Food preferences ofthe vlei rat (Otomys irroratus) and the four-striped mouse (Rhabdomys pumilio)

B.A. Curtis and M.R. Perrin Department of Zoology and Entomology, Rhodes University, Grahamstown

The food preferences of O. irroratus and R. pumilio have been Stomach content analyses of the two most common rodent examined in laboratory tests. O. irroratus prefers green species of the Fish River Valley Scrub community have vegetation including herbs, shrubs and grasses whereas revealed that the vlei rat, Otomys irroratus, is a strict her­ R. pumilio prefers fruits and seeds. Both species eat insects in bivore with its trophic niche contained within that of the laboratory, but they are more important in the natural diet an opportunistic omnivore, the four-striped mouse, of R. pumilio than O. irroratus. O. irroratus readily eats grasses Rhabdomys pumilio, (Perrin In press). Although there was which are not consumed by R. pumilio. Partial dietary overlap occurs in the laboratory which suggests that some degree of a possible 30 - 50% dietary overlap in winter, it seemed competition for fruits and seeds will occur at certain seasons in unlikely that competition between the two species was great.

The aim of this study was to demonstrate some particular

. natural populations. ) food preferences in each species, and hence to provide more 0 s. Afr. J. Zool. 14:224-229(1979) 1

0 information on competition for food. Food preference tests 2 Die voedselvoorkeure van O. irroratus en R. pumilio is in were employed (Drozdz 1966; Zemanek 1972) to supple­ d e laboratoriumtoetse ondersoek. O. irroratus verkies groen t ment previous information from stomach content analyses a gewasse met inbegrip van kruidgewasse, struike en grassoorte d (Perrin In press). Such tests indicate food preference or (

terwyl R. pumilio vrugte en sade verkies. Albei spesies eet r selectivity rather than what was eaten out of necessity. e insekte in die laboratorium, maar insekte is belangriker in die h s

i dieet van R. pumilio as van O. irroratus. O. irroratus eet l b geredelik grassoorte wat nie deur R. pumilio geeet word nie. Methods u

P In die laboratorium kom die dieet van die twee gedeeltelik Rodents and food plants for the preference tests were e ooreen wat daarop dui dat in sekere seisoene daar by h collected from a study area located in the Andries Vosloo t

natuurlike bevolkings 'n mate van mededinging om vrugte en y Kudu Reserve, 30 km north-east of Grahamstown in the b sade sal voorkom.

d S.-Afr. Tydskr. Dierk. 14:224-229(1979) eastern Cape (33°8' S, 26°39' E) during February and e t March. The reserve is situated in the Fish River Valley n a Scrub which, in its undamaged state is an extremely dense, r g

semi-succulent, thorny veld type (Acocks 1975) but over­ e c grazing has opened up the vegetation on some parts of the n e

c Reserve, which has been invaded by prickly pear, Opuntia i l ficus-indica, and Euphorbia bothae. As this study was r e designed to demonstrate the extent of dietary overlap d n between two rodent species rather than provide a u

y comprehensive survey of all the foods eaten, a random re­ a

w presentative sample of the most abundant plants occurring e t on the reserve was collected. Plants were identified by com­ a

G parison with specimens in the Albany Museum Herbarium. t e Collected food items were separated into categories n i based on their approximate nutrient content (or quality) b a (Perrin In press) and their size and position in the habitat S

y (availability). The six categories recognized were (1) grass b stems and leaves; (2) grass seeds; (3) herbs (excluding fruits d B.A. Curtis· and M.R. Perrin e c Department of Zoology and Entomology, Rhodes University, or seeds); (4) shrubs (excluding fruits or seeds); (5) fruits u P.O. Box 94, Grahamstown 6140, South Africa d and seeds (of herbs and shrubs); and (6) insects. During the o ·To whom all correspondence should be addressed r presentation of results and in the discussion it was p e Accepted 1 May 1979 convenient to combine groups (1) & (2); and (3) & (4), R I S. Afr. J. Zoo!. 1979, 14 (4) 225

which can be regarded as graze and browse respectively. taken in total after three days, or 10% taken on the first Drupes consisting of a hard stone and soft flesh were day, but thereafter not touched. These species are not likely analyzed as separate entities. Herbs were offered as whole to be consumed in the wild and would only be selected in plants, including leaves, stems and occasionally flowers. cases of extreme food shortage. They may contain unpala­ However, only the leaves oflarge shrubs were offered to the table toxins or physical defence mechanisms such as spines rodents. The consumption of grass seeds was determined or numerous silica bodies. separately from that of stems and leaves. A range offreshly The preferences of both rodent species were ranked linearly killed insects was offered to the rodents when available. for all food items tested within a dietary category, in order Owing to its larger size O. irroratus was given larger to employ Mann-Whitney U tests (Sokal & Rohlf 1969: p. samples of food than R. pumilio. Water and laboratory rat 391 - 4). These tests are based on the sequence or ordering pellets were available ad libitum during the feeding trials to of observations, in this case dietary preferences; they are eliminate eating of unpalatable foods due to excessive semigraphical and non parametric. Due to the small sample hunger. sizes, categories (1) and (2) were combined for ranking and Food preferences were determined using the cafeteria test analysis. method of Drozdz (1975). Several plant species within each category, but not between categories, were offered to each Results of ten animals of each species at the start of a three-day test period. After 24, 48 and 72 h the amount of the original The individual results of the food preference tests are pre­ food consumed was recorded using an arbitrary four-point sented in Tables 2 and 3, and demonstrate the potential scale and later converted to percentages (Drozdz 1975) food of each rodent species. Figure 1 shows the percentage (Table 1). The mean consumption of each food item was of food consumed in each dietary category after one day of calculated daily and categorized or ranked as follows. the feeding trials. It can be seen that there was an inverse Class 1 relationship between the preferences of O. irroratus and R. Preferred: 50% or more consumed on the first day of the pumilio. O. irroratus had a marked preference for grass test. These species would be taken most readily in the wild. seeds, stems and leaves, herbs and shrubs, but also ate insects, seeds and fruits. R. pumilio showed greatest Class 2 preference for insects, seeds and fruits, with a lower pre­ Palatable: 30 - 40% consumed on the frrst day or more ference for grass seeds, herbs and shrubs. Grass leaves and than 50% taken in total after three days. These species are stems were not eaten by R. pumilio. The results of Mann­

. less palatable than those of Class 1, but would be likely to ) Whitney U-tests (Table 4) substantiate these findings but 0

1 form a substantial part of the natural diet. fail to demonstrate the preference shown by R. pumilio for 0 2

Class 3 insects over O. irroratus. d

e Unpalatable: 10 - 20% taken on the frrst day or 30 - 50% t

a taken in total over the three day period. This food would

d Seeds and fruits ( only be eaten if species of the above two categories were not r

e Most of these foods were classified as either preferred or available. h

s palatable to both O. irroratus and R. pumilio. O. irroratus i l Class 4

b showed a strong preference for the fleshy fruit of drupes

u Inedible: None eaten on the first day and less than 30%

P over their stones. This preference was not so marked for R.

e pumilio which had an equally high preference for the stones h t and flesh of Jasminum angulare and Pappea capensis. The y Table 1 An example of individual variation in food b

high preference of both rodents for Lycium campulatum,

d preference in four-striped field mice (Rhabdomys

e May tenus capitata, Lantana camara, Rhigozum obova­ t pumilio) in 10 separate tests. Food consumption was n tum, Asparagus africanus and the flesh of Jasminum angu­ a determined, using the cafeteria test and a four-point r lare, Pappea capensis and Grewia robusta suggest that g scale. Mean values and percentage consumption has e there is overlap between the two rodent species for prefer­ c been determined n red fruits and seeds. Only two species, Opuntiaficus-indica e c

i Rhabdomys pumilio

l and Diospyros dichrophylla, were classed as unpalatable

r foods of and only stones

e O. irroratus, Zizyphus mucronata Plant species 1 2 3 4 5 6 7 8 9 10 X % d and Cyphostemma quinata were inedible. O. ficus-indica n u

Acacia karoo 1 0 1 1 1 0 1 1 1 0,8 20 was a preferred fruit of R. pumilio, but D. dichrophylla was y

a Grewia robusta F 2 3 3 2 1 3 1 3 3 2,2 60 unpalatable; Ziyphus stones and Cyphostemma were w

e Grewia robusta S 1 2 3 I I I 1 0 I 2 1,4 40 inedible. t

a May tenus capitata 2 3 3 2 2 3 2 3 2 3 2,5 70 G I I t Ruschia sp. 3 3 1 3 3 3 3 3 2,5 70

e Herbs and shrubs

n Diospyros dichrophylla 0 0 0 3 0 0 0 0 0 0 0,3 10 i Most of the herbs and shrubs were preferred by O. irroratus b May tenus heterophylla I 0 2 1 I 3 I I 3 I 1,5 40 a with only Kalanchoe rotundifolia, Tecomaria capensis and S

Lantana camara 3 3 3 3 3 3 3 3 3 3 3 90

y Portulacaria afra being classed as palatable rather than pre­

b Phy/lanthus verrucosus 3 3 3 3 3 3 3 3 3 3 3 90 ferred. There were no unpalatable or inedible species for O. d 1 1 0,4 10 e Cyphostemma quinata 0 0 0 0 0 2 0 0

c irroratus. The only preferred species of R. pumilio was

u Putterlickia pyracantha 3 1 0 2 0 1 2 0 1,1 30

d Hermania althacoides, 14 were palatable, five unpalatable

o Rhigozum obovatum 2 2 3 2 2 0 1,6 50 r and only K. rotundifolia was inedible. These results show p e S = Stone. F=Flesh. that all of the green plant material offered, with the excep- I R 226 S.-Afr. Tydskr. Dierk. 1979, 14 (4)

Table 2 Food preference of Otomys irroratus and Rhabdomys pumilio. Each test was conducted over a three-day period and the percentage of each food species consumed per day was recorded. Preference categories (Column 4 for each species) are as described in the text

Otomys i"oratus Rhabdomys pumilio

One Two Three Prefer- Rank­ One Two Three Prefer- Rank­ Food species day days days ence ing day days days ence ing

Fruits and seeds Lycium campanulatum 90 2 90 1 May tenus capitata 80 10 6 70 10 10 12 lasminum angulare F 80 10 1 8 80 10 10 9 lasminum angulare S 40 20 20 2 26 60 10 10 21 Ehretia rigida 80 10 49 Lantana camara 80 11 90 3 May tenus heterophylla 70 10 10 13 40 20 10 2 28 Rhigozum obovatum 70 10 16 .. 50 10 24 Asparagus africanus 70 10 17 70 18 Pappea capensis F 60 30 19 80 10 7 Pappea capensis S 20 30 10 2 39 80 10 1 5 Grewia robusta F 60 10 10 1 22 60 20 1 20 Grewia robusta S 30 20 10 2 35 40 10 10 2 30 Acacia karoo 50 10 10 1 23 20 10 30 2 40 Putterlickia pyracanth a 40 20 20 2 27 30 30 20 2 33 Zizyphus mucronata F 40 20 10 2 29 30 10 20 2 36 Zizyphus mucronata S 10 10 4 47 10 10 3 43 Ruschia sp. 1 40 10 10 2 31 70 10 14 Passi/1ora coerulea 40 10 2 32 70 10 15 Phyllanthus verrucosus 30 30 10 2 34 90 1 4

.

) Combretum caffrum 30 10 20 2 37 30 10 10 2 38 0

1 Scutia myrtina 20 10 20 2 41 50 0 2

Opuntiajicus-indica 20 10 3 44 50 25 d

e Diospyros dichrophylla 10 20 3 45 10 4 46 t a Cyphostemma quinata 20 4 48 10 10 10 3 42 d (

r Herbs e

h Cyperus teneriIJae 90 1 40 20 2 15 s i l Crassula tetragona 90 2 10 20 10 3 24 b u Anthospermum sp. 90 3 40 20 2 16 P 4 10 2 17 e Stachys kuntzei 90 40 10 h t Crassula lycopodioides 80 10 5 30 10 10 2 20 y Delosperma sp. 80 10 6 20 20 10 2 22 b

d Commelina africana 80 10 7 40 10 2 18 e t Pentzia incana 70 20 8 20 30 2 21 n a Mohria caffrorum 9 10 3 25

r 70 10 g Ruschia sp. 2 60 20 10 10 30 20 10 2 19 e c Helichrysum rosum 60 10 10 11 20 10 10 3 23 n e c Crassula trachysantha 50 20 10 1 12 40 30 10 2 13 i l 4 26 r Kalanchoi rotundifolia 40 20 20 2 14 10 e d Shrubs n u

Rhigozum obovatum 90 1 30 10 20 2 12 y

a Asparagus suaveolus 80 10 2 20 10 10 3 15 w 3 3 16 e Grewia robusta 80 20 10 t a Hermania althaeoides 70 10 10 4 50 10 1 7 G Phyllanthus verrucosus 70 20 1 5 20 20 20 2 14 t e 1 n Hypoesta verticil/ata 50 10 10 6 40 10 2 10 i b Tecomaria capensis 40 20 2 9 20 30 10 2 13 a S

Portulacaria afra 30 20 20 2 11 40 20 10 2 8 y b

d Moss 70 40 20 2 e c u d

o Grewia stems 50 10 20 20 2 r p Delosperma stems 80 10 40 20 10 2 e R I S. Afr. J. Zool. 1979, 14 (4) 227

Table 3 Food preference of Otomys irroratus and Rhabdomys pumilia. Tests were conducted over a period of one day for insects and two days for grasses. Figures indicate the percentage of each food eaten per day. Preference categories are as described in the text

Otomys irroratus Rhabdomys pumilio

One Two Prefer- Rank- One Two Prefer- Rank- Food species day days ence ing day days ence ing

Insects Grasshoppers (Acrididae) 90 3 90 Termites (Hodotermes mossambicus) 50 I 6 90 I I Cockroaches (Periplaneta americana) 30 2 7 30 2 8 Mealworms (Tenebrio molitor) 30 2 9 90 4 Toktokkies (Psammodes sp.) 20 3 10 4 II Crickets (Gryllidae) 12 90 5 Grass seeds Panicum sp. cf. coloratum 70 8 30 10 2 12 Eragrostis obtusa 80 4 30 20 2 II Digitaria sp. 70 10 6 40 20 2 9 Setaria sp. 80 5 40 20 2 10 Grass stems and leaves Panicum sp. cf. coloratum 70 10 7 4 13 Panicum denstum 90 I 4 14 Sporobolus sp. 90 2 4 IS Cynodon dactylon 90 3 4 16

100 o Irrorotus 100 R pumilia Table 4 Results of Mann-Whitney U-tests for the food preferences shown by Otomys irroratus and Rhabdomys

. ) r- pumilia 0 80 80

1 .--

0 c I---r-- Preferred Mann-Whitney Signi-

2 Q 'ii Food category by statistics ficance d r-- e § 60 60 t C a 0 Fruits and seeds R.pumilio 331 p<0,OO2

d u (

Herbs O. irroratus 168 p

r r-J--- " 40 e 0'" 40 Shrubs O. irroratus 59 p

h C

s u r--- i " Insects Neither 26 l ;;; r-- b "- I-- Grasses O. irroratus 64 p

e h t 0 0 y

b 3 4 6 3 4 6 Insects

d Food Category Food Cotegory The values indicated in Table 3 for insect preferences may e t

n Fig. I Food preferences of O. irroratus and R. pumilio. Histograms be misleadingly high because insects were offered with only a

r indicate mean consumption offood after 24 h oftesting. Food category I rat pellets as an alternative food. Since insects were usually g = grass stems and leaves, 2 = grass seeds, 3 = herbs, 4 = shrubs,S = e offered dead, no prey capture was necessary. When offered c fruits and seeds, 6 = insects. n live cockroaches and beetles, neither rodent would catch or e c

i kill them. Live grasshoppers and crickets were readily l

r caught and eaten by R. pumilio but were rejected by O. e tion of K. rotundifolia, is a potential food source for both d i"oratus. Both rodents preferred grasshoppers and ter­ n rodents. However, O. i"oratus shows a significant pre­

u mites as insect food, possibly because they are abundant in

y ference for herbage over R. pumilio (Table 4). the study area. Insects were more readily eaten by R. a The stems of two shrub species were offered as food; w pumilio than O. i"oratus. e

t both were readily eaten by O. i"oratus but not by R. a

G pumilio. Both rodents tended to strip bark from stems

t

e rather than eat the whole stem. The soft succulent stems of Grasses n i Deiosperma sp. were consumed more extensively than the All grass stems, leaves and seeds offered to O. i"oratus b

a woody Grewia robusta stems. When herbs were offered as

S were consumed, but only grass seeds were eaten by R.

y whole plants, O. i"oratus ate the flowers and stems as well pumilio. This represents a marked difference in the feeding b as the leaves, while R. pumilio ate only leaves. It appears

d habits of these two coexisting rodents. e

c therefore that the smaller rodent species is a more selective u

d feeder within this food category. Moss was offered on one o

r occasion and was readily eaten by O. i"oratus but not by Discussion p

e R.pumilio. Dietary overlap occurs for certain of the natural foods eaten I R 228 S.-Afr. Tydskr. Dierk. 1979, 14 (4)

by O. i"oratus and R. pumilio, but their relative propor­ Regional as well as seasonal changes in diet are known to tions ditTer. O. i"oratus is predominantly herbivorous aod exist. In the Transvaal, R. pumilio's diet consists of pre­ selects large Quantities of grasses and herbs in preference to dominantly of seeds throughout the year, but in February fruits and seeds. Although insects were eaten during labora­ and March much of the diet consists of white vegetable tory tests they do not form a substantial part of the natural matter and few insects (Brooks 1974). In Uganda, grass diet (Davis 1973; Perrin In press). R. pumilio has a high probably forms an important component of the diet of R. preference for insects, seeds, and fruits but not for herbage. pumilio (Delany 1975), and in Malawi, large Quantities of When eating herbage it has a greater preference for shrub green vegetation are taken (Hanney 1965), which contrasts leaves than herbs and shows considerable selectivity. O. strongly with the current results. i"oratus is catholic in eating many types of green vegeta­ Seasonal changes in the composition and nutritive values tion but unlike R. pumilio has a higher preference for herbs of the diet are believed to be very important in the timing of than shrubs, possibly because the whole herbaceous plant is breeding, reproductive tactics and the dynamics of these edible. Grass, which forms a significant part (45%) of the two coexisting rodents (Perrin In press), and in other rodent natural diet of O. i"oratus (perrin In press) and is readily communities (Field 1975). The seasonality and intensity of eaten in laboratory tests, is not consumed by R. pumilio. breeding in opportunistic feeders appears to be correlated One cause of food unpalatability may be due to toxins. with the availability of insects or seeds in the diet (Perrin In For example, Kalanchoe rotundifolia is known to contain press; Field 1975). Changes in the availability of nutrient picrotoxins which attack the mammalian central nervous foods (insects and seeds) are likely to influence foraging system causing paralysis (Steyn 1949). The leaves of this behaviour, diet Quality, maternal nutrition and hence species, which are less toxic than the flowers and seeds, juvenile recruitment and population dynamics. In a her­ were avoided by R. pumilio but were palatable to O. bivorous species such as O. irroratus which has i"oratus. This might indicate some degree of adaptation to morphological specializations to herbivory (Curtis 1978) plant toxicity by the herbivorous O. i"oratus. and where green vegetation forms the major part of the diet, Although food selection tests indicate food preferences seasonal or year round breeding is likely. However, the low they cannot demonstrate the composition of a natural diet. energy content of the diet might affect the lengths of gesta­ However, it is most valuable to compare food preferences tion and weaning, litter size and hence reproductive success with the results of stomach content analyses. Often the two and rates of increase. Obviously differences in food pre­ approaches correlate well (Drozdz 1966, 1967; Zemanek ferences and availability can have very profound affects

upon the coexistence and abundance of O. irroratus and R.

. 1972). Stomach content analyses (Perrin In press) from the ) pumilio. 0 study area revealed a mean abundance of 90 - 100% of 1 Food preference tests at other seasons would demon­ 0 green plant material throughout the year, but only neg­ 2 strate whether seasonal changes in stomach contents

d ligible amounts of seeds and essentially no insects. Thus, O. e t i"oratus will eat seeds and insects, as demonstrated by (Perrin In press) are due to changes in food availability a and/or palatability. Studies on the nutritive values, toxi­ d laboratory tests, but these items do not form a large or (

r important part of the natural diet. Both approaches show cology and digestability of natural foods would be of great e

h value in determining food selectivity by these two co­

s that O. i"oratus eats a substantial amount of woody mate­ i l existing rodents.

b rial. Grass was a prominent dietary item in stomach u analyses and in food tests. Sections of grass stems were P

e found in O. i"oratus runways, from which Panicum sp. and

h Acknowledgements t

Digitaria sp. were identified, confrrming that these grases y We wish to thank Mrs Brink and Ms Britten of the Albany b are eaten in the wild. Museum for help with plant identification and the CSIR and d e t Davis (1973) mentioned that O. i"oratus ate nearly all Rhodes University for financial support. We also wish to n plant species common to its distribution on a grid in the a express our thanks to N. Bruton for typing this paper. r g Transvaal. Although not all species from our study area e

c were offered to O. i"oratus in this study, those shrubs and n e herbs offered were readily eaten. O. i"oratus does not References c i l appear to be a selective feeder within the category of green ACOCKS, J.P.H. 1975. Veld types of South Africa. Mem. Bot. Surv. r e plant material. Perrin (In press) noted from numerous SA. 40: 1-28. d BROOKS, P.M. 1974. The ecology of the four·striped grass mouse, n stomach content analyses that when leaves formed a large u part of the diet the diversity ofleaves eaten was low, sugges­ Rhabdomys pumilio (Sparrman 1784), with particular reference to a y population on the Van Riebeeck Nature Reserve, Pretoria, D.Sc. a ting selective feeding. w Thesis, University of Pretoria, Pretoria, South Africa. e t The results presented here indicate that R. pumilio selects CURTIS, B.A. 1978. Comparative morphology of the digestive system a

G fruits and seeds preferentially but readily eats insects and of 19 Southern African Myomorph rodents. B.Sc. Hons project, t Rhodes University, Grahamstown, South Africa. e the leaves of some shrubs. The preference for herbage is low n i and grasses are not eaten. These fmdings correspond well DAVIS, R.M. 1973. The ecology and life history of the vlei rat, Otomys b irroratus (Brants 1827), on the Van Riebeeck Nature Reserve, a with those from stomach content analyses from the same S

Pretoria. D.Sc. Thesis, University of Pretoria, Pretoria, South Africa. y locality and time of year (Perrin In press). Marked changes b DELANY, M.J. 1975. The Rodents of Uganda. Trustees of British in the composition and quality of the diet occur seasonally d Museum (Nat. Rist.), London. e c (Perrin In press) and R. pumilio appears to be an oppor­ DROZDZ, A. 1966. Food habits and food supply ofrodents in the u d tunistic feeder. In summer when insects and seeds are abun­ beach forest. Acta Theriol.ll: 363-384. o r dant these are taken in large quantities but during winter R. DROZDZ, A. 1967. Food preference, food digestability and the natural p e pumilio is compelled to eat less preferred herbs and shrubs. food supply of small rodents. In: Secondary Productivity of R I S. Afr. J. Zoo!. 1979, 14 (4) 229

Terrestrial Ecosystem. (ed.) Petrusewicz, K., I: 323 -330. Polish Zool. 146: 577-633. Science Publisher, Warszawa-Krakow. PERRIN, M.R. In press. The feeding habits of two co-existing rodents, DROZDZ, A. 1975. Feeding and Nutrition. In: Methods for Ecological Rhabdomys pumilio and Otomys irroratus, in relation to rainfall and Bio-energetics. (eds) Grodzinski, W., Klekowski, R.Z. & Duncan, A., reproduction. Mammalia. IBP Bo. 24: 325-351. Blackwells, Oxford. STEYN, D.G. 1949. Vergiftiging van mens en dier. J.L. van Schaik Ltd, FIELD, A.C. 1975. Seasonal changes in reproduction, diet and body Pretoria, South Africa. composition of two equatorial rodents. E. Afr. Wildl. J. 13: 221-325. ZEMANEK, M. 1972. Food and feeding habits of rodents in a HANNEY, P. 1965. The Muridae of Malawi (Africa:Nyassaland). J. deciduous forest. Acta Theriol. 17: 315 -325.

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