Observaciones de umueller

This Atta texana colony used to be about 100 meter to the west at this road, but was poisoned in November 2020 at its former location by the highway department (see here https://www.inaturalist.org/observations/65826122), and the colony then migrated to this new location here. Migrations of texana colonies are triggered by disturbances, like natural flooding or poisoning by humans.

observation UGM230113-30
elevation 109 meter

Easternmost Atta texana mound that I currently know at this latitude. This is also the first texana mound that I have found in Winn Parish, but I have not searched here much. There were no active workers at the time when I found the mound late afternon because the temperature was only 51°F and the mound had been shaded for some time during the afternoon, but the fresh excavate at the exits indicates that excavating workers must have been active at this mound earlier that same day. The mound is visible at this exact location at Google StreetView from July 2016 and April 2014, so this colony is at least 10 years old.

observation UGM230114-37
elevation 54 meter

Mycetosoritis hartmanni colonies are very abundant throughout the Coastal Sand Plain in South Texas. Hartmanni nests are easiest to find here in March and April, when colonies seem to do most of the excavation during the year. I post photos of two different hartmanni colonies, showing a volcano-type mound where the nest entrance is in a shallow depression in the center of the excavation crater, and the more common pointy mound-type ("turriform crater" sensu Wheeler 1907) where the nest entrance is at the apex of a steep mound. Both types of mounds measure about 4-10 centimeter across at the base in mature mounds, and the mound shapes shown here are characteristic for hartmanni (there is no other ant species here that produces these kinds of mounds; some Dorymyrmex mounds can look similar, but Dorymyrmex mounds are typically asymmetrical in shape, with more excavate to one side of the nest entrance, whereas hartmanni mounds are always symmetrical when viewed from above). Hartmanni colonies have been consistently very abundant in this area since I first collected ants here 20 years ago.

Whereever I have found hartmanni in sandy areas in South Texas, I have also found closeby two other fungus-growing ant species, Mycetomoellerius turrifex and Atta texana. The distribution of hartmanni further north in Texas is more spotty, but always in white-sand soil.

The correct spelling is hartmanni (Wheeler 1907), not hartmani as currently used at iNAT.

observation UGM220406-00
elevation 13 meter

Atta texana constructs and maintains noticeable foraging trails this time of year, but it is rare to see one of these trails branch, like in the first and third images here. This is different from the more permanent trails of tropical Atta species, which construct wider and longer trails than texana, and trails that branch frequently into subtrails. In contrast, a texana trail is typically one long trail that becomes narrower and narrower with distance from the mound or foraging exit (fourth image), and then a trail either ascends a tree, or peters out on the ground near where workers are dispersed near the trail to cut vegetation.

It is always possible to tell from the shape of these branchings in the trail system where the foraging exit is located to which the main trail connects: the direction of the foraging exit is always such that the foragers don't have to make any sharp turns at the intersection. In this case here, the foraging exit is towards the right, and returning foragers coming from the left can join smoothly from two trails into a single trail without having to make sharp turns. This is exactly how human highways are contructed when traffic from two lanes has to join smoothly without interrupting the flow in either lane. I don't think it is known how the communal decisions of thousands of Atta foragers building trails result predictably in the construction of trail intersections that optimize flow of foragers, without a central architect planning these trails, like humans do.

observation UGM220210-15B
elevation 34 meter

The population of Atta texana in the sandy areas of northern Hidalgo County is unusual for two reasons. First, texana colonies are very abundant, with one of the greatest mound densities that I have seen anywhere in the range of texana. Second, texana colonies seem to migrate (relocate mounds) at a greater rate than in typical texana populations. I believe the greater rate of migrations, and possibly regular colony fissioning, are reasons why texana colonies never grow huge in this population (never much more than about 10 meter mound diameter).

The reasons for the greater rate of colony migrations in this texana population are unclear. Perhaps the limited shade in the relatively open, scrubby habitat requires regular colony relocation as the ground dries in summer; or the possible presence of predators like Nomamyrmex army ants causes colonies to relocate as an escape response whenever a mound is attacked. Atta texana overlaps with Nomamyrmex esenbeckii army ants only in southern Texas, and I have found dead Nomamyrmex workers on Atta mounds of this population here (https://www.inaturalist.org/observations/108890321), but texana does not have to deal with these ferocious predators elsewhere in the texana range (i.e., north of the Nueces River).

Atta texana mounds are actually very abundant throughout the coastal sand plains of south Texas (see map, 4th image), whereever the ground is well-drained and not seasonally flooded. Much of this area is fenced off by cattle ranches, and I have followed the fate of texana colonies in the coastal sand plains during the last 20 years therefore mostly along highways, like shown here in the 2nd image.

observation UGM220211-31
elevation 24 meter

The population of Atta texana in Anzalduas Park at the Rio Grande River is interesting because the entire area was flooded for weeks in July 2010 following Hurricane Alex, presumably killing any texana colonies in flooded areas. In 2013, I found only a single large texana colony in this park, at the elevated road leading to Anzalduas Dam, but no texana mound in the lower areas of the Park that were flooded for weeks in 2010. In February 2022, I found two large texana mounds in these lower areas (each mound about 9 meter diameter), and a few smaller satellite mounds linking to these two larger mounds. I could search only half of the Park, however, because several areas in the Park are now fenced off for storage of equipment and vehicles from border patrol, so there may be a few additional texana colonies in the Park. All these texana colonies must have established after the flood of 2010.

The mound shown here measured 9 meter X 8 meter in total (only a 1.5 meter area is shown in the first photo), with much soil excavated by the workers during the last year. The size of this mound is therefore an indication of the size attainable by an Atta colony in south Texas within about 10 years time, starting with colony founding sometime after the flood of 2010.

observation UGM220210-14
elevation 34 meter

There used to be several large Atta texana colonies along Hwy77 near Yturria, but all these colonies were bulldozed or paved over in 2019 when the highway was expanded into the new interstate I-69E.

observation UGM150406-03
elevation 7 meter

Large foraging exit of the leafcutter ant Atta texana, with two large foraging trails radiating out in different directions from this exit, one trail going towards the top and the other trail towards the bottom-right corner of the first image. The workers were foraging mostly on forbs. The foraging exit was 49 meter distant from the mound of this texana colony, and I found an additional 5 other such foraging exits within a 20 meter radius of the exit shown here. Each of these foraging exits had at least one busy foraging trail on this warm afternoon (about 19°C). This is the time of the year when colonies are getting ready to rear alate brood, to be ready for the main mating flights in mid-April to May (in central Texas), so texana colonies typically forage well throughout winter to grow enough gardens for the thousands of alates dispersing from a colony during a series of mating flights in spring.

The trails shown here are about 5-8 centimeter wide, among the widest that I have seen for Atta texana. Tropical Atta can have foraging trails that are 15-20 centimeter wide and over 150-200 meter long, but Atta texana has much narrower trails, and the longest trails that I have seen were only about 50-100 meters long for texana.

observation UGM220128-03
elevation 139 meter

Foraging exit of Atta texana surrounded by a ring of leaf fragments cut by foragers, then deposited by the foragers at the entrance to this foraging tunnel. I discuss the possible function of such leaf accumulations here: https://www.inaturalist.org/observations/104916820

elevation 133 meter

Workers of Atta texana cutting bluewood Condalia (= Brasil = Brasilwood = Bluewood), Condalia hookeri. Because Condalia leaves are relatively small, the workers typically cut the leaves whole, cutting each paddleshaped leaf at the base. Texana foragers seem to cut Condalia more in winter, and then frequently build up accumulations of fresh Condalia leaves around foraging exits (4th image) or along foraging trails (5th image).

Here, the workers also accumulated a massive pile of Condalia leaves at the base of the Condalia tree (3rd image) on which the workers were foraging. Accumulations of leaf fragments occur most frequently at junctures along a foraging trail with some traffic congestion, like at the base of a tree where workers have to re-orient from a trail descending a tree vertically to a trail running horizonally, or a traffic congestion at the entrance of a foraging tunnel (4th image, the entrance to the foraging tunnel is surrounded by a large ring of Condalia leaves). Significant accumulations of leaf forage along trails of texana are more rare. The accumulations of leaves dry over time, but can persist throughout the year, even if a texana colony ceases using a trail or a foraging exit. Leaf material accumnulated on top of a mound is used by texana to thatch the openings on the mound (the thatch is easily moved to open or close entrances), but the reason for the accumulations of leaf material at foraging exits away from the mound or along trails is unclear, because foraging exits at some distance from a mound are typically left open by the ants and are not closed with thatch when a foraging exit is not used. Here is another foraging exit from a different texana colony nearby, also with a ring of leaf fragments surrounding a foraging exit, but in that case the leaf material is from a different plant species: https://www.inaturalist.org/observations/104943419

One hypothesis that has been discussed is that the accumulations of leaf and plant debris (e.g., small sticks) around exits reduce the chance that rain will cause soil to be flushed into an entrance, but I don't know of any experiments that have tested this idea. The accumulations of leaf material at the base of trees and along trails occurs also in tropical leafcutter-ant species, and such accumulations are there sometimes called caches, as if the ants store leaf material along trails. But "cache" is actually a misnomer because most of the accumulated leaf material is never retrieved by the workers.

I counted a subsample of the leaf accumulation shown here in the 2nd image, and I estimated that the workers accumulated at least 5000 Condalia leaves here, which seems like massive wasted foraging effort.

elevation 129 meters

Here is another observation of piles of Condalia leaves at a foraging trail of Atta texana, from the Medina River Greenway nearby: https://www.inaturalist.org/observations/105073112

I found three large mounds of Atta texana in Franklin Nature Park, adjacent to Franklin Ranch Community Park. The soil in the Nature Park is well-drained white sand, ideal for Atta texana. This is different from the red-clay soil in the adjacent Franklin Ranch Community Park, and no surprise, I was not able to find any Atta texana colonies there.

observation UGM211226-19
elevation 144 meter

This is the only Atta texana colony that I know from Madison County TX, but I expect that texana is relatively abundant in the sandy areas in the northwest of the County. I can see at GoogleMaps other texana mounds in the pastures near this particular colony here. This is the first record at iNAT of texana in Madison County, and I post this to help define the distribution of Atta texana.

observation UGM211224-08
elevation 83 meter

Crossing of an Atta texana trail with a trail of the harvester ant Pogonomymrex barbatus. The two photos posted here are identical, but in the first photo I marked the Atta trail in yellow, and the Pogonomyrmex trail in red. The direction of the leafcutter mound is towards the bottom of the photo, and the leafcutter workers carry forage to their mound about 90 meter distant from this intersection of trails. The direction of the Pogonomyrmex mound is towards the right about 12 meter distant.

I have seen crossings of busy trails of these two species only a few times; the two species seem to avoid each other on the ground, although mounds can be in close proximity. It is more frequent to see crossing trails of these two species where the trail of one species is inactive (no foraging workers) and only one trail is therefore active. At this particular crossing here, interestingly, trails from both species were very busy. Forager ants from the two species frequently bumped into each other at the intersection. There were no fights, no carnage, no pile of mutilated workers as can happen in the tropics when a leafcutter-ant trail and an army-ant trail intersect. Here, any disoriented workers from each species quickly found their own species' exit from the intersection, without veering off into the trail of the other species. This is because workers from each species are following the unique pheromone trail of their own species.

The exact location of the leafcutter mound to which the leafcutter workers belong is here: https://www.inaturalist.org/observations/103186710

Foraging trails of Atta texana sometimes specialize on a particular kind of forage, like juniper berries here. Interestingly, a second foraging trail of the same texana colony, but leading to a different foraging exit within 15 meters of this observation here, specialized on grass and forbs as forage (no photo of this second trail, unfortunately, I forgot).

Flavio Roces' research group at the University of Würzburg has shown that leafcutter-ant specialization on a particular type of forage emerges because outgoing leafcutter workers receive odor information from incoming workers about the type of forage carried, whenever outgoing workers bump into incoming workers on the foraging trail. Outgoing workers then use this odor information to search upstream (up-trail?) for the kind of forage that is brought in by the incoming workers. That is, a postitive feedback dynamics operates on trails, such that a small bias in type of forage carried by incoming workers can build up over time into a large bias, manifested then as a pronounced forage specialization of a particular trail. A small initial bias can therefore lead to a major preference over short time. Foraging dynamics and foraging specialization of ant and honeybee colonies actually emerges because of interactions between positive-feedback and negative-feedback loops. The observed difference here in specialization between the two adjacent trails from the same texana colony presumably arose because of a difference in the positive-feedback dynamics between the two trails.

This trail was linked to colony/mound UGM211215-12 here: https://www.inaturalist.org/observations/103186710

In the Kreutzberg Canyon Natural Area, I found only two large Atta texana colonies. This here is the smaller colony near the western Park bounday. The texana colonies constructed their mounds in the deep riverine soil along the Guadalupe River, but above the high floodline; this is typical for texana populations along rivers in central Texas. The ground away from the River in the Park is mostly limestone with little soil, unsuitable for Atta texana to dig their deep nests. The two texana colonies at Kreutzberg are almost 300 meter apart. I tought initially that there may be a third texana colony halfway between the western and the eastern colony, but I searched that area carefully, and I did not find a third colony. By recording the locations of foraging exits, it was possible to estimate the size of each colony's tunnel system. Each colony has a tunnel system extending 100-150 meter from the mound, running here mostly parallel to the River and not extending much into the limestone areas away from the River; this too is typical for texana colonies in central Texas. Each texana colony at Kreuzberg therefore occupies a foraging territory of about 200-300 meter diameter along the River.

observation UGM211215-12
elevation 358 meter

There are two Atta texana colonies in James Kiehl River Bend Park, one at the northern end of the Park right at the parking lot, and this second colony near the southern end of the Park. I have observed both of these colonies since 2009. While the first colony has not relocated the mound much, even when the parking lot was reconstructed several years ago, this second colony has moved around somewhat, relocating the mound every few years by 20-30 meter. These two mounds are about 375 meters apart, so the southern mound is not a satellite mound of the northern colony; the two colonies are independent neighbors. I have noticed this at many other locations too, that between two neighboring Atta texana colonies, one stays put and maintains a mound on the same exact spot for several decades, whereas a neighboring colony relocates the mound regularly within its territory. It is as if there are two types of texana colonies, anchored colonies and rover colonies.

observation UGM211215-10
elevation 416 meter

This Atta texana mound has been at this exact spot for many years. You can see this mound at Google StreetView since 2008 at this spot, and I am including (third image) a screenshot of this colony at StreetView from 2013. In most other towns, this leafcutter colony would have been poisoned many years ago, because this colony generates some extra work for the groundkeepers to remove every year the excavated soil that the ants amass at the side of the road (see third image). I think it is a tribute to the groundkeepers and the citizens of Comfort TX that they keep their native biodiversity alive.

observation UGM211215-07 = UGM190828-24
elevation 439 meter

Atta texana mound in an unusual sun-exposed, dry location. In May 2020, this colony had a mound located about 40 meter to the west, in an even more exposed and rocky/limestome spot. It may be that this colony will continue to migrate until it finds some partial shade and more suitable soil.

observation UGM211207-17
elevation 383 meter

Large mound of Atta texana at forest edge in Cedar Hill Park. I found two other colonies in this park along trails, and I expect there are additional colonies in the park.
https://www.inaturalist.org/observations/102478559
https://www.inaturalist.org/observations/102478709
The ground in much of the park is quite waterlogged and likely floods regularly after heavy rains. I was surprised to find several Atta texana colonies here.

observation UGM211202-03
elevation 14 meter

Large mound of Atta texana at trail in Cedar Hill Park. I found two other colonies in this park along trails, and I expect there are additional colonies in the park.
https://www.inaturalist.org/observations/102478559
https://www.inaturalist.org/observations/102478709
The ground in much of the park is quite waterlogged and likely floods regularly after heavy rains. I was surprised to find several Atta texana colonies here.

observation UGM211202-02
elevation 24 meter