Foraging in the dark–integrating echolocation and smell to find a meal

Chestnut short-tailed bat (Carollia castanea). (Image: Carlos Nivelo)

The ability to detect and respond to the world around us is a marvel undertaken by all living things to perform the tasks necessary for life. From primitive sight and smell to more complex color vision and echolocation, a biological arms race has pressed on for millions of years with an ultimate goal of acquiring an advantage in staying alive and reproducing. 

A good example of complex sensory adaptation with consequences for finding nutrition to stay alive can be found within bat species.  Employing a number of unique sensory abilities such as vision, smell, and echolocation, bats are able to navigate diverse environments and hunt for a variety of food types. Doing this however requires a level of synchronization to utilize these senses efficiently given the parameters of the environment and the specific traits of the organism.

The manner in which bats modify and use these systems in the scope of food acquisition is the subject of interest for L. B. Leiser-Miller and S. E. Santana,  whose recent work:

(https://academic.oup.com/iob/advance-article/doi/10.1093/iob/obaa007/5803075?searchresult=1)

provides insights on how bats can modulate their use of sensory information in the face of different environmental cues.

Chestnut short-tailed bat (left), Piper plant (right). (Image:  S. E. Santana)

Chestnut short tailed bats are frugivores located throughout the forests of Central and South America. Between August and September of 2016, Leiser-Miller and Santana set out to La Selva Biological Station in Sarapiquı, Heredia Province, Costa Rica to investigate the relative influence of varying scent cues on behavioral foraging decisions, especially integrating their use of echolocation and olfactory (smell) techniques. 

By using 3D printed models to control the availability of sensory cues, such as the scent and texture of Piper plant vegetation and fruit, a favorite food item for these bats, Leiser-Miller and Santana could independently test the bats’ varying reliance on echolocation and smell. Experimental trials were held in flight cages where bats were able to forage freely, according to several treatments: 1) the presence of 3D printed fruit with fruit scent from fresh pulp, 2) 3D printed fruit with fresh vegetation, 3) 3D printed fruit with fruit scent and vegetation, and 4) 3D printed fruit with no scent or vegetation.  Echolocation and flight patterns were observed using visual and auditory recordings to determine behavioral responses to the treatments. Researchers also analyzed the chemical components of piper vegetation and ripe fruit scents to understand the possible differences in olfactory cues.

Figure 2. From Leiser-Miller and Santana demonstrating the experimental set up. A-C microphone to record echolocation, D food choice treatment, E infrared camera.

The authors expected that bats would primarily be motivated by the scent of ripe fruit, and secondarily by vegetation scents, since they are specialized frugivores. They also expected that in the absence of scent cues, bats will more readily echolocate in an attempt to replace missing information.

The data in fact did support these postulates. Bats readily made food selections (prey was “captured”) if there was a ripe fruit scent, regardless of vegetation presence. On the contrary, in the absence of fruit scent, neither vegetation nor 3D shape elicited a single fruit selection event. 

Figure 3. In the presence of fruit scents, Chestnut short-tailed bats more readily identify and select food targets (gray bars).From  Leiser-Miller and Santana 

These results support a primary role of olfaction (smell), and secondary role of echolocation for positively identifying food sources. It also suggests these bats use ripe fruit scent, as opposed to a combination of fruit/vegitative scents and shape, to ultimately determine if a target is worthy of selection. Chemical analysis of the volatile organic compounds (VOC’s) of fruit and vegitative scents further support this notion. Although the profile of both scents are similar, the abundance of specific VOC’s within ripe fruit scents may explain the ability to distinguish the presence and absence of fruit.

Figure 4: Summary of durations of echolocation calls (a) and intervals of echolocation calls (b) among treatments: 1) preference for fruit scent in the presence of vegetation scent, 2) preference between vegetation or fruit scent, 3) preference for vegetation scent in the presence of fruit scent, 4) preference for vegetation scent. Results suggest shorter duration calls at longer intervals with the presence of fruit scents and the opposite in the absence of fruit scents. From Leiser-Miller and Santana

Auditory data also supports that in the absence of scent cues, bats utilized longer echolocation calls in shorter intervals suggesting a heavier reliance on these systems when scent information is lacking. However, in the presence of fruit scents, shorter echolocation calls with longer intervals indicate modulation of sensory use toward olfactory systems. This circumstance is likely due to the dual role the nasal cavity plays in not only scent sensitivity but also in contributing to the auditory component of echolocation. 

These results illustrate a trade-off between usage of these olfactory and auditory sensory systems and suggest a hierarchical ranking of information when determining the identity and quality of a potential food source. Repeatable selection of targets with a fruit scent, but never without it, despite credible vegetative scents and fruit shapes may be advantageous in complicated and saturated environments full of background vegetation and unripe or other scents. By switching between sensory modes in the presence or absence of fruit, bats are able to filter olfactory and auditory noise to more efficiently gather and utilize information to determine the whereabouts of potential meals.

This work builds upon our understanding of system integration and performance in the realm of sensory ecology. This study also motivates future research on what key volatiles, or combinations of sensory cues initiate acceptance of fruit ripeness among the vegitative and environmental assemblage. Finding these results would help us have a more thorough understanding behind the mechanics and behavioural motivation of forging techniques initiated by Chestnut short-tailed bats. Knowledge such as this is applicable to similar species, and furthermore can help us protect the constituents of the ecosystem that these bats rely on for survival so that they can forage through the nighttime for years to come. 

William Ray is a Senior Undergraduate Biology Major at Georgia Southern University. He is a member of Kane Lab GSU under Emily Kane (@kanelabgsu) and his research interests include wildlife ecology and conservation of marine and fluvial environments. You can follow him on twitter/instagram: @willray35 , and can contact him via email: Willray35@gmail.com.

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