Optimal diet models and rodent food consumption

Optimal diet models and rodent food consumption

Anim. Behav., 1984, 32, 340-348 OPTIMAL DIET M O D E L S AND RODENT F O O D C O N S U M P T I O N BY WILLIAM L. VICKERY* Department of Zoology, Univ...

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Anim. Behav., 1984, 32, 340-348

OPTIMAL DIET M O D E L S AND RODENT F O O D C O N S U M P T I O N BY WILLIAM L. VICKERY*

Department of Zoology, University of Western Ontario, London, Ontario, N6A 5B7 Canada Abstract. Generalizations derived from simple optimal diet models were field-tested at forest feeding stations where a choice of two foods was offered. The relationship between food availability and consumption by three species of rodents (Peromyseus manieulatus, Clethrionomys gapperi and Napaeozapus insignis) was examined in five separate experiments. In all five cases rodents preferred the food which yielded the highest caloric gain per unit handling time, as predicted by the simple models. However, in all five cases, less preferred items were sampled even when they were not part of the optimal diet. In three cases the consumption of the non-preferred food type varied with its abundance and in one case food preference changed with its availability. The latter three results indicate that these rodents do not use simple optimal diet strategies. Introduction Why do animals eat the foods they do? An extensive body of theory has been erected to answer this question (see reviews by Schoener 1971; Pyke et al. 1977; Krebs 1978). In order to increase the mathematical tractability of their arguments, these authors, without exception, make certain explicit and implicit assumptions about food value, handling times, encounter rates of predators with their foods and other parameters which influence the predator's ability to identify, attain and devour its food. Given these assumptions, optimal diets are readily defined. Testing can proceed in two ways. One can test all the assumptions implicit in the model. If they are all true, and no logical or mathematical errors have been made, then the logical conclusion deduced from them must also be true. This is not a feasible approach with optimal diet models, for two reasons. First, one implicit assumption of all optimal foraging models is that no assumptions other than those explicitly formulated need be made. If hidden assumptions exist, they are not falsifiable. Second, most models contain explicit assumptions which are demonstrably invalid (e.g. that animals can always estimate encounter rates precisely, that handling times are constant, that search images change immediately when a new diet becomes optimal, and so on). Nevertheless, the models may be good explanations of the predation process if the assumptions are sufficiently accurate for the predictions of the model to be good approximations of reality. Thus, a second and more,feasible

way of testing optimal diet models is to test their ability to predict the actual diet. To date, formal tests of optimal diet theory (Smith & Dawkins 1971; Werner & Hall 1974; Emlen & Emlen 1975; Goss-Custard 1977; Krebs et al. 1977) have taken this approach. What testable hypotheses do these models yield ? To answer, let us consider first the simplest optimal diet models (MacArthur & Pianka 1966; Schoener 1971; Timin 1973; Pulliam 1974; Krebs et al. 1977). These models make the following assumptions: (1) The diet is optimized subject to only one nutrient constraint. (2) Food items are defined as having constant value for the critical nutrient. (Here I define a food item as one unit of food, one berry, one nut or one insect, which might be eaten by the animal. A food type is a collection of similar items, such as all berries of a certain plant species.) (3) The encounter rate with food items is constant over the optimization period. (4) The forager always has accurate knowledge of food values and encounter rates. (5) Time required to handle each item of a given food type is constant. (6) The environment is fine-grained. (7) No other processes occur which add other parameters to the model. (8) The choice of an optimal diet over some time period will maximize fitness. This set of assumptions leads to a set of predictions: (A) Preference for different food items can be ranked by the ratio of their value (Ei), in some appropriate currency, to the time needed to

*Present address: D6partement des Sciences biologiques, Universit6 du Qu6bec ~ Montr6al, C.P. 8888, Succ. '%', Montr6al, Qu6bec H3C 3P8, Canada. 340

VICKERY: OPTIMAL DIETS AND RODENTS handle them (hi). (Henceforth, I shall call this the energy maximization hypothesis, as the currency used in this study is energy.) (B) The food with the highest Ei/hi ratio should always be preferred, even if it becomes rare relative to other foods (the constant preference hypothesis). (C) Food with lower Ei/hi ratios should only be eaten if their Ei/hi ratios are greater than the rate at which the nutrient is already being consumed. The abundance of any food type should have no bearing on its own inclusion in the diet (the independence of relative availability hypothesis). (D) Any food type should be either eaten whenever it is encountered or ignored every time it is encountered (the all-or-nothing hypothesis). These conclusions differ from those of more complex models (Emlen 1966; Rapport 1971; Marten 1973; Pulliam 1975; Hughes 1979; McNair 1979; Stenseth t981; Owen-Smith & Novellie 1982), which allow for violations of the assumptions. These latter models do not lead to simple gener.alizations about the optimal diet. Nor do they permit easy prediction of the optimal diet, because of the many parameters to be estimated. Thus, the power with which optimal foraging theory explains the predation process cart be measured by its ability to predict, despite its false assumptions, observed food choice. This is the context of most previous tests of optimal diet theory. Such tests have showed that simple models do predict food choice in bluegill sunfish, Lepomis macrochirus (Werner & Hall 1974), great tits, Parus major (Krebs et al. 1977), redshanks, Tringa totanus (Goss-Custard 1977), wagtails, Motacilla alba (Davies 1977a), spotted flycatchers, Muscicapa striata (Davies 1977b), sticklebacks, Spinachia spinachia (Kislalioglu & Gibson 1976) and starfish, Leptasterias hexactis (Menge 1972) - - with the exception that the allor-nothing hypothesis is never true. By contrast, Lehman (1976), Wolf et al. (1975), Jaeger & Barnard (1981), Belovgky (1981), Erlinge (1981) and Colgan & Cross (1982) showed that simple models were inadequate for filter feeders, nectarfeeding birds (Trochilidae and Nectariniidae), salamanders (Methodon cinereus), moose (Aloes alces), stoats ( Mustela erminea) and algae-eating fish (Gyrinocheilusayo'~aonieri),respectively. Wolf & Hainsworth (1983) have examined numerous parameters not considered in simple optimality models, and the ways in which nectar-feeding birds respond to them.

341

Relatively few studies have field-tested the hypotheses generated by optimal diet theory (Goss-Custard 1977; Belovsky 1978; Werner & Mittelbach 1981). This study attempts such field tests with three rodent species (Peromyscus maniculatus, Clethrionomys gapperi and Napaeozapus insignis). The conditions of the field study will be such that some if not all of the assumptions of simple optimal diet models will be unavoidably false. My objective is to test the validity of the four general predictions of simple optimal diet models, despite the falsity of their assumptions. Some effort was made to ensure the validy of one assumption, that there was only one nutrient constraint. The foods used in the tests were ripe berries, whose high energy content and similar chemical composition (Golley 1961) made it likely that energy would be the simple optimizing constraint. Also, free access to all other food types naturally available allowed the mice to attain their normal ration of arty necessary nutrients, thus avoiding any spurious multiple-nutrient constraints on fruit choice. Finally, the brief duration of tests using natural fruit minimized the chance that varying ripeness would alter food choice. Although optimal diet models are defined in terms of food items, I analysed the choice of food types. This is because one must destroy a food item in order to measure its caloric content, and thus the value of food items used in feeding trials is not known but inferrred from values of similar items of the same type. I chose to define a type as a species because of the similarity of visual, olfactory and taste cues within a species which may lead to food choice at the species level, and because this simplifies the manipulation of food in the field. Methods All the field work reported here was done at Lac Cart6, Qu6bec during the summers of 1977 and 1979. The study area, about 10 ha of a much larger forest, varies in elevation from 310 m to 320 m and is crossed by a small stream. The forest is dominated by maples (Acer spp.), with balsam fir (Abies balsamea) in lower, damper areas. Thorough habitat descriptions are given by Bider (1968) and Vickery (1979, 1981). The timing of the five tests was as follows: (1) 9 July to 15 August 1977; (2) 18 August to 24 August 1977; (3) and (4) 1 to 30 August 1979; and (5) 15 to 30 August 1979.

342

ANIMAL

BEHAVIOUR,

Food consumption was measured at feeding stations (Vickery 1979) in the forest. Each station consisted of a 2.4x 1.2-m piece of styrofoam, fixed firmly to the ground with form ware and covered with fine-grained sand. The stations had plastic canopies, 0.6-0.9 m high, which protected them from rainfall. The number of stations used varied from test to test. There were three groups of stations. Group A (area 4 in Vickery 1981) contained five stations; group B (areas 7, 8 and 9 in Vickery 1981) contained 15 stations; and group C (partly in area 10 of Vickery 1981)contained 15 stations. The average spacing between stations was about 15 m in groups A and B and 25 m in group C. The distance from group A to group B was about 200 m; from group B to group C, 100 m. Steep slopes separated A from B and B from C. Food was put on the stations at about sundown. The stations were revisited either 1.5 h later, at midnight, or between 0600 and 0900 hours the next morning. The choice of time depended on the rate at which the food disappeared. On revisiting the station, I recorded the amount of food taken from the station and the species of rodent which took it. Species were identified by their footprints in the sand (Murie 1974). Zapus hudsonius and Mierotus pennsylvanieus have rarely been trapped on the study area and are unlikely to have been confused with N. insignis and C. gapperi. The individual which came closest to, or which stopped at, a marked spot where a food item had been placed was assumed to have taken that item. I tried to time my visits so that the food present was not completely consumed, while still giving the rodents ample time to eat. This method permits the assumptions that food abundance reflects encounter rates (as all food is clearly visible on the stations) and that mice search for all food types simultaneously (as they find food by finding feeding stations whose contents change randomly each night). Test 1

Two very palatable foods, raisins and shelled, unsalted peanuts (Vickery 1979), were offered on 14 stations (groups A, B and C). On the first four nights, five pieces of each food were offered in order to determine which food the rodents preferred. For the next 14 nights, raisin availability was kept constant at 5 per-station while the number of peanuts per station was increased from 0 to 10 (0 six nights; 1 once; 3 once; 5 thrice; 7 twice; 10 once). For the next

32, 2

nine nights, peanut availability was kept constant at 25 per station while raisins were increased from 0 to 50 per station (0 thrice; 3 twice; 7 twice; 25 once; 50 once). In the last case, only the 12 stations in groups B and C were used. Test 2

This was an attempt to repeat test 1 using different foods: raisins and blueberries. The 12 stations in groups B and C were used. The test lasted only five nights, with the numbers of raisins and blueberries respectively per station being (25, 7), (25, 25), (25, 50), (10, 25) and (0, 25), in that order. Test 3

Berries of blueberry (Vaccinium sp.) and false Solomon's seal (Smilacina raeemosa) were offered on the five stations in group A. Five of each food type were offered per station per night for 14 nights. For the following 16 nights the number of Smilaeina was increased to 15. Test 4

Five each of choke cherries (Prunus virginiana) and bunchberries (Comus canadensis) were offered on the 14 remaining stations of group B for five nights. For the following 16 nights the number of Comus was increased to 15 per station. On the final two nights, no Prunus were available. Test 5

Five berries each of rose twisted-stalk (Streptopus roseus) and yellow clintonia (Clintonia borealis) were offered on eight of the stations in group C for 14 nights. On the next 16 nights, the number of Clintonia was increased to 10 per station. On the last two nights no Streptopus were available. The first two tests assessed whether the rodents could forage according to simple optimal diet models. They used very palatable (Vickery 1979) but novel foods between which there was a clear difference in preference and food value. The final three tests used foods known to be in the rodent's diets (Vickery 1979) and assessed whether the rodents do naturally forage according to the simple optimal diet models. Random samples of each wild food type were weighed, dried by sunlight in open air for 7 days and reweighed. Caloric densities were measured using art adiabatic bomb calorimeter. Handling time for each food type was determined by timing consumption rates of rodents caught near the study site. Animals were held for

VICKERY: OPTIMAL DIETS AND RODENTS 7 days and fed each food type. After this period, each animal was timed while eating in its own cage and in the absence of any alternative food source. Two stations were watched from a distance of 20 m using a 'Sniperscope' (U.S. A r m y stocklist No. 18-2804.720-200) to see how animals treated the food. In all cases, animals took the food from the station to eat or cache it elsewhere. At times more than one animal was seen at the station, but no agonistic behaviour was observed during 3 h of observation. Results

From the results of the handling time trials and measurements of caloric density (Table I), simple optimal foraging models predict that the preferred foods in the five trials should be: peanuts, raisins, blueberries, choke cherries and Streptopus. C. gapperi was rarely trapped, but it is assumed that its preference would be the same as that of the other two rodents. Test 1

P. maniculatus preferred peanuts to raisins, as predicted (Table II). N o significant preference was detected for N. insignis or C. gapperi. Increasing raisin availability increased raisin consumption, even though there was an excess of peanuts available (Fig. 1). Furthermore, increasing peanut availability did not eliminate raisins from the rodents' diets (Fig. 2). Thus, the independenee of relative availability hypothesis and the all-or-nothing hypothesis were falsified.

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stations was so intense that for some food items I was unsure which rodent species was the consumer. Therefore, only total consumption is presented here. Raisins were preferred to blueberries, as was predicted for both N. insignis and P. maniculatus. Thus the energy maximization hypothesis is not falsified here. When raisin availability increased, blueberries did not drop out of the rodents' diets (Table III): instead, consumption declined gradually. The consumption of blueberries increased with their availability, while raisin availability was constant and abundant. Thus, the independence of relative availability hypothesis and the all-or-nothing hypothesis were falsified for at least one rodent. In these first two tests the rodents often consumed all of the favoured food, leaving only the lower-ranking type available. However, this is not the only reason why the lower-ranking type was consumed. Analyses based on stations at which not all the preferred food had been eaten yielded the same results. Analysis of covariance, accounting for the number of preferred items eaten, reconfirmed the results. Thus the hypothesis of independence of relative availability and the all-or-nothing hypothesis were falsified. Test 3

Only P. maniculatus ate food in this area. As predicted, blueberries were preferred to false Solomon's seal (Table IV). Consumption of false Solomon's seal did not increase when its relative abundance increased. However, P. maniculatus continued to eat a few berries of false Solomon's seal, thus falsifying the all-or-nothing hypothesis.

Test 2

Test 4

During this test and when food was most available in test 1, animal activity at the feeding

Both P. maniculatus and N. insignis preferred choke cherries to bunchberries. Consumption of

Table I. Mean (sE; N) Caloric Density, Mean Percentage Moisture, Mean Handling Rates and Mean Caloric Consumption Rates for Foods Used as Test Materials Mean handling rates

(rag wet weight/s)

Food Peanuts Raisins Vaccinium Smilacina Prunus Comus Streptopus Clintonia

Mean caloric

Mean %

density*

moisture

6.56 ( - - t ; 1) 2.91 ( - - ; 1) 3.67 ( - - ; 1) 3.99 (0.10; 2) 4.18 (0.13; 3) 4.08 (0.03; 2) 4.22 (0.07; 2) 4.46 (0.05; 3)

--82.2 (0.8; 20) 78.2 (1.9; 45) 57.0 (1.0; 18) 85.4 (1.0; 44) 87.4 (1.0; 37) 85.2 (0.9; 43)

Peromyscus 1.21 (0.13; 13) 2.68 (0.42; 16) 2.96 (0.26; 32) 0.85 (0.10; 25) 1.64 (0.18; 27) 1.47 (0.15; 25) 1.50 (0.20; 25) 0.97 (0.12; 25)

*Cal/g dry weight, except for peanuts and raisins (cal/g fresh weight). tDashes indicate the parameter was not estimated.

Napaeozapus 0.93 (0.07; 26) 1.51 (0.16; 25) 5.79 (2.21; 3) -2.42 (0.34; 26) 2.44 (0.24; 27) 2.11 (0.29; 25) 1.55 (0.15; 25)

Mean caloric consumption rates (cal/s)

Peromyseus Napaeozapus 7.94 7.79 1.94 0.74 2.95 0.88 0.80 0.64

6.10 4.39 3.79 -4.35 1.46 1.13 1.03

344

ANIMAL

BEHAVIOUR,

bunchberries by both rodents was independent of bunchberry availability. However, both rodents ate a few bunchberries, thus falsifying the all-or-nothing hypothesis. That bunchberries are acceptable rodent food is shown by the consumption observed in the absence of choke cherries. Test 5

Both P. maniculatus and N. insignis preferred false Solomon's seal, as predicted when the food types were equally available. However, doubling the availability of Clintonia led not only to an increase in Clintonia consumption but, for N. insignis, to a change in preference: the consumption of Clintonia increased to more than twice that of Streptopus, implying that Streptopus was rejected more often than Clintonia. This constitutes a falsification of the constant preference hypothesis, which was not rejected

32,

in the previous four tests. The independence of relative availability hypothesis and the all-ornothing hypothesis were also falsified here. Discussion

In all five tests, when foods were equally available, rodents preferred the food type which provided the most energy per unit handling time. Thus P. maniculatus and N. insignis do maximize the rate of energy (or some factor correlated with energy) intake in choosing the fruit component of their diet: the energy maximization hypothesis is true. The all-or-nothing hypothesis was falsified in all five tests. Rodents of both species continued to eat some items of the lower-ranking food type even when preferred foods were abundant. The all-or-nothing hypothesis predicts that rodents should eat either no lower-ranking items or that the ratio of lower-ranking to preferred items consumed should equal the ratio of lower-

Table II. Mean Consumption of Raisins and Peanuts (per station per night) on 14 Feeding Stations over Four Nights

Raisins

Peanuts

SE

~

SE

Peromyscus

1.2

0.23

2.5

0.28

Clethrionomys

1.0

0.21

1.0

0.19

Napaeozapus

0.3

0.11

0.4

0.13

20

Z

o_

8

g 6 0~Oz

42~

]RAISINS ]i. 2 4, 6 B 10 PEANUTAVAILABILITY

t t-

Fig. 2. Peanut and raisin consumption (per station per night) as a function of peanut availability (per station per night) in test 1 (mean • sE). Table III. Availability and Mean Consumption (per station) of Raisins and Yaecinium on 12 Stations per Night

Z O t~

Availability

Mean consumption Raisin

Raisin Vaccinium

0 7 25 RAISINAVAILABILITY

50

Fig. 1. Peanut and raisin consumption (per station per night) as a function of raisin availavility (per station per night) in test 1 (mean • s~).

25 10 0 25 25

25 25 25 7 50

Vaccinium

.~

SE

~

SE

23.0 9.8 -22.4 20.6

1.8 1.8 -0.9 2.0

6.0 7.4 12.7 1.7 3.6

1.3 1.7 2.0 0.6 0.8

VICKERY: OPTIMAL DIETS AND RODENTS ranking to preferred items available. This was clearly not true, as in most cases consumption of the lower-ranking type was less than that of the preferred type even when the lower-ranking type was two to three times as abundant as the preferred type. Numerous reasons why this hypothesis has been falsified by virtually every test of optimal foraging are reviewed by Krebs (1978). One possibility which would be advantageous for these rodents, whose food supply varies seasonally and whose life span is often less than a full year, is that they sample novel foods to assess their desirability (energy content, ripeness, etc.) as a future food source. The constant preference hypothesis was falsified in one of the five tests. The preference between Streptopus and Clintonia varied with availability. Streptopus was preferred at equal availability, while Clintonia was preferred when it was twice as abundant as Streptopus. While this falsifies the constant preference hypothesis as I have stated it, it may be consistent with simple optimal foraging models which deal with values of each food item rather than food type; for example, see Emlen" (1966). The value of food items within each type varies (Table I). As mean caloric consumption rates of Streptopus and Clintoria, by both Perornyscus and Napaeozopus, are quite similar, variance within types may mean that some items of Clintonia will yield energy faster than some items of Streptopus. If a rodent bases its choice on the value of an item rather than a food type, then some items of both types should be eaten. Furthermore, as the relative availability of the less preferred type increases, its consumption should increase, even to the point of becoming the preferred type. Unfortunately, I cannot evaluate this hypothesis with my data. Food items were destroyed in order to determine their caloric value and consumption rate and thus were not available for Table IV. Availability and Mean Consumption (per station per night) of Smilacina and Vaccinium on Five Stations

Availability

Smilacina Vaccinium

Consumption*

Smilacina

Vaccinium

X

X

SE

SE

5

5

0.06 0.04

0.37 0.09

15

5

0.06 0.04

0.94 0.19

*By Peromyscusonly at this site.

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feeding trials. Also, the variance of caloric consumption rate cannot be estimated from my data as this variable is the product of three other variables (Table I). Variance in caloric consumption rate within food types would also result in the failure of the all-or-nothing hypothesis and the independence of relative availability hypothesis. If within-foodtype variance is the cause of the failure of the latter hypothesis, one will expect that hypothesis to be falsified when mean consumption rates are similar and true when the means are far apart (assuming constant variance). The independence of relative availability hypothesis was rejected where preferred to non-preferred caloric consumption rates had ratios of 1.02, 1,10, 1.16, 1.25, 1.39 and 4.02 (Table I). This hypothesis was not rejected for ratios of 2.62, 2.98 and 3.35. This follows the predicted pattern except for one case, Peromyseus in test 2. In test 2, I did not separate consumption by species, and thus the failure of this hypothesis may be due to increased consumption by Napaeozapus or Clethrionomys and not Peromyscus. However, Peromyseus was the only species whose activity increased with increasing blueberry availability in test 2. Thus, it is likely that Peromyseus did increase blueberry consumption as blueberry availability increased. Therefore, within-food-type variance is probably not the only explanation for the failure of this hypothesis. In order to field-test optimal foraging hypotheses, I have made a number of assumptions not normally made in laboratory tests of these hypotheses. First, I assumed that all rodents of a given species have the same caloric consumption rates. This was necessary in order to accept total consumption by one species as a test criterion. This assumption can be tested using data from the feeding trials in captivity. Caloric consumption rates were estimated for each individual that was fed both food types of a given test. For Napaeozapus, there was no evidence of differences in caloric consumption rates b e t w e e n individuals. However, there was evidence for this in Peromyscus. Despite this variance, all individuals had the highest consumption rate for the same food in tests 2, 3 and 4. Thus optimal foraging theory predicts the same preference for all individuals. Furthermore, this was the same preference as that of Napaeozopus, thus justifying the lumping of data in test 2. However, three of six Perornyscus in test 1 and four of seven in test 5 had higher caloric consumption rates for the food that was, on the average, lower-ranking.

346

ANIMAL

BEHAVIOUR,:

32,

2

Table V. Availability and Mean Consumption (per station per night) of Comus and Prunus on 14 Feeding Stations

Consumption Peromyscus

Availability

Comus Cornus Prunus

Napaeozapus Prunus

Comus

Prunus

3?

s~

3?

sE

3?

sE

X

s~

5

5

0.10

0.06

0.64

0.14

0.07

0.04

2.43

0.24

15

5

0.00

0.00

0.75

0.15

0.11

0.08

4.42

0.09

15

0

0.35

0.15

--

--

1.97

0.23

--

--

Table VI. Availability and Mean Consumption (per station per night) of Streptopus and Clintonia on Eight Feeding Stations

Consumption Peromyscus

Availability

Streptopux

Napaeozapus

Clintonia

Streptopus

Clintonia

Streptopus

Clintonia

37

sE

37

sn

37

x~

3?

s~

5

5

0.21

0.07

0.13

0.07

0.22

0.07

0.12

0.07

5

10

0.62

0.15

0.70

0.19

0.64

0.11

1.64

0.22

0

10

--

--

0.50

0.50

--

--

1.44

0.41

This observation could explain the apparent failure of the independence of relative availability hypothesis and the all-or-nothing hypothesis in these two tests and the constant preference hypothesis in test 5. Second, although animals carried food away to eat it, I assumed that this w a s not a central place foraging problem. Specifically, one food was not transported more easily than another. Observations by 'sniperscope' and the pattern of footprints on the stations confirmed that food was carried away one item at a time, so there was n o question of loading several small lowerranking food items rather than one large preferred item. Also, in all but one of the tests, the food types were of roughly the same mass and volume so there should have been no advantage to carrying away one food rather than the other. The exception to this, test 3, involved Prunus, which was much larger than Smilaeina; but rodents clearly did not carry away Smilaeina, almost completely ignoring them. Third, I assumed that each rodefft had available to it all the food which I put on a station. However, this was untrue as food was not replaced on the stations once eaten. As the

preferred food was consumed, the lower-ranking type was most likely to be included in the optimal diet. T o avoid this, I returned to the stations as quickly as possible after consumption began in order to record the data before substantial amounts of preferred types had disappeared. Also, I re-analysed the data using only stations where the preferred food type was not completely consumed, and I re-analysed the data controlling for consumption of the preferred food type in analysis of covariance. These analyses yielded the same qualitative conclusions as those presented here under the assumption that all the food was available. Thus failure to meet this assumption did not cause the failure of the optimal diet hypotheses. Finally, in order to treat each station as a replicate, I assumed that individuals fed at one feeding station only. In fact, rodents could easily visit several stations in the time period available. This could Cause deviations from my null hypothesis without falsifying optimal diet hypotheses if food availability varied between stations and rodents used availability at one station to determine consumption at the next. This is unlikely, since all stations had the same initial food

VICKERY: OPTIMAL DIETS AND RODENTS densities a n d n e i g h b o u r i n g stations generally showed similar rates o f f o o d c o n s u m p t i o n . I n total, m y results show t h a t r o d e n t s d o n o t always c o n s u m e the o p t i m a l diet defined b y a simple m o d e l b a s e d o n f o o d types. This conclusion m a y be due, in part, t o v a r i a t i o n a m o n g f o o d items within a t y p e a n d t o v a r i a t i o n a m o n g rodents within a species. However, these d o n o t explain t h e results o f test 2, where neither o f these p h e n o m e n a was i m p o r t a n t . M y results differ f r o m those o f l a b o r a t o r y tests o f o p t i m a l diet m o d e l s ( W e r n e r & H a l l 1974; K r e b s et al. 1977), which ensured t h a t as m a n y assumptions as possible were true a n d t h e n tested the hypothesis t h a t o r g a n i s m s are c a p a b l e o f choosing diets which m a x i m i z e energy i n t a k e p e r unit h a n d l i n g time. Such tests d o n o t test whether a n i m a l s actually d o c h o o s e such o p t i m a l diets in the field. H o w e v e r , field tests o f o p t i m a l diet models, such as the one p r e s e n t e d here, will be u n a b l e t o assure t h a t the m a n y a s s u m p t i o n s o f such models are true. Thus field tests will test whether o r n o t m o d e l s a p p l y in n a t u r e despite the falsity o f s o m e o f the assumptions. T h e utility o f such m o d e l s w i l l be m e a s u r e d b y h o w often they do predict o b s e r v e d diets in field situations. There have been few field tests o f simple o p t i m a l diet models. G o s s - C u s t a r d (1977) a n d Davies (1977a, 1977b) s h o w e d some birds to follow m o s t o f the predictions o f simple models, as d o bluegill sunfish ( M i t t e l b a c h 1981). M y research suggests t h a t rodents follow t h e m only s o m e o f t h e time, as d o s o m e birds ( S u t h e r l a n d 1982; T u r n e r 1982). M o r e c o m p l e x m o d e l s are often necessary to describe feeding b e h a v i o u r . Tests o f such m o d e l s ( E m l e n & E m l e n 1975; E s t a b r o o k & D u n h a m 1976; Stenseth et al. 1977; Elner & Hughes 1978; Stenseth & H a n s s o n 1979; W e s t o b y 1974) in the l a b o r a t o r y a n d the field will be n e e d e d to assess their utility, in the same way t h a t simple m o d e l s have been tested.

Acknowledgments D r J. R. Bider allowed m e the use o f his l a n d a n d research facilities at L a c Carr6, Quebec. K i m Asquith, Bill S a l m o n a n d J u d i t h N o w l a n assisted m e in collecting data. V. J. D e G h e t t a n d two a n o n y m o u s reviewers p r o v i d e d helpful c o m ments. This research was f u n d e d by an o p e r a t i n g g r a n t f r o m the N a t i o n a l R e s e a r c h C o u n c i l o f Canada.

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(Received 18 January 1983 ; revised 7 July 1983 ; MS. number: A4006)