More bang for your buck: the effect of caller position, habitat and chorus noise on the efficiency of calling in the spring peeper
Introduction
Animals such as insects, fish, frogs, birds and mammals produce a wide range of acoustic signals that serve a diversity of biological and social purposes. These include attracting and bonding with mates, protecting territories, maintaining spacing between groups, communicating distress and hunger, and warning of danger from predators (e.g. MacKinnon, 1974, Waser, 1977, Duellman and Trueb, 1986, Strahan, 1995, Bradbury and Vehrencamp, 1998, Poole, 1999). Male anuran amphibians (frogs and toads) produce advertisement calls to attract conspecific females for mating, and to announce to other males that a territory is occupied (Wells, 1977, Duellman and Trueb, 1986). In experimental studies, females of a wide range of anuran species have demonstrated a preference for higher calling effort, such as louder calls, higher call rates and longer duration of calling (Sullivan et al., 1995). The production of an acoustic signal can be energetically expensive (Prestwich, 1994, Wells, 2001); for example, the rate of oxygen consumption by male frogs (a measure of aerobic metabolism) can be 20 times greater when calling than when resting (Taigen and Wells, 1985). An animal will only benefit from the production of a signal if it is successfully transmitted to the intended receiver or receivers (Brenowitz, 1986). The distance over which an acoustic signal is effective depends on the characteristics of the signal, the habitat through which it travels, the level of ambient noise, the proximity of the sender and receiver to a boundary such as the ground or forest canopy, and the ability of the receiver to extract information from the transmitted sound (Forrest, 1994, Larom et al., 1997, Bradbury and Vehrencamp, 1998).
Spatially explicit models of the movement of entities such as plants, animals, diseases and fire across heterogeneous landscapes have been developed in recent years (e.g. Boone and Hunter, 1996, Lavorel et al., 1999, Vos, 1999, White et al., 1999, Hargrove et al., 2000, Williams and Liebhold, 2000, Nathan et al., 2001). However, the movement or propagation of acoustic signals has not previously been modelled in this way. In this paper, I present a spatially explicit model of the propagation of an anuran advertisement call across a landscape. The model calculates the geographic area in which a call can be detected by male and female frogs of the same species, as a function of call intensity, the hearing threshold of the receivers, and the caller's position in the landscape. It uses the example of the northern spring peeper Pseudacris crucifer crucifer, as both physiological and environmental aspects of acoustic communication in this species are well understood (e.g. Brenowitz et al., 1984, Wilczynski et al., 1984, Taigen et al., 1985, Schwartz and Gerhardt, 1998). Here, I use the model to explore the effect of habitat, caller position and chorus noise on the effective distance of a spring peeper call, the energetics and efficiency of calling at different intensities and from different locations in the landscape, and the expected behavioural consequences.
The amplitude of a sound can be measured in pressure or intensity. Pressure is proportional to the square root of intensity, and both can be expressed in decibels (dB). As a sound propagates, its energy is spread over a greater sphere of disturbance. The inverse square law predicts that both the intensity and pressure of a sound will decrease (attenuate) by 6 dB for each doubling of distance from the source (Forrest, 1994, Bradbury and Vehrencamp, 1998). However, in natural habitats, sound generally attenuates at a higher rate than that predicted by the inverse square law. This excess attenuation can be caused by atmospheric absorption, scattering or boundary interference. Vegetation, topography and atmospheric turbulence can scatter sound waves, deflecting sound energy from the path of propagation and causing interference between the direct wave and the scattered waves. Wavelengths shorter than or equal to the dimensions of environmental obstacles (e.g. leaves, tree trunks, boulders) will be scattered more than longer wavelengths. As the frequency of a sound is the inverse of its wavelength, high frequencies will attenuate more rapidly as a result of scattering than will low frequencies (Brenowitz, 1986).
The propagation of sound at boundaries such as the surface of the ground or a lake depends on the acoustic impedances of the adjoining media, the spatial arrangement of the sender and receiver, and the wavelength of the sound. The greater the difference between the acoustic impedances of the adjoining media, the greater the proportion of sound that will be reflected at the boundary, and the smaller the proportion that will enter the second medium (Bradbury and Vehrencamp, 1998). Softer (more porous) surfaces such as loose soil tend to have low acoustic impedance and will attenuate sound faster than hard surfaces (such as water) with higher acoustic impedance, more dissimilar to that of air (Forrest, 1994). When both the source of a signal and the receiver are on the ground, excess attenuation over soft surfaces with low acoustic impedance can be very large, and the effective range of the signal will be diminished. However, when the source or the receiver is elevated above the ground, the attenuating effect of a soft ground surface is reduced (Marten and Marler, 1977, Forrest, 1994). Thus, calling from an elevated perch can dramatically increase the effective distance of a signal. For example, male short-tailed crickets calling on elevated perches have an effective signal area 14 times that of crickets calling on the ground (Paul and Walker, 1979). Because of the large difference between the acoustic impedances of air and water, the surface of a water body can act as an effective sound reflector. If the sender and receiver are on the ground but the acoustic signal travels over still water, signal amplitude can be increased by 6 dB above that expected from the inverse square law, effectively doubling the distance of propagation (Forrest, 1994).
The northern spring peeper P. crucifer crucifer is a small brown-to-grey frog (≤3.7 cm; snout–vent length), with a geographic range extending through eastern North America from Nova Scotia to South Carolina (Harding, 1997). Spring peepers breed in temporary and permanent ponds, marshes and ditches, and choruses of calling males can be heard at breeding sites in spring. The species can be classed as a prolonged breeder, with individual populations breeding over a period of 1–2 months (Wells, 1977). The advertisement call of the northern spring peeper is a single, high-pitched peep repeated up to 1.5 times/sec (Wells et al., 1996). This call is designed to attract conspecific females for mating, and may also serve to regulate spacing between males in a chorus (Brenowitz et al., 1984, Gerhardt et al., 1989). Spring peepers call from the ground or from elevated perches on grass, shrubs and trees (Brenowitz et al., 1984).
Wilczynski et al. (1984) analysed the call of the northern spring peeper, and investigated the physiological characteristics of the peripheral auditory system of males and females of the species. They determined that in their study population near Ithaca, New York, the call of the peeper was a simple, nearly tonal signal with a mean dominant frequency of 2895 Hz (range 2588–3212 Hz). This is approximately 120 Hz lower than the mean dominant frequency of the advertisement call observed in a population of spring peepers from Missouri (Doherty and Gerhardt, 1984, Diekamp and Gerhardt, 1992). The mean amplitude of the call measured 50 cm in front of the caller was 86.6 dB root mean squared (RMS) sound pressure level (SPL—Wilczynski et al., 1984), with a range of 75.5–93.5 dB (Brenowitz et al., 1984). In the study of Wilczynski et al. (1984), the threshold (lowest) SPL at which the frequency of the advertisement call could be detected differed considerably between male and female spring peepers. Among males, the lowest threshold to the mean dominant frequency of the advertisement call was 69.5 dB SPL, while for females the lowest threshold to this frequency was 58 dB SPL (Wilczynski et al., 1984). Thus in this population, the advertisement call of the northern spring peeper is audible to females at much greater distances from the caller than it is to the males. These thresholds are considerably higher than those observed in the above-mentioned population from Missouri, where the minimum thresholds to 2900 Hz were 50 dB for females and 51 dB for males (Diekamp and Gerhardt, 1992, Schwartz and Gerhardt, 1998). Gerhardt (1975) investigated directionality in anuran advertisement calls and found that the amplitude of the call of northern spring peepers is approximately uniform around the caller.
Section snippets
Model description
An interactive, spatially explicit model of the propagation of the advertisement call of the northern spring peeper was created, using information on call characteristics and hearing thresholds (Wilczynski et al., 1984, Gerhardt et al., 1989, Diekamp and Gerhardt, 1992, Schwartz and Gerhardt, 1998), equations describing attenuation of the call in a lightly wooded grassland (Brenowitz et al., 1984), and data on attenuation of an acoustic signal propagating over soft and hard surfaces (Forrest,
Effect of caller height, habitat and chorus noise on call propagation
In the example run of the model presented, the vertical position of the calling male and its location in the landscape had a dramatic effect on the area in which the call could be detected by other spring peepers (Table 1; Fig. 1). For example, the effective call area increased by a factor of 10 for male receivers and 40 for female receivers when the calling male was elevated 50 cm above the ground in the grassy habitat (Fig. 1a,b). The superior propagation of sound across still water resulted
Effect of caller height and habitat on call propagation
The simple example shown in Fig. 1 demonstrates the effect of caller height and the habitat through which the signal passes on call propagation and acoustic communication in the northern spring peeper. Elevation of the caller by just 50 cm substantially increases the effective area of the call by reducing the attenuating effect of soft ground (Forrest, 1994). This positive effect of caller height on signal propagation was also observed by Kime et al. (2000) in their field study of the
Acknowledgements
I thank Bill Reiners for his encouragement and enthusiasm during this project, and Phil Polzer, Ken Driese and Emma Seager for their help with the user interface for the model. Michael McCarthy provided much assistance with c programming, and he and Mark McDonnell gave helpful comments on the manuscript. I thank David Dubbink, Josh Schwartz and Kentwood Wells for their assistance and advice. Work underlying this paper was supported by the Andrew W. Mellon Foundation.
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