Hydric balance and locomotor performance of an anuran (Rhinella marina) invading the Australian arid zone

Invasive species often encounter environmental conditions well outside those found in their native geographic ranges, and thus provide ideal model systems with which to explore responses to novel abiotic challenges. Within Australia, the invasive cane toad Rhinella marina has colonized areas that are considerably more arid than those found within its native range. Has the colonization of these novel environments been accompanied by shifts in physiology and/or locomotor performance? We measured rates of evaporative water loss, water gain, and effects of desiccation on locomotor performance of cane toads from two invasion fronts: one mesic (the wet‐dry tropics) and one arid (the semi‐desert). The two populations diverged substantially. Contrary to intuition (but consistent with intra‐specific comparisons between other toad populations from mesic vs arid areas), rates of evaporative water loss were lower (not higher) in toads from the mesic population. However, arid‐zone toads gained water more rapidly through their ventral surfaces, and rates of water loss and gain were highly correlated within individual toads from the arid‐zone population. Rates of water exchange in laboratory‐acclimated toads from the semi‐arid zone did not differ from those of free‐ranging conspecifics from the same population, suggesting that divergences between mesic and semi‐arid toads reflect genetic changes that have occurred during the species’ Australian invasion. Mesic and semi‐arid toads showed similar locomotor performance (endurance, distance per hop) when fully hydrated, but locomotor performance declined much more rapidly with desiccation in the mesic toads. Thus, within the short (decades‐long) timespan of the cane toad's Australian invasion, there has been substantial population divergence in the ability to withstand desiccating conditions. If we are to accurately predict the distributions (and hence impacts) of invading organisms, we will need to include adaptation potential in risk assessment schemes.     

Excluding access to invasion hubs can contain the spread of an invasive vertebrate

Many biological invasions do not occur as a gradual expansion along a continuous front, but result from the expansion of satellite populations that become established at 'invasion hubs'. Although theoretical studies indicate that targeting control efforts at invasion hubs can effectively contain the spread of invasions, few studies have demonstrated this in practice. In arid landscapes worldwide, humans have increased the availability of surface water by creating artificial water points (AWPs) such as troughs and dams for livestock. By experimentally excluding invasive cane toads (Bufo marinus) from AWP, we show that AWP provide a resource subsidy for non-arid-adapted toads and serve as dry season refuges and thus invasion hubs for cane toads in arid Australia. Using data on the distribution of permanent water in arid Australia and the dispersal potential of toads, we predict that systematically excluding toads from AWP would reduce the area of arid Australia across which toads are predicted to disperse and colonize under average climatic conditions by 38 per cent from 2,242,000 to 1,385,000 km(2). Our study shows how human modification of hydrological regimes can create a network of invasion hubs that facilitates a biological invasion, and confirms that targeted control at invasion hubs can reduce landscape connectivity to contain the spread of an invasive vertebrate.     

On the fringe of the invasion: the ecology of cane toads in marginally-suitable habitats

Understanding how invasive species flourish under climatic conditions outside those found within their native range can inform management. In southeastern Australia, cane toads (Rhinella marina) are spreading into montane areas cooler than have been predicted to be suitable. We monitored the presence of active toads in two high-elevation sites (750–1010 m above sea level [asl]) and two adjacent low-elevation sites (150–210 m asl) in northeastern New South Wales over spring and summer. We radio-tracked 28 field-collected adult toads (n = 5–9 toads per site) and quantified their thermoregulatory opportunities and body temperatures. Toads were active at low-elevation sites in spring, but were not seen in high-elevation sites until late summer. At low elevations, toads had access to a wide range of temperatures and selected cool diurnal refugia. In montane sites, toads had less control over their temperatures, because thermal differentials between exposed and sheltered microhabitats were smaller. Overall though, body temperatures of toads at high-elevation sites in summer were not different to those of conspecifics at lower elevations in spring. As a result, toads at high elevations moved as far (mean daily displacement around 50 m) as did low-elevation conspecifics. Toads in high elevations spent the day in superficial shelter, often partly exposed. Thus, although toads only appear in high-elevation sites seasonally, their behaviour at those sites (spending the day exposed; moving extensively at night) likely exacerbates their ecological impact by bringing them to the attention of vulnerable native predators.     

Interacting impacts of invasive plants and invasive toads on native lizards

The ecological impacts of an invasive species may be reduced by prior invasions if selective pressures imposed by earlier events preadapt the native biota to deal with the newer arrival. In northwestern Australia, invasion of the cane toad (Rhinella marina) kills many native predators if they ingest the highly toxic toads. Remarkably, the toads' defensive toxins (bufadienolides) are chemically similar to those of another invasive species: an ornamental plant from Madagascar, Bryophyllum spp. (Crassulaceae, mother-of-millions). Omnivorous lizards (bluetongue skinks, Tiliqua scincoides) are imperiled by the invasion of toads in northwestern Australia, but conspecifics from other areas of the continent (those where exotic plants were introduced and including areas where toads have yet to invade) are less affected because they exhibit higher physiological tolerance of toad toxins (and also of plant toxins). The willingness of captive bluetongues to consume both toads and these plants and the high correlation in the lizards' sensitivity to toad toxins versus plant toxins suggest that exotic plants may have imposed strong selection on the lizards' physiological tolerance of bufadienolides. As a result, populations of lizards from areas previously exposed to these alien plants may be preadapted to deal with the toxins of the more recent anuran invader.     

Native Australian frogs avoid the scent of invasive cane toads

Invasive species can affect the ecosystems they colonize by modifying the behaviour of native taxa; for example, avoidance of chemical cues from the invader may modify habitat use (shelter site selection) by native species. In laboratory trials, we show that metamorphs of most (but not all) native frog species on a tropical Australian floodplain avoid the scent of invasive cane toads (Bufo marinus Linnaeus 1758). Cane toads also avoid conspecific scent. This response might reduce vulnerability of metamorph frogs and toads to larger predatory toads. However, similar avoidance of one type of pungency control (garlic), and the presence of this avoidance behaviour in frogs at the toad invasion front (and hence, with no prior exposure to toads), suggest that this may not be an evolved toad‐specific response. Instead, our data support the simpler hypothesis that the metamorph anurans tend to avoid shelter sites that contain strong and unfamiliar scents. Temporal and spatial differences in activity of frogs versus toads, plus the abundance of suitable retreat sites during the wet season (the primary time of frog activity), suggest that avoiding toad scent will have only a minor impact on the behaviour of native frogs. However, this behavioural impact may be important when environmental conditions bring toads and frogs into closer contact.     

Quantifying Anuran Microhabitat Use to Infer the Potential for Parasite Transmission between Invasive Cane Toads and Two Species of Australian Native Frogs

Parasites that are carried by invasive species can infect native taxa, with devastating consequences. In Australia, invading cane toads (Rhinella marina) carry lungworm parasites (Rhabdias pseudosphaerocephala) that (based on previous laboratory studies) can infect native treefrogs (Litoria caerulea and L. splendida). To assess the potential of parasite transmission from the invader to the native species (and from one infected native frog to another), we used surveys and radiotelemetry to quantify anuran microhabitat use, and proximity to other anurans, in two sites in tropical Australia. Unsurprisingly, treefrogs spent much of their time off the ground (especially by day, and in undisturbed forests) but terrestrial activity was common at night (especially in anthropogenically modified habitats). Microhabitat overlap between cane toads and frogs was generally low, except at night in disturbed areas, whereas overlap between the two frog species was high. The situations of highest overlap, and hence with the greatest danger of parasite transmission, involve aggregations of frogs within crevices by day, and use of open ground by all three anuran species at night. Overall, microhabitat divergence between toads and frogs should reduce, but not eliminate, the transmission of lungworms from invasive toads to vulnerable native frogs.     

Skin resistance to water gain and loss has changed in cane toads (Rhinella marina) during their Australian invasion

The water‐permeable skin of amphibians renders them highly sensitive to climatic conditions, and interspecific correlations between environmental moisture levels and rates of water exchange across the skin suggest that natural selection adapts hydroregulatory mechanisms to local challenges. How quickly can such mechanisms shift when a species encounters novel moisture regimes? Cutaneous resistance to water loss and gain in wild‐caught cane toads (Rhinella marina) from Brazil, USA (Hawai''i) and Australia exhibited strong geographic variation. Cutaneous resistance was low in native‐range (Brazilian) toads and in Hawai''ian populations (where toads were introduced in 1932) but significantly higher in toads from eastern Australia (where toads were introduced in 1935). Toads from recently invaded areas in western Australia exhibited cutaneous resistance to water loss similar to the native‐range populations, possibly because toads are restricted to moist sites within this highly arid landscape. Rates of rehydration exhibited significant but less extreme geographic variation, being higher in the native range than in invaded regions. Thus, in less than a century, cane toads invading areas that impose different climatic challenges have diverged in the capacity for hydroregulation.     

Effects of an invasive species on refuge‐site selection by native fauna: The impact of cane toads on native frogs in the Australian tropics

Invasive species can induce shifts in habitat use by native taxa: either by modifying habitat availability, or by repelling or attracting native species to the vicinity of the invader. The ongoing invasion of cane toads (Rhinella marina) through tropical Australia might affect native frogs by affecting refuge‐site availability, because both frogs and toads frequently shelter by day in burrows. Our laboratory and field studies in the wet‐dry tropics show that native frogs of at least three species (Litoria tornieri, Litoria nasuta and Litoria dahlii) preferentially aggregate with conspecifics, and with (some) other species of native frogs. However, the frogs rarely aggregated with cane toads either in outdoor arenas or in standardized experimental burrows that we monitored in the field. The native frogs that we tested either avoided burrows containing cane toads (or cane toad scent) or else ignored the stimulus (i.e. treated such a burrow in the same way as they did an empty burrow). Native frogs selected a highly non‐random suite of burrows as diurnal retreat sites, whereas cane toads were less selective. Hence, even in the absence of toads, frogs do not use many of the burrows that are suitable for toads. The invasion of cane toads through tropical Australia is unlikely to have had a major impact on retreat‐site availability for native frogs.     

A review of ecological interactions between native frogs and invasive cane toads in Australia

Translocated from their native range in the Americas in 1935, cane toads (Rhinella marina, Bufonidae) have now spread through much of tropical and subtropical Australia. The toad's invasion and impact have attracted detailed study. In this paper, I review information on ecological interactions between cane toads and Australian anurans. The phylogenetic relatedness and ecological similarity between frogs and toads creates opportunities for diverse interactions, ranging from predation to competition to parasite transfer, plus a host of indirect effects mediated via impacts of toads on other species, and by people's attempts to control toads. The most clear‐cut effect of toads on frogs is a positive one: reducing predator pressure by fatally poisoning anuran‐eating varanid lizards. However, toads also have a wide range of other effects on frogs, some positive (e.g. taking up parasites that would otherwise infect native frogs) and others negative (e.g. eating frogs, poisoning frogs, competing with tadpoles). Although information on such mechanisms predicts intense interactions between toads and frogs, field surveys show that cane toad invasion has negligible overall impacts on frog abundance. That counter‐intuitive result is because of a broad balancing of negative and positive impacts, coupled with stochastic (weather‐induced) fluctuations in anuran abundance that overwhelm any impacts of toads. Also, the impacts of toads on frogs differ among frog species and life‐history stages, and depend upon local environmental conditions. The impacts of native frogs on cane toads have attracted much less study, but may well be important: frogs may impose biotic resistance to cane toad colonization, especially via competition in the larval phase. Overall, the interactions between native frogs and invasive toads illustrate the diverse ways in which an invader's arrival can perturb the native fauna by both direct and indirect mechanisms, and by which the native species can curtail an invader's success. These studies also offer a cautionary tale about the difficulty of predicting the impact of an invasive species, even with a clear understanding of mechanisms of direct interaction.     

The accelerating invasion: dispersal rates of cane toads at an invasion front compared to an already-colonized location

Evolutionary theory predicts that individuals at an expanding range edge will disperse faster than conspecifics in long-colonized locations, but direct evidence is rare. Previous reports of high rates of dispersal of cane toads (Rhinella marina) at the invasion front have been based on studies at a single site in the Northern Territory. To replicate the earlier work, we radio-tracked free-ranging toads in the Kimberley region of northwestern Australia (at the westward-spreading invasion front) and 500 km northeast, on the Adelaide River floodplain of the Northern Territory (where toads had already been present for 6 years). For comparison, we also radio-tracked native frogs (Litoria caerulea and L. splendida) at the same sites. Consistent with the earlier reports, invasion-front cane toads travelled further per day, were more highly directional, and re-used refuge sites less frequently, than did conspecifics from an already-colonized site. In contrast, native frogs showed similar movement patterns in the two study areas. Our results confirm previous reports, and suggest that accelerated dispersal may be a common feature of individuals at the vanguard of a biological invasion.     

Do invasive cane toads (Chaunus marinus) compete with Australian frogs (Cyclorana australis)?

Abstract Despite widespread concern about the ecological impacts of invasive species, mechanisms of impact remain poorly understood. Cane toads (Chaunus [Bufo] marinus) were introduced to Queensland in 1935, and have now spread across much of tropical Australia. One plausible impact of toad invasion concerns competition between toads and native frogs, but there has been no previous experimental evaluation of this possibility. We examined interactions between toads and a morphologically similar species of native frog (Cyclorana australis) by manipulating toad and frog densities within large outdoor enclosures beside a floodplain in the wet‐dry tropics of the Northern Territory. Toads differed from frogs significantly in dietary composition and feeding rates, even in comparisons controlling for body‐size differences between these two taxa. Perhaps reflecting the abundant insect biomass, manipulating anuran densities or the presence of the putatively competing species did not influence food intake or dietary composition. However, the presence of toads suppressed activity levels of native frogs. The degree to which the invasion of cane toads influences attributes such as the activity levels, food intake and dietary composition of native frogs warrants further study, but our study suggests that competitive effects are likely to be minor compared with other pathways (such as direct poisoning during ingestion attempts) by which toads can affect frog populations.     

Locomotor performance in an invasive species: cane toads from the invasion front have greater endurance, but not speed, compared to conspecifics from a long-colonised area

Cane toads (Bufo marinus) are now moving about 5 times faster through tropical Australia than they did a half-century ago, during the early phases of toad invasion. Radio-tracking has revealed higher daily rates of displacement by toads at the invasion front compared to those from long-colonised areas: toads from frontal populations follow straighter paths, move more often, and move further per displacement than do toads from older (long-established) populations. Are these higher movement rates of invasion-front toads associated with modified locomotor performance (e.g. speed, endurance)? In an outdoor raceway, toads collected from the invasion front had similar speeds, but threefold greater endurance, compared to conspecifics collected from a long-established population. Thus, increased daily displacement in invasion-front toads does not appear to be driven by changes in locomotor speed. Instead, increased dispersal is associated with higher endurance, suggesting that invasion-front toads tend to spend more time moving than do their less dispersive conspecifics. Whether this increased endurance is a cause or consequence of behavioural shifts associated with rapid dispersal is unclear. Nonetheless, shifts in endurance between frontal and core populations of this invasive species point to the complex panoply of traits affected by selection for increased dispersal ability on expanding population fronts.     

The enduring toxicity of road-killed cane toads (<Emphasis Type="Italic">Rhinella marina</Emphasis>)

The primary ecological impact of invasive cane toads (Rhinella marina) in Australia is mediated by their powerful toxins, which are fatal to many native species. Toads use roads as invasion corridors and feeding sites, resulting in frequent road-kills. The flattened, desiccated toad carcasses remain highly toxic despite being heated daily to >40°C for many months during the tropical dry-season. In controlled laboratory experiments, native tadpoles (Cyclorana australis, Litoria rothii), fishes (Mogurnda mogurnda) and leeches (Family Erpobdellidae) died rapidly when we added fragments of sun-dried toad to their water, even if the native animals had no physical access to the carcass. Given the opportunity, native tadpoles and fishes strongly avoided the vicinity of dried toad fragments. Hence, long-dead toads may contaminate roadside ponds formed by early wet-season rains and induce avoidance and/or mortality of native anuran larvae, fishes and invertebrates. Our studies show that the toxicity of this invasive species does not end with the toad’s death, and that methods for disposing of toad carcasses (e.g., after culling operations) need to recognize the persistent danger posed by those carcasses.     

Frogs in the spotlight: a 16‐year survey of native frogs and invasive toads on a floodplain in tropical Australia

Although widespread declines in anuran populations have attracted considerable concern, the stochastic demographics of these animals make it difficult to detect consistent trends against a background of spatial and temporal variation. To identify long‐term trends, we need datasets gathered over long time periods, especially from tropical areas where anuran biodiversity is highest. We conducted road surveys of four anurans in the Australian wet–dry tropics on 4637 nights over a 16‐year period. Our surveys spanned the arrival of invasive cane toads (Rhinella marina), allowing us to assess the invader's impact on native anuran populations. Our counts demonstrate abrupt and asynchronous shifts in abundance and species composition from one year to the next, not clearly linked to rainfall patterns. Typically, periods of decline in numbers of a species were limited to 1–2 years and were followed by 1‐ to 2‐year periods of increase. No taxa showed consistent declines over time, although trajectories for some species showed significant perturbations coincident with the arrival of toads. None of the four focal frog species was less common at the end of the study than at the beginning, and three of the species reached peak abundances after toad arrival. Survey counts of cane toads increased rapidly during the initial stage of invasion but have subsequently declined and fluctuated. Distinguishing consistent declines versus stochastic fluctuations in anuran populations requires extensive time‐series analysis, coupled with an understanding of the shifts expected under local climatic conditions. This is especially pertinent when assessing impacts of specific perturbations such as invasive species.     

Foraging Responses of the Larvae of Invasive Bullfrogs(Lithobates catesbeianus): Possible Implications for Bullfrog Control and Ecological Impact in China

The predatory behavior of invasive species can affect their ecological impact, and offer opportunities for targeted control. In Australia, tadpoles of invasive cane toads(Rhinella marina) do not consume eggs of native anurans, but are strongly attracted to(and consume) newly-laid eggs of conspecifics; chemical cues from such eggs(or adult secretions) thus can be used to attract toad tadpoles to traps. Do other invasive anurans show similar selectivity? Our laboratory trials on a Chinese population of invasive American bullfrogs(Lithobates catesbeianus) revealed similar behaviors as exhibited by Australian cane toads. Bullfrog tadpoles rarely consumed the eggs of native anurans, but were attracted to both bullfrog eggs and bullfrog skin secretions. Although the attraction response was less intense in bullfrogs than in cane toads, it might nonetheless enable selective removal of bullfrog tadpoles from invaded sites.     

Sex and age differences in habitat use by invasive cane toads (Rhinella marina) and a native anuran (Cyclorana australis) in the Australian wet–dry tropics

Although generalized habitat use may contribute to the success of invasive taxa, even species that are typically described as habitat generalists exhibit non‐random patterns of habitat use. We measured abiotic and biotic factors in 42 plots (each 100 × 10 m) along a 4.2‐km long unpaved road in tropical Australia, at a site that had been invaded by cane toads (Rhinella marina Bufonidae) seven years previously. We also counted anurans at night in each of these plots on 103 nights during the tropical wet season, over a five‐year period, beginning soon after the initial toad invasion. Spatial distributions differed significantly among adult male toads (n = 1047), adult female toads (n = 1222), juvenile toads (n = 342) and native frogs (Cyclorana australis Hylidae, n = 234). Adult male toads were closely associated with water bodies used as calling and/or spawning sites, whereas adult female toads and native frogs were most commonly encountered in drier forested areas on sloping ground. Juvenile toads used the margins of the floodplain more than conspecific adults did, but the floodplain itself was rarely used. Understanding which components of the habitat are most important to specific age and sex classes within a population, or how invasive species differ from native species in this respect, can clarify issues such as the spatial and temporal location of ecological impact by an invader, and the most effective places for control of the invader with minimal collateral effects on the native biota.     

Experimental evidence for the use of artificial refugia to mitigate the impacts of invasive <Emphasis Type="Italic">Gambusia holbrooki</Emphasis> on an endangered fish

Habitat degradation has a major impact on freshwater ecosystems and also facilitates biological invasions thus intensifying the problem. The survival of native species under threat from invaders can be improved either by eradicating the invading species or by providing resources or conditions that benefit the native species. One such resource is shelter and in degraded habitats artificial refugia may be a viable option. However, the use of artificial refugia to promote coexistence between native and invasive species remains poorly understood. We assessed the potential for artificial refugia to ameliorate the disruption of social interactions in an endangered native Iberian toothcarp, Aphanius iberius, by the invasive mosquitofish, Gambusia holbrooki. We found that mosquitofish do not compete for access to refugia despite their higher level of aggression compared to native fish. Native fish use refugia more overall, particularly in the presence of mosquitofish. Despite this, the benefits of refugia are not clear cut: increases in refuge use by male toothcarp induced by mosquitofish aggression correspond to decreases in attention to conspecifics. However, changes in refugia use over time together with constant attention to conspecifics indicates that it is not refugia use itself that disrupts social interactions but the interrelated effects of mosquitofish aggression. Provision of artificial refugia in degraded freshwater ecosystems may thus be a viable management tool to protect native populations under imminent threat of invasion.     

Cues for cannibalism: cane toad tadpoles use chemical signals to locate and consume conspecific eggs

Although animals of many species kill and consume conspecifics, most such cases probably involve serendipitous encounters between the individuals concerned. In some taxa, however, cannibalism is an active process, with predatory individuals searching out and consuming specific types of conspecific prey items. Although anuran tadpoles often have been reported to consume conspecific eggs, this behaviour has been interpreted as a by‐product of usual foraging behaviours rather than a result of targeted searching. Our field and laboratory studies in tropical Australia show that the tadpoles of invasive cane toads Bufo marinus are strongly attracted to chemical cues from conspecific eggs; the effective cues are released late in embryonic development, as the jelly coat breaks down. Tadpoles of native Australian frog species were attracted to the eggs of toads only rarely. If deployed as bait in traps, chemical cues from toad eggs could provide a way to selectively remove toad larvae from waterbodies.     

Toads in the backyard: why do invasive cane toads (Rhinella marina) prefer buildings to bushland?

Like many invasive species, cane toads (Rhinella marina) in Australia concentrate in the disturbed habitats created by human activity, rather than in pristine areas. We surveyed cane toads in the wet–dry tropics of the Northern Territory to assess the abundances, body sizes, sexes, behaviour, hydration state and feeding rates of toads around buildings compared to those in areas remote from buildings, and conducted experimental trials to assess the effects of building-related variables (lights and increased toad densities) on the foraging success of toads. Toads around buildings were smaller than bushland conspecifics, and adult sex-ratios were female-biased. Toads were more sedentary around buildings than in the bush, but their feeding rates (based on direct observations and faeces production post-capture) were similar. That similarity, despite twofold-higher densities of competing toads around building, reflected the strong enhancement of feeding rates due to artificial lights attracting insects (in our experimental trials, a threefold increase regardless of the number of competing toads). Toads collected from around buildings were apparently in better hydric condition. Thus, access to water also may attract toads to buildings. The relative scarcity of adult male toads around buildings likely reflects waterbody-centred reproductive activities, whereas the concentration of females and juveniles around buildings is driven largely by access to the insects attracted by artificial light. We conclude that buildings enhance the persistence of cane toad populations and may facilitate their spread.     

Call Transmission Efficiency in Native and Invasive Anurans: Competing Hypotheses of Divergence in Acoustic Signals

Invasive species are a leading cause of the current biodiversity decline, and hence examining the major traits favouring invasion is a key and long-standing goal of invasion biology. Despite the prominent role of the advertisement calls in sexual selection and reproduction, very little attention has been paid to the features of acoustic communication of invasive species in nonindigenous habitats and their potential impacts on native species. Here we compare for the first time the transmission efficiency of the advertisement calls of native and invasive species, searching for competitive advantages for acoustic communication and reproduction of introduced taxa, and providing insights into competing hypotheses in evolutionary divergence of acoustic signals: acoustic adaptation vs. morphological constraints. Using sound propagation experiments, we measured the attenuation rates of pure tones (0.2–5 kHz) and playback calls (Lithobates catesbeianus and Pelophylax perezi) across four distances (1, 2, 4, and 8 m) and over two substrates (water and soil) in seven Iberian localities. All factors considered (signal type, distance, substrate, and locality) affected transmission efficiency of acoustic signals, which was maximized with lower frequency sounds, shorter distances, and over water surface. Despite being broadcast in nonindigenous habitats, the advertisement calls of invasive L. catesbeianus were propagated more efficiently than those of the native species, in both aquatic and terrestrial substrates, and in most of the study sites. This implies absence of optimal relationship between native environments and propagation of acoustic signals in anurans, in contrast to what predicted by the acoustic adaptation hypothesis, and it might render these vertebrates particularly vulnerable to intrusion of invasive species producing low frequency signals, such as L. catesbeianus. Our findings suggest that mechanisms optimizing sound transmission in native habitat can play a less significant role than other selective forces or biological constraints in evolutionary design of anuran acoustic signals.