International Association for Ecology

Digestion Time and Reemergence in the Desert Grassland Scorpion Paruroctonus utahensis(Williams) (Scorpionida, Vaejovidae)Author(s): Richard BradleySource: Oecologia, Vol. 55, No. 3 (1982), pp. 316-318Published by: Springer in cooperation with International Association for EcologyStable URL: http://www.jstor.org/stable/4216836 .

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Oecologia (Beri) (1982) 55:316^318 OCCOlOgltt ? Springer-Verlag 1982

Digestion Time and Reemergence in the Desert Grassland

Scorpion Paruroctonus utahensis (Williams)

(Scorpionida, Vaejovidae)

Richard Bradley

Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131, USA

Summary. The desert grassland scorpion Paruroctonus uta- hensis spends most of its life in its burrow. During the active season, only about 5% of the individuals in a popula- tion appear on the surface each night. Individuals do not

appear on the surface for several nights following a meal. To determine if physiological digestion time could account for this delay in reemergence after eating, I measured chan-

ges in oxygen consumption immediately following a meal.

Oxygen consumption exceeded 125 ?? ?2g~1h"1 just after

completion of a meal, then dropped to normal levels (53 ?? 02g~ 1h~ *) within 6 h. I also measured the interval between

completion of the meal and subsequent defecation. All indi- viduals defecated by 72 h following ingestion (median 12 h). In field enclosures, scorpions returned to the surface after a mean of 20.3 days (median = 16) following a successful

pr?dation event. Lack of correspondence between estimates of physiological digestion time and the reappearance inter- val lead me to reject the idea of a long digestive pause in Paruroctonus utahensis. This conclusion lends support to the hypothesis that scorpions remain in their burrows to minimize exposure to pr?dation.

Introduction

The desert grassland scorpion Paruroctonus utahensis like

many other scorpions is notable for its relative inactivity. Individual P. utahensis spend most of their lives in a shallow

(20-30 cm deep) spiral burrow. In central New Mexico the

species is nocturnally active between the first warm nights of spring (Feb. or Mar.) and late fall (Nov.). When individ- uals appear on the surface they remain still, near the en- trance to their burrows, and await the approach of prey. Some scorpions can be found on the surface during most of the season, but rarely more than 5% of the population is up on any specific night. Similar patterns have been de- scribed for Paruroctonus mesaensis (Polis 1980a).

In my initial study of activity patterns, I noted that an individual typically did not appear on the surface for

many nights following a meal. Scorpions possess very low

resting metabolic rates (MR) (Hadley and Hill 1969, Ander- son 1970). Paruroctonus utahensis is no exception (Riddle 1978). I hypothesized that the long interval between surface

appearances of individuals that had just eaten might be related to a long digestion time. I used two approaches to test this hypothesis. The first test incorporates the idea

of the calorigenic effect of the meal. In mammals resting oxygen consumption rises during digestion, and then de- clines to "basal" levels when assimilation is complete. This effect has been documented for a few insects (Gourevitch 1928, Aidley 1976, Taylor 1977). Hagstrom (1970) reported a "3-fold increase" in MR after eating in Tarentula kochi

(Araneae, Lycosidae). For my test, I measured oxygen con-

sumption in scorpions after a meal, and monotored decreas-

ing MR until normal "resting" rates were attained. The second method was to determine the length of time between

completion of a meal and subsequent defecation (gut pas- sage time).

I considered the hypothesis of a long physiological di-

gestion time to be rejected if the two measures, gut passage time and length of a demonstrable calorigenic effect, dif- fered significantly from the surface appearance interval fol-

lowing a meal.

Methods

Adult and subadult (124-480 mg) Paruroctonus utahensis were collected about 7 km west of Albuquerque, Bernalillo Co., New Mexico, (Jun. 1980-Oct. 1981). They were kept in individual 700 cm3 plastic boxes with 200 cm3 of natural soil. Relative humidity in the boxes ranged from 72.6% to 82.3%. Containers were exposed to a 12:12 light, dark schedule. Box temperature varied from 19.5? C (dark) to 28.3? C (light) (similar to summer field values). Scorpions were fed mealworms (Tenebrio molitor), and crickets Ammo- baenetes phrixocnemoides or Acheta assimilis. Ammobaen- etes (natural prey) were used for the feeding tests. Experi- ments were conducted 1 h after onset of the dark period. Runs were made in a lighted room, so the scorpions re- mained inactive. Standard manometric methods were used with a Gilson Respirometer (Riddle 1978). Scorpions were denied food for at least one week before each experiment. Immediately after eating, scorpions were moved from

plastic box to respirometer flask. At least 30 min was al- lowed for equilibration. Metabolic rates presented here are 1 h means (microliters (??)) oxygen consumed per g live

weight per h barometric pressure corrected, for 20? C). Control (fasting) rates are means of the first 6 h (no time- related patterns were present). To measure gut passage time

scorpions were transferred to beakers after eating, and sub-

sequently checked at 4-12 h intervals until feces were seen. To estimate the interval between feeding and subsequent

reemergence, I set up field enclosures. Polyethylene 18.91

0029-8549/82/0055/0316/$ 01.00

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317

cross sectional view

view from above

Fig. 1. Field enclosures (buckets) in place. The 18.9 1 buckets are buried to a depth of about 35 cm. The scorpions are unable to climb the plastic sides of the buckets (5 cm), be, burrow entrance; sc, fiberglass window screen (about 1.5 mm mesh); dh, drainage holes (about 4 mm diameter). The soil (inside and out) is sandy loam

(5 gallon) buckets were buried to a depth of about 35 cm

(Fig. 1). About 250 4 mm holes were drilled in the bottom for drainage. Buckets were filled with soil and covered with fine screening, held with a rubber band (to prevent prey entrace). Continuous thermograph readings verified that the soil temperatures were similar to natural soil. One scor-

pion was placed in each bucket. Individuals were randomly selected, fed, and subsequent emergence patterns recorded each night.

Results

The influence of digestion on MR of experimental Paruroc- tonus was substantial. Mean oxygen consumption for the first hour after feeding was 126.9 ?? 02g~ xh~1. This is over twice the MR for fasting (control) scorpions (53.4 ?? 02g_1h_1). Using a Duncan's Multiple Range test I deter- mined that the readings for the first 3 h after feeding were

significantly higher than the fasting MR (P<0.01). For h 4 and 5 following completion of the meal, MR were signifi- cantly higher at ? < 0.05. The MR in the period following feeding declined to the pre-meal (fasting) MR level after about 6 h (Fig. 2).

There was more variability between individuals in the defecation experiments. Over 50% defecated 9-12 h after

completion of the meal, and about 75% of the scorpions had defecated after 36 h. By 48 h after eating, 87% of the

experimental animals had defecated. All remaining individ- uals defecated by 72 h (Fig. 3).

Fed scorpions in buckets return to the surface after a mean interval of 20.3 days (median = 16 days, ? = 62 feed-

ings). Some individuals remained underground for very long periods (8 individuals between 34 and 76 days). These

long post feeding inter-appearance intervals correspond well to my observations on marked individuals in the field.

However, field observations may include erroneously long intervals, since I may have missed individuals in the field.

Discussion

A large discrepancy is evident when digestion time estimates obtained here are compared with the post feeding emer-

gence interval. The estimate of assimilation time was 6 h. All individuals defecate in less than 72 h. A reappearance interval of 20.3 days is much longer than either digestion

time estimate. I conclude that the hypothesis of a long phys- iological digestion time, causally related to the long post- feeding reappearance interval, must be rejected.

An alternative explanation for the high metabolic rates measured here is that they reflect a circadian rhythm in MR. Circadian rhythms in metabolic parameters have been demonstrated many times in scorpions (Dresco-Derouet 1960, Naidu and Naidu 1976, Masthanaiah et al. 1977, Jayaram et al. 1978, Masthanaiah ?tal. 1978, Raghavaiah ?tal. 1978, Reddy ?tal. 1978, Uthaman and Reddy 1979, and Constantinou 1980). Despite this, I believe that my controls should have eliminated this factor. I measured

150

125

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75 o ?

d 50

25-

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8 12

Elapsed Time

16 20

Fig. 2. Results of the metabolic rate experiments. Oxygen consump- tion is in ?? 02g live wt" 1h~1. F is the reading for fasting (control) individuals, and represents the "basal" levels before initiation of the experiments. The elapsed time is expressed in h since comple- tion of a meal, mean for each hour period, for the first reading 0.5-1.5 h, for the second 1.5-2.5 h etc. The points represent mean values the vertical lines are 95% confidence intervals about the mean. Two stars indicate statistically significant deviation from the resting mean at ? < 0.01 (Duncan's multiple range test, a Bart- let's test verified similar variances). One star indicates significance at ? < 0.05. The samples sizes range from 10-36

100?1

< o

o

S Z> o

25-

12 ? ???

36 ?I? 48

??? 60

"T~ 24 36 48 60 72

HOURS SINCE COMPLETION OF MEAL

Fig. 3. Results of defecation experiments. No scorpions had defe- cated by 9 h, however 52% had defecated by 12 h. Scorpions were transferred to clean glass beakers after completion of a meal, and time of first appearance of fecal material noted. Results are lumped into 12 h intervals. ?'=27 individuals

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318

unfed scorpions during the same portions of the diel cycle, and there were no temporal changes in MR for these control runs. In addition, the magnitude of the feeding effect was

larger than would have been predicted if a circadian factor had been the cause.

Gut passage times reported here agree fairly well with those reported for other arthropods. Von Metzenauer

(1981) determined the schedule of movement of food within the gut of the ground beetle Pterostichus nigrita (Paykull) (Carabidae). Food was transferred into the midgut within 5 h. Some food reached the hindgut 1 h after ingestion, and defecation occurred "about 25-30" h after eating. Kanungo et al. (1970) studied the excretory products of the scorpion Palamnaeus bengalensis. It can be inferred from their methods that defecation occurred within 3 days of the meal.

My results lead me to reject the digestive pause explana- tion for the long post-feeding delay in emergence of Parur- octonus. What then can be proposed to explain this phe- nomenon? Scorpion activity patterns are constrained by the danger of cannibalism (Polis 1980 b) and pr?dation. On

my study areas in central New Mexico a number of verte- brate predators are known to eat scorpions. These include the palid bat Antrozous pallidus (Vespertilionidae), the

grasshopper mouse Onychomys torridus (Cricetidae), and the Burrowing Owl Athene cunicularia (Strigidae). I also have records of cannibalism in Paruroctonus utahensis as well as interspecific pr?dation by the sympatric scorpion Vaejovis russelli (Williams). The apparent "time minimiza- tion" strategy of Paruroctonus utahensis may be a response to the cumulative pressure from such predators. Presum-

ably, well-fed scorpions can "afford" the luxury of remain-

ing underground for long periods, thus sparing themselves

exposure to pr?dation or cannibalism.

Acknowledgments. I thank Eric Toolson and Cliff Crawford for the use of equipment, as well as advice on the design of my study. I also thank Sarah George for assistance in the laboratory. I thank John Anderson, Cliff Crawford, Neil Hadley, Larry Marshall, Gary Polis, Eric Toolson, John Wiens, and Bruce Woodward for

reading one or more early drafts of this paper and providing many helpful suggestions. Financial assistance was provided by the Grad- uate Research Allocations Committee and the Student Research Allocations Committee, University of New Mexico. Computer time was provided by the Computer Sciences Department, University of New Mexico.

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Received June 1, 1982

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