Linköping University | Department of Physics, Chemistry and Biology

Master Thesis, 60 hp | Educational Program: Applied Ethology and Animal Biology

Spring 2016 to spring 2017 | LITH-IFM-x-EX—17/3362--SE

Gustatory responsiveness of West African Chimpanzees (Pan troglodytes verus) to seven substances tasting sweet to humans

Desirée Sjöström

Examinator, Mats Amundin Tutor, Matthias Laska, Linköping University, Daniel Roth, Borås Zoo.

Rapporttyp Report category

Licentiatavhandling Examensarbete

C-uppsats

D-uppsats Övrig rapport

_____________

Språk Language

Svenska/Swedish

Engelska/English

________________

Titel

Title

Gustatory responsiveness of West African Chimpanzees (Pan troglodytes verus) to seven

substances tasting sweet to humans

Författare

Author

Desirée Sjöström

Sammanfattning Abstract

Comparative studies of taste perception have found that primates may differ markedly in their sensitivity for substances perceived

as sweet by humans. These findings raise questions about the reason that may underlie these differences in sweet-taste sensitivity

between species. The aim of the present study was to assess the taste responsiveness of chimpanzees (Pan troglodytes verus) to

seven substances tasting sweet to humans and to compare the results with those of other primate species. Using a two-bottle

preference test (1 min) I found that the taste preference thresholds of the chimpanzees for five food-associated carbohydrates

ranged between 20-30 mM for sucrose, 20-50 mM for fructose, 60-80 mM for glucose, 50-80 mM for maltose, and 30-80 mM for

lactose. Taste preference thresholds for two steviol glycosides ranged from 0.04-0.05 mM for stevioside, and 0.03-0.05 mM for

rebaudioside A. The chimpanzees displayed clear preferences for all sweet-tasting substances presented. In line with data obtained

in other primates, the taste preference threshold of the chimpanzees for sucrose was lower compared to the other carbohydrates

presented and the taste preference thresholds for stevioside and rebaudioside A were lower compared to sucrose. In general, the

taste sensitivity of the chimpanzees fell into the range of data reported in other nonhuman primate species. Interestingly, the taste

preference thresholds of the chimpanzees reported here are similar to the taste detection thresholds obtained in humans, despite the

fact that the former are only a conservative approximation of an animal’s taste sensitivity. This suggests that chimpanzees may be

as sweet-taste sensitive as humans.

ISBN

ISRN: LITH-IFM-x-EX--17/3362--SE _________________________________________________________________

Serietitel och serienummer ISSN

Title of series, numbering ______________________________

Nyckelord Keyword

chimpanzees, fructose, lactose, glucose, maltose, rebaudioside A, stevioside, sucrose, taste

preference threshold

Datum

Date

2017-05-25

URL för elektronisk version

Avdelning, institution

Division, Department

Department of Physics, Chemistry and Biology

Linköping University

Content

1 Abstract ............................................................................................... 2

2 Introduction ......................................................................................... 2

3 Material & methods ............................................................................ 4

3.1 Animals and housing .................................................................... 4

3.2 Taste stimuli ................................................................................. 5

3.3 Experimental set-up ..................................................................... 5

3.4 Experimental procedure ............................................................... 7

3.5 Data analysis ................................................................................ 8

4 Results ................................................................................................. 8

4.1 Carbohydrates .............................................................................. 8

4.2 Steviol glycosides ...................................................................... 10

5 Discussion ......................................................................................... 10

5.1 Comparison of the sweet-tasting carbohydrates ........................ 10

5.2 Comparison of the two steviol glycosides ................................. 14

5.3 Ethical and societal considerations ............................................ 15

5.4 Conclusions ................................................................................ 16

6 Acknowledgement ............................................................................ 16

7 References ......................................................................................... 17

2

1 Abstract

Comparative studies of taste perception have found that primates may

differ markedly in their sensitivity for substances perceived as sweet by

humans. These findings raise questions about the reason that may

underlie these differences in sweet-taste sensitivity between species. The

aim of the present study was to assess the taste responsiveness of

chimpanzees (Pan troglodytes verus) to seven substances tasting sweet to

humans and to compare the results with those of other primate species.

Using a two-bottle preference test (1 min) I found that the taste

preference thresholds of the chimpanzees for five food-associated

carbohydrates ranged between 20-30 mM for sucrose, 20-50 mM for

fructose, 60-80 mM for glucose, 50-80 mM for maltose, and 30-80 mM

for lactose. Taste preference thresholds for two steviol glycosides ranged

from 0.04-0.05 mM for stevioside, and 0.03-0.05 mM for rebaudioside A.

The chimpanzees displayed clear preferences for all sweet-tasting

substances presented. In line with data obtained in other primates, the

taste preference threshold of the chimpanzees for sucrose was lower

compared to the other carbohydrates presented and the taste preference

thresholds for stevioside and rebaudioside A were lower compared to

sucrose. In general, the taste sensitivity of the chimpanzees fell into the

range of data reported in other nonhuman primate species. Interestingly,

the taste preference thresholds of the chimpanzees reported here are

similar to the taste detection thresholds obtained in humans, despite the

fact that the former are only a conservative approximation of an animal’s

taste sensitivity. This suggests that chimpanzees may be as sweet-taste

sensitive as humans.

2 Introduction

The primate taxon is a group of species which varies not only regarding

their appearance, but also with regard to their feeding ecology. Among

the primates, there are several different dietary specializations such as

frugivores (fruit-eating) e.g. spider monkey (Ateles geoffroyi) (Laska et

al., 1999b), folivores (leaf-eating) e.g. howler monkeys (Alouatta) (Ungar

et al., 2016), herbivores (plant-eating) e.g. western lowland gorilla

(Gorilla gorilla gorilla) (Remis et al., 2001), omnivores (plant- and

animal-eating) e.g. baboon (Papio hamadryas anubis) (Laska et al.,

1999b), insectivores (insect-eating) e.g. squirrel monkeys (Saimiri)

(Ungar et al., 2016) and gummivores (plant exudate-eating) e.g.

marmosets and tamarins (Callithrix) (Ungar et al., 2016). Therefore,

comparative studies in primates are a suitable method to assess whether

dietary specialization correlates with physiological features such as taste

perception (Breslin 2013). Previous studies reported that sweet-tasting

3

carbohydrates are important for primates regarding their need for

metabolic energy (Kare and Brand, 1986; Leonard and Robertson, 1992).

Thus, it seems reasonable to assume that primates should be sensitive to

these taste substances.

Several studies have assessed the responsiveness of non-human primates

to substances tasting sweet to humans (e.g. Glaser et al., 1996; Laska,

1996; Nofre et al., 1996; Simmen and Hladik, 1998; Simmen et al.,

1999a). Notable differences in the taste sensitivity for sugars have been

found between species of primates using the two-bottle preference test

(Laska et al., 2001; Simmen and Hladik, 1998). These results raise the

question if these differences in sweet-taste sensitivity are due to

differences in the dietary specialization, allometric relationships, or

phylogenetic relatedness (Simmen et al., 1999a; Simmen et al., 1999b;

Simmen and Hladik, 1998).

Chimpanzees (Pan troglodytes) include a large variety of food items of

both plant and animal origin into their diet. However, depending on

season they spend 31-88 % of their feeding time foraging on various

fruits (Wrangham, 1977). Fruits are rich in carbohydrates and are often

low on other essential nutrients, such as minerals, fats and protein

(Milton, 1999). This may explain why chimpanzees also feed on, for

instance, leaves, seeds, flowers, underground storage organs, pith, insects,

and small vertebrates (Hockings et al., 2010; McLennan, 2013; Morgan

and Sanz, 2006; Potts et al., 2011; Watts et al., 2012). Nevertheless,

chimpanzees are considered to be ripe fruit specialists (Wrangham,

1977). Previous studies reported that chimpanzees not only use visual and

olfactory cues to select food items, but also gustatory cues. Chimpanzees

inspect potential food items thoroughly and select individual fruits that

contain low levels of fibre and higher contents of soluble carbohydrates

(Conclin-Brittain et al., 1998). So far, only one previous study

investigated the responsiveness of chimpanzees to sweet tastants,

(Simmen and Charlot, 2003), in which the taste preference threshold for

fructose of two chimpanzees was reported.

The five carbohydrates sucrose, glucose, lactose, maltose and fructose

used in the present study are naturally occurring food-associated

carbohydrates. Sucrose, glucose and fructose are the three quantitatively

predominant soluble carbohydrates found in fruits (Kinghorn and

Soejarto, 1986) and usually make up more than 90 % of the total sugar

content in fruits (Nagy and Shaw, 1980). Lactose is the predominant

carbohydrate found in milk which is the first diet of mammals. In

primates, milk can contain lactose at concentrations as high as 6-9 g/100

4

ml depending on the species (Hinde and Milligan, 2011). Chimpanzee

milk contains 7.4 g/100 ml of lactose, corresponding to a concentration of

216 mM (Hinde and Milligan, 2011). Maltose contributes to a sweet taste

sensation when the animal is feeding on starch-containing plants during

mastication and via enzymatic degradation starch is converted to sweet-

tasting maltose (Beck and Ziegler, 1989).

The two glycosides stevioside and rebaudioside A used in the present

study are two compounds found in the neotropical plant species Stevia

rebaudiana which is endemic to Paraguay in South America, and belongs

to the plant family Asteraceae (Soejarto, 2002). Stevioside and

rebaudioside A are the two quantitatively predominant substances found

in the leaves of the stevia plant and they are commercially used as low-

calorie sweeteners (DuBois and Prakash, 2012). Further, both substances

have been found to be 100-300 times sweeter than sucrose for humans.

Depending on which reference concentration and psychophysical method

was used, this number may vary (DuBois et al., 1991; Carakostas et al.,

2012; Upreti et al., 2012). Nonetheless, when presented to humans at high

concentrations, these two substances have been reported to have a bitter

side taste (Schiffman et al., 1995).

There are, to my knowledge, no previous studies which assessed the

responsiveness of chimpanzees to any of the sweet tastants mentioned

above except fructose. It was therefore the aim of the present study to

assess the taste responsiveness of chimpanzees to five carbohydrates

(sucrose, fructose, lactose, maltose and glucose) and two steviol

glycosides (stevioside and rebaudioside A) as well as to compare these

results to those of other primate species. To this end, I determined the

taste preference thresholds of Pan troglodytes verus for these seven

substances.

3 Material & methods

3.1 Animals and housing

The present study took place at Borås Zoo in Sweden and included three

West African Chimpanzees (Pan troglodytes verus), one male, Jonathan,

and two females, Pippi and Kankan. Pippi was 48, Kankan 33 and

Jonathan 27 years old at the start of the study. They were housed together

with an additional female which was not included in the study.

The chimpanzees were kept in both an indoor and an outdoor enclosure.

During the nights they were kept indoors in a room measuring 450 m3 (70

m2) and during daytime they were kept in a room of 750 m3 (142 m2).

5

During daytime they were also allowed to go outside on an island with

natural vegetation measuring 560 m2. A narrow corridor connected the

daytime enclosure and the outdoor enclosure and next to it, there was a

small room measuring 100 m3 (12 m2) which was the room Jonathan and

Pippi were tested in.

The chimpanzees were fed three times a day with a large variety of

vegetables and fruits such as carrots, tomatoes, cucumber, salad, onions,

strawberries, blueberries, grapes, oranges, and apples. In addition, the

chimpanzees were fed commercial primate chow pellets once a day and

provided with water ad libitum.

Figure 1. The female chimpanzee Kankan included in the study at Borås Zoo.

3.2 Taste stimuli

Seven sweet-tasting substances were used in this study, the five

carbohydrates sucrose (CAS# 57-50-1), glucose (CAS# 50-99-7), lactose

(CAS# 63-42-3), maltose (CAS# 6363-53-7) and fructose (CAS# 57-48-

7), and the two steviol glycosides stevioside (CAS# 57817-89-7) and

rebaudioside A (CAS# 58543-16-1). The five carbohydrates were

obtained from Sigma-Aldrich Sweden AB, Stockholm, Sweden. The two

steviol glycosides were obtained from Shanghai Xunxin Chemical Co.

(Shaoxing, China), and Xinghua Green Biological Preparation Co.

(Jiangsu, China). All substances were of the highest purity available

(≥99.5%).

3.3 Experimental set-up

A two-bottle preference test of short duration (1 min) was used to

determine the taste preference thresholds of the chimpanzees (Richter and

Campbell, 1940). Three different drinking stations were built and placed

in three different places of the enclosure, one for each test subject. The

6

reason for this was to avoid distraction and competition between the

individuals. The drinking stations will be referred to as 1, 2 and 3.

Drinking station 3 was placed on a gate in the daytime enclosure where

Kankan was tested. The other two drinking stations were placed in the

smaller room between the daytime enclosure and the outdoor enclosure.

The smaller room was divided in two compartments and one drinking

station was placed in each compartment. Jonathan and Pippi were tested

in the two-compartment room at drinking station 1 and 2, respectively

(Fig.2).

Figure 2. The daytime enclosure with drinking station 3 and the smaller compartments

with drinking stations 1 and 2.

The drinking stations were constructed in a way so that the chimpanzees

were not able to see the content in the bottles (Fig 3.).

Figure 3. Drinking station from the experimenter’s side, without bottles (left), and

from the animal’s side, with drinking spouts pointing towards the animal (right).

7

3.4 Experimental procedure

Firstly, the chimpanzees were familiarised with the bottles and trained to

participate in the two-bottle preference test. The training started in

February 2016. In the beginning the carbohydrate sucrose dissolved in tap

water was used as well as juice to make the chimpanzees interested in the

bottles. Each individual was also trained to go to his/her own drinking

station in every test. When data collection started in May 2016, the

animals had learned to cooperate and the carbohydrates were presented in

the following order: sucrose, glucose, lactose, maltose, fructose,

stevioside, rebaudioside A. For testing the three chimpanzees in parallel,

a couple of caretakers aided in every test.

In each trial the chimpanzees were simultaneously presented with two

graduated bottles of 700 mL with metal drinking spouts for 1 minute. One

bottle contained tap water and the other bottle contained a solution of one

of the sweet-tasting substances diluted with tap water at a given

concentration. The bottles were weighed before and after each trial and

the consumption of liquid was calculated for each bottle.

With the five carbohydrates (sucrose, glucose, lactose, maltose, fructose)

testing started at a concentration of 200 mM and proceeded with 100

mM, 50 mM, 20 mM and 10 mM until the individual failed to show a

significant preference for a concentration. Thereafter, the individuals

were presented with intermediate concentrations, in order to determine

the taste preference threshold more accurately. Regarding stevioside and

rebaudioside A, testing started at a concentration of 1 mM and continued

with 0.1 mM and 0.01 mM and subsequently intermediate concentrations.

The reason for presenting stevioside and rebaudiosode A at such low

concentrations was due to the fact that they are known to taste a lot

sweeter than the carbohydrates to human subjects. The lowest

concentration of a sweet-tasting substance that the animals preferred over

the bottle with tap water was considered as the taste preference threshold

value.

However, the concentrations were not presented in a strict descending

order but instead presented pseudo-randomized. The reason for this was

to keep the animals motivated and willing to participate in the tests. Also,

the bottle containing a sweet-tasting solution was not placed on the same

side in every trial, but rather in a pseudo-randomized sequence on either

the left or the right side, in order to avoid a preference for a certain side.

This procedure was also performed during the training period before data

collection started. The animal had to sample each bottle at least once in

order to count it as a complete trial.

8

Four to six such 1-minute trials were performed each testing day per

animal, if possible, two in the morning, two after lunch and two in the

afternoon. The number of trials performed with each individual depended

on the willingness of the animal to participate in the testing.

3.5 Data analysis

Each animal was presented with a set of a given concentration of a taste

substance and tap water for ten trials. The amount of liquid consumed

from each of the two bottles summed over ten trials was calculated and

converted to percentages. A two-tailed binomial test was performed for

each experiment (over 10 trials) and to determine the taste preference

threshold two criteria had to be met. Firstly, the taste preference value in

percent had to be >66,7 % as well as the taste stimulus had to be

preferred over the water in at least 8 out of 10 trials (P <0.05). For each

concentration of a tested substance. The mean value across the three

individuals was built by summing up the individual values and dividing

them by the number of animals.

4 Results

4.1 Carbohydrates

The taste preference threshold regarding sucrose was 20 mM for both

Jonathan and Kankan and 30 mM for Pippi (P <0.05, binomial test) (Fig.

4). The mean taste preference threshold for sucrose was calculated to be

20 mM for the three chimpanzees combined. With glucose, the taste

preference threshold was 80 mM for both Jonathan and Kankan and 60

mM for Pippi (P <0.05, binomial test) (Fig. 4). The mean taste preference

threshold for all three chimpanzees combined was 80 mM for glucose.

The taste preference threshold for lactose was 80 mM for both Pippi and

Kankan and 30 mM for Jonathan (P <0.05, binomial test) (Fig. 4). The

mean taste preference threshold regarding lactose including all three

chimpanzees combined was 80 mM. Regarding maltose, the taste

preference threshold was 50 mM for both Jonathan and Pippi and 80 mM

for Kankan (P <0.05, binomial test) (Fig. 4). The mean taste preference

threshold for maltose for the three chimpanzees combined was 80 mM.

The taste preference threshold for fructose was 50 mM for Kankan, 40

mM for Jonathan and 20 mM for Pippi (P <0.05, binomial test) (Fig. 4).

The mean taste preference threshold for all three chimpanzees combined

regarding fructose was 40 mM.

9

Figure 4. Taste preferences of the three

chimpanzees when tested with sucrose,

glucose, lactose, maltose and fructose

presented along with tap water. Each

data point represents the mean value of

10 trials of 1 min per individual. The

horizontal lines at 66.7 % and 50 %

illustrate the criterion of preference and

the chance level, respectively.

30

40

50

60

70

80

90

100

10 15 20 30 50 80 100 200

% p

refe

rence f

or

sucro

se

Concentration (mM)

Sucrose vs. WaterJonathanPippiKankan

30

40

50

60

70

80

90

100

20 50 60 70 80 100 200

% p

refe

rence f

or

glu

cose

Concentration (mM)

Glucose vs. Water

30

40

50

60

70

80

90

100

20 30 40 50 70 80 100 200

% p

refe

rence f

or

lacto

se

Concentration (mM)

Lactose vs. Water

30

40

50

60

70

80

90

100

20 40 50 70 80 100 200

% p

refe

rence f

or

maltose

Concentration (mM)

Maltose vs. Water

30

40

50

60

70

80

90

100

15 20 30 40 50 100 200

% p

refe

rence f

or

fructo

se

Concentration (mM)

Fructose vs. Water

10

4.2 Steviol glycosides

The taste preference threshold concerning stevioside was 0.05 mM for

Pippi and Kankan, and 0.04 mM for Jonathan (P <0.05, binomial test)

(Fig. 5). The mean taste preference threshold combined for all three

chimpanzees was 0.05 mM for stevioside. The taste preference threshold

regarding rebaudioside A was 0.05 mM for Jonathan, 0.04 mM for Pippi

and 0.03 mM for Kankan (P <0.05, binomial test) (Fig. 5). The mean

taste preference threshold for all three chimpanzees combined was 0.03

mM for rebaudioside A.

Figure 5. Taste preferences of the three chimpanzees when tested with stevioside and

rebaudioside A presented against tap water. Each data point represents the mean

value of 10 trials of 1 min per individual. The horizontal lines at 66.7 % and 50 %

illustrate the criterion of preference and the chance level, respectively.

5 Discussion

All seven substances tasting sweet to humans were clearly preferred over

water by the chimpanzees. Further, the taste preference thresholds of the

chimpanzees ranged from 20-30 mM for sucrose, 20-50 mM for fructose,

60-80 mM for glucose, 50-80 mM for maltose, and 30-80 mM for lactose.

Taste preference thresholds for two steviol glycosides ranged from 0.04-

0.05 mM for stevioside, and 0.03-0.05 mM for rebaudioside A.

5.1 Comparison of the sweet-tasting carbohydrates

Table 1 compares the taste preference thresholds of the chimpanzees for

the five food-associated carbohydrates found in the present study to all

published data reported in other primate species. Except for sucrose, there

30

40

50

60

70

80

90

100

0.0

1

0.0

2

0.0

3

0.0

4

0.0

5

0.0

8

0.1 1

% p

refe

rence f

or

rebaudio

sid

e A

Concentration (mM)

Rebaudioside A vs. Water

30

40

50

60

70

80

90

100

0.0

1

0.0

3

0.0

4

0.0

5

0.0

8

0.1 1

% p

refe

rence f

or

ste

vio

sid

e

Concentration (mM)

Stevioside vs. Water

Jonathan

Pippi

Kankan

11

is in general only little information concerning taste responsiveness in

nonhuman primates regarding naturally occurring food-associated sugars.

Within the Hominoidea, chimpanzees are the only species except for

humans which have been tested with all five naturally occurring food-

associated sugars.

The chimpanzees tested here have a mean taste preference threshold of 20

mM for sucrose. Several nonhuman primate species have been tested for

sucrose and the taste preference thresholds range between 10-20 mM for

the majority of them. However, some species have extremely high taste

preference thresholds, that is, a lower taste sensitivity compared to the

chimpanzees. For instance, the cotton-top tamarin (Saguinus oedipus)

belonging to the Platyrrhini and the mongoose lemur (Eulemur mongoz)

belonging to the Strepsirrhini have a taste preference threshold of 125

mM for sucrose. Further, the sunda slow lori (Nycticebus coucang) also

belonging to Strepsirrhini primates has a taste preference threshold as

high as 330 mM for sucrose. In contrast, black-handed spider monkeys

(Ateles geoffroyi) belonging to the Platyrrhini have a taste preference

threshold of 3 mM for sucrose, which is a higher taste sensitivity for

sucrose compared to the chimpanzees.

The chimpanzees have a mean taste preference threshold of 40 mM for

fructose. Nonhuman primates within the Hominoidea that have been

tested for fructose in addition to chimpanzees are western lowland

gorillas (G. gorilla gorilla) and bornean orangutans (Pongo pygmaeus).

The chimpanzees show a higher taste sensitivity for fructose compared to

the western lowland gorilla which has a taste preference threshold of 75

mM for fructose. However, the chimpanzees show a lower taste

sensitivity compared to the bornean orangutan, which has a taste

preference threshold of 15 mM for fructose and is also one of the lowest

taste preference thresholds noted in primates for fructose. Additionally,

the mongoose lemur (E. mongoz) has a taste preference threshold as high

as 110 mM which is the highest taste preference threshold for fructose

found in primates.

The mean taste preference threshold of the chimpanzees for glucose was

found to be 80 mM. Few nonhuman primate species have been tested for

glucose. However, a couple of species have taste preference thresholds

ranging between 20-25 mM, belonging both to the Cercopithecidae and

the Platyrrhini primates. Furthermore, the black-handed tamarin

(Saguinus midas niger) belonging to the Platyrrhini has been reported to

have a taste preference threshold as high as 330 mM, that is, a much

lower taste sensitivity for glucose compared to the chimpanzees.

12

As in the case with glucose, only few nonhuman primate species have

been tested for maltose. The mean taste preference threshold of the

chimpanzees is 80 mM for maltose. Of those primate species tested for

maltose, the squirrel monkey (Saimiri sciureus) belonging to the

Platyrrhini has the highest taste preference threshold of 90 mM, which

does not differ notably from the taste preference threshold of the

chimpanzees. The remaining taste preference thresholds of nonhuman

species tested ranged between 10-50 mM for maltose.

The taste preference threshold of the chimpanzees for lactose was 80

mM. Again, few nonhuman species have been tested for lactose with

regard to their taste preference thresholds. However, as stated for glucose,

the black-handed tamarin (S. midas niger), again, has the highest taste

preference threshold of the nonhuman species tested, with a concentration

of 250 mM. Additionally, the pygmy marmoset (Cebulla pygmaea) has a

taste preference threshold of 125 mM and the squirrel monkey (S.

sciureus) a taste preference threshold of 100 mM, both belonging to the

Platyrrhini primates. On the other hand, the lowest taste preference

threshold, that is, the highest taste sensitivity, reported for lactose has

been reported in the black-handed spider monkey (A. geoffroyi) with 10

mM, also belonging to the Platyrrhini.

Humans and chimpanzees do not differ markedly in their taste sensitivity.

However, when comparing the taste thresholds of the chimpanzees to

those of humans one must keep in mind that the values for humans are so

called taste detection thresholds and not taste preference thresholds. The

two-bottle method as used in the present study only allows for

determining a taste preference threshold and is therefore a conservative

estimate of the chimpanzees’ taste sensitivity whereas the psychophysical

signal detection methods used for human testing allow for determining a

taste detection threshold which is indeed the lowest concentration that

humans can detect. Therefore, it is quite possible that the chimpanzees

can still detect lower concentrations of a sweet tastant in the two-bottle

preference test but these concentrations may be so weak that they do not

display a preference anymore.

13

Table 1. Taste preference thresholds (mM) of the food-associated carbohydrates,

sucrose, fructose, glucose, maltose and lactose in the tested chimpanzees and in

various other primate species, based on published data. Note that the values for H.

sapiens are taste detection thresholds.

Species Sucrose Fructose Glucose Maltose Lactose Ref.

Hominoidea Homo sapiens 10 40 80 31 72 12

Pan troglodytes 20 40 80 80 80 1, 11 Gorilla gorilla 75 11

Pongo pygmaeus 15 11 Cercopithecidae

Macaca nemestrina 10 20 20 10 30 8 Macaca mulatta 6 3 Macaca radiata 10 10 9 Macaca fuscata 10 10 13

Papio hamadryas anubis 10 20 25 20 20 10 Cercopithecus pygerythrus 11 3

Cercopithecus nictitans 11 3 Platyrrhini

Ateles geoffroyi 3 15 20 20 10 5 Saimiri sciureus 10 40 90 90 100 6

Saguinus midas niger 66 66 330 250 3 Saguinus fuscicollis 50 3

Saguinus oedipus 125 16 2, 3 Cebulla pygmaea 33 50 100 125 3 Callithrix jacchus 25 29.5 3, 7

Callithrix geoffroyi 41 2 Callithrix argentata 19.5 2

Leontopithecus rosalia 19.5 2 Leontopithecus chrysomelas 21.5 2

Callimico goeldii 31 2 Cebus apella 8 2

Aotus trivirgatus 17 3 Strepsirrhini

Varecia variegata variegata 25 25 50 50 50 1 Eulemur coronatus 21 2

Eulemur fulvus 9 22.5 2 Eulemur macaco 8 14 2 Eulemur mongoz 125 110 3 Hapalemur simus 17.5 18.5 2

Hapalemur griseus 16.5 2 Phaner furcifer 65 2

Microcebus murinus 167 47.5 3, 4 Microcebus coquereli 90 2

Cheirogaleus major 50 2 Cheirogaleus medius 143 3

Propithecus verreauxi 52.5 2 Loris tardigradus 50 3

Nycticebus coucang 330 3 Galago senegalensis 66 3

14

1Present study; 2Simmen and Hladik (1998); 3Glaser (1996); 4Simmen et al. (1999b); 5Laska et al. (1996); 6Laska (1996); 7Simmen (1994); 8Laska (2000); 9Sunderland and

Sclafani (1988); 10Laska et al. (1999); 11 Simmen and Charlot (2003); 12 van Gemert

(2011); 13Nishi (2016).

The taste preference thresholds for sucrose in the three chimpanzees

tested in the present study were overall lower compared to those for the

other four sweet-tasting carbohydrates. This is in line with the vast

majority of nonhuman primate species that have been tested with

carbohydrates other than sucrose (see Table 1). Three species of

Cercopithecoid primates, pigtail macaque (Macaca nemestrina), bonnet

macaque (Macaca radiata) and Japanese macaque (Macaca fuscata) are

as sensitive for maltose as they are for sucrose. This can be explained by

the notion that taste sensitivity for maltose is linked to an evolutionary

adaptation of the gustatory system in order to be able to generate

nutritional energy from feeding on starch-rich plants (Laska, 2000). Old

world monkeys are considered as herbivores and they feed on leaves,

seeds and fruits. The three Macaca species mentioned above are known

to feed on starch-rich plants (Nishi et al., 2016). Similar to these Macaca

species, granivorous rodents, such as mice and rats show the same pattern

which makes sense considering that they also feed on starch-rich grains

(Laska, 1997).

5.2 Comparison of the two steviol glycosides

Table 2 compares the taste preference thresholds of the chimpanzees for

the two substances stevioside and rebaudioside A which are

commercially used as low-calorie sweeteners. The taste preference

threshold for stevioside and rebaudioside A has previously only been

established in A. geoffroyi and V. variegata variegata. However, for

humans, the taste detection thresholds of these two substances have been

reported. Chimpanzees are more taste sensitive to both stevioside and

rebaudioside A compared to V. variegata variegata, that is, they have a

lower taste preference threshold. Regarding A. geoffroyi, the taste

sensitivity for stevioside is the same as in the chimpanzees, whereas the

chimpanzees are less taste sensitive for rebaudioside A compared to A.

geoffroyi, that is, they have a higher taste preference threshold. Human

taste detection thresholds were one order of magnitude lower than those

of the chimpanzees. However, one should keep in mind that the

difference in taste perception between humans and chimpanzees might

not be that big because taste preference thresholds are only a conservative

estimate of a species’ taste sensitivity.

Considering that chimpanzees are endemic to Africa and the stevia plant

to South America, it is unlikely that chimpanzees have encountered this

15

plant throughout their evolutionary history. However, as shown in Table

2, nonhuman primates belonging to the Hominoidea, the Plathyrrhini,

and the Strepsirrhini suborders, respectively, are able to detect these two

steviol glycosides. Consequently, a plausible explanation for the fact that

primate species of all these taxa are able to detect these two substances

may be that this was also the case regarding their common ancestor.

However, another plausible explanation could simply be that a co-

evolution between animal and plant species has not been necessary to

occur regarding the ability or inability to perceive sweet-tasting

substances between different species. This explanation is supported by

previous studies which reported that the artificial sweeteners aspartame

and neotame are only detectable for Catarrhine primates but not for

Prosimian and Platyrrhine primates (Glaser et al., 1992). A further

explanation could be that plant species containing substances that are

structurally similar to those of stevioside and rebaudioside A have co-

evolved with catarrhine and prosimian primates. This, in turn, may have

resulted in modifications of the receptors responsible for sweet taste in

the primates and thus allowed for the recognition and binding of

stevioside and rebaudioside A to the sweet-taste receptors (Niklasson et

al., 2017).

Table 2. Taste preference thresholds (mM) for stevioside and rebaudioside A in V.

variegata variegata, P. troglodytes verus, A. geoffroyi, and H. sapiens. Note that

values for H. sapiens are taste detection thresholds.

Species Stevioside Rebaudioside A Reference

Hominoidea

Homo sapiens 0.0053 0.0046 2

Pan troglodytes verus 0.05 0.03 1

Plathyrrhini

Ateles geoffroyi 0.05 0.01 3

Strepsirrhini

Varecia variegata variegata 0.1 0.1 4

1Present study; 2 van Gemert (2011); 3 Niklasson et al. (2017); 4 Sandra Niklasson

(2015) Master thesis.

5.3 Ethical and societal considerations

The experiments reported here comply with current Swedish laws and the

Guide for the Care and Use of Laboratory Animals (National Institutes of

Health Publication no. 86-23, revised 1996). This study was approved by

Gothenburg’s Animal Care and Use Committee (Göteborgs

djurförsöksetiska nämnd, protocol #75-2016.

16

The experiments conducted in the present study were all performed

depending on the willingness of the chimpanzees to cooperate. The

animals were not in any way forced to partake and could leave the

drinking stations at all times. The substances used in the present study

were all safe and approved for human consumption and were not

considered as harmful to the chimpanzees in any of the presented

concentrations. The caretakers were present during every test session and

monitored the health and behaviour of the chimpanzees on a daily basis,

during the test period.

Studies like the present one provide knowledge about taste perception of

captive animals which can help to improve their welfare and it gives

information on how to improve their diet. For instance, with such

information, obesity may be avoided in chimpanzees which is a common

health condition among captive chimpanzees. When the chimpanzees are

keen on sweet tastes, this makes it easier to modify the diet so that they

are not so keen on eating certain stuff.

5.4 Conclusions

All three chimpanzees included in the present study displayed a clear

preference for all seven sweet-tasting substance presented. Further, the

chimpanzees did not differ markedly in their taste sensitivity from

humans, however it is important to distinguish between taste preference

threshold as for chimpanzees, and taste detection threshold as for

humans. Hence, it is possible that the chimpanzees detect even lower

concentrations of the sugars used in the present study. The chimpanzees

displayed a lower taste preference threshold for sucrose compared to

glucose, lactose, maltose and fructose. This result is in line with studies

on other primates. In addition, the chimpanzees in the present study

displayed a preference for both stevioside and rebaudioside A, which is

interesting as the stevia plant is endemic to South America and the

chimpanzees to Africa. To better understand how the gustatory sense has

evolved among primates and how preferences for food-associated sugars

correlate with dietary specialization, more studies are needed on

gustatory responsiveness including different primate species and different

food-associated substances.

6 Acknowledgement

I want to thank my supervisors Matthias Laska and Daniel Roth who

gave me the opportunity to perform this study and I am truly grateful to

the caretakers at Borås Zoo who trained the Chimpanzees to willingly

participate in the testing and who assisted me throughout the whole study.

17

7 References

Beck, E., Ziegler, P. (1989) Biosynthesis and degradation of starch in

higher plants. Annual Review of Plant Physiology, 40, pp. 95-117.

Breslin, P. A. S. (2013) An Evolutionary Perspective on Food and Human

Taste. Current Biology, 23, R409-R418.

Carakostas, M., Prakash, I., Kinghorn, A. D., Wu, C. D., Soejarto, D. D.

(2012) Steviol glycosides. In: O´Brien Nabors L (ed) Alternative

Sweeteners, 4th ed. CRC Press, Boca Raton, pp. 159-180.

Conclin-Brittain, N. L., Wrangham, R. W., Hunt, K. D. (1998) Dietary

response of chimpanzees and cercopithecines to seasonal variation in fruit

abundance. II. Macronutrients. International Journal of Primatology, 19

(6), pp. 971-991.

DuBois, G. E., Prakash, I. (2012) Non-caloric sweeteners, sweetness

modulators, and sweetener enhancers. Annual Review of Food Science

and Technology, 3 (1), pp. 353-380.

Dubois, G.E., Walters, D.E., Schiffman, S.S., Warwick, Z.S., Booth, B.J.

(1991) Concentration-response relationships of sweeteners: A systematic

study. Sweeteners: Discovery, Molecular Design and

Chemoreception, pp. 261-276. Walters D.E., Orthoefer, F.T., DuBois,

G.E., Eds.; American Chemical Society: Washington, DC, USA.

Glaser, D., Tinti, J.-M., Nofre, C. (1996) Gustatory responses of non-

human primates to dipeptide derivatives or analogues, sweet in man.

Food Chemistry, 56 (3), pp. 313-321.

Glaser, D., van der Wel, H., Brouwer, J. N., DuBois, G. E., Hellekant, G.

(1992) Gustatory responses to the sweetener aspartame and their

phylogenetic implications. Chemical Senses, 17, pp. 325-335.

Hinde, K., Milligan, L.A. (2011) Primate milk: Proximate mechanisms

and ultimate perspectives. Evolutionary Anthropology, 20 (1), pp. 9-23.

Hockings, K.J., Anderson, J.R., Matsuzawa, T. (2010) Flexible feeding

on cultivated underground storage organs by rainforest-dwelling

chimpanzees at Bossou, West Africa. Journal of Human

Evolution, 58 (3), pp. 227-233.

18

Kinghorn, A.D., Soejarto, D.D., Inglett, G.E. (1986) Sweetening agents

of plant origin. Critical Reviews in Plant Sciences, 4 (2), pp. 79-120.

Kare, M. R., Brand, J. G. (eds.) (1986) Interaction of the Chemical

Senses with Nutrition. Academic Press, New York, pp. 247-268.

Laska, M. (1996) Taste preference thresholds for food-associated sugars

in the squirrel monkey (Saimiri sciureus). Primates, 37 (1), pp. 91-95.

Laska, M. (1997) Taste preferences for five food-associated sugars in the

squirrel monkey (Saimiri sciureus). Journal of Chemical Ecology, 23 (3),

pp. 659-672.

Laska, M., Scheuber, H.-P., Carrera Sanchez, E., Rodriguez Luna, E.

(1999) Taste difference thresholds for sucrose in two species of

nonhuman primates. American Journal of Primatology, 48 (2), pp. 153-

160.

Laska, M., Kohlmann, S., Scheuber, H.-P., Salazar, L.T.H., Luna, E.R.

(2001) Gustatory responsiveness to polycose in four species of nonhuman

primates. Journal of Chemical Ecology, 27 (10), pp. 1997-2011.

Laska, M. (2000) Gustatory responsiveness to food-associated sugars and

acids in pigtail macaques, Macaca nemestrina. Physiology and

Behavior, 70 (5), pp. 495-504.

Leonard, W. R., Robertson, M. L. (1992) Nutritional requirements and

human evolution: a bioenergetics model. American Journal of Human

Biology, 4 (2), pp. 179-195.

McLennan, M. R. (2013) Diet and feeding ecology of chimpanzees (Pan

troglodytes) in Bulindi, Uganda: foraging strategies at the forest–farm

interface. International Journal of Primatology, 34 (3), pp. 585-614.

Milton, K. (1999) Nutritional characteristics of wild primate foods: do the

diets of our closest living relatives have lessons for us?. Nutrition 15 (6),

pp. 488–498.

Morgan, D., Sanz, C. (2006) Chimpanzee feeding ecology and

comparisons with sympatric gorillas in the Goualougo Triangle, Republic

of Congo. In: Hohmann G, Robbins MM, Boesch C, editors. Feeding

ecology in apes and other primates: ecological, physiological, and

behavioural aspects. Cambridge: Cambridge Studies in Biological and

Evolution-ary Anthropology, pp, 97–122.

19

Nagy, S., Shaw, P. E. (1980) Tropical and subtropical fruits:

Composition, nutritive values, properties and uses. Westport CT: Avi

Publishing.

Niklasson, S., Sjöström, D., Amundin, M., Roth, D., Hernandez, L.T.,

Laska, M. (2017) Taste responsiveness to two steviol glycosides in three

species of nonhuman primates. Current Zoology,

DOI:10.1093/cz/zox012.

Nishi, E., Tsutsui, K., Imai, H. (2016) High maltose sensitivity of sweet

taste receptors in the Japanese macaque (Macaca fuscata). Scientific

Reports, 16 (6):39352.

Nofre, C., Tinti, J.M., Glaser, D. (1996) Evolution of the sweetness

receptor in primates. II. Gustatory responses of non-human primates to

nine compounds known to be sweet in man. Chemical Senses, 21 (6), pp.

747-762.

Potts, K.B., Watts, D.P., Wrangham, R.W. (2011) Comparative Feeding

Ecology of Two Communities of Chimpanzees (Pan troglodytes) in

Kibale National Park, Uganda. International Journal of

Primatology, 32 (3), pp. 669-690.

Remis, M.J., Dierenfeld, E.S., Mowry, C.B., Carroll, R.W. (2001)

Nutritional aspects of western lowland gorilla (Gorilla gorilla gorilla) diet

during seasons of fruit scarcity at Bai Hokou, Central African Republic.

International Journal of Primatology, 22 (5), pp. 807-836.

Richter, C.P., Campbell, K.H. (1940) Taste thresholds and taste

preferences of rats for five common sugars. Journal of Nutrition, 20, pp.

31-46.

Simmen, B., Charlot, S. (2003) A comparison of taste thresholds for

sweet and astringent-tasting compounds in great apes. Comptes Rendus -

Biologies, 326 (4), pp. 449-455.

Simmen, B., Hladik, A., Ramasiarisoa, P. L., Iaconelli, S., Hladik, C. M.

(1999a) Taste discrimination in lemurs and other primates, and the

relationships to distribution of plant allelochemicals in different habitats

of Madagascar. New Directions in Lemur Studies, pp. 201-219. In:

Rakotosamimanana, B, Rasamimanana H, Ganzhorn JU, 20.

20

Simmen, B., Josseaume, B., Atramentowicz, M. (1999b) Frugivory and

taste responses to fructose and tannic acid in a prosimian primate and a

didelphid marsupial. Journal of Chemical Ecology, 25 (2), pp. 331-346.

Simmen, B., Hladik, C.M. (1998) Sweet and Bitter Taste Discrimination

in Primates: Scaling Effects across Species. Folia

Primatologica, 69 (3), pp. 129-138.

Soejarto, D. D. (2002) Botany of Stevia and Stevia rebaudiana. In:

Kinghorn AD (ed) Stevia: The genus Stevia. Taylor & Francis, London,

pp. 18-39.

Schiffman, S.S., Booth, B.J., Losee, M.L., Pecore, S.D., Warwick, Z.S.

(1995) Bitterness of sweeteners as a function of concentration. Brain

Research Bulletin, 36 (5), pp. 505-513.

Ungar, P.S. Healy, C., Karme, A., Teaford, M., Fortelius, M. (2016)

Dental topography and diets of plathyrrhine primates. Historical Biology,

pp. 1-12.

Upreti, M., Dubois, G., Prakash, I. (2012) Synthetic study on the

relationship between structure and sweet taste properties of steviol

glycosides. Molecules, 17 (4), pp. 4186-4196.

Watts, D. P., Potts, K. B., Lwanga, J. S., Mitani, J. C. (2012) Diet of

chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National

Park, Uganda, 1. Diet composition and diversity. American Journal of

Primatology, 74 (2), pp. 114–129.

Wrangham, R.W. (1977) Feeding behaviour of chimpanzees in Gombe

National Park, Tanzania. Primate Ecology, pp. 503–538.

Figure 1, 2 and 3. Ownership by Desirée Sjöström.