The brain limit

How fast the brain and muscles can respond to information about prey location constrains visual and echolocating predators in similar ways.

Keeping an ear out: size relationship of the tympanic bullae and pinnae in bandicoots and bilbies (marsupialia: Peramelemorphia)

Bandicoots and bilbies (Order Peramelemorphia) occupy a broad range of habitats across Australia and New Guinea, from open, arid deserts to dense forests. This once diverse group has been particularly vulnerable to habitat loss and introduced eutherian predators, and numerous species extinctions and range retractions have occurred. Understanding reasons for this loss requires greater understanding of their biology. Morphology of the pinnae and tympanic bullae varies markedly amongst species. As hearing is important for both predator avoidance and prey location, the variability in ear morphology could reflect specialisation and adaptation to specific environments, and therefore be of conservation relevance. We measured 798 museum specimens representing 29 species of Peramelemorphia. Controlling for phylogenetic relatedness and head length, pinna surface area was weakly negatively correlated with average precipitation (rainfall being our surrogate measure of vegetation productivity/complexity), and there were no environmental correlates with effective diameter (pinna width). Controlling for phylogenetic relatedness and skull length, tympanic bulla volume was negatively correlated with precipitation. Species that inhabited drier habitats, which would be open and allow sound to carry further with less obstruction, had relatively larger pinnae and tympanic bullae. By contrast, species from higher rainfall habitats, where sounds would be attenuated and diffused by dense vegetation, had the smallest pinnae and bullae, suggesting that low-frequency hearing is not as important in these habitats. Associations with temperature did not reach statistical significance. These findings highlight linkages between hearing traits and habitat that can inform conservation and management strategies for threatened species.

Effects of vessel sound on oyster toadfish Opsanus tau calling behavior

In coastal waters, anthropogenic activity and its associated sound have been shown to negatively impact aquatic taxa that rely on sound signaling and reception for navigation, prey location, and intraspecific communication. The oyster toadfish Opsanus tau depends on acoustic communication for reproductive success, as males produce ‘boatwhistle’ calls to attract females to their nesting sites. However, it is unknown if in situ vessel sound impacts intraspecific communication in this species. Passive acoustic monitoring using a 4-hydrophone linear array was conducted in Eel Pond, a small harbor in Woods Hole, MA, USA, to monitor the calling behavior of male toadfish. The number of calls pre- and post-exposure to vessel sound was compared. Individual toadfish were localized, and their approximate sound level exposure was predicted using sound mapping. Following exposure to vessel sound, the number of calls significantly decreased compared to the number of calls pre-exposure, with vessel sound overlapping the frequency range of male toadfish boatwhistles. This study provides support that anthropogenic sound can negatively affect intraspecific communication and suggests that in situ vessel sound has the ability to mask boatwhistles and change the calling behavior of male toadfish. Masking could lead to a reduction in intraspecific communication and lower reproductive efficiency within the Eel Pond toadfish population.

Brown rats and house mice eavesdrop on each other’s volatile sex pheromone components

Mammalian pheromones often linger in the environment and thus are particularly susceptible to interceptive eavesdropping, commonly understood as a one-way dyadic interaction, where prey sense and respond to the scent of a predator. Here, we tested the “counterespionage” hypothesis that predator and prey co-opt each other’s pheromone as a cue to locate prey or evade predation. We worked with wild brown rats (predator of mice) and wild house mice (prey of brown rats) as model species, testing their responses to pheromone-baited traps at infested field sites. The treatment trap in each of two trap pairs per replicate received sex attractant pheromone components (including testosterone) of male mice or male rats, whereas corresponding control traps received only testosterone, a pheromone component shared between mouse and rat males. Trap pairs disseminating male rat pheromone components captured 3.05 times fewer mice than trap pairs disseminating male mouse pheromone components, and no female mice were captured in rat pheromone-baited traps, indicating predator aversion. Indiscriminate captures of rats in trap pairs disseminating male rat or male mouse pheromone components, and fewer captures of rats in male mouse pheromone traps than in (testosterone-only) control traps indicate that rats do eavesdrop on the male mouse sex pheromone but do not exploit the information for mouse prey location. The counterespionage hypothesis is supported by trap catch data of both mice and rats but only the mice data are in keeping with our predictions for motive of the counterespionage.

The long-range echo scene of the sperm whale biosonar

Sperm whales use their gigantic nose to produce the most powerful sounds in the animal kingdom, presumably to echolocate deep-sea prey at long ranges and possibly to debilitate prey. To test these hypotheses, we deployed sound recording tags (DTAG-4) on the tip of the nose of three sperm whales. One of these recordings yielded over 6000 echo streams from organisms detected up to 144 m ahead of the whale, supporting a long-range prey detection function of the sperm whale biosonar. The whale navigated this complex acoustic scene by maintaining a stable, long-range acoustic gaze suggesting continual resource evaluation. Less than 10% of the echoic organisms recorded by the tag were targeted for capture and only 18% of the buzzes were emitted within the 50 m depth interval of maximum organism encounter rate, demonstrating echo-guided prey selection. Buzzes were initiated more than 20 m from the prey, showing that sperm whales do not debilitate their prey with sound, but trade echo levels for reduced forward masking and rapid updates on prey location in keeping with the lower manoeuvrability of these large predators. We conclude that the powerful biosonar of sperm whales enables long-range echolocation and selection of prey, but not acoustic debilitation.

Dynamics of gaze control during prey capture in freely moving mice

Many studies of visual processing are conducted in constrained conditions such as head- and gaze-fixation, and therefore less is known about how animals actively acquire visual information in natural contexts. To determine how mice target their gaze during natural behavior, we measured head and bilateral eye movements in mice performing prey capture, an ethological behavior that engages vision. We found that the majority of eye movements are compensatory for head movements, thereby serving to stabilize the visual scene. During movement, however, periods of stabilization are interspersed with non-compensatory saccades that abruptly shift gaze position. Notably, these saccades do not preferentially target the prey location. Rather, orienting movements are driven by the head, with the eyes following in coordination to sequentially stabilize and recenter the gaze. These findings relate eye movements in the mouse to other species, and provide a foundation for studying active vision during ethological behaviors in the mouse.

A new species of frog-biting midge from Papua New Guinea with a key to the described Corethrellidae of the Australopapuan region (Diptera, Corethrellidae, Corethrella)

CorethrellaoppositophilaKvifte & Bernal,sp. n.is described based on one male and six female specimens collected at 2200 m a.s.l. on Mount Wilhelm, Papua New Guinea. The species is the fourth species of frog-biting midge described from this country and appears similar toCorethrellasolomonisBelkin based on pigmentation of legs and abdominal tergites. It differs fromC.solomonis, however, in the shape of female flagellomeres I–III, and in the thorax which has a dark brown vertical stripe. The new species is named for its sexually dimorphic flagellomeres, which are short and squat in the female and elongate in the male. These differences in morphological characters are discussed in light of the likely sexual differences in functional uses of the antennae, as males use them for mating only whereas females use them both for mating and prey location. An emended key is presented to the described Australopapuan species of Corethrellidae.

Are wild insectivorous birds attracted to methyl-jasmonate-treated Pyrenean oak trees?

Plants emit herbivore-induced plant volatiles (HIPVs) when they are attacked by herbivorous arthropods to attract natural enemies of the herbivores. Previous studies have suggested that insectivorous birds can detect the HIPVs for prey location. The production of HIPVs is partly mediated by the jasmonic acid signalling pathway. Methyl jasmonate is one compound involved in the jasmonic acid signalling pathway. Therefore, spraying of methyl jasmonate (MeJa) solution on plants can induce the emission of volatiles similar to the HIPVs induced by herbivory. We performed two field experiments to test whether insectivorous adult birds are attracted to Pyrenean oak trees (Quercus pyrenaica) treated with MeJa solutions. We used artificial larvae to measure bird predation rate. There were no differences in the predation rates of plasticine larvae between control and MeJa treatments, suggesting that insectivorous birds were not attracted to trees treated with MeJa. We also tested whether blue tit nestlings without previous experience in foraging respond to the scent of Pyrenean oak tree leaves treated with MeJa, to test whether the attraction to MeJa-treated trees is innate. A similar number of nestlings performed begging responses when exposed to the scent of leaves of MeJa-treated trees compared to the scent of leaves of control untreated trees. Furthermore, the duration of the first begging was similar in response to the volatiles emitted by MeJa-treated trees and untreated trees. Despite previous evidence suggesting that insectivorous birds are attracted to HIPVs, our results show that neither insectivorous adult birds nor inexperienced nestlings were attracted to trees treated with MeJa. Further studies are needed to disentangle whether the differences in the emission of volatiles between herbivore-infested trees and MeJa-treated trees can explain this lack of attraction to MeJa-treated Pyrenean oak trees.