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Males of the little known crabronid wasp Tachysphex menkei Pulawski engage in hilltopping behavior at the peak of Usery Mountain in central Arizona. Males are active at midday in the late spring at this location. Individuals perch on and launch out and back flights from small rocks near prominent plants growing at the highest parts of the undulating ridgeline that makes up the peak. The same set of sites attracted two generations of males (in 2009 and 2010). If site-faithful males are territorial (and if size influences territorial success), resident males (those that returned to their perches over at least two days) should be larger on average than the males that replace them after the residents have been removed. This expectation was met. In keeping with the hypothesis that hilltopping is a mating system of last resort, only a few males were seen on any given day and no females were observed, suggesting that the population of the species is small and dispersed, at least in central Arizona.
Crabronidae, hilltopping, Hymenoptera, mating system, Tachysphex
Many butterflies, flies and ants employ hilltopping
mating systems in which males fly to conspicuous landmarks to await the
arrival of receptive females (
In this report, I provide an account of the hilltopping behavior of a small and inconspicuous crabronid, Tachysphex menkei Pulawski, a little known species from southern California and Arizona (
The study was conducted on 7 days in May 2009 and on 20
days from late April to early June 2010 at Usery Peak, near Mesa, AZ.
The site lies within the Upland Sonoran Desert and attracts many
hilltopping insects (
On May 19, 2009 and on May 16 and May 20, 2010, a total of 6 marked males known to have returned to a particular perching location over a period of at least two days (i.e., “resident” males) were captured and placed in vials within a chilled cooler. The site was then monitored for up to 70 min. If a replacement male arrived and perched on the vacant rock, it too was captured and placed in the cooler. Subsequently, all males taken in this fashion were transported to a home freezer. On a following day, the “residents” and “replacements” were weighed on a Mettler Balance accurate to 0.1 mg. The data collected in this manner were used to test the prediction that resident territory holders should be larger (heavier) than males that had yet to acquire a territory of their own.
ResultsIn May of both years of the study, males of Tachysphex menkei were consistently found perched on certain small rocks (Fig. 1) on Usery Peak. From their perches, males launched frequent flights out and back over a distance of no more than 50 cm. Four males watched between May 13 and May 16, 2009 engaged in 10 to 17 such flights per 2 min. Males appeared as early as 1012 and stayed as late as 1525, although the wasps were most often observed between 1100 and 1300. Many individuals were present over periods of 60 min or more, with one male recorded at a site for 2.25 hr.
The small rocks used by males as perch sites (Fig. 1) were found at a total of nine spots scattered along the 300 m ridgeline transect. Although it was common for some of these perch sites to be left unoccupied on any given day, two sites were nearly always taken with a male present on 19 of 20 days of observation at each location in the late spring of 2010.
At one place, two males sometimes perched on rocks separated by only 2 m but the next closest pair of nearest neighboring perch sites was 8 m apart. Each occupied site was near a prominent plant growing on a higher than average point along the ridge. In 2009, a total of six sites were used by perching males over the course of the study; in 2010, the same six sites plus three others were taken on occasion by perching males.
On some days at some sites, more than one male was recorded during the observation period, almost always because a male seen on one census was absent on a subsequent one and in his place was a different individual. In 2009, 21 records of this sort were accumulated with another 31 records gathered in 2010. Thus, the number of male wasps visiting the peaktop on a given day was often greater than the number of occupied perches on that day.
Males did not often interact with other individuals. In 2009, just two chases were seen in which a resident pursued a departing intruder. In 2010, six chases were recorded and one case in which two males grappled together on the ground after one male briefly captured a visitor to his perching area.
Males of Tachysphex menkei tended to be site faithful. In 2009, 27 males were marked and released; 14 returned on the same or subsequent day up to 7 days after marking. In 2010, 24 males were marked and released; 12 returned on same or subsequent day up to 4 days after marking, all but one to the site where they had been originally marked.
If it is true that at least some males were territorial
as suggested both by their site fidelity and by occasional observations
of chases between conspecifics, then we can predict that removal of an
established resident would lead to his replacement by a smaller male,
given that large body size is generally believed to confer an advantage
in aggressive competition between male insects and other animals (
A male of Tachysphex menkei perched on a rock on Usery Peak that attracted a series of territory holders in the spring in two consecutive years (2009 and 2010).
Although the genus Tachysphex contains hundreds of species (
In species in which nesting and emerging females are probably dispersed, males either perch by themselves (
Tachysphex menkei
is apparently the first known hilltopping member of the genus with
larger males more likely to control lookout perches. The behavior of
males of this species is very similar to that of another summer-active
species of hilltopping crabronid on Usery Peak, Astata boharti F. Parker (
The wasp was kindly identified by Wojciech Pulawski at the California Academy of Sciences where specimens have been deposited. The research was facilitated by Jennifer Johnson of the Usery Mountain Regional Park; she provided permission to park in an area adjacent to Usery Mountain.