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on a barrel cactus flower |
nectar at a barrel cactus EFN |
On this page (which is divided into three parts to speed up downloading) I will provide a brief species description and then discuss in turn:
Wheeler and Wheeler (1986) describe this species as being large to very large. Indeed, some major workers are over 1 cm in length putting them among the largest ants in the United States.
As indicated, C. ocreatus workers are polymorphic. Morphologically, major and minor workers differ in a number of ways. In general, mean body length of majors is 1.4 times that of minors (Lamon and Topoff 1981). A more striking difference, though, is the difference in the size and shape of the head: the heads of majors are twice as wide as those of minors and they are chordate in shape compared to the more rounded heads of minors. Finally, there are also color differences in the two castes (Wheeler and Wheeler 1986). The heads of major workers are reddish black while those of the minors are a lighter brown. The bodies of the majors are likewise generally darker in color.
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on a barrel cactus flower |
nectar at a barrel cactus EFN |
These ants are found in a number of different habitats. In addition to my desert site, Blom and Clark (1980) recorded them at their Rancho Santa Inés field site in Baja California. While Wheeler and Wheeler (1986) found them in both the Mojave Desert they also found them in the Pinyon-Juniper biome of Nevada. Interestingly, Andersen (1997) did not find any C. ocreatus workers at the two lowest elevation sites he sampled near Portal, Arizona. He did, however, find them at the next four higher sites that included desert scrub dominated by mesquite, oak-juniper woodland, and pine-oak woodland.
While there is little more to say about C. ocreatus, it is interesting to note that species within the genus Camponotus are among the most common collectors of extrafloral nectar (Olivera and Brandão 1991). In fact, many have specialized endosymbiotic bacteria that may help them more fully use this food (Schröder et al. 1996). Camponotus species also frequently make up a large fraction of the arboreal arthropod biomass in the tropics (Davidson 1996). Despite this, they are frequently behavioral subordinants in their interactions with other tree-dwelling ants (Andersen 1991, 1997; Davidson and McKey 1993).
In the main, C. ocreatus is a nocturnal species. Workers at my field site left their nests soon after sunset and most returned well before sunrise. Occasionally, though, I did find a worker visiting a cactus during the day. But, marked workers who did this did it for only a few days before they invariably disappeared altogether.
Most of the workers leaving a nest visited EFN-bearing cacti, although at times they would also visit saguaros to collect pulp from recently opened fruits. Some majors, though, would hang around the nest entrance presumably to guard it (LaMon and Topoff 1981). Also, there were always a few workers who wandered around on the ground near the nest returning to it every now and then.
Workers visiting a particular plant seemed to travel independently of one another. They didn't travel as a group, took slightly different paths, and removing one ant didn't seem to affect the others. But occasionally I found pairs of ants engaging in something known as tandem running. In tandem running one ant usually leads another to a source of food or a new nest site. Tandem running has been reported before for C. ocreatus, but those who reported it did not elaborate on its function (Hölldobler and Wilson 1991, pg 273 table 7.7). I didn't study it in any detail either, but I can say the following of it. First, I saw minors leading both majors and minors. I never saw majors lead. Second, tandem running occurred very infrequently and I only saw any particular pair tandem run for a single night. Third, the leader of a tandem pair never led the follower directly to a food source or a new nest site. On one occasion a pair did eventually reach an EFN-bearing cactus but it was by a most circuitous route. Most of the time I gave up watching pairs before they reached any recognizable destination. Often such pairs would travel up and down small desert plants then head out in one direction only to return over the same ground a short time later.
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Given these observations, one possible explanation for tandem running in this species is that it plays some role in familiarizing new workers to their colony's landscape. Perhaps the "aimless" wandering of tandem pairs allows the trailing ant to familiarize itself with features of its landscape in relation to EFN-bearing cacti and the nest site. Among several possible tests of this hypothesis, an obvious one would be to determine if lead ants are older workers while the trailing ones are young and inexperienced. While I did mark some C. ocreatus workers, I didn't do it to the degree necessary to test this prediction.