Browsing by Author "Glander, Kenneth"

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    Audiograms of howling monkeys: are extreme loud calls the result of runaway selection?
    (2019-02-05) Ramsier, Marissa; Cunningham, Andrew; Patiño, May; Glander, Kenneth; Dominy, Nathaniel
    Abstract The eponymous vocalizations of howling monkeys (genus Alouatta ) are associated with territorial defense and male-male competition, yet the extreme loudness of howls, which are among the loudest vocalizations of any terrestrial mammal, have yet to be fully explained. Loudness facilitates long-distance sound propagation but the effectiveness of any vocal signal depends in part on the auditory capabilities of the intended receiver, and the auditory sensitivities of howling monkeys are unknown. To better understand the evolution of loud calls, we used the auditory brainstem response (ABR) method to estimate the auditory sensitivities of Alouatta palliata . The mean estimated audiogram of four wild-caught adults displayed a w-shaped pattern with two regions of enhanced sensitivity centered at 0.7-1.0 and 11.3 kHz. The lower-frequency region of auditory sensitivity is pitched moderately higher than the fundamental frequencies of howling, whereas the higher-frequency region corresponds well with harmonics in an infant distress call, the wrah-ha . Fitness advantages from detecting infants amid low-frequency background noise, including howling, could explain the incongruity between our ABR thresholds and the fundamental frequencies of howling. Attending to infant calls is expected to enhance reproductive success within an infanticidal genus, and we suggest that the extraordinary loudness of male howling is an indirect (runaway) result of positive feedback between the selective pressures of hearing infant distress calls and deterring infanticide.
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    When top-down becomes bottom up: behaviour of hyperdense howler monkeys (Alouatta seniculus) trapped on a 0.6 ha island.
    (PLoS One, 2014) Orihuela, Gabriela; Terborgh, John; Ceballos, Natalia; Glander, Kenneth
    Predators are a ubiquitous presence in most natural environments. Opportunities to contrast the behaviour of a species in the presence and absence of predators are thus rare. Here we report on the behaviour of howler monkey groups living under radically different conditions on two land-bridge islands in Lago Guri, Venezuela. One group of 6 adults inhabited a 190-ha island (Danto) where they were exposed to multiple potential predators. This group, the control, occupied a home range of 23 ha and contested access to food resources with neighbouring groups in typical fashion. The second group, containing 6 adults, was isolated on a remote, predator-free 0.6 ha islet (Iguana) offering limited food resources. Howlers living on the large island moved, fed and rested in a coherent group, frequently engaged in affiliative activities, rarely displayed agonistic behaviour and maintained intergroup spacing through howling. In contrast, the howlers on Iguana showed repulsion, as individuals spent most of their time spaced widely around the perimeter of the island. Iguana howlers rarely engaged in affiliative behaviour, often chased or fought with one another and were not observed to howl. These behaviors are interpreted as adjustments to the unrelenting deprivation associated with bottom-up limitation in a predator-free environment.