Archive | October 2013

In our last post we provided details about the size of the home range of P33 and P27. When graphed against time since release and calculated at the point when the range size appeared stable, estimates were 20ha and 15ha respectively.

However, looking at space use within the home range highlights further differences between the two individuals. We have observed both individuals a) using a den site for several consecutive nights and b) returning to previously used den sites. However, the picture is not as clear cut as this.

The map below shows that over the past 3 weeks P27’s movements have been contained in a specific part of his home range, approx. 4ha in area. Over these 3 weeks he used the same den site on 3 separate occasions (for 8, 2 and 4 consecutive nights)  using a total of 4 different den sites. It will be interesting to see how long resources can support such fidelity to this area, especially with the approach of the dry season.

ImageLocation fixes of P27, the diameter of the circle is related to the frequency of fixes at that location. The colour of the dots represents that number of weeks after release (red-yellow-green-blue)

P33, however has been recorded returning to previously utilised areas after prolonged periods away. For example, the area surrounding the release site was used for two weeks before she moved away. She was then found back in this area 10 weeks later: cameras caught 4 photographs of her within a 2 week period.

ImageLocation fixes of P33, the diameter of the circle is related to the frequency of fixes at that location. The colour of the dots represents that number of weeks after release (red-yellow-green-blue)

This information is vital in recommending ways to gather presence/absence/occupancy data through the provision of information about how big survey sites should be and for how long/how often an area should be surveyed to determine if Sunda pangolins may be present.

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Interesting results about home range size and den site selection

Having spent a few days really looking at the data we have gathered, we have discovered some interesting results, that have given us lots to think about . 

Firstly, before presenting some results, I want to stress that when estimating the location of an animal using radio tracking (and triangulation) there is a certain amount of error involved that needs to be taken into consideration, however, this should not detract from the biological significance of the data. For reference, the mean difference between the GPS location and an estimated value is 78m (based on the a pilot study and the fact that each time we located a den site and obtain an accurate GPS location, we also generate an estimate through triangulation).

For fear of missing things of biological significance we re-looked at P33’s home range size. Previously, this had only been calculated using GPS locations of den sites. However, on some occasions we were unable to locate the den site and took only a triangulation estimate, these were originally excluded from the home range size. The graphs below give a more show the home range sizes of P33 (female, top graph) and P27 (male, bottom graph) and how strictly excluding data could bias results. 

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The gaps in the graph for P33 are days where no locations were gathered. The days where we couldn’t accurately locate/access the den site correlate with the days there was a substantial increase in home range size. 

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‘A’ stands for estimates taken when P27 was active. There are four activity estimates that results in a sharp increase in home range size and three of these are on nights he went to a new den site. However, the den site was no more than 150m from the last one. 

As was mentioned in a previous post, more activity has been recorded for P27 compared to P33, despite a similar sampling effort. Activity estimates seem to have a comparatively large impact on home range size for P27. This could be a combination of him being active even when he returns to the same den site, and him moving around more when changing den sites. This was not seen for P33, despite the mean distance between an old and new den site being 89m for P27 and 76m for P33. 

Interestingly, though, after our reevaluation of the data, the home range estimate for P33 increased considerably to 20ha, to 17 ha for P27. Originally, based on GPS locations of den sites alone, they stood at 2.5ha and 4.7ha respectively. This warns that den site location alone may not be enough to accurately estimate home range size in this species, but could be enough to indicate when the home range is stable. 

Another observation that we have made with P27 is that the selection of den sites are a lot different to that of P33 and P34 (for the short time she was tracked). Whereas both females used very small tree hollows, often climbed higher up inside the trunk, or even climbing the outside of the trunk, P27 prefers large, open, often rotting tree trunks, where there is potential for him to dig a burrow. There are some examples in the photos below.

ImageP33 tended to select small tree hollows at the base of the tree

ImageIn the above photo P27 may have dug a burrow into the soil inside the rotting trunk as the signal was at ground level and with torches peering inside, we could not see him. 

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On a few occasions P27 has been very easy to spot

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At this den site P27 climbed up the mud and dug himself into a burrow at the top.

These differences in behavior maybe due to the provenance of the animal and differences in the habitat it lived in. For example released Loris have been know to travel large distances and found in a habitat the matches that the were originally taken from. However, as well as raising interesting ecological questions about the species, it is also reassuring to know that so far all the release candidates seem to be surviving and doing well. 

 

 

 

 

 

 

P27 activity patterns

In this release program there are two sets of data we are collecting to evaluate the success of the project: survival and the time it takes for the animal to establish a stable home range.

Our first female, P33, seemed to establish a stable home range of around 2 hectares after 22 days. This was regardless of whether the estimate was using only GPS den site co ordinates; only den site triangulation estimates; or triangulation estimates of den site and activity locations (although triangulation estimates did produce larger estimates of home range size).

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The above graph shows the changes in home range size after release for P33 using den site GPS coordinates

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The map above shows all the locations P33 (green stars = homed den sites; green circles = camera trap photos; red stars = triangulated den sites; and red circles = triangulated locations during activity).

P27, a larger male is currently has a home range size of 4.7 hectares. Over the first week he was showing much lower den site fidelity, moving every night. However, more recently he has been utilising den sites for two or three consecutive nights and the dens are a lot nearer. This insinuates that his home range is stabilising.

However, unlike the female P33, even if P27 use the same den site, he still appears to leave and then return. Last night we obtained camera trap photos of him leaving the den site at around 6pm and returning at around 9pm. With P33, if she used the same den site on consecutive nights, we never recorded her leaving the den, but changes in the radio signal suggested she was moving within the tree hollow.

At night we monitor activity by noting changes in the strength and/ or direction of the signal bearing, most nights the times we have concluded he is active using radio tracking accord with times the photos have been taken.

ImageThis photo shows P27 leaving his den site at 6.40pm and was photographed returning at 9pm. This matches what we concluded while we were camped 150m away listening to changes in the signal strength and direction.