Perch exposure and predation risk: a comparative study in passerines

Singing birds optimize signal transmission by perching in exposed locations. However, conspicuous singing may be risky, and previous research has found that individuals trade off singing performance with song perch exposure. We studied the relationship between predation risk (degree of concealment, height in tree or shrub, and distance to the forest edge) and time allocated to singing and vigilance in a group of 13 passerine species living in an East African savanna. Concealed birds sang more and were less vigilant. Vigilance increased as distance to the forested edge increased, but distance had no effect on time allocated to singing. Body size was significantly correlated with vigilance but not singing; larger passerines were more sensitive to both relative concealment and the distance to the forest edge, while song was influenced by neither of these factors. Perch height had no effect on either behavior. Our results suggest that birds modify vigilance and, to some extent, singing behavior to minimize their exposure to predators.     

The Sound of Arousal: The Addition of Novel Non-linearities Increases Responsiveness in Marmot Alarm Calls

Vocal structure should reflect vocal function. While much attention has focused on quantifying attributes of harmonic vocalizations, the vocalizations of many species also may contain non-linear phenomena such as warbles, subharmonics, biphonation, and deterministic chaos or noise. The function of these non-linearities remains enigmatic. In some species, harmonic vocalizations abruptly become 'noisy' when individuals are physiologically aroused and the sudden onset of these non-linearities could signal arousal or fear to receivers. One untested functional hypothesis is that vocalizations containing non-linearities are more variable from one rendition to the next, and thus are harder to habituate to. In some situations, reducing the likelihood of habituation could be important. Signals that are highly evocative are more difficult to habituate to. Thus, we conducted playback experiments to foraging yellow-bellied marmots ( Marmota flaviventris ) to determine whether the addition of white noise (a non-linear acoustic phenomenon) to alarm calls elicited a greater response than control calls without the non-linearity or control calls with silence, rather than noise, added to them. Marmots spent less time foraging after hearing calls that included noise than after normal or control calls. This result is consistent with the unpredictability hypothesis and suggests that the adaptive value of non-linearities is to prevent habituation.     

Faecal glucocorticoid metabolites and alarm calling in free-living yellow-bellied marmots

When individuals of a variety of species encounter a potential predator, some, but not all, emit alarm calls. To explain the proximate basis of this variation, we compared faecal glucocorticoid metabolite concentrations in live-trapped yellow-bellied marmots (Marmota flaviventris) between occasions when they did and did not emit alarm calls. We found that marmots had significantly higher glucocorticoid levels when they called than when they did not call, suggesting that stress or arousal may play an important role in potentiating alarm calls. Marmots are sensitive to variation in the reliability of callers. The present finding provides one possible mechanism underlying caller variation: physiological arousal influences the propensity to emit alarm calls.     

Anthropogenic noise affects risk assessment and attention: the distracted prey hypothesis

Many studies have focused on the effects of anthropogenic noise on animal communication, but only a few have looked at its effect on other behavioural systems. We designed a playback experiment to test the effect of noise on predation risk assessment. We found that in response to boat motor playback, Caribbean hermit crabs (Coenobita clypeatus) allowed a simulated predator to approach closer before they hid. Two hypotheses may explain how boat noise affected risk assessment: it masked an approaching predator''s sound; and/or it reallocated some of the crabs'' finite attention, effectively distracting them, and thus preventing them from responding to an approaching threat. We found no support for the first hypothesis: a silent looming object still got closer during boat motor playbacks than during silence. However, we found support for the attentional hypothesis: when we added flashing lights to the boat motor noise to further distract the hermit crabs, we were able to approach the crabs more closely than with the noise alone. Anthropogenic sounds may thus distract prey and make them more vulnerable to predation.     

Effect of predation risk on the presence and persistence of yellow-bellied marmot (Marmota flaviventris) colonies

Habitat selection may have population level consequences and ultimately may influence a population's local persistence or extinction. We capitalized on a long-term study (1962–2004) of yellow-bellied marmots Marmota flaviventris in and around the Rocky Mountain Biological Laboratory, Gothic, Colorado, USA, and compared habitat characteristics associated with food availability and predation risk to explain variation in persistence of marmots at 27 sites, and their absence at 22 additional, randomly selected sites. We classified sites as persistent, intermittent or null based on whether there was a history of extinction; intermittent sites periodically went extinct and null sites never had marmots. Logistic regression analyses revealed that environmental variables associated with visibility and safety, rather than food, correctly classified sites as persistent or non-persistent as well as persistent or intermittent. Discriminant function analysis that included the null sites revealed that the same visibility-related characteristics predicted where marmots were found. These results highlight the importance of variation in safety among sites in predicting long-term population persistence, as well as a species' distribution.     

Testing alternative hypotheses for evolutionary diversification in an African songbird: rainforest refugia versus ecological gradients

Geographic isolation in rainforest refugia and local adaptation to ecological gradients may both be important drivers of evolutionary diversification. However, their relative importance and the underlying mechanisms of these processes remain poorly understood because few empirical studies address both putative processes in a single system. A key question is to what extent is divergence in signals that are important in mate and species recognition driven by isolation in rainforest refugia or by divergent selection across ecological gradients? We studied the little greenbul, Andropadus virens, an African songbird, in Cameroon and Uganda, to determine whether refugial isolation or ecological gradients better explain existing song variation. We then tested whether song variation attributable to refugial or ecological divergence was biologically meaningful using reciprocal playback experiments to territorial males. We found that much of the existing song variation can be explained by both geographic isolation and ecological gradients, but that divergence across the gradient, and not geographic isolation, affects male response levels. These data suggest that ecologically divergent traits, independent of historical isolation during glacial cycles, can promote reproductive isolation. Our study provides further support for the importance of ecology in explaining patterns of evolutionary diversification in ecologically diverse regions of the planet.     

Are blue land crabs (Cardisoma guanhumi) attracted to falling fruit?

Many species rely on multiple modalities to acquire information about predation risk, potential mates, and food. We studied the sensory modalities of blue land crabs, Cardisoma guanhumi, used for food detection. We isolated the acoustic and seismic cues of falling fruit and measured latency to emerge from their burrows after hearing the sound of falling fruits, seismic signals associated with fruit drop, and a combination of both modalities. In contrast to a previous study, we found no support that either substrate-born vibration or sound-enhanced emergence time. In fact, the actual fruit drop caused slower emergence times at one site. This crab lives in a seismically variable environment and perhaps such species are likely to rely more on other modalities to identify food.     

Masculinized female yellow-bellied marmots initiate more social interactions

The presence of male siblings in utero influences female morphology and life-history traits because testosterone transferred among foetuses may masculinize females. Similarly, litter sex composition might alter the display of sexually dimorphic behaviour, such as play and allogrooming, since they are modulated by androgens. We explored whether masculinization alters the frequency of play and sociopositive behaviour in female yellow-bellied marmots (Marmota flaviventris). We found that masculinized juvenile females were more likely to initiate play and allogrooming, but yearling females exhibited higher levels of oestrogen-modulated sociopositive behaviours. Additionally, the more they interacted, the greater number of different partners they interacted with. Our results suggest that masculinization increases the rate of age-dependent social behaviour. This probably works by increasing exploration that predisposes individuals to higher encounter rates. Further support comes from previous findings showing that masculinized females were more likely to disperse. Our study stresses the importance of considering litter sex composition as a fitness modulator.