Ontogeny

Early Development

After hatching, baby platypus resemble pink blobs with differentiated limbs. The platypus's head is relatively large in comparison to its body, with no apparant neck region (Manger et al., 1998). The future area of the bill is also indistingishable from the rest of the head area. However, the bill area becomes distinguishable at 2 days post-hatching (p.h.), continuing to grow until about 6 months p.h., when the length of the bill is greater than that of the width, and resembles, in proportion, the same ratio as an adult's.

 

summary of platypus electro

Figure 4. Summary of the changes in electroreceptors over the course of platypus development, spanning a) 10 days p.h.; (b) 12 days p.h.; (c) 24 days p.h.; (d) 28 days p.h.; (e) 5 weeks p.h.; ( f ) 6 weeks p.h.; (g) 11 weeks p.h.; (h) 14 weeks p.h.; (i) 6 months p.h.; ( j) adult. The colors represent stripes that have been added or altered, with blue representing those stripes that are found 12 days p.h. and that persist during development. These changes continue to occur until the platypus has reached adulthood (Manger et al., 1998).

 

Over the course of development, the periphery of the platypus's electroreceptive system also undergoes great changes. Within 10 days of hatching, pores begin to appear on the upper cutaneous surface of the platypus' bill. At 12 days p.h., the striped pattern of distribution of the pores becomes apparent, and two types of pores, one bigger than the other, are readily discernable. The larger pores are presumed to be mucous gland electroreceptors. The arrangement of stripes is continually modified during development, with additions and divisions of stripes, until an adult pattern of parasaggital striation along the upper and lower surfaces of the bill develops at around 6 months p.h. (Manger et al., 1998). The number of electroreceptors increase rapidly until around 24 and 28 days p.h., in which there is a massive cell death of 60% of electroreceptors (see Figure 4.). This occurs concurrently with the appearance of other sensory structures on the bill skin, however, including the push-rod mechanoreceptors and sensory serous glands, and a drastic change in bill morphology.

The axons of the neurons that innervate the electroreceptive system of the developing platypus remain unmyelinated, and do not demonstrate neural sheathing, until around 11 weeks p.h., and it is not until 6 months p.h. that any myelinated nerve terminals are found to be associated with the electroreceptor organ (Manger et al., 1998). The anatomical array of the developing electroreceptors suggests, at least until the platypus is at least 3-4 months of age, that the electroreceptive system of the developing platypus is not functional in the same manner as an adult.

Enviromental Interaction

platypus patrolling behavior

Source: Manger & Pettigrew, 1995

Platypus swim vigorously as they forage for food, swinging their heads widely from side to side (see Figure 5.). It is logical to assume that the electroreceptive system will experience interference problems stemming from the fluctuating electric fields generated by the platypus' own muscles and changes in its body orientation as it swims, but Pettigrew argues that the platypus morphology compensates for this (1999).

The electroreceptive nerve endings in the bill, for instance, are naked at their exposed tips where they project into the pore, in contrast to fish, where the nerve ending is capped by a sensory cell. This does not directly help combat the problem of self-generated noise, but likely aids in the general problem of achieving sensitivity while reducing noise. In addition, platypus fur has also has very high impedance, or resistance to an electric circuit, rivaling sea otter fur for density of hairs, sans bill (Pettigrew, 1999). Having high impedance reduces the contribution of the platypus' own activity to the ambient field in the electroreceptive bill region.

In addition, the platypus is responsive to both steady potential electrical gradients and to rapidly changing potentials, and this sensitivity to alternating potentials is considered by many to be the key to the platypus' ability to detect moving prey with such accuracy. The platypus' sensitive electroreceptors, for example, are able to pick up on potentials caused by the tail flicks the tiny freshwater shrimp they eat (Gregory et al., 1989).