09 de junio de 2020

Achieving the full potential of a scale item in scat observations

This post supplements the following one that I posted in Sep 2018:

https://www.inaturalist.org/projects/scats-dung-s-afr/journal/27547-submitting-inat-observations-of-animal-scats-droppings-some-suggestions

Sometimes it is possible to ID a scat with reasonable confidence just knowing its approximate size and, of course, such things as habitat, the scat contents, its exact location (buried, elevated, in midden), its shape, likely occurrence in that area and so on. But this is often not possible. This is when knowing the exact size of the scat can be useful.

The problem

A challenge with animal scats is that the average sizes for different species can be very similar. Consider this example. There are 22 non-antelope species with an average scat width between 10 and 15 mm. How about this: There are 20 antelope species with an average pellet width less than 10 mm.

I know these numbers because I have a database of thousands of scat measurements covering many southern African species.

It’s not difficult to imagine that if average widths are very similar then the ranges (e.g., smallest width to largest width) for the species overlap. Hugely. That is the real problem.

How I use the scale item

I can measure on your photo on screen (i) the scat width and (ii) the size of the scale item. (Say they are 45 mm and 60 mm respectively.) Using the actual size of the scale item (say a South African 50 cent coin so its diameter is 22 mm) I calculate the exact width of the actual scat as follows:

Scat width = 22 X (45/60) = 16.5 mm.

I will sometimes calculate the average of multiple measurements of the scat or pellets in the photo. I’m most likely to do this for the antelope because the average width of the pellets is more representative than the width of a single pellet.

Producing the shortlist

You may well wonder how I can possibly establish anything useful from ranges that overlap so much - a very reasonable question.

Applying a statistical approach to analyse the data in the database gives me my best chance. I use (i) the average scat width and (ii) the width range (represented by its standard deviation) for each species in the database. Specifically, I calculate the statistical likelihood of each species being your species.

I’m most interested in the shortlist of species with the highest likelihoods. Your species is most likely in that list.

However, “The problem” described above is acutely manifest in these lists. If the width of the scat in your photo is not accurate – because you or I had to guess it - I get one list. An accurate width will give a different list. This is when the full potential of a scale item with accurately known size is realised.

Scale item recommendations

A ruler as scale item makes things easy, for me at least. It also works for any size scat or pellets, a 10 cm ruler length being quite sufficient.

A coin is good, placed so that the value is clear in the photo. The exact dimensions of most coins on the planet are available on the web, if I don’t happen to have your chosen one to hand.

You can measure the scat yourself though I don’t advise this, for various reasons. It’s simpler for you to display some scale item.

If the exact length of the scale item is not obvious (like it is for a coin), then please measure it exactly and state the size.

If you don’t want an ugly distracting unnatural scale item in your photo, then just submit two photos, the first without and the second with the scale item.

Don’t use fingers unless you (i) specify their width, (ii) where on your finger the with applies and (iii) you take note of the suggestions below on how to take the scale item photo. I understand perfectly why fingers are often used as scale items – you are always sure to have some with you no matter where you are. But as scale items, they are problematic.

[ I said in the post mentioned above that “a finger is not ideal although it’s better than nothing”. This is true. However, a finger can also be downright misleading. If I use the above method to determine a scat width, what value do I use for the width of your finger? I can only base it on the width of my own finger, i.e., assume that your finger is the same size as mine (or maybe guessing that your finger is a little wider or narrower than mine). It has become evident that I can easily be out by tens of percent! Not that surprising, in retrospect. ]

Please use scale items of an equivalent scale to the size of the scats or pellets. Even if you state the exact maximum width of your shoe, it won’t help much if I need to calculate the width of tiny antelope pellets. Use your shoe (with stated width) for the big stuff. Conversely, using a tiny coin for a megaherbivore dropping doesn’t help much either. Just use your common sense remembering that I’m not looking for a rough idea of how big the scat is – I need it exactly.

Any other scale item can be used but please state what its exact size is. It can help me and therefore you.

Taking the scale item photo

The following is worth noting: Cameras can distort perspective. For scale item photos, wide angle photos are the worst. Zooming in is best.

I also suggest the following, in the interests of accuracy:

Place the scale item as close as possible to the scat or pile of pellets.

Take the photo straight down onto the surface of the ruler or coin (or finger, whatever).

The graduation lines on the ruler should be parallel (not look like they will converge somewhere in the background).

A coin should be perfectly circular, not oval.

The same applies to any scale item for which you have stated the size: it shouldn’t be distorted in any way.

The return on investment

I understand entirely that the quest for better accuracy using a scale item with known or stated size requires effort from you, the observer. The only return on that investment I can offer is an increased confidence in the ID of the animal that produced the scat.

Also, don’t forget that the supplementary information (habitat, contents, etc.) can also clinch the deal.

Kevin

Ingresado el 09 de junio de 2020 por kevinatbrakputs kevinatbrakputs | 4 comentarios | Deja un comentario

18 de septiembre de 2019

Submitting iNat observations of animal scats/droppings – some suggestions

The ID process – playing detective

It goes something like this: Knowing the size will suggest a series of possible animals (almost never just a single species). The next clue, say that it occurs in a latrine/midden, will suggest a subset of the first list. If the scat contains, say, bones and hair, then a further subset is produced. And so on, until one has some degree of confidence in the final single species ID (if you’re lucky).

In other words, it involves detective work (hence the questions I often bombard observers with). You, the observer, need to look for clues in situ and you need to record as many as possible. Record the clues you observe in (i) your pics and (ii) associated written descriptions.

Consider taking these pics:

(1) The whole deposit or latrine

It should show the overall volume if there’s only one deposit (i.e., the place has only been used once) or the whole latrine/midden if the place has been used many times.

(2) Closeup of the individual pieces with a scale item

The closeup pic should show size, shape, and colour.

Important: Choose a scale item proportional in size to the smallest scat pieces, i.e., a coin or ruler for anything up to some tens of millimetres. A finger is not ideal although it’s better than nothing. You can use your shoe for big stuff like zebra to rhino or elephant. A shoe is useless if they are small scats.

Don’t underestimate the usefulness of an accurate measurement. It can help distinguish, for example, between the various cats, from Black-footed Cat through to Lion. However, each cat’s scat width range overlaps with that of the next smaller and next larger cat. So, unfortunately, this means that, even with an accurate measurement, you may not be able to distinguish between them (when it falls between the average sizes for two cats). Hence, again, the importance of other clues.

(3) Contents, revealed, if necessary, by breaking open the scat with a stick, can provide very useful clues.

(4) Habitat (if you don’t get this in your “whole deposit or latrine” pic)

(5) Other signs of the animal, e.g., spoor, nearby den entrance, diggings, prominence of deposit (on a rock, clump of grass, on rhino droppings, in fork of tree, or buried or partially so).

Observations you could record in your written description:

(1) Any obvious odour.

(2) Anything relevant that you observed but were not able to photograph.

(3) Any information you might have on what animals are known to occur in the area. This is often possible for protected areas which usually have species lists. (Yes, one does need to use such lists with caution.)

Suggest a possible ID (even if it’s just a comment)

Do your best to justify your suggestion. It can help a lot. If you’re uncertain, say so. I have found that sometimes people don’t realise they have useful information (that’s not in their pics or written record).

Join the iNat “Scats & Dung (s Afr)” project

Linking your scat observation to this project will make things more efficient. I monitor this project regularly so I will get your scat pics sooner.

Posted by kevinatbrakputs September 16, 2019 10:27

Tony Rebelo added the following:

Thanks: most useful.
Please join the project here: https://www.inaturalist.org/projects/scats-dung-s-afr

Examples from the project here:
https://www.inaturalist.org/observations?place_id=113055&project_id=scats-dung-s-afr&subview=grid&verifiable=any

to see any particular group just add the taxon into the species box.e.g.
buck - 22 species to date
cats - 5 spp
dogs - 2 spp
rodents - 5 spp
afrotheria - 3 spp
primates - 2 spp
herps - 9 spp
you can even look at a species or subspecies.

Remember if there are Dung Beetles or flies visiting the dung, to please add an interaction to the dung, so that we ca see if some species only visit one type of dung, or many ...

Posted by tonyrebelo 2 days ago

Hail our leading Scatologist! Keep up the kak Kevin - great stuff :-)

Posted by bushboy about 5 hours ago

Ingresado el 18 de septiembre de 2019 por kevinatbrakputs kevinatbrakputs | 4 comentarios | Deja un comentario

16 de octubre de 2017

Scats & Dung

Scats, droppings, pellets, dung: faeces of any animal, including middens and their derivatives.
Also regurgitations, pellets and hairballs.

Ingresado el 16 de octubre de 2017 por tonyrebelo tonyrebelo | 0 comentarios | Deja un comentario

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