COLONIC ELECTROLYTE FLUX AND GUT COMPOSITION

AS SEEN IN FOUR SPECIES OF SUB-HUMAN PRIMATES

E. T. CLEMENS and G. M. 0. MALOIY

Department of Veterinary Physiology, University of Nairobi, P.O. Box 30197 Nairobi, Kenya, East Africa

(Received 21 August 1980)

Abstract-l. Studies compare the composition of gastrointestinal contents and transmucosal electrolyte flux in four species of subhuman primates.

2. The bushbaby, an insectivore, had significantly drier stomach contents, potassium and chloride concentration than the baboon, vervet or Sykes monkey.

3. While the caecum of all primates appeared to be the major site of microbial fermentation, the Sykes monkey, a herbivore, indicated the highest level of stomach fermentative activity.

4. Transmucosal electrolyte flux occurred primarily within the proximal colon of the baboon, vervet and Sykes monkey.

5. Volatile fatty acids were absorbed from the proximal colon of these animals.

INTRODUCTION

Gastrointestinal electrolytes vary within the gut of an animal according to diet, feeding regime, intestinal secretion or absorption and site along the length of tract, yet maintain an osmotic gradient which is generally hypertonic to plasma. A wealth of infor- mation is available on the electrolyte concentrations within the human gut (Fordtran & Locklear, 1966; Windgate et al., 1973; Field, 1974) and on species far removed from the order Primate (Alexander, 1962, 1965; Kay & Pfeffer, 1969; Maloiy & Clemens, 1980b). However, since gut electrolytes may be de- rived from both dietary sources and gut secretion, a major question is whether observed species differences are due to dietary habit or to actual differences in their respective digestive physiology.

The studies presented herein describe the electro- lyte composition at each site along the gastrointesti- nal tract of four species of sub-human primates; i.e. an insectivore (bushbaby), an omnivore (vervet mon- key), a herbivore (Sykes monkey) and one of the more carnivorous of sub-human primates (the baboon). Ad- ditional studies were conducted to compare the net coionic electrolyte flux in three of these species (i.e. the vervet, baboon and Sykes monkey). All animals were fed identical diets so that a clearer understand- ing of species differences could be obtained.

METHODS AND MATERIALS

Electrolyte composition studies

Sixteen adult bushbabies (G&go crassicaudatus), 12 ver- vet monkeys (Cercopithecidak pygerythrus), 12 baboons (Pupio cynocephalus) and 8 Sykes monkeys (Cercopithecus

mitis) weighing an average of 0.89 + 0.09 kg, 3.74 +

* Composition of diet. Ingredients are as a percent of dry matter: crude fibre, 8.3; crude protein, 19.8; ether extract, 10.9; nitrogen-free extract, 53.1; ash, 7.8; Ca, 1.18; P, 0.88; Na, 0.42; K, 1.00 and Mg, 0.02. The moisture content was 10.1 percent.

1.15 kg, 9.82 i 0.97 kg and 6.20 + 0.53 kg (+SEM). re- spectively, were used in the studies. All animals were indi- vidually caged and fed a commercial, pelleted primate diet*. Each animal was preconditioned to the diet for 4 wk before beginning the experiment. Animals were fed the diet twice daily at 12 hr intervals for a 1 hr feeding period to enable measurement of post-feeding changes related to meal-feeding. All animals were given free access to drinking water.

On the day of the experiment the animals were sacrificed at 2,4, 8 and 12 hr after feeding. Four bushbabies, 3 vervet monkeys, 3 baboons and 2 Sykes monkeys were sacrificed at each time period. Since the animals were fed at 12 hr intervals, the 12 hr post-feeding period also represents the 0 hr sample, or time just before the next meal.

Immediately after sacrifice the abdominal cavity was opened and clamps were used to close the gastro-esophe- geal and rectal-anal junctions. The gastrointestinal tract of each animal was removed and separated by ligatures into 10 segments. These consisted of the cranial and caudal halves of the stomach, 3 equal segments of the small intes- tine, the caecum and 4 segments of the colon. Total con- tents were removed from each segment, weighed and im- mediately refrigerated. Dry matter content was determined by drying samples from each segment of tract in a forced air oven at 105°C to a constant weight. The remaining contents from each segment of tract were centrifuged and the supernatant collected for analysis. Volatile fatty acids were determmed on the day of the experiment of dupli- cate samples by steam distillation (Markham, 1942). The osmolality of each sample was determined on a laboratory osmometer. Additional samples were analyzed for the concentrations of sodium and potassium by flame spectro- photometry, and chloride was determined with the aid of an automatic chloridometer.

The movement of ingesta, gastrointestinal pH and or- ganic acid production observed in each species has been reported earlier (Clemens & Phillips, 1980; Clemens & Maloiy, 1980). Analysis of variance and Duncans’s multiple range test were used to determine significant differences (Snedecor & Cochran, 1967).

Electrolyte transport studies

Four animals of each species (i.e. vervet, baboon and Sykes monkey) were used in the study. Each animal was

543

544 E. T. CLEMENS and G. M. 0. MALOIY

individually caged and ud libirum fed the same diet as the previous study. All animals were fed for a 4 wk adjustment period prior to beginning the experiment. Drinking water was available at all times. Ten days prior to sacrifice polyethylene glycol (PEG) was added to the drinking water at a concentration of 2% and maintained at this level throughout the remainder of the experiment. Animals were sacrificed and the gastrointestinal tract was removed and the large bowel (caecum and colon) separated by ligatures into the segments described in the earlier study. Total con- tents were removed from each segment, weighed and im- mediately refridgerated. Sample analysis was as described above. In addition, samples were analysed for lactic acid according to the methods of Barker & Summerson (1941). PEG determinations were by the methods of HydCn (1956) and the calculated net absorption or secretion of electro- lytes were as outlined by Kotb & Luckey (1972). using PEG as the external, steady-state indicator. An approxi- mation of net water absorption along the length of colon of each species was determined by the methods of Staaland (1975). Extraction ratios were derived from the caecum electrolyte concentration and net flux differences.

RESULTS

Electrolyte composition studies

Table 1 gives the mean values for dry matter, fluid volume and osmolality observed at each site along the gastrointestinal tract., For the vervet monkey, baboon and Sykes monkey the percent dry matter of gut con- tents remained reasonably constant throughout the stomach and small intestine, becoming increasingly drier from caecum to rectal regions. The dry matter percentage within the bushbaby’s stomach was signifi- cantly ‘greater (P < 0.05) than that observed in the stomach of the other three species and significantly greater than that observed in the bushbaby’s proximal small intestine. Statistically significant differences (P > 0.05) were not observed at any point along the gastrointestinal tract from proximal small intestine to rectum when comparing the four species of, sub- human primates. Diurnal variations in the dry matter content was evident only in the stomach and proxi- mal small intestine. Highest values were observed in the baboon, vervet and Sykes monkey stomach con- tents 2 hr after the meal (24%) and were similar (16 to 17%) for the 4, 8 and 12 hr post-feeding periods. Cyc- lic variations were not evident in the dry matter con- tent of the bushbaby’s stomach.

The fluid volume within each segment of tract rep- resents variation due to differences in the size of the ai,‘nal and anatomical features of their respective ;astrointestinal tracts, as well as physiological differ- ences relating to absorption and secretion mechan- i_,.ns. Therefore, stati ;tical analysis was not applied to this set of data. The data is presented for the readers benefit and can be used for calculating quantative values for the parar?eters measured.

Osmolality of LU.. stomach contents within the Sykes monkey were hypotonic to plasma and signifi- cantly lower (P < 0.05) than tliat observed in the other species. However, the intestinal contents were hypertonic to plasma and similar in all species for each segment of tract from proximal small intestine to rectum.

The results from the measurement of major gas- trointestinal electrolytes are presented in Table 2. Sig- nificant differences in the sodium concentrations

(P < 0.05) were observed in the proximal small intes- tine, caecum and mid to distal colon. The primary difference being between the bushbaby and Sykes monkey. A diurnal variation in sodium concentration was evident within the stomach of the baboon, vervet and Sykes monkey. Sodium concentrations were high- est before the meal and decreased for the 2, 4 and 8 hr post-feeding periods. A diurnal variation in the stomach sodium concentration was least evident in the bushbaby, with highest concentration observed 2 hr after the meal.

Potassium concentrations were similar for the baboon, vervet and Sykes monkey along the entire length of their gastrointestinal tract. Diurnal vari- ations were not detected at any site along the tract. However, the bushbaby demonstrated a significantly higher (P < 0.05) potassium concentration in the colon, when compared to the other three species investigated. The bushbaby also demonstrated a defi- nite diurnal variation in the concentration of potas- sium in the stomach, reaching peak concentration (78 mmol/l) 2 hr after the meal, and lowest value (39 mmol/l) 8 hr after the meal.

Statistically significant differences (P < 0.05) in the chloride content were most evident when comparing the Sykes monkey to the other species of primates. In each instance chloride ion concentrations were less than that observed in the bushbaby, vervet and baboon. In addition, the chloride concentration of the bushbaby’s stomach contents was significantly greater than that of the baboon and vervet monkey.

The organic anion contribution to gut electrolytes (i.e. the volatile fatty acid (VFA)) was primarily within the caecum and colon of all species investigated. Sig- nificant differences were observed in the cranial stomach, caecum and distal regions of the colon.

Table 3 gives the mean osmolality and electrolyte composition of plasma and urine for each of the four primate species.

Electrolyte transport studies

The fluid volume per 1OOg of intestinal contents observed at each site along the animals caecum and colon is presented in Fig. 1. Calculated net absorption of intestinal fluid is also presented in this figure. The data indicates a drying of the intestinal contents in all species as the ingesta moves in an aboral direction. Baboons and Sykes monkeys produced faecal material of similar consistancy (=35% dry matter) while the vervet monkey’s was approximately 30%. Net water flux per 1OOg of dry faecal contents indicated that recovery of fluid was most efficient in the Sykes mon- key (46 ml/lOOg) and least efficient in the baboon (23 ml/100 g). In each instance a greater portion of fluids were absorbed in the proximal segments of the colon. Mean concentration of cations and anions ob- served in the caecum of each primate (i.e. the ion composition presented to the colon for absorption) is shown in Table 2. For the three species in the trans- port studies (vervet, baboon and Sykes monkey) the composition was similar except for chloride (Sykes) and VFA (vervet). The absorption of sodium occurred primarily within the proximal colonic areas of each species (Fig. 2). Smaller quantities of sodium were ab- sorbed throughout the remainder of the tract. Appar- ent absorption of potassium also occurred within the

Tab

le

I.

Mea

n ( +

SE

M)

dry

mat

ter,

fl

uid

volu

me

and

osm

olah

ty

of g

astr

oint

estin

al

cont

ents

m

easu

red

alon

g th

e tr

acts

of

fou

r sp

ecie

s of

sub

-hum

an

prim

ates

*

Sect

ion

of

Dry

m

atte

r tr

act

f%)

Flui

d vo

lum

e

(ml)

Osm

olal

ity

tmO

sm/k

g)

Cra

nial

stom

ach

Cau

dal

stom

ach

Smal

l

int-

I Sm

all

int-

2 Sm

all

int-

3 C

aecu

m

Col

on-

1

Col

on-2

Col

on-3

Col

on-4

Bus

hbab

y 31

.3”

(2.8

) 37

.6”

(2.5

) 15

.5

(2.0

) 23

.9

(2.2

) 27

.4

(1.5

) 26

.3

(2.8

) 21

.1

(1.4

) 30

.6

(1.3

) 32

.1

(2.0

) 31

.1

(0.8

)

Ver

vet

19.4

b

(3.4

) 20

.5b

(2.1

) 14

.6

(2.4

) 15

.3

(3.1

) 19

.7

(0.8

) 18

.8

(2.0

) 25

.4

(2.0

) 23

.0

(1.5

) 28

.6

(1.1

) 29

.2

(0.8

)

Bab

oon

18.2

b

(3.1

) 19

.1b

(2.5

) 14

.4

(2.3

) 16

.3

(2.4

) 16

.7

(3.9

) 29

.1

(2.1

) 33

.4

(1.8

) 37

.0

(1.2

) 38

.5

(1.5

) 38

.1

(1.1

)

Syke

s 18

.2b

(9.4

) 18

.5b

(6.3

)

(E,

13.4

(3.0

) 11

.2

(5.1

) 20

.7

(2.6

) 30

.8

(1.7

) 37

.3

(5.1

) 35

.3

(5.7

) 39

.9

(3.7

)

Bus

hbab

y 5.

4

(0.8

) 4.

1

(0.5

) 1.

5

(0.2

) 3.

1

(0.4

) 4.

3

(0.5

) 3.

6

(0.4

) 2.

6

(0.4

) 5.

0

(0.8

) 3.

5

(0.7

) 4.

4

(0.9

)

Ver

vet

19.4

(7.6

) 10

.3

(4.3

) 2.

4

(0.5

) 3.

0

(1.0

) 5.

0

(0.8

) 10

.9

(1.7

) 9.

2

(1.4

) 14

.8

(2.8

) 11

.3

(3.2

) 9.

8

(1.8

)

Bab

oon

48.1

(10.

7)

39.1

(9.1

) 9.

9

(2.6

) 9.

6

(1.8

) 18

.0

(3.4

) 30

.4

(4.9

) 27

.1

(4.0

) 25

.4

(4.6

) 25

.0

(4.2

) 18

.8

(3.6

)

Syke

s 11

.3

(5.7

) 7.

0

(4.3

) 9.

9

(2.3

) 8.

4

(4.1

) 6.

0

(2.7

) 19

.9

(2.2

) 28

.8

(3.8

) 22

.8

(6.0

) 15

.9

(2.3

) 15

.9

(1.6

)

Bus

hbab

y 35

5”

(42)

33

Pb

(46)

43

2

(25)

38

1

(24)

36

9

(33)

35

3b

(33)

39

2

(20)

39

1

(32)

42

7

(31)

31

6

(51)

Ver

vet

348”

(33)

33

0”

(37)

49

2

(43)

43

5

(49)

40

2

(21)

33

9b

(27)

40

2

(18)

38

6

(23)

38

0

(22)

33

3

(20)

Bab

oon

326”

(24)

33

2”

(25)

43

0

(47)

53

4

(42)

37

9

(22)

42

1ab

(31)

47

2

W)

416

(37)

44

5

(31)

37

6

(39)

Syke

s 22

4b

(45)

23

2b

(42)

36

1

(81)

38

5

(59)

40

3

(39)

46

8”

(8)

4577

(21)

46

2

(17)

41

0

(29)

41

3

(30)

* V

alue

s w

ith

diff

eren

t su

pers

crip

ts

are

sign

ific

antly

di

ffer

ent

(P

< 0

.05)

.

Tab

le

2.

Mea

n ( I

SE

M)

conc

entr

atio

ns

of s

odiu

m,

pota

ssiu

m,

chlo

ride

an

d or

gani

c an

ions

m

easu

red

alon

g th

e ga

stro

inte

stin

al

trac

ts

of

four

sp

ecie

s of

sub

-hum

an

prim

ates

*

Sect

ion

of

trac

t So

dium

(m

-equ

iv/l)

Po

tass

ium

(m

-equ

iv/l)

C

hlor

ide

Org

anic

an

ion

(m-e

quiv

/l)

(mm

ol/l)

Cra

nial

st

omac

h C

auda

l st

omac

h Sm

all

int-

1 Sm

all

int-

2 Sm

all

int-

3 C

aecu

m

Col

on-

I

Col

on-2

Col

on-3

Col

on-4

Bus

hbab

y 67

.0

(6.6

) 65

.2

(7.4

) 99

.3”

(8.3

) 11

8.0

(6.6

) 99

.4

(4.8

) 55

.5b

(7.7

) 74

.6

(7.6

) 83

.9”

(5.6

) 73

.9”

(8.6

) 64

.0”

(7.9

)

Ver

vet

Bab

oon

58.3

52

.6

(9.5

) (9

.4)

55.8

48

.8

(11.

2)

(8.7

) 94

Ph

(9:l)

74

Fh

(10.

3)

103.

8 10

0.4

(11.

4)

(11.

6)

98.9

99

.8

(8.0

) (9

.4)

85.9

” 92

.3

(5.6

) (7

.7)

19.5

60

.1

(7.1

) (4

.1)

64.6

” 50

.9b

(7.2

) (5

.4)

57 6

”b

(7:3

) 52

.2b

(9.4

) 54

.Pb

37.8

”

(4.3

) (7

.1)

Syke

s 41

.6

(16.

9)

54.6

(1

3.0)

58

.5”

(17.

9)

91.2

(1

0.1)

87

.3

(7.4

) 83

.3”

(8.4

) 54

.1

(9.3

) 37

.0”

(5.2

) 29

.2”

(5.7

) 33

.1b

(5.7

)

Bus

hbab

y V

erve

t 56

.6”

35.0

b

(5.7

) (6

.9)

55.1

” 3s

.4b

(6.2

) (7

.0)

47.0

” 30

2”b

(6.0

) (4

:o)

29.9

24

.3

(4.1

) (2

.5)

19.4

b 34

.8”

(3.5

) (4

.0)

22.5

’ 42

.4”

(2.8

) (5

.0)

22.7

b 66

.2”

(3.8

) (8

.8)

23.3

b 71

.9”

(4.0

) (5

.1)

37.8

” 10

2.1”

(4.4

) (6

.7)

52.6

b 10

6.7*

(5.4

) (7

.0)

Bab

oon

24.7

b

(9.6

) 22

.0b

(8.7

) 25

.8b

(5.3

) 19

.7

(3.5

) 14

.Sb

(8.3

) 28

.0”

(5.3

) 60

.1”

(7.6

) 81

.2”

(8.5

) 10

6.7”

(9.8

) 10

9.3”

(1

1.7)

Syke

s B

ushb

aby

16.9

b 15

1.1”

(7.3

) (1

0.8)

21

.1b

162.

3”

(5.1

) (6

.4)

22.0

b 84

.7”

(7.0

) (8

.1)

17.6

45

.5

(4.7

) (3

.7)

19.4

b 30

.1”

(2.7

) (2

.6)

36 l

abc

(4:6

) 14

W

(1.8

) 53

.p

13.7

”

(3.0

) (1

.6)

64.1

” 14

.0”

(5.3

) (1

.7)

81.6

” 19

.4”

(11.

7)

(3.9

) 83

.0”

16.6

” (1

4.9)

(3

.9)

Ver

vet

109.

Sb

(9.3

) 1

10.7

b (1

1.9)

95

.4”

(12.

3)

59.5

(9.6

) 28

.eb

(8.9

) 20

.4”

(4.4

) 13

.3”

(2.0

) 14

.8”

(2.7

) 20

.2”

(4.4

) 12

.7”

(2.0

)

Bab

oon

108.

2b

(9.4

) 1

10.S

b

(8.8

) 96

.6”

(8.1

) 44

.6

(7.4

) 27

0”

(4.2

) 17

.0”

(2.6

) 13

.9”

(1.2

) 15

.6”

(0.9

) 16

.0”

(1.2

) 14

.1”

(1.8

)

Syke

s 67

.4

(17.

6)

74.3

b (2

0.0)

SO

.lb

(11.

2)

38.2

(7.4

) ll.

4b

(3.5

)

,;:t

b

(E,

4.4b

(1.3

) 4.

8b

(1.6

) 2.

Sb

(0.7

)

Bus

hbab

y V

erve

t B

aboo

n 19

.6

19.2

20

.7

(2.3

) (2

.0)

(4.2

) 21

.9b

32.6

” 1 S

.@

(3.6

) (7

.1)

(2.4

) 34

.1

20.6

24

.0

(4.7

) (3

.3)

(5.1

) 23

.1

30.9

30

.2

(3.1

) (8

.1)

(6.0

) 29

.Sb

21.0

b 37

.36

(4.7

) (2

.3)

(7.0

) 15

6.4”

11

3.0

b 17

1.6”

(1

2.6)

(1

1.3)

(1

2.2)

12

1.8

132.

1 15

7.1

(12.

8)

(14.

5)

(12.

2)

138.

8 12

2.6

141.

9 (1

5.5)

(1

6.7)

(1

5.5)

19

1.6’

12

8.8b

13

9.lb

(1

4.9)

(1

3.3)

(1

2.4)

17

3.4”

13

S.6”

b lO

l.Sb

(14.

0)

(12.

1)

(15.

7)

Syke

s 31

.1

(10.

1)

n 49

.0”

-i

(9.8

) R

31

.5

G

(9.7

) L

36

.4

z

(11.

3)

B

59.9

”

(7.9

) :

179.

i*

;s:

(15.

1)

143.

7 P

(10.

9)

138.

7 f

(8.9

) s

139.

2’

(14.

3)

132Y

(1

2.2)

* V

alue

s w

ith

diff

eren

t su

pers

crip

ts

are

sign

ific

antly

di

ffer

ent

(P

< 0.

05).

Primate colonic electrolytes 547

Table 3. Mean (*SEM) values for the osmolality and electrolyte composition observed in the plasma and urine of the baboon and Sykes monkey

Plasma

Urine

Osmolality Sodium Potassium Chloride

(mOsm/kg) (m-equiv/l) (m-equiv/l) (m-equiv/l)

Baboon Sykes Baboon Sykes Baboon Sykes Baboon Sykes 310 311 98.2 122.5 3.4 10.8 86.5 91.8

(10) (5) (18.9) (10.3) (0.4) (2.1) (8.5) (2.8) 939 428 79.5 112.0 21.6 33.0 46.8 41.5

(118) (62) (21.7) (27.7) (6.4) (5.2) (17.0) (10.1)

proximal colon of the baboon and vervet monkey, being re-secreted into the more distal regions of the large bowel. The sykes monkey showed a net se- cretion of potassium ions from proximal to mid colon with an apparent re-absorption in the most distal colonic regions.

Chloride ions were neither absorbed nor secreted along the colon of the Sykes monkey (Fig. 2). Small quantities of chloride (approx 40 mg/lOO ml) were ab- sorbed within the most proximal segments of the baboon’s and vervet monkey’s colon.

Like sodium ions, the net flux of organic anions (VFA and lactic acid) occurred primarily within the proximal colonic areas for all species of primates (Fig. 3). Small quantities of VFA, but not lactic acid, were lost (absorbed or metabolized) in the mid and distal colon of the baboon and Sykes monkey. The quantity of lactic acid lost from the colon was mini- mal.

DISCUSSION

The ionic composition of gastrointestinal contents varies markedly from one segment of tract to the next, and is associated with the effects of meal feeding and diet. Obvious differences are most evident when com- paring species of a more diverse dietary background (Alexander, 1962, 1965; Maloiy & Clemens, 1980). The present study demonstrates that dissimilarities

.cl go[ Baboon 0, Vervet 0, Sykeso.

‘c Ce Cl c2 c3 c4

Section of Tract

Fig. I. Mean fluid volume per 100 g of intestinal contents, and the calculated net water absorption per 1OOg of dry contents, as seen at each site along the colon of three sub- human primates. Symbols along the abscissa represent the sections of tract, caecum (Ce) and 4 equal segments of

colon (CL-C4).

BaboonO. Vervet e. SykesO.

‘c 7 z Ce Cl cz c3 c4

Section of Tract

Fig. 2. Mean sodium, potassium and chloride flux as seen at each site along the colon of three sub-human primates. Absorption indicated as positive values and secretion as negative values. Values are expressed as the change relative to the quantity observed in caecal fluids. Symbols along the abscissa represent the sections of tract: caecum (Ce)

and 4 equal segments of colon (C,-C,).

Baboon 0. Vervet 0, Sykes 0.

r; * . 2’ Ce Cl c2 c3 c4

Section of Tract Fig. 3. Mean volatile fatty acid (VFA) and lactic acid (LA) flux as seen at each site along the colon of three sub- human primates. Absorption indicated as positive values and secretion as negative values. Values are expressed as the change relative to the quantity observed in caecal fluids. Symbols along the abscissa represent the sections of tract; caecum (Ce) and 4 equal segments of colon (Cr-C.,).

548 E. T. CLEMFNS and G. M. 0. MAL~IY

may also exist between closely related species when considerable importance. From the VFA data in the feeding program and diets are identical. The most Table 2 it appears that the caecum of primates, par- prominent difference was noted when comparing the ticularly the Sykes monkey, rather than the proximal bushbaby, an insectivore, to the omnivorous baboon colon is the site of major microbial activity. There is, and the vervet monkey, and the herbivorous Sykes however, a significant degree of VFA production

monkey. within the stomach of the Sykes monkey as well. The bushbaby has been shown to have anatomical,

as well as physiological differences in their gastro- intestinal tract when compared to the other species investigated (Hill, 1966; Clemens & Maloiy, 1980). Direct consumption of fluid by these animals is mini- mal (Charles-Dominique, 1977) which may account for the deviation in foregut dry matter and electro- lytes. Unlike the other primates, the caecum of the bushbaby selectively retains fluid while preventing the entrance of large particulate matter (Clemens & Maloiy, 1980), accounting for the major variation at this site.

Acknowledgements-Appreciation is expressed for the technical assistance of Mr Shadrack Ojwang Orwa, Mr James Gatihi and Mr Simon Mungai. Also to the animal attendants Mr Paul Opil and Mr William Muiruri.

The studies were supported by the University of Nairobi. Research and Publication Grant.

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