Rats (Rattus norvegicus) modulate eating speed and vigilance to optimize food

consumption: Effects of cover, circadian rhythm, food deprivation, and individual

differences

Whishaw, Dringenberg & ComeryJ. Comparative Psychology 1992

Vol 106 (4): 411-419

1st Author and PI: Ian Whishaw, Ph.D.

Canadian Center for Behavioural Neuroscience

University of Lethbridge, Canada

Listed as author on 296 papers in PubMed from 1970 to Jan 2008

T. A. Comery currently associated with Discovery Neuroscience, Princeton NJ (9 papers beginning in 1992)

H. C. Dringenbery currently at Queens University Psychology & Neuroscience, Ontario (27 papers beginning in 1992)

Introduction

• Foraging animals behave so as to avoid being eaten by predators.

Behavioral Strategies from Previous Studies

• Not eating while predators are present

• Moving food to safer location

• Hiding food for later consumption

• Rushing to move or hide food

• Threaten conspecifics who attempt theivery

• More Behavioral Observations from Previous Studies– Birds, squirrels and rats vary eating speed– Within a species, individual animals exhibit a

wide range of eating speeds• Some individuals eat fast, others eat slowly

Hypothesis: Animals optimize food intake by altering their eating speed.

• Obviously: eating slowly increases risk of predation.

• Possibly: Eating too fast causes problems with digestion.

• Therefore: Eating speed is adjusted to be fast enough but not too fast (i.e. optimized)

Hypothesis (not explicitly stated in the paper):

Animals vary their eating speed in response to environmental

conditions.

The Big Question: Why do animals (or individuals) vary

their eating speed?

“Environmental conditions” tested in this paper:

• Exposed or Protected – Prediction: Exposed animals eat faster than sheltered

animals.

• Daytime/light vs nighttime/dark (Circadian Rhythm)– Prediction: Animals eat faster in the light than in the

dark.

• Degree of Food Deprivation – (fed ad libitum, or mildly deprived to 80% or 90% of

ad libitum intake)– Prediction: Hungrier animals eat faster.

• Also monitored individual animal eating speeds.

Justification for this research

• To understand why rats change their eating speed (What is the adaptive value? What is the advantage of eating fast or slow?)

• To obtain a more complete understanding of foraging behavior in general

• To understand how central mechanisms (the brain) adjusts eating speed

My opinion: Authors did not convincingly explain the importance of this research.

My suspicion: Whishaw did not receive grant money from Natural Sciences and Engineering Research Council of Canada specifically for this project.

• Experiment 1 = 4 tests• 20-26 adult female Long Evans rats • (potential problem with estrus cycle

and variation in eating behaviors)• Group housing (4-6 rats per wire

mesh cage)• 12:12 Light/Dark cycle• Weighed & handled daily• Fed Purina Rat Chow to keep body

mass at 90% of age-matched controls (mildly food deprived rats will eat the test food pellets)

Overview of Experiment 1

Methods• Videotaped and timed rats eating food

pellets of 5 different sizes in various conditions:

• Also counted “head scans” while eating

00:00:01

Definitions

• Eating time: “Time from when rat adopted a sitting position to begin eating the food pellet to the time when the last piece of food was gone from its paws.”

• Scanning: “Any movement of the head away from its normal position during eating.” = “Vigilance”

Statistical Analysis

• Student’s t tests• Repeated measures

analysis of variance (RM ANOVA)

• Neuman-Keuls post hoc tests

Not stated in text: p<0.05 is considered significantly different

26 rats in Experiment 1 Test 120 rats in Experiment 1 Test 2-410 rats in Experiment 2

Exp 1 Test 1 ProcedureEffect of Environmental Security

• Purpose: to see if eating time and vigilance are affected by the “security” of the environment.

• Carried rats to another lab room in a bucket• Placed into test cage for ~ 3 minutes• Then videorecording begun with rat

OR

Exposed Test Covered TestInverted cage

5 pellets in random order

ExposedCovered

*

*

*

*

Rats eat small food pellets faster than large pellets.

Rats eat food pellets faster in the “exposed” condition for the 4 larger pellet sizes.

Rats make more scanning movements while eating larger pellets in the “exposed” condition.

Rats make very few scanning movements while eating in the dark “covered” condition.of

hea

d sc

ans

Exp 1 Test 1 ResultsEffects of Environmental Security

Figure 1

Additional observations fromExperiment 1 Test 1 Effects of Cover

• Scanning movements could occur at any time during eating.

• Rats continued to chew while scanning.

Exp 1 Test 2 ProcedureFamiliar and Novel Environments

• Purpose: To see if eating speed and vigilance were affected by a novel environment

• 2 tests daily for each rat for 4 days

• Measured Eating Time, Scans, & Latency

OR

New Room

Test cage

1 large pellet 1 large pellet

Cage moved to different position in room for each of the 4 tests

Home Room

Exp 1 Test 2 ResultsFamiliar and Novel Environments

Not Significantly Different

Rats take the same time to consume a large food pellet whether they’re in a familiar environment or a new environment.

Rats make more scans (are more vigilant) in a novel environment.

*Latency to begin eating:

In home room…… ~4 seconds

In novel room…… ~10 seconds

* Significantly different

Rats take significantly longer to begin eating in a novel environment.

Home roomNew room

Figure 2

Exp 1 Test 3 Serial Presentation Test

• Purpose: To see if eating speed remains constant

• Measured Eating Time for each of 5 large pellets presented sequentially

Home Room

Test cage

Eating time:

1st pellet…… 45.2 seconds

5th pellet…… 56.7 seconds

* Significantly different

Exp 1 Test 3 Results (no figure)

Rats slow down their eating speed as they consume more food.

Exp 1 Test 4 Day Vs Night Test

• Purpose: To see if eating speed is affected by light or dark (time of presentation during the photoperiod.)

• Measured Eating Time & Scans for 1 large pellet presented between 1300 and 1700 (lights on) and 2000 to 2400 (lights off)

Test cage

Eating time:

Dark period……. 38.1 seconds

Light period…… 43.7 seconds

* Significantly different

Exp 1 Test 4 Results (no figure)

Rats eat faster in the dark! An UNEXPECTED result!

OR

No difference in head scans under light or dark condition (data not shown) Another UNEXPECTED result!Home Room

Discussion of Experiment 1

• Rats (animals) eat slower in secure/covered/familiar conditions than in exposed/novel conditions.

• In other words, rats (animals) eat faster when exposed or in novel conditions where they might be subject to predators.

• Furthermore, faster eating was accompanied by more scanning (greater vigilance) but scanning didn’t slow down eating. Authors say that rats chewed while scanning and that rats must be chewing faster under exposed or novel conditions.

• Rats eating behavior differs from birds: bird stop eating to scan for predators (Lima, 1987)

• Authors propose that rats have an advantage over birds.

Discussion of Experiment 1 continued• Eating time was shorter in the DARK home cage than

the LIGHTED home cage.• An UNEXPECTED result• Prediction: Rats are more “secure” in the dark and

should thus eat slower.• Previous studies: Rats normally do most of their

eating in the dark. • Could it be that they were more hungry in the dark? • Could it be that rats are more “sleepy” in the light? • Could it be that rats aren’t as hungry in the light

(having eaten to fullness during the dark.)• Authors don’t mention that in the wild, rats may be

eaten by owls at night!

Minor Concerns with Exp 1

• Most tests were videotaped for analysis.• Tests conducted under dark conditions were not

videotaped. Observers used stopwatches. • Light intensity could have been measured under

“dark conditions” where door was left partially open “6 m away… and behind two other racks of cages.”

• Some variation due to using female rats whose feeding may change through the estrus cycle.

Experiment 2

• Purpose: “To examine whether eating time does vary as a function of the rat’s circadian rhythm.”

Important changes for Experiment 2

• 10 adult MALE Long Evans rats (no issue of estrus cycle) HOUSED INDIVIDUALLY in cages

• Rats handled daily and given a large food pellet to overcome neophobicity for 2 weeks preceding experiments

• Maintained on Purina Rat Chow in 4 states• Ad libitum (no food deprivation)• 10% Body weight reduction (90% of ad libitum food)• 20% Body weight reduction (80% of ad libitum food)• Then restored to Free Feeding (ad libitum)

• All timing with a stopwatch• Data collected over 6 days under each of 4 states

Given 1 large food pellet each hour during the 24 hour cycle.

Time points chosen randomly except a rat never given food pellets on two successive 1 hr intervals.

No more than 4 pellets per day.

Thus, 6 days required to test during each hour of the 24 hr cycle.

WK 0 WK 4WK 2 WK 6 WK 8 WK 14

0 Depr Free Feeding

80% Depr

90% Depr

Free Feeding

Ambient Lighting Change Test

Reminder:

• Purpose of Experiment 2: “To examine whether eating time varies as a function of the rat’s circadian rhythm.”

Time (hours)

Tim

e (s

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nd

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at 1

lar

ge

pel

let

Figure 3Text: Lights on at 0800, off at 2000

Figure: Lights on at 0700, off at 1900

Result from Figure 3

• Eating speed depends on the hour of the feeding.• Rats eat faster in the dark period (confirmation of

unexpected results in Experiment 1.)• Food deprivation increases the speed of eating in

proportion to the degree of deprivation.• Circadian variations persisted throughout the deprivation

period.• When deprived rats were allowed to eat ad libitum, they

continued to eat faster than originally even 6 weeks after they had been freely feeding on rat chow. Thus, food deprivation has long-lasting effects on feeding behavior.

Each data point on Figure 3 represents the average time to eat one large pellet for 10 rats. There is no indication of individual variation (which could have been represented by error bars on each data point.

Next question: How much variation is there for eating times for individual rats?

Figure 4 Average time to eat 1 large pellet for each of the 10 rats across 24 hours on day 1

Should plot Standard Deviations too.

Additional Observations

• “…Eating times for each rat (across all 5 tests) were highly correlated” which means that “fast eaters” tend to eat fast in every condition and “slow eaters” consistently eat slowly in every condition.

Purpose: to determine how eating speed is affected by a change in the lighting conditions

WK 0 WK 4WK 2 WK 6 WK 8 WK 14

0 Depr Free Feeding

80% Depr

90% Depr

Free Feeding

Ambient Lighting Change Test

Experiment 2 Test 2Effects of Ambient Lighting Changes

During the Circadian Cycle

Protocol for Ambient Lighting Changes

Figure 5Rats eat slower during the light period (Confirms Experiment 1.)

Rats do not change eating speed when the lights are turned on during the dark photoperiod.

Turning the lights on during the dark portion of the photoperiod did not result in a change in eating time.

Discussion of Experiment 2

• Previous studies: rats eat mostly during the dark period.

• Variations in eating time are related to circadian rhythms, so eating speed depends on when the test is given.

• Previous study (Bolles 1967): food deprivation leads to shorter latency to eat.

• Didn’t measure latency in this study.

Discussion of Experiment 2 Continued

• Deprivation leads to long-term (maybe permanent?) reduction in eating time.

• Previous studies: food deprivation subsequently potentiates food consumption and hoarding (Marx, 1950 & 1952)

• Authors suggest a persistent change in motivation after deprivation implying that some brain mechanisms are “permanently” altered.

• Eating times for individual rats varied but for a given rat tended to be consistent. Authors suggest that this variation may be genetic and yet eating behaviors can be clearly altered by environmental conditions.

General Discussion

• Rats alter their eating speed in response to a variety of conditions:– Environment– Food deprivation experience– Circadian rhythm

• Rats normally forage & feed in the dark…. All conditions which favor faster eating.

General Discussion continued

• Rats normally forage & feed in the dark…. All conditions which favor faster eating.

• Eating speeds may correlate with – Suceptibility to addition– Eating disorders

• So maybe eating time could be an easy-to-measure index of vulnerability.

“Environmental conditions” tested in this paper:

• Exposed or Protected – Prediction: Exposed animals eat faster than sheltered

animals.

• Daytime/light vs nighttime/dark (Circadian Rhythm)

– Prediction: Animals eat faster in the light than in the dark.

• Degree of Food Deprivation – (fed ad libitum, or mildly deprived to 80% or 90% of

ad libitum intake)– Prediction: Hungrier animals eat faster.

Justification for this research

• To understand why rats change their eating speed (What is the adaptive value? What is the advantage of eating fast or slow?)

• To obtain a more complete understanding of foraging behavior in general

• To understand how central mechanisms (the brain) adjusts eating speed

My opinion: Authors did not convincingly explain the importance of this research.

My suspicion: Whishaw did not receive grant money from Natural Sciences and Engineering Research Council of Canada specifically for this project.

Relevance to our Research

• Eating time for each food unit

• Does eating time slow during a meal

• Latency

• Chronic vs acute deprivation

• Our tests during the light period