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A LONG-TERM STUDY OF MALE TERRITORIALITY IN THE TARANTULA HAWK WASP (HEMIPEPSIS USTULATA; POMPILIDAE) IN CENTRAL ARIZONA.

The tarantula hawk wasp Hemipepsis ustulata is a lek-forming insect whose males defend certain conspicuous shrubs and trees growing on mountain ridges in Arizona, even though these plants provide no resources of utility to males or females (Alcock, 1979, 1981). From mid-March to early May, depending on the year, one can find males perched in certain shrubs and trees on hilltops, one male to a plant. Males fly out to meet incoming males (and females with which they mate). Most intruder males quickly depart but, on occasion, the male in possession of the territory engages his opponent in an ascending flight in which the two individuals fly more or less straight up together for a distance that can range from about 5 m to much higher in the sky. At some point, the two wasps break off their ascending flight and dive down to circle about the territorial plant. They often repeat the maneuver, sometimes continuing for approximately an hour but generally for much shorter periods. The male that initially held the tree or shrub has a very large advantage in these contests, but occasionally a territory takeover occurs after some ascending flights (Alcock and Bailey, 1997). For 6 y starting in 1987, one mountain ridge (Usery Peak) has been surveyed during the flight season to census the male wasps present and to collect data on the relationship between male head-width (a correlate of body size) and territorial behavior. This report provides an update based on observations of the wasp made in Spring 2016 for comparison with earlier studies of the tarantula hawk. Long-term studies of animals, especially of insects, are relatively rare despite the fact that they permit an analysis of the repeatability of major findings, a point that will be explored here using the data collected in 2016.

Materials and Methods-On 57 days from March 13-May 25, 2016, I climbed to Usery Peak (elevation = 901 m; coordinates = 33[degrees]30'07.8"N, 111[degrees]38'23.9"W) in the Tonto National Forest near Mesa, Arizona to census the tarantula hawks that were occupying and defending certain of the foothills' palo verdes (Parkinsonia microphylla), creosote bushes (Larrea tridentata), wolfberries (Lycium species), and jojoba (Simmondsia chinensis). The ridge censused was about 175 m in length, and all plants along the ridge that had been previously occupied had been numbered. On 16 days I made two such censuses, one at around 0830-0930h Mountain Standard Time (MST) and the other at 1430h to as late as 1600h MST. I attempted, usually with success, to capture any perched male with an insect net, after which I removed the wasp. If it was unmarked, I measured its head-width to 0.05 mm with a pair of Helios dial calipers. I then marked the insect with a distinctive color, and number or letter, on the thorax with a Uniposca paint pen. The now-individually marked male was then released. At each census, perched males were captured and the location of any previously marked individual was recorded.

Any male captured at the same location on at least two consecutive days of censusing was considered a resident male (i.e., a territorial individual that had successfully defended his perch tree or shrub from intruders that visited the territory and would occupy it, if the site were not occupied already).

Results-Daily Activity Period of Wasps-My earlier studies had been invariably terminated on a daily basis around 1100h MST after up to 5 h of flight activity (Alcock, 1979), largely for the convenience of the observer. In 2016, I became aware that during at least a part of the 2.5-mo flight season some males continued to occupy territories long into the afternoon. On the 16 days between 21 March and 2 May 2016, when I visited the study site both in the morning and again in the mid- to late afternoon, I found an average of 4.06 [+ or -] 1.98 males occupying territories during afternoon censuses. This number was less than half the average for the morning period (10.50 [+ or -] 3.65 males; paired t-test, t = 7.86, P <0.001, df = 30) but, nevertheless, some wasps continued to occupy the same popular territories in the afternoon that were used in the morning by site-defending males.

When only two or three sites were occupied in the afternoon, these locations always included palo verde 2 (site identified as pv 2), palo verde 17 (pv 17), and creosote 4 (c4) (n = 6; afternoon census days from 22 March-2 May 2016). These were the most-frequently occupied territories during this and other studies (see below). The trees were scattered along the undulating ridgeline where they grew at the higher points of the peak.

Head Widths of Territory-Occupying Males-During the course of the study in 2016, I captured 214 males as they perched in a tree of their choice. The mean head-width of this sample of males was 4.29 [+ or -] 0.48 mm, a figure similar to that recorded in previous years at this site and at a lower ridge in the Usery Mountains beginning in 1980 (Table 1). Head-widths are highly correlated with body weight (Alcock, 1983).

Although there was no trend over years toward larger mean body sizes (Table 1), nevertheless large males appeared to have an advantage in securing 'preferred' territories. This, despite the fact that the head-widths of resident males in 2016 as a group were actually slightly smaller on average (4.24 [+ or -] 0.84 mm, n = 43) than males that made up the entire sample (4.29 [+ or -] 0.48 mm, N = 214). However, mean head-width of males (4.62 [+ or -] 0.30 mm, n = 17) that secured the top three territories (pv 2, pv 17, and c 4), as measured by the relative number of censuses in which the sites were taken by male defenders, was significantly larger than that of males that were residents elsewhere (4.13 [+ or -] 0.57 mm, n = 26; t = 3.27, P < 0.003; df = 41). Pv 2, pv 17, and c4 were occupied on 75, 62, and 64 censuses, respectively, far more often than the average for the remainder of the sites (mean = 14.46 [+ or -] 12.6 daily censuses; range = 3-41). Further evidence that pv 2, pv 17, and c4 were preferred by territorial wasps comes from the finding that they were the first three sites to be taken by territorial males after the study began on 13 March. Moreover, these three plants were the last sites to be occupied at the end of the study on 25 May. As noted above, they were also the sites most likely to be held during afternoon censuses in which only two or three territories were occupied by territorial males.

Comparison of Key Parameters of Male Territorial Behavior: 1997-2016-Over the 2 decades during which observations were made at Usery Peak itself, pv 2, pv 17, and c4 were always among the four, most-frequently occupied sites during the 6 y of censuses. (The other most-often taken site, palo verde 1, blew down in an intense storm in December 2009 or January 2010 and therefore is not considered here.) Thus, the males of many generations of wasps greatly preferred the same limited number of territorial sites across years. The same was true for a number of other key features of territoriality by H. ustulata (Table 2), such as the total number of sites occupied over the flight season, the maximum daily counts of defended sites and resident males, the mean tenure of resident males, and the total number of resident males recorded during the censuses at the peak. Although in 1 y (1998) all figures were higher (except for the average tenure of resident males), in the other 5 y of the study the population of tarantula hawk wasps behaved very similarly, such that the various parameters measured were highly similar from year to year.

Discussion-Long-term studies help overcome the risk of error arising from data collected during a short period that is atypical in terms of the ecology of the species under investigation (Wiens, 1977). Such studies are particularly helpful when one must follow the behavior of known individuals for several years to determine whether these individuals change their behavior and, if so, what might cause a shift in their behavior (e.g., Hughes and Hyman, 2011; Parker and Maniscolo, 2014). For example, studying populations of social animals for multiple years has been highly useful in determining what factors influence the decision of a bird to become a nonbreeding helper at the nest rather than a breeding adult (e.g., Brown et al., 1997; Hatchwell et al., 2013; Stacey and Koenig, 1990).

However, most studies of this sort have been done with long-lived birds and mammals and not with insects whose adult lives usually span a year or less. Nonetheless, repeat studies of the tarantula hawk H. ustulata have resulted in useful findings. For example, one result of the 2016 study was the discovery that some of the sites were defended by male tarantula hawks well into the afternoon, with some males not departing around midday but staying on until 1600h MST. It is true that the territorial activity of the wasps declines as the day progresses in March and April, but certain plants continue to attract defender males until late in the afternoon. Territories held by males in the afternoon are those that are consistently occupied by males earlier in the day.

Another result of this work is to confirm the conclusions presented earlier by Alcock (2008), namely that there is consistency in the numbers of males that become resident defenders of plant perches on Usery Peak (except for 1998 when wasps were especially abundant), the mean head-width of the wasps captured and measured during these studies, the maximum number of territories occupied during the flight season, the mean tenure of residents, and several other factors. The same individual plants that were highly "popular" in one year on Usery Peak, as defined by the number of days a male or males occupied the plant, remained much the same over 3 decades, probably in part because the dimensions of these plants (with larger individuals favored) stay much the same from year to year. in addition, the location of the hilltop plants (another key factor determining the attractiveness of potential territories) is also unchanged except when a plant dies (Alcock, 2008). The most-frequently taken territories were those on the highest points along the ridge, and these were generally held by larger males (Alcock and carey, 1988; this study). This finding suggests that body size, which varies greatly in this species, is important in male-male competition, despite the apparent constancy in average male head-widths across study years. Similarity in the distribution of male sizes probably stems from a similar distribution in prey sizes selected by female tarantula hawk wasps from year to year, as the weight of the offspring is almost certainly largely a function of the amount of provisions provided by its mother (Dow, 1942; Klostermeyer et al., 1973; Roulston and Cane, 2000). Thus, multiple factors are involved in the maintenance of territorial preferences and other territorial behaviors in the lekking pompilid wasp, Hemipepsis ustulata.

Literature Cited

Alcock J. 1979. The behavioural consequences of size variation among males of the territorial wasp Hemipepsis ustulata (Hymenoptera: Pompilidae). Behaviour 71:322-335.

Alcock, J. 1981. Lek territoriality in a tarantula hawk wasp Hemipepsis ustulata (Hymenoptera: Pompilidae). Behavioral Ecology and Sociobiology 8:309-317.

Alcock, J. 1983. Consistency in the relative attractiveness of a set of landmark territorial sites to two generations of male tarantula hawk wasps (Hymenoptera: Pompilidae). Animal Behaviour 31:74-80.

Alcock, J. 2008. Territorial preferences of the hilltopping wasp Hemipepsis ustulata (Pompilidae) remain stable from year to year. Southwestern Naturalist 53:190-195.

Alcock, J., and W.J. Bailey. 1997. Success in territorial defence by male tarantula hawk wasps Hemipepsis ustulata: the role of residency. Ecological Entomology 22:377-383.

Alcock, J., and M. Carey. 1988. Hilltopping behaviour and mating success of the tarantula hawk wasp, Hemipepsis ustulata (Hymenoptera: Pompilidae), at a high elevation peak. Journal of Natural History 22:1173-1178.

Brown, J. L., E. R. Brown, J. Sedransk, and S. Ritter. 1997. Dominance, age, and reproductive success in a complex society: a long-term study of the Mexican jay. Auk 117:279-286.

Dow, R. 1942. The relation of the size of the prey of Sphecius speciosus to the size and sex of the adult wasp (Hymenoptera: Sphecidae). Annals of the Entomological Society of America 35:310-317.

Hatchwell, B. J., S. P. Sharp, A. P. Beckerman, and J. Meade. 2013. Ecological and demographic correlates of helping behaviour in a cooperatively breeding bird. Journal of Animal Ecology 82:486-494.

Hughes, M., and J. Hyman. 2011. Should I stay or should I go now: late establishment and low site fidelity as alternative territorial behaviors. Ethology 117:979-991.

Klostermeyer, E. C., S.J. MechJr., and W. B. Rasmussen. 1973. Sex and weight of Megachile rotundata (Hymenoptera: Megachilidae) progeny associated with provision weights. Journal of the Kansas Entomological Society 46:536-548.

Parker, P., and J. M. Maniscolo. 2014. A long-term study reveals multiple reproductive strategies among territorial adult male Steller sea lions (Eumetopias jubatus). Canadian Journal of Zoology 92:405-415.

Roulston, T. H., and J. H. Cane. 2000. The effect of diet breadth and nesting ecology on body size variation in bees (Apiformes). Journal of the Kansas Entomological Society 73:129-142.

Stacey, P. B., and W. D. Koenig. 1990. Cooperative breeding in birds: long-term studies of ecology and behavior. Cambridge University Press, Cambridge, United Kingdom.

Wiens, J. A. 1977. On competition and variable environments. American Scientist 65:590-597.

Submitted 25 June 2016. Accepted 20 March 2017.

Associate Editor was Jerry Cook.

John Alcock

School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501

Correspondent: j.alcock@asu.edu
Table 1--Mean (-1 SD) head-widths (a correlate of body
size) of male Hemipepsis ustulata at the Usery Mountains in
Arizona sampled between 1980 and 2016. Data for 1980 and
1981 were gathered from males lower in the Usery Mountains;
the remaining data came from males collected specifically at
Usery Peak.

Year    N        Head width

1980   210   4.41 [+ or -] 0.51
1981   125   4.40 [+ or -] 0.42
1987   142   4.32 [+ or -] 0.42
1998   272   4.37 [+ or -] 0.45
2001   119   4.30 [+ or -] 0.41
2004   151   4.41 [+ or -] 0.46
2005   424   4.38 [+ or -] 0.46
2016   214   4.29 [+ or -] 0.48

Table 2--A comparison of key features of the territorial behavior
of male tarantula hawk wasps (Hemipepsis ustulata) observed from
1997 to 2016 on Usery Peak, Arizona.

Key parameter                            1997               1998

Total sites occupied                      23                 42
Maximum daily count of occupied
  sites                                   19                 35
Maximum daily count of resident
  males                                   13                 25
Mean tenure in days of resident
  males                            9.4 [+ or -] 7.1   9.7 [+ or -] 7.2
Number of residents                       47                 92
Days of census data                       25                 35

Key parameter                            1999               2006

Total sites occupied                      24                 16
Maximum daily count of occupied
  sites                                   13                 15
Maximum daily count of resident
  males                                   9                  9
Mean tenure in days of resident
  males                            8.2 [+ or -] 6.2   8.9 [+ or -] 7.2
Number of residents                       47                 41
Days of census data                       38                 51

Key parameter                            2007

Total sites occupied                      16
Maximum daily count of occupied
  sites                                   14
Maximum daily count of resident
  males                                    8
Mean tenure in days of resident
  males                            10.5 [+ or -] 6.5
Number of residents                       28
Days of census data                       32

Key parameter                            2016

Total sites occupied                      23
Maximum daily count of occupied
  sites                                   16
Maximum daily count of resident
  males                                    8
Mean tenure in days of resident
  males                            10.2 [+ or -] 7.9
Number of residents                       43
Days of census data                       57
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Author:Alcock, John
Publication:Southwestern Naturalist
Geographic Code:1U8AZ
Date:Jun 1, 2017
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