In contrast to the cries of human infants, sounds made by non-human infants in different stressful behavioral contexts (hunger or physical discomfort, isolation, capture by humans or predators) are usually treated as distinct types of vocalizations. However, if distress vocalizations produced by different species and in different contexts share a common motivational state and associated neurochemical pathways, we can expect them to share a common acoustic structure and adaptive function, showing only limited variation that corresponds to the infant’s level of arousal. Based on this premise, we review the acoustic structure and adaptive function of two types of distress calls, those given when infants were isolated from their mothers (isolation calls) or captured by humans (capture calls). We conducted a within-context comparison examining the two call types across a diverse selection of mammalian species and other vertebrate groups, followed by a comparison of how acoustic structure and function differs between these contexts. In addition, we assessed acoustic traits that are critical to the response of caregivers. Across vertebrate species, distress vocalizations produced in these two behavioral contexts tend to be tonal with a simple chevron, flat or descending pattern of frequency modulation. Reports that both isolation and capture calls of vertebrate infants serve to attract care-givers are universal, and the fundamental frequency of infant vocalizations is often critical to this response. The results of our review are consistent with the hypothesis that differences in the acoustic structure of isolation and capture distress vocalizations reflect differences in arousal, and not discrete functions. The similarity in acoustic structure and caregiver response observed across vertebrates adds support to the hypothesis that the production and processing of distress vocalizations are part of a highly-conserved system of social vocal behaviour in vertebrates. Bioacoustic research may move forward by recognizing the commonality among different forms of infant solicitations that attract caregivers, and the commonality of these solicitations with vocalizations that attract conspecifics in still other behavioral contexts.

It is well known that prey of different size and morphology often use different antipredator strategies. The prevailing notion is that this occurs because size, morphology and weaponry determine the relative effectiveness of alternative strategies, and nowhere is this assumption more entrenched than in our view of the basic decision to stay, fight or flee. Here, we use observations of coyote (Canis latrans) packs hunting deer in winter to show that two ungulates of similar size and morphology, white-tailed deer (Odocoileus virginianus) and mule deer (O. hemionus), use different antipredator strategies when encountered or attacked. Mule deer typically responded by holding their ground and aggressively defending conspecifics, and were at high risk of being attacked and killed if they fled or were undefended. White-tails always fled when pursued or attacked by coyotes. Coyotes pursued fewer white-tails than mule deer they encountered regardless of prey response. Once pursued or attacked, white-tails faced a risk of attack and capture, respectively, that was intermediate between the high and low risk mule deer groups. The overall risk of capture per encounter for white-tails was similar to that facing mule deer that confronted coyotes, which was much lower than risk facing mule deer that fled and were undefended. Contextual variables such as the opportunity to improve one's position by joining another group, moving to rugged terrain, or the presence of companions that are willing to provide defense may explain why a mixed strategy is maintained in mule deer, despite the apparently detrimental effects of flight. These examples illustrate the value of including prey behavior in models of hunting success in so far as prey defenses may not be coupled with differences in size and morphology.

Acoustic structure, behavioral context, and caregiver responses to infant distress vocalizations (cries) are similar across mammals, including humans. Are these similarities enough for animals to respond to distress vocalizations of taxonomically and ecologically distant species? We show that mule deer (Odocoileus hemionus) and white-tailed deer (Odocoileus virginianus) mothers approach a speaker playing distress vocalizations of infant marmots (Marmota flaviventris), seals (Neophoca cinerea and Arctocephalus tropicalis), domestic cats (Felis catus), bats (Lasionycteris noctivagans), humans (Homo sapiens), and other mammals if the fundamental frequency (F0) falls or is manipulated to fall within the frequency range in which deer respond to young of their own species. They did not approach to predator sounds or to control sounds having the same F0 but a different structure. Our results suggest that acoustic traits of infant distress vocalizations that are essential for a response by caregivers, and a caregiver's sensitivity to these acoustic traits, may be shared across diverse mammals.

No abstract

Predation has been proposed as a major factor maintaining segregation among species of ungulates, but predator-prey interactions have not been observed to test this idea directly. Here, observations of coyote (Canis latrans) packs hunting deer are used to show that mule deer (Odocoileus hemionus), which typically stand high on slopes and on rugged terrain, increased both their risk of being encountered and attacked by coyotes by standing low rather than high on slopes. The risk incurred at certain heights was not fixed: a mule deer's risk of being approached by coyotes was also affected by the height of other mule deer groups present during the same hunt. White-tailed deer (O. virginianus), which typically use gentle terrain, were not similarly disadvantaged by remaining low on slopes or on flat terrain. When confronted by coyotes, mule deer moved to and up slopes, whereas white-tails moved down and away from slopes. Species differences in behavior were independent of starting position and were observed for animals in mixed-species groups. Unlike their response to coyotes, feeding preferences or competition did not lead mule deer to use rugged habitats: mule deer moved down and left slopes to feed, bringing them closer to white-tails. These results suggest that coyote predation contributes to the habitat segregation of white-tails and mule deer (1) by selective predation against mule deer but not white-tails in gentle habitats and (2) by eliciting differing antipredator behavior that increases their segregation. Unlike prey involved in other examples of predator-mediated resource partitioning, white-tails and mule deer are similar in size and morphology. Contrasting antipredator strategies, specifically, their ability to avoid predation using flight or confrontation, are likely to explain why the species differ in their behavior and in their risk on gentle terrain.

Summary 1.Ungulates are viewed as being highly susceptible to predation during the initial weeks or months of life. Yet aggressive defence by adult females is common in many ungulates and has the potential to reduce the vulnerability of the young significantly. 2. We observed naturally occurring predatory encounters between coyotes Canis latrans Say and deer fawns to test the hypothesis that a difference in aggressive defence leads to the differential vulnerability of mule deer Odocoileus hemionus Rafinesque and whitetailed deer O. virginianus Zimmermann fawns in summer, when fawns are 0-14 weeks in age. Whitetail fawns suffer higher levels of coyote predation than do mule deer fawns at that time. The two species of deer are similar in size, but are known to differ in their antipredator behaviour in winter when fawns are older. 3. Coyotes were less likely to attack mule deer than whitetail fawns they encountered, and were less likely to kill mule deer than whitetail fawns they attacked. 4. The presence of a mule deer, but not a whitetail, female with a fawn deterred coyotes from attacking the fawn. Once attacked, fawns of both species were less likely to be killed when females defended them, but mule deer females were far more likely to defend fawns. 5. Mule deer females defended fawns that were not their own offspring, including heterospecific fawns. Mule deer fawns were more likely to be defended if they had a larger number of females nearby when encountered. These observations raise the possibility that mule deer, and even whitetail, fawns may have improved survival in areas with higher densities of mule deer females. 6. These results show that higher levels of defence by mule deer females reduced the vulnerability of mule deer fawns, contributing to the lower predation rates reported for mule deer than for whitetail fawns of this age group.

White‐tailed deer (Odocoileus virginianus) and mule deer (O. hemionus) are closely related species of similar size that differ in their anti‐predator behavior. White‐tails flee from coyotes (Canis latrans), whereas mule deer typically stand their ground and attack this predator. I used observations of coyotes hunting deer to identify: (i) changes in group structure made in response to coyotes; and (ii) the relationship between group structure and the risk of predation for each species. In response to coyotes, groups of mule deer merged with other groups and individuals bunched together. Predation attempts were more likely to escalate when groups split and individuals failed to bunch. Coyotes typically attacked mule deer that were in outlying positions, and these deer had to move to central positions to end attacks. Due to the high frequency of attacks on small groups as well as to the level of dilution of risk, individuals in small mule deer groups were at high risk of being attacked compared with those in larger groups. In contrast to mule deer, white‐tails made no consistent changes in group size or formation, and coyotes attacked individuals in central as well as in outlying positions. Variation in aspects of group cohesion was not related to the vulnerability of white‐tails, and there was no obvious difference in the risk of attack facing individuals in groups of different size. These results suggest that coyote predation selects for relatively large, cohesive groups in mule deer, apparently because this type of group improves their ability to deter coyotes. Coyote predation does not have similar effects on groups formed by white‐tails, which use flight rather than deterrence to avoid predation. The benefits of responding cohesively, occupying certain positions within groups, and forming groups of a certain size can vary widely depending on the anti‐predator strategies used by an animal.

Predation has been proposed as a major factor maintaining segregation among species of ungulates, but predator-prey interactions have not been observed to test this idea directly. Here, observations of coyote (Canis latrans) packs hunting deer are used to show that mule deer (Odocoileus hemionus), which typically stand high on slopes and on rugged terrain, increased both their risk of being encountered and attacked by coyotes by standing low rather than high on slopes. The risk incurred at certain heights was not fixed: a mule deer's risk of being approached by coyotes was also affected by the height of other mule deer groups present during the same hunt. White-tailed deer (O. virginianus), which typically use gentle terrain, were not similarly disadvantaged by remaining low on slopes or on flat terrain. When confronted by coyotes, mule deer moved to and up slopes, whereas white-tails moved down and away from slopes. Species differences in behavior were independent of starting position and were observed for animals in mixed-species groups. Unlike their response to coyotes, feeding preferences or competition did not lead mule deer to use rugged habitats: mule deer moved down and left slopes to feed, bringing them closer to white-tails. These results suggest that coyote predation contributes to the habitat segregation of white-tails and mule deer (1) by selective predation against mule deer but not white-tails in gentle habitats and (2) by eliciting differing antipredator behavior that increases their segregation. Unlike prey involved in other examples of predator-mediated resource partitioning, white-tails and mule deer are similar in size and morphology. Contrasting antipredator strategies, specifically, their ability to avoid predation using flight or confrontation, are likely to explain why the species differ in their behavior and in their risk on gentle terrain.