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Some observations on the mating behavior of the giant walking stick, Megaphasma dentricus (Orthoptera: phasmidae).

Sexual dimorphism is a prominent feature throughout the animal kingdom, from birds to ungulates to arthropods (Blanckenhorn et al. 2007). In many stick insects, sexual dimorphism is two-fold. First, the males may bare enlarged femoral spines, perhaps used as weapons in male-male competitions (Sivinski 1978; Brock 1999). Second, the males can be smaller than females, which may be a benefit to both males and females, as during mating, the male hangs on to the female, and a smaller male may allow the female to be more mobile for feeding and/or have more energy to invest in reproduction (Sivinski 1978). However, the idea that relatively small males have a higher fitness than large males is contradictory to a body of sexual selection theory that suggests that larger males, specifically in species that have weapons and engage in male-male competitions, often obtain more matings (Andersson 1994). The preliminary observations on the male-male and male-female interactions in the giant walking stick Megaphasma denticrus (Stal; also referred to as Megaphasma denticrum), while too few for statistical analyses, suggest that future investigations of the following hypotheses could be profitably pursued: (1) males compete with each other over access to females or resources, (2) large males are more likely to mate than small males, (3) males missing legs (autotomy) experience any mating disadvantages, (4) males prefer large females, and (5) females prefer small males.

The giant walking stick is one of the largest phasmid species in North America and is found in the south central United States (Figure 1; Hebard 1943; Wilkings & Breland 1951). Both sexes have femoral spines on their mid and hind legs, but males have a single large spine and females a row of small spines. Both sexes are green and brown in color, and females are generally larger than males (females in this study ranged from [approximately equal to] 105-135 mm and males ranged from [approximately equal to] 90-125 mm). Females drop their eggs to the ground, and can lay up to three eggs per hour and thirteen per day for several months. Like some other phasmids, males have a clasping organ--modified cerci used to wrap around a female's abdomen during copulation and/or mate guarding (Figure 2; Wilkins & Breland 1951; Sivinski 1978).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In May 2007, 29 females and 30 males were collected in Blanco County, Texas (mostly found on mesquite, Prosopis glandulosa Torr.). Individuals were brought to the lab, measured for body size (anterior of head to tip of abdomen), inspected for missing legs, and given a unique color band for identification (Figure 1). All animals were fed hackberry leaves ad libitum (Celtis laevigata Willdenow) and kept in cages in a large greenhouse. Groups of individuals (see Table 1) were placed in a 37.85 L fish tank and recorded for 12 h with a red light/ lamp and Sony Handycam DCR-SR42 ([approximately equal to] 7:00 p.m. to [approximately equal to] 7:00 a.m.; like many insects, phasmids are thought to be unable to see red light). No individuals were used in observations more than once, and once used, individuals were put together in a large holding cage where their mating behaviors were recorded approximately every twelve hours. During filming, six categories of behaviors were noted: mating effort, mating, grappling, feeding, walking, and motionless. 'Mating effort' was defined as either a male simply hanging on a female (Figure 1) or a male attempting to wrap his clasping organ around the female. 'Mating' was defined as a male having his clasping organ wrapped around the female (Figure 2; actual copulation was not determined in this study because it was difficult to tell whether the genitalia were in physical contact or whether the male simply has his claspers around the female). 'Grappling' was defined as two individuals who engaged in extensive touching of their legs and is considered to be mildly aggressive (Sivinski 1978).
Table 1. Mating behavior in Megaphasma dentricus. Small males were
shorter than 101.3 mm, and large males were longer than 115.7 mm.
Similarly, small females were shorter than 110.2 mm, and large females
were longer than 127.7 mm. All small and large pairs were at least 15
mm difference in size. Average males were [approximately equal to] 105
mm and average females were [approximately equal to] 117 mm.

Individuals Trial Successful [approximately Time spent on mating
 Mating equal to] effort by 'loser' male
 Pair Minutes until (out of [approximately
 claspers equal to] 720 min)
 attached

Small [male] 1 [female] + 1 70.1% (505 min)
 Small
 [male]

Large [male]

Average 2 [female] + < 1 97.2% (700 min)
[female] Large
 [male]

 3 [female] + 4 3.5% (25 min)
 Small
 [male]

 4 [female] + 2 0.0% (0 min)
 Large
 [male]

 5 [female] + 2 29.2% (210 min)
 Large
 [male]

[male] * 1 [female] + < 1 47.9% (345 min)
 [male] (-)
 1 leg

[male] (-) 1
leg

[female] 2 [female] + 2 3.5% (25 min)
 [male]

 3 [female] + 2 91.0% (655 min)
 [male]

[male] * 1 [female] + < 2 39.6% (285 min)
 [male] (-)
 2 legs

[male] (-) 2
legs

[female] 2 [female] + 1 41.7% (300 min)
 [male] (-)
 2 legs

Small 1 [male] + 2 N/A
[female] Large
 [female]

Large
[female]

Average 2 [male] + < 1 N/A
[male] Small
 [female]

 3 [male] + 3 N/A
 Small
 [female]

* Body size in males was controlled for in each of these
experiments/trials (e.g., the male with all six legs and the male
missing legs were within two mm of each other).


Males do not appear to compete with each other over food or females, as grappling behavior occurred less than 2% of the time ([bar.x] = 10.9/720 min, SD [+ or -] 6.3). This was true for all trials involving females (listed in Table 1) and trials with only two males (e.g., small vs. small, large vs. large, small vs. large, and average vs. average--one trial each, not listed in Table 1, same filming methods as above). With or without a female present, males often came in contact without grappling, and simply walked over or around each other with no signs of aggression. In addition, they spent many minutes feeding in close proximity, and on several occasions, one male hung off the other.

These preliminary observations suggest that mating is opportunistic and on a 'first-find, first-mate' basis (Table 1). Large males were no more likely to find and successfully attach to females first, and males missing appendages appeared to be at no mating disadvantage. Females may show little preference for small males, and males no preference for large females; a male clasped to the first female he encountered, and remained attached for at least the next twelve hours, even if the other individual approached the mating pair. In each of the observations, a male had his claspers around a female in four minutes or less (most within two minutes), and he remained there for at least the next twelve hours. That is, there was no instance where a male successfully clasped around a female and then abandoned that female, even with no other male present. On many occasions within each of the 12h observations, the 'loser' male also got his claspers around the mating female, in which scenarios he attached anterior to the other male, further away from the female's genitalia. This did not appear to be very stable, as this second male would repeatedly slip off the female. However, 'second' males spent up to 90% of their time attempting to mate with a paired male and female ([bar.x] = 305/720 min, SD [+ or -] 253.7), often hanging or following the mating pair around the cage and/or swinging their abdomen around in an attempt to attach. On average, both males and females spent [approximately equal to] 15% of their time feeding ([bar.x] = 105.2/720 min, SD [+ or -] 71.7), and [approximately equal to] 85% of their time walking or motionless ([bar.x] = 614.8/720 min, SD [+ or -] 71.7). The exceptions to this were single females; in the three observations with a male, a large female, and a small female, the unmated female spent up to 95% of her time feeding ([bar.x] = 490/720 min, SD [+ or -] 190.8).

Other interesting observations arose after the experimental animals were placed together in one large cage. First, two mating pairs were together for over 60 hours (but < 72), presumably extensive mate guarding (Sivinski 1983; Brock 1999). Second, five males changed partners at least three times in a 36-hour period, and most males changed partners once every 24-36 hours. Moreover, their mate switching did not follow any pattern of preferring smaller or larger individuals. Third, available males and females did not always form a mating pair (e.g., just because there was an available mate did not mean a mating pair formed). These observations provide more support that both males and females do not show strong preferences for body size or overall morphology of their mate.

In conclusion, mating in Megaphasma dentricus appears to be opportunistic. Although there are strong adaptive hypotheses for both sexes related to their sexual dimorphism, our limited observations illuminated no apparent patterns of behavior involved in male-male competition or female choice. Other North American phasmids, such as Diapheromera veliei and D. covilleae, are different; larger males win contests over access to females and larger males mate longer than small females (Sivinski 1978). Perhaps the femoral spines function for crypsis and/or predator defense (Bedford 1978), and/or their sexual dimorphism is an evolutionary relic of other closely related species. Future work will reveal more details about the mating behavior seen in M. dentricus, and if it is typical of other phasmids.

LITERATURE CITED

Andersson, M. 1994. Sexual selection. Princeton University Press, Princeton, NJ, 624 pp.

Bedford, G.O. 1978. Biology and ecology of the phasmatodea. Ann. Rev. Entomol., 23:125-49.

Blanckenhorn, W.U., A.F.G. Dixon, D. J. Fairbairn, M. W. Foellmer, P. Gibert, K. van der Linde, R. Meier, S. Nylin, S. Pitnick, C. Schoff, M. Signorelli, T. Teder & C. Wiklund. 2007. Proximate Causes of Rensch's Rule: Does Sexual Size Dimorphism in Arthropods Result from Sex Differences in Development Time? Am. Nat., 169(2):245-257.

Brock, P. 1999. The amazing world of stick and leaf insects. The amateur entomologist, 26. Cravitz Printing Company Ltd., Brentwook, UK, 165 pp.

Hebard, M. 1943. The dermaptera and orthoperous families blattidae, mantidae and phasmidae of Texas. Trans. Amer. Ent. Soc., 68:239-313.

Sivinski, J. 1978. Intrasexual selection in the stick insects diapheromera veliei and D. covilleae and sexual dimorphism in the phasmatodea. Psyche, 85:395-405.

Sivinski, J. 1983. Predation and sperm competition in the evolution of coupling durations, particularly in the stick insect Diapheromera veliei. Pp, 147-162, in Gwynne, D. T. and G. K. Morris (eds.), Orthoptera mating systems: Sexual competition in a diverse group of insects. Wesview Press, CO, 376 pp.

Wilkins, O. P. & O. P. Breland. 1951. Notes on the giant walking stick Megaphasma denticrus (Stal) (Orthoptera: Phasmida). Texas J. Science, 2:305-310.

TLM at: taram@stedwards.edu

Tara L Maginnis, Christina L Cool and Jana L Muniz

Department of Biology, St. Edward's University Austin, Texas 78704
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Title Annotation:GENERAL NOTES
Author:Maginnis, Tara L.; Cool, Christina L.; Muniz, Jana L.
Publication:The Texas Journal of Science
Date:Feb 1, 2008
Words:1863
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