February 2020 Natural Mystery Answered

Identifying which side gets tricky. Opossum front tracks can appear highly symmetrical. Kirsten Welge suggested that this is a left front track and offered the following analysis:"1) Toes 3 and 4 share a metacarpal pad. This shared pad is seen between the toes 2nd and 3rd from the left of the picture - which would make the toe nearest the ruler Toe 1.

2) In this track, it looks like the leftmost toe registered more strongly than the rightmost toe, and there is a stronger, clearer imprint connecting the leftmost toe and metacarpal. We can conjecture that as a plantigrade walker, opossums might carry more weight towards the outside edge of the foot, similar to bears and beavers."

Kim Cabrera proposes that this is a right track. She did side-by-side comparisons to numerous images of opossum tracks from iNaturalist which showed both front and hind feet and offered the following points for us to consider:

"There is a small indentation in the snow around the 6" mark on the ruler, which is in the right place for the carpal pad below toe 5.

The divide between toes 2 and 3 on the metacarpal pads is really close. Toe 3 often appears right in between the two metacarpal pads. If you look at an opossum foot, toe 3 appears to be right in the middle of those two pads.

Typically, the toes toes 3, 4, and 5 curve outward. I think it's just due to the way they walk. Those toes sometimes sort of stick to the substrate and show up as curved rather than straight toes.

Also, toe 5 often appears shorter than toe 1 on opossum front feet, but that may be due to the curling of the toes when they walk. Their front feet don't hit the ground like a stamp, perfectly up and down, with all toes equally splayed and flat on the ground. So, sometimes, toe length can seem to vary due to the animal having their toes a bit more curled on some steps than others. The angle at which the foot strikes the ground can also cause this to vary. This is also the reason that the toes on opossum tracks sometimes appear to be curved."

When I posted this Natural Mystery, I had confidence we could determine the side of an opossum front track just from subtle clues in the morphology. After reading the responses, and having a number of follow-up conversations with very experienced trackers, I have lost that confidence. It appears that all of the clues that can help us identify right vs. left are weak indicators.

Weak indicators are features that can help us with identification, but which are not reliable by themselves. The lack of claws in a feline track is a weak indicator. While it is true that felines often do not show claws, it is not especially rare that they do. Weak indicators can be help support an identification, but cannot be relied on to make an identification. If all we have are weak indicators, and they all line up with one another, we may have a good case. But if week indicators point us in different directions, we may not be able to come to a conclusion.

In opossum front tracks, it seems that many of the indicators for side are a function of how the animal typically places its foot on the ground, rather than morphology of the foot itself. Here are the weak indicators we can use to help guide us in determining right vs. left.

• • Typically, toes 3-5 curve outward. But sometimes the toes curve inward.

  • Typically, the weight is set slightly to the outside of the foot, making the pads at the bases of toes 2-5 register clearly, and sometimes be the clearest components in the tracks. But sometimes toe 1 and the pad at the base of toe 1 register more deeply than toe 5.

  • Typically, toe 3 appears to intersect the palm overlapping the metacarpal pad at the base of toe 4 more than it overlaps the pad at the base of toe 2. But sometimes toe 3 appears exactly centered between the two pads and, occasionally, toe 3 appears offset slightly toward toe 2.

  • Typically, toe 5 appears shorter than toe 1. But sometimes the appear the same length and occasionally toe 5 appears longer.

  • Occasionally, the carpal pad will register below the metacarpal pad for toe 5. An additional "heel" pad may also register below toe 1.

All of these features are strongly affected by how the animal places and moves its foot. In the track above, some of these indicators suggest a right foot, and some suggest a left foot. Unfortunately, we cannot verify the side from the track pattern. This track was the only clear print in a jumble of tracks made by an investigating opossum. The fact that this track was not part of a normal walking pattern may help explain why the weak, movement based indicators are inconsistent with one another. Morphology is all we have to go on. And based on that morphology, I don't think we can say with certainty.

In a follow up conversation I had with Kim Cabrera, she added this reflection: "We had this discussion with Mark Elbroch on an eval once. I asked him this exact question. In the absence of a hind track, could one tell a left from right front track of an opossum? He said he didn't know of a way to tell for sure."

So congratulations once again to Brendan, David, Kim, Kirsten, Leah, Liz, Mike, Rachel and an anonymous reader for all correctly identifying this as the front track of an opossum!

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