Many species of birds rely on their sense of touch to forage in conditions where other senses fail— think of Mallards dabbling in murky water, or Oystercatchers prying open their prey on a moonless night. We study tactile foraging birds to increase our understanding of general principles of touch (to benefit humans), and how birds interact with their habitat (to benefit birds). Our current focus is on using ducks to study the sense of touch.
Ducks as a model for studying touch
The most touch-sensitive organs in humans are covered with glabrous (hairless) skin, densely innervated by light touch-sensing neurons called low threshold mechanoreceptors (LTMRs). The most acutely mechanosensitive LTMRs are the rapidly-adapting LTMRs (RA-LTMRs). Terminals of RA-LTMRs interact with somatic cells in the glabrous skin to form cutaneous end organs: Meissner and Pacinian corpuscles – the principal detectors of transient touch and vibration. Mechanical stimulation of the corpuscles triggers transient firing in RA-LTMRs, signaling physical contact. Yet much about the development and function of RA-LTMRs is understudied. For instance, both the sensory neuron and corpuscle are mechanosensitive— yet how they each contribute to touch transduction is not fully understood. It is also known that sensory neurons are necessary for the development of Pacinian corpuscles. However, Pacinian corpuscles persist without sensory afferents present for up to a year after nerve injury. The differential requirements for development and maintenance of Pacinian corpuscles remain somewhat mysterious.
Tactile-foraging species of duck are a useful model for studying these questions and others. The duck’s bill is covered with glabrous skin containing homologs of both Pacinian and Meissner corpuscles called Herbst and Grandry corpuscles. The density of Herbst and Grandry corpuscles in the bill is comparable to the corpuscle density in human fingertips. Vibration sensitivity of LTMRs in the bill skin closely resembles responses in human LTMRs complexed with Pacinian corpuscles. Ducks use their exquisitely sensitive bill to find food in murky water, and can discriminate between objects 1-2 mm in size using touch alone. The majority of neurons in the duck trigeminal ganglia (TG), which innervates the bill, are LTMRs, and that the sensitivity of mechano-activated (MA) current of their dissociated cell bodies is greater than that of neurons from non-specialized species such as mice and chickens. Finally, duck embryos are accessible during development for observation and manipulation in ovo, and much of the anatomy underlying touch perception has matured by the time of hatching. These unique characteristics of ducks allow us to address a number of biological questions such as:
- How does the anatomy underlying the sense of touch differ in tactile foragers from closely-related species that are not tactile specialists?
- How do sensory neurons contribute to the development of corpuscles?
- What is the relative contribution of sensory neurons and corpuscles to touch transduction in RA-LTMRs?
- How is tactile information integrated with other senses during foraging?
We approach these questions using a variety of techniques including: 2-photon microscopy, electrophysiology, in ovo electroporation, transsynaptic viral tracing, cell culture, molecular biology, histology, and behavior.