Tests of Alternative Models of Prey Consumption by Predators, Using Ant-Lion Larvae Author(s): David Griffiths Source: Journal of Animal Ecology, Vol. 51, No. 2 (Jun., 1982), pp. 363-373 Published by: British Ecological Society Stable URL: http://www.jstor.org/stable/3970 . Accessed: 07/05/2014 20:01 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. http://www.jstor.org This content downloaded from 169.229.32.136 on Wed, 7 May 2014 20:01:02 PM All use subject to JSTOR Terms and Conditions
Transcript
Tests of Alternative Models of Prey Consumption by Predators, Using Ant-Lion LarvaeAuthor(s): David GriffithsSource: Journal of Animal Ecology, Vol. 51, No. 2 (Jun., 1982), pp. 363-373Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/3970 .
Accessed: 07/05/2014 20:01
Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp
.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].
.
British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal ofAnimal Ecology.
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TESTS OF ALTERNATIVE MODELS OF PREY CONSUMPTION BY PREDATORS, USING ANT-LION
LARVAE BY DAVID GRIFFITHS
Zoology Department, P.O. Box 35064, University ofDar es Salaam, Tanzania
SUMMARY
(1) When fed one ant/day the amount of food extracted per prey by larvae of the ant-lion Macroleon quinquemaculatus was independent of predator size or hunger level. Feeding time was constant within an instar but varied between instars.
(2) When fed prey at one hour intervals extraction efficiency per prey again remained constant but feeding time declined with successive prey. This pattern is consistent with the hypothesis that the rate of prey digestion determines feeding time. Another experiment showed that the rate at which the contents were extracted from the body did not determine feeding time, in line with the hypothesis.
(3) Differences in the extraction efficiency of larvae feeding on different prey species are shown to be due to the presence in some prey of defensive secretions which the larvae do not ingest. Variation in extraction efficiency also results from incomplete consumption of the ant by the ant-lion.
(4) It is concluded that there is little difference in the rates of energy intake predicted by the gut-limitation and optimal foraging hypotheses; the factors affecting the level of gut fullness are of greater importance in determining the rate of prey consumption.
INTRODUCTION
Cook & Cockrell (1978) recently extended optimal foraging theory to consider the amount of food extracted from prey (the extraction efficiency when expressed as a percentage of initial prey weight) and the time spent in that extraction (feeding time). They argued that if some parts of a prey were easier (or better) to consume than others a predator could maximize the net rate of energy intake at high prey densities by feeding selectively on the best parts. Hence this model predicts that feeding time and extraction efficiency will be functions of prey density. They contrast this behaviour with that predicted by the gut-limitation model; this states that the predator continues feeding until its 'gut' is full. According to this model feeding time and extraction efficiency will not vary with prey density, provided the predator's gut is not full. At low prey densities both models make the same predictions; extraction efficiency should be at a maximum and feeding time should be constant.
Many ant-lion larvae are sessile, and thus dependent on active prey for their food. Encounters with prey are infrequent (Griffiths 1980b); both models predict that feeding efficiency should not vary with hunger level. Comparison of the food extraction efficiency and feeding times of starved and fed Morter obscurus Ramb. larvae, a small species, were consistent with the predictions (Griffiths 1980a). Here I examine the effect of prey size, prey species, predator size and predator hunger level on the feeding efficiency of larvae of
0021-8790/82/0600-0363 $02.00 ? 1982 British Ecological Society
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the large ant-lion Macroleon quinquemaculatus (Hagen). Preliminary observations showed that neither model adequately explained the feeding efficiency of this species. Forty milligram larvae ingested about 15 mg from prey presented at 1 h intervals with no detectable decline in extraction efficiency. The corresponding figures of 60-80 and 80-100 mg larvae were 20 and 25 mg respectively. Clearly ant-lion larvae are capable of ingesting large amounts of food (the figures are not upper limits as I gave up before the ant-lions did). However the results are not consistent with either model because the larvae showed consistent declines in feeding time on successive prey.
I propose here, and test, a modified gut-limitation model, the digestion rate limitation (DRL) hypothesis. In ant-lion larvae digestion is extra-oral and the food liquefied (or broken down into small particles) before being ingested. I suggest that the rate at which food can be digested in the prey determines the feeding time. The DRL hypothesis makes a number of predictions:
(1) Rates of digestion depend, among other things, on the quantity of enzyme present; release of enzyme is usually stimulated by the presence of food (Davson & Segal 1975). Accordingly one would expect the quantity of enzyme present and the rates of digestion and extraction to increase with successive prey.
(2) Feeding time should depend on the amount of material to be digested and not the quantity to be extracted (i.e. the rate at which food is sucked out of the prey is not limiting).
(3) Feeding time should consist of three periods; (a) injection of enzymes, (b) followed by a digestive pause and (c) extraction of the food.
(4) Extraction efficiency should be more or less constant and not show any systematic variation with prey density or hunger level.
METHODS
All experiments were carried out in the laboratory, where the larvae were kept separately partitioned in trays of fine sand. Air temperature showed little variation about the mean of 27 ?C (range 26-29 ?C). All weights presented are wet weights. The amount of food extracted from a prey (We) was determined by weighing the victim before presentation to the predator (Wa) and weighing the carcass (We) after it was ejected from the pit. These latter weighings were always done within 15 min of carcass ejection so that weight loss by evaporation was small. Ant-lion larvae have three instars, easily separable by head capsule size but body weights were also recorded. The ants Camponotus sp. and Anoplolepis sp. were the principal prey. The former showed little size variation whereas workers of the latter were highly variable. Griffiths (1980a) divided handling time into three periods, the greatest of which was always the time spent extracting food from the prey (Te). Macroleon usually drags its prey beneath the substratum so that the three periods cannot be easily separated. As an approximation to feeding time I measured the time between prey capture and ejection of the carcass.
RESULTS
Extraction efficiency, measured as the percentage of whole ant weight extracted, was independent of predator instar and weight (Fig. 1). Figure 2 shows that the amount of food extracted was not affected by hunger level for larvae feeding on Anoplolepis; similar results were obtained for predation on Camponotus. Separate regressions for hungry and fed
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FIG. 1. Extraction efficiency, as a function of predator size and instar, for larvae feeding on Camponotus sp. The arrow indicates the size at which most larvae moult from instar 2 to 3.
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eye.
larvae for these data showed no differences in slopes or intercepts for each species (Camponotus F1,115 slopes 0.24 NS, intercepts 2.80 NS; Anoplolepis F1,44 slopes 1.53
NS, intercepts 0.04 NS). However there were differences in predator extraction efficiency between prey species. Mean extraction efficiency (arcsin transformed data) on
Camponotus was 60.0% (n = 113), on a very small Pheidole sp. 59.9% (n = 20), and on
Anoplolepis 49.3% (n = 116). The reason for the much lower extraction efficiency of Macroleon larvae when feeding on Anoplolepis will be considered later.
Figure 3(a) shows mean feeding times (based on eight daily feedings) on Camponotus for larvae that had previously been well fed (i.e. fed one 3 + mg ant/day for the previous 7
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FIG. 3. (a) Mean feeding times (?95% C.L.) for individual ant-lions feeding on Camponotus sp. Mean feeding times for each instar are shown by horizontal lines. These ant-lions had previously been well-fed. (b) Individual feeding times for previously starved instar 3 larvae feeding on
Camponotus.
days). Feeding times between instars were significantly different (instars 1 v. 2, t = 7.66, d.f. = 12P < 0.001; instars 2 v. 3, t = 10.06, d.f. = 20P < 0.001) but within an instar feeding time was independent of predator weight (slopes + sb, Il 0 11 + 0-83; 2 -0.18 + 0.22; 13 0-05 + 0-13). This is in marked contrast to Morter where feeding times declined continuously with increasing predator size and where there were no discontinuities between instars (Griffiths 1980a). Figure 3(b) shows feeding times for larvae that had been starved for 4-16 days. Feeding times for small third instar larvae clearly increased after starvation, approaching those of instar 2 larvae, but there was no detectable hunger effect for larvae in excess of 80-90 mg.
DRL hypothesis To test the DRL hypothesis seven large (90-170 mg) third instar larvae were fed one
medium-sized (3-34 + S.E. 0 185 mg, n = 56) Anoplolepis every hour for eight feedings The ant-lions took between 12-45 min to consume their prey; the remaining time was used to weigh ants and carcasses. Figure 4 shows mean extraction efficiency and feeding time for successive prey. Extraction efficiency was constant (Table 1), as found in the preliminary experiment. Extraction rate was independent of prey size but feeding time declined significantly with feedings. As indicated earlier this reduction in feeding time with a constant extraction efficiency is consistent only with the DRL hypothesis.
The presence of water carriers in Camponotus suggested a further test of the DRL hypothesis. These ants have their abdomens distended by considerable quantities of water which they are transferring to the nest. Hence the weight of extractable material is much greater than in normally foraging individuals. However since water carriers appear to be similar in all other respects to normal foragers the feeding times for ant-lions should be the same for both prey types if the DRL is correct. Table 2 presents the results of this comparison. The carcass weights of water carriers and normal ants were not significantly
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different, consistent with the notion that these were otherwise similar individuals with similar amounts of digestible material. Despite the fact that ant-lions feeding on water carriers ingested, on average, 2.7 times more material than those feeding on normal foragers there was no difference in feeding times (i.e. the rate at which the body contents were sucked out of the prey was not limiting).
D. GRIFFITHS 367
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The DRL hypothesis suggests that feeding can be divided into three periods as outlined earlier. I allowed Macroleon to feed on Camponotus ants for varying times and determined the weight of food extracted. The weights extracted after different feeding durations are shown as percentages of the total weight extractable in Fig. 5, this total weight being calculated as 60% of ant live-weight (see earlier). The results are consistent with the hypothesis. There is an initial weight increase while the enzymes are injected but within a few minutes of the start of feeding the animal begins to extract food. The ant-lion's behaviour supports this interpretation. For 2-3 min after the death of the ant the predator does not move, but then it starts manipulating the prey with its mandibles as extraction occurs. Cook & Cockrell (1978) found that in Notonecta glauca (L.) there appeared to be immediate extraction of food from the prey with no evidence of a digestive period. This is probably partly due to their use of dry weights (which would make enzyme injection difficult to detect) but the possibility exists of a real difference between Notonecta and Macroleon in extraction techniques. Cook & Cockrell suggested two explanations for varying extraction rates:
(a) It becomes more difficult to extract food from the prey as feeding progresses; this is a consequence of the feeding process.
(b) Some parts of the prey are more profitable than others; this is either because these parts are easier to consume or because they have a higher food value.
Note that these possibilities are not necessarily mutually exclusive. One might expect the second explanation not to apply to Notonecta feeding on mosquito larvae simply on morphological grounds; the prey consist of a more or less straight tube. In contrast, this second explanation seems likely for Macroleon feeding on morphologically complicated ants; the gaster tends to be more or less spherical but the thorax is much more convoluted and extraction time should therefore be greater. To test this I measured the amount of time spent extracting food from the gaster and from the head and thorax. Ant-lions spent 71% (range 55-87%, n = 8) of the feeding time extracting food from the head and thorax
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FIG. 5. Actual weight extracted (as % maximum weight extractable) for larvae allowed to feed for varying times on Camponotus. Values shown are means + 95% C.L. Line fitted by eye.
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but only obtained 57% of the extractable food from these regions, in agreement with the second explanation.
If feeding time is determined simply by the rate of digestion, then one would expect no differences in feeding times between instars. Figure 3(a) showed that this was not the case for larvae feeding on Camponotus while Fig. 6 extends this conclusion to other prey species. Differences in feeding times between instars are small when prey are small but get relatively larger (note the logarithmic ordinate) as prey size increases. These differences are consistent with the notion that mandible size also affects feeding time. Whereas a large ant-lion should be able to reach all parts of the thorax, for example, of a large prey a first instar larva, with its small mandibles, would have to insert these on several occasions to reach these parts. If each insertion (or any process associated with it, e.g. digestion) takes time then the greater the number of insertions the greater the feeding time. When feeding on small prey, however, small larvae would be expected to reach all parts as efficiently as large larvae and feeding time differences should be less.
Variation in extraction efficiency
All the models predict that when prey are scarce the larvae should extract all the food available and we should therefore expect little variation in extraction efficiency on prey of a given species. Figure 7 shows that extraction efficiency is in fact quite variable. Part of this variation can be attributed to measurement errors and, more importantly, to variation in the ants themselves and their crop contents. There are however two additional sources of variation. Dissection of the carcasses showed that in some ants (particularly Anoplolepis) some of the body contents were left in the gaster, whereas the head and thorax appeared clear of food material. Anoplolepis ants carry appreciable quantities of defensive secretion which makes their collection (by aspirator) unpleasant while Camponotus have much less defensive secretion. I suggest (a) that the difference in mean extraction efficiency for larvae feeding on the two species is due to the presence of this secretion which the ant-lions do not ingest and (b) that part of the variation in extraction efficiency while feeding on
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I' I I I I 1
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FIG. 6. Mean feeding times (+ 95% C.L.) for the three instars feeding on the prey species Pheidole (solid points), Anoplolepis (triangles) and Camponotus (circles).
D. GRIFFITHS 369
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FIG. 7. Histograms of percentage extraction efficiency for ant-lions feeding on Camponotus (shaded) and Anoplolepis.
Anoplolepis is due to variation in the quantity of secretion present. To test the hypothesis I compared extraction efficiency on individual Anoplolepis with the quantity of secretion left in the carcass. This quantity was estimated by squashing the gaster of the carcass against litmus paper and assessing the size of the resulting red patch (the body contents of the prey did not react with litmus). Similar sized ants (7.99 ? S.E. 0.43 mg, n = I) were used to reduce the effect of size-related differences in the quantity of secretion carried. I ranked the quantity in terms of the area covered on the litmus paper and compared this with the corresponding extraction efficiencies. If these efficiencies were within 1% of each other they were treated as tied rankings, Kendalls rank correlation coefficient gave a highly significant negative correlation, as predicted (r = -0-692, P = 0-002, n = I, one-tailed test). Similarly Griffiths (1980a) found that Morter larvae have a lower extraction efficiency when feeding on Crematogaster nigriceps Emery than on other ants; this species also contains large quantities of defensive secretion.
Variation also results from ant-lions missing a part of an ant's body when feeding. I divided Camponotus carcasses into head, thorax, and gaster and compared the weights of these sections with those of whole ants. Of thirty prey examined one ant-lion had missed the head, while another had missed the gaster. While such errors occur only infrequently they account for the long left tail of the extraction efficiency distributions. Table 3 shows coefficients of variation for the three body sections for whole ants and carcasses (unconsumed sections were omitted from the latter calculations). The gaster is by far the
TABLE 3. Coefficients of variation for whole ants, carcasses and weight extracted for the three body sections of Camponotus sp.
* Determined as difference between carcass and whole ant values.
compared extraction efficiency on individual Anoplolepis with the quantity of secretion left in the carcass. This quantity was estimated by squashing the gaster of the carcass against litmus paper and assessing the size of the resulting red patch (the body contents of the prey did not react with litmus). Similar sized ants (7.99 + S.E. 0.43 mg, n = 11) were used to reduce the effect of size-related differences in the quantity of secretion carried. I ranked the quantity in terms of the area covered on the litmus paper and compared this with the corresponding extraction efficiencies. If these efficiencies were within 1% of each other they were treated as tied rankings, Kendalls rank correlation coefficient gave a highly significant negative correlation, as predicted (r = -0 692, P = 0.002, n = 1 1, one-tailed test). Similarly Griffiths (1980a) found that Morter larvae have a lower extraction efficiency when feeding on Crematogaster nigriceps Emery than on other ants; this species also contains large quantities of defensive secretion.
Variation also results from ant-lions missing a part of an ant's body when feeding. I divided Camponotus carcasses into head, thorax, and gaster and compared the weights of these sections with those of whole ants. Of thirty prey examined one ant-lion had missed the head, while another had missed the gaster. While such errors occur only infrequently they account for the long left tail of the extraction efficiency distributions. Table 3 shows coefficients of variation for the three body sections for whole ants and carcasses (unconsumed sections were omitted from the latter calculations). The gaster is by far the
TABLE 3. Coefficients of variation for whole ants, carcasses and weight extracted for the three body sections of Camponotus sp.
Head Thorax Gaster n Whole ants 7.9 7.8 24.4 10 Carcasses 19.7 16.2 35.6 30 We* 11.8 8.4 11.2
* Determined as difference between carcass and whole ant values.
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most variable section of the body but the coefficient of variation in food extracted is similar for all three, i.e. the ant-lions are equally proficient at extracting food from each section. If all body sections were always consumed, extraction efficiency should vary between 50-80% on Camponotus.
DISCUSSION
Is it possible to distinguish between the gut-limitation (and DRL) and optimal foraging hypotheses for predators that normally operate at low prey densities? I believe the answer is 'No'. The optimal foraging hypothesis predicts a decline in extraction efficiency when prey densities are so high that additional prey frequently enter the pit when the predator is feeding. Only in these circumstances could the animal do better, by lowering extraction efficiency, than by filling its gut. Griffiths (1980a) showed that there was no decline in extraction efficiency for Morter when a second prey was introduced midway through the feeding period, in apparent contradiction of the optimal foraging model. However Griffiths (1980b) showed that contacts with two or more prey during feeding were likely to be very infrequent for Morter so the selective pressure for optimal foraging behaviour to evolve is probably low.
The contrasts between the gut-limitation (and DRL) and optimal foraging models have been emphasized but it must be stressed that the former describe mechanisms while the latter is more concerned with strategy. Frequently the models make similar predictions about the net rate of energy intake, contrary to Cook & Cockrell's belief. As discussed above the predictions of the hypotheses are indistinguishable at low prey densities. At high prey densities a gut limited predator can also do as well as an 'optimal forager'. Consider predators whose prey are of uniform profitability. A predator which keeps its gut full will then be maximizing energy intake and behaving as an optimal forager. Johnson, Akre & Crowley (1975) and Nakamura (1977) have described such behaviour in damselfly nymphs and the wolf spider Pardosa laura Karsch respectively. Johnson, Akre & Crowley termed the behaviour wasteful killing because the predator did not necessarily consume all of a prey item. They suggested that hunger (space) in the midgut motivated attack behaviour but that fullness of the foregut could prevent eating. If prey have 'best bits' a predator behaving as Cook & Cockrell describe could do better than one which simply fills its gut if (but only if) it consumed these best bits first. However, unless the differences in profitability are gross (which I suspect they often are not) the energy intake of both types of predator will be similar. Optimal foragers would seem to have the greatest advantage at intermediate prey densities (i.e. in situations where the gut contains only small quantities of food but where prey encounters are, on average, fairly common). Prey abandonment before the gut is full (which is an essential feature of the optimal foraging model) might well be more commonly found in fluid feeders than in particle consumers. In the former, 'eating' consists of digestion and ingestion of food while in the latter only ingestion occurs. Accordingly feeding times for particle consumers should be less than for fluid feeders and the advantage of discarding prey reduced. Cook & Cockerell suggest that prey abandonment at low prey densities, just because the gut is full, is unlikely to be advantageous; the predator should continue to feed but at a rate which matches the rate of evacuation of the gut. Nakamura (1977) has shown that Pardosa laura can behave in this manner. The spider is capable of multiple prey captures but does not abandon previously captured items and will return to feed on them. While Cook & Cockrell might be correct
D. GRIFFITHS 371
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for predators that cannot accumulate prey, discarding them because of gut limitation seems unlikely to produce a large departure from optimality. At low prey densities the gut is, on average, unlikely to be full and so the loss, on average, from such behaviour should be small, though as Taylor (1976) has shown the discrepancy will also depend on prey aggregation patterns and gut size.
This discussion then suggests that the important question is not whether predators are maximizing the net rate of energy intake but why some predators fill their guts, e.g. damselfly nymphs and others, apparently, do not, e.g. Notonecta (I say apparently because Cook & Cockrell's experiments on Notonecta were for 4 h feeding periods after 48 h starvation; the possibility exists that under natural conditions the gut is normally full). Gut filling results from high ingestion and/or low digestion and absorption rates. Nakamura (1977) concluded that spiders have low digestion/ingestion ratios and hence normally full guts while in insects the ratio is greater and gut limitation less important. He related the low ratios in spiders to the presence of gut diverticula in which partially digested food is stored and suggested that the ability of spiders to resist starvation was correlated with this storage. Other factors also affect gut fullness. Clearly fullness depends on prey availability; predators living at low prey densities will not normally have full guts (e.g. ant-lion larvae). Secondly feeding mechanisms can affect gut fullness; a large particle feeder will ingest more than the corresponding fluid feeder which leaves the exoskeleton. Finally physiological constraints on food processing rates could affect gut fullness. Johnson, Akre & Crowley, for example, suggest that the rate of production of the peritrophic membrane in the midgut (itself under hormonal control) limits the rate of passage of food through the midgut and hence the rate of prey consumption.
The DRL and gut-limitation hypotheses argue that physiological (physical) constraints operate on the rates of prey consumption in some predators. The maximum extraction rates for Macroleon and Morter larvae with similar sized mandibles are very close (instar 2 Macroleon 0.051 mg/min, instar 3 Morter 0.052 mg/min) but the rates for Morter are weight dependent and, in general, lower than those of the larger species, suggesting the operation of some additional constraint. In Macroleon feeding time seems to be constrained only by the rate at which food can be digested. Why should selection have favoured the removal of other constraints (such as the one apparently operat- ing in Morter) i.e. what is the advantage of short handling times? The presence of an ant carcass in the pit when a new prey enters would probably reduce prey capture success; by reducing feeding time to a minimum the food loss from this cause should also be reduced. Comparison of prey consumption rates and the factors determining them in pit building and free-living ant-lion larvae would be interesting in this respect.
Addendum After this manuscript had gone to press I discovered two papers which provide evidence
for the DRL hypothesis. Mayzaud & Poulet (1978) showed that the quantities of six digestive enzymes in five species of copepods were correlated with food availability. The ingestion rates of the copepods were also correlated with food availability, i.e. ingestion rate is related to enzyme concentration, as in the DRL hypothesis. Giller (1980) carried out similar experiments to Cook & Cockrell, using two Notonecta species. He found some support for a gut-filling model but was unable to explain declining mean handling times and intercatch intervals through the catch sequence. The former decline is predicted by the DRL hypothesis and the latter decline is also to be expected, with more rapid gut clearing
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as successive prey are consumed and at high prey densities; resort to search image and memory window explanations seems unnecessary.
ACKNOWLEDGMENTS
My thanks to P. C. Barnard and B. Bolton of B.M.(N.H.) for identifying ant-lion and ants respectively and to J. H. Lawton for comments which greatly improved the manuscript.
REFERENCES
Cook, R. M. & Cockrell, B. J. (1978). Predator ingestion rate and its bearing on feeding time and the theory of optimal diets. Journal of Animal Ecology, 47, 529-547.
Davson, H. & Segal, M. B. (1975). Introduction to Physiology. Vol. 1. Basic Mechanisms. Academic Press, London.
Giller, P. S. (1980). The control of handling time and its effects on the foraging strategy of a heteropteran predator, Notonecta. Journal of Animal Ecology, 49, 699-712.
Griffiths, D. (1980a). The feeding biology of ant-lion larvae: prey capture, handling and utilization. Journal of Animal Ecology, 49, 99-125.
Griffiths, D. (1980b). The feeding biology of ant-lion larvae: growth and survival in Morter obscurus. Oikos, 34,364-370.
Johnson, D. M., Akre, B. G. & Crowley, P. H. (1975). Modelling arthropod predation: wasteful killing by damselfly naiads. Ecology, 56, 1081-1093.
Mayzaud, P. & Poulet, S. A. (1978). The importance of the time factor in the response of zooplankton to varying concentrations of naturally occurring particular matter. Limnology and Oceanography, 23, 1144-1154.
Nakamura, K. (1977). A model for the functional response of a predator to varying prey densities; based on the feeding ecology of wolf spiders. Bulletin of the National Institute of Agricultural Sciences, Series C, 31, 29-89.
Taylor, R. J. (1976). Value of clumping to prey and the evolutionary response of ambush predators. American Naturalist, 110, 13-29.
(Received 7 January 1981)
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