. 2010), and fighting ability (Mager et al. 2007a). Flyovers constitute a lesser threat to a resident male versus a close intruder. Males yodel less often (~1 of every 3 flyovers), and always assume the low-risk, low-cost 'crouch' posture. Not only is the 'crouch' posture less costly physiologically to assume versus the 'vulture' posture, but it also places the signaler in a less vulnerable posture for receiver attack (although no attack could possibly occur during a flyover) and a more effective posture to transmit information over great distances (McIntyre 1988).

It is likely that both residents and intruders obtain more reliable or perhaps additional information about each other's fighting ability and motivation when intruders approach within 20 m of the resident pair and engage in social gatherings. Residents, in response, yodel more often (yodeling approximately once for each such intrusion), and give more repeat phrases in each yodel. Longer yodels likely provide intruders more reliable information about the identity, health, and vigor of the signaler. However, this information comes with considerable costs to the signaler. Physiologically, longer yodels necessarily are more energetically expensive to produce. A male that produces longer yodels must be able to endure these costs. Producing longer yodels could also place the signaler at a greater risk of retaliation from the receiver. Our results have shown that longer yodels evoke quicker responses by, and more tremolos (signaling greater alarm) and yodels (signaling greater threat) from conspecific receivers. These receiver-independent and receiver-dependent costs of producing longer yodels ultimately could prevent males from bluffing about their motivation, and act as selective processes to maintain signal honesty (Vehrencamp 2000, Hurd and Enquist 2001). We feel both types of costs are likely more than offset by the significant costs of fighting, as aggressive pursuits and fights (Rummel and Goetzinger 1975, McIntyre 1988, Paruk 2006) can lead to a resident's eviction or death (McIntyre 1988, Paruk 1999, Piper et al. 2008). However, more detailed analyses are needed to consider these factors regarding fitness costs associated with producing longer yodels.

We found loons yodel in the vulture posture most often when contests escalate into social gatherings and actual chases/fights. The vulture posture may amplify/reinforce the communication of a higher motivational state and/or it may serve as a separate signal of aggression (Rummel and Goetzinger 1978, Johnstone 1996). The posture is clearly more costly for males to assume, as they must vigorously paddle their feet to 'stand' upright upon the water surface and extend their wings outward while 'pointing' their bills toward the potential receiver (McIntyre 1988).

We provide empirical support of Barklow's (1979) contention that male Common Loons signal greater aggressive motivation by adding repeat phrases to territorial yodels. However, signaling motivation by lengthening territorial vocalizations is not the conventional way birds signal greater motivation (e.g., Morton 1977, 1982; Ripmeester et al. 2007). Many species that sing longer songs tend to communicate greater fighting ability (e.g., Lambrechts and Dhondt 1986, 1987; Appleby and Redpath 1997). In contrast, male Common Loons communicate fighting ability through the dominant acoustic frequencies of yodels: larger males of better fighting ability produce lower-frequency yodels and smaller males of poorer fighting ability produce higher-frequency yodels (Mager et al. 2007a). Past studies have shown that males that signal lower fighting ability produce yodels with more repeat phrases (Mager et al. 2007a). we believe that males of poorer fighting ability must be more aggressively motivated (and produce longer yodels) to successfully defend their territories based on our empirical support of Barklow's (1979) hypothesis that males communicate greater aggressive motivation by lengthening yodels. This strategy may not be limited to loons, as other species similarly lengthen threat signals when more motivated to attack. Many examples are non-oscines (e.g., Martin-Vivaldi et al. 2004), including some waterbirds (e.g., Nelson 1984, Ewins and Weseloh 1999, Mowbray et al. 2002) living in open water environments where acoustic scatter and absorption are minimal. This raises the question regarding those selective factors associated with signal production, propagation, and reception among birds that call over open water that would favor this means of honest communication of aggressive motivation.

ACKNOWLEDGMENTS

Parts of this manuscript were presented as partial fulfillment of requirements of J. N. Mager's Doctor of Philosophy degree, in agreement with guidelines of IACUC approval 97-12-02 at Cornell University. Support was provided by Cornell University (Cornell Laboratory of Ornithology Walter Benning Fellowship, Department of Neurobiology and Behavior Student Research Grant, Edna Bailey Sussman Fellowship, Kieckhefer Adirondack Fellowship, Local Sigma Xi Grants-in-Aid-of-Research Grant, University Travel Grant), the Sigurd Olson Environmental Institute Loon Research Award, the Denison University Visiting Scholar Program, and the Ohio Northern University Faculty Summer Research Grant. We thank A. A. Dhondt, H. K. Reeve, and S. L. Vehrencamp for constructive recommendations in study design and manuscript preparation, Karen Grace-Martin and F. M. Vermeylen for statistical consulting, A. R. Lindsay, M. W. Meyer for assistance, the many field assistants for their hard work, and private landowners for access to lakes, encouragement, and support.

Received 31 January 2011. Accepted 15 July 2011.

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JOHN N. MAGER III, (1, 4) CHARLES WALCOTT (2), AND WALTER H. PIPER (3)

(1) Department of Biological and Allied Health Sciences, Ohio Northern University, Ada, OH 45810, USA.

(2) Department of Neurobiology and Vehavior, Cornell University, Ithaca, Ny 14858, USA.

(3) Department of Biological Sciences, Chapman University, One University Drive, Orange , CA 92866, USA.

(4) Corresponding author; e-mail: j-mager@onu.edu
TABLE 1. Vocal responses of territorial pairs (n = 38) of
Common Loons to playback yodels having one, four, and seven
repeat phrases.

Response                     One repeat        Four repeats

Univariate response
  # Vocalizations        9.68 [+ or -] 2.96     17.80 [+ or -] 5.06
  # Tremolos             5.92 [+ or -] 2.79     14.72 [+ or -] 4.91
  # Wails                3.95 [+ or -] 1.14      2.44 [+ or -] 0.79
  # Yodels               0.18 [+ or -] 0.08      0.36 [+ or -] 0.14
  Latency before 1st   128.20 [+ or -] 52.72   131.72 [+ or -] 49.12
  vocalization (sec)

                                               Individual growth
Response                   Seven repeats       model [F.sub.1,112]

Univariate response
  # Vocalizations      26.76 [+ or -] 4.85            6.89
  # Tremolos           20.08 [+ or -] 4.53            5.26
  # Wails               4.54 [+ or -] 0.91            0.12
  # Yodels              0.73 [+ or -] 0.20            6.51
  Latency before 1st   62.87 [+ or -] 17.27          12.37
  vocalization (sec)

Response                 P

Univariate response
  # Vocalizations      0.0099
  # Tremolos           0.0237
  # Wails              0.7288
  # Yodels             0.0121
  Latency before 1st   0.0006
  vocalization (sec)
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