Heart rate values for beaver, mink and muskrat

Camp. Biochem. Physrol. Vol. 73A. No. 2. pp. 249 to 251. 1982 Printedin Great Britain.

HEART

RATE

0300-9629/82/100249-03$03.Cil/O 0 1982 PergamonPress Ltd

VALUES AND

of Zoology

and Clinical

Studies,

BEAVER,

MINK

MUSKRAT

FREDERICK F. GILBERT*

Departments

FOR

(Received

NORMAN GOFTON

and

University

of Guelph.

5 Fe/mm-v

Guelph,

Ontario.

Canada

NIG 2Wl

1982)

Abstract-l. Implanted ECG transmitters were used to determine heart rates for several activities of beaver (Castor canadensis). mink (Mustela bon). and muskrat (Ondutru zihrthicus) under free-ranging laboratory conditions within an aquatic tank. 2. All three species exhibited bradycardia when diving but mink heart rates returned to pre-dive levels if the dive lasted > 30 sec. 3. Heart rates for all other behaviours were significantly (P < 0.05) higher than for diving and averaged about 120/min (beaver), 265/min (mink) and 240/min (muskrat). 4. Mink heart rate values were higher than would be expected based on general energetic equations if we assume heart rate to be reflective of energy costs. This was considered to be a function of this species’ fusiform body shape.

INTRODUCTION

aquatic species have diving responses which result in bradycardia (e.g. Blix, 1975; Clausen and Ersland, 1970; Drummond & Jones, 1979; Kooyman & Campbell, 1972). However, there is some contro\‘ersy regarding the biological significance of the bradycardic responses reported for a number of species, particularly birds, because of restrictive experimental conditions (Kanwisher et ul., 1981; West, 1981). The current research was designed to determine heart rates for certain behavioural responses including diving, in unrestrained individuals of three species of semi-aquatic mammals and at least partially to compare the degree of physiological adaptation to sub-surface activity and potentially to derive the relative energy costs of the behaviours observed.

Certain

MATERIALS AND METHODS

A fibre glass over wood frame aquatic tank 2 m x 2 m x 3 m with a plexiglass viewing port on each side was used for the aquatic testing. A moveable platform (106cm x 106cm) at one end supported the nest box and served as a feeding platform. Water temperature was either as directly supplied (IO~ISC) or was &l’C when cooled by a refrigeration unit (General Refrigeration, Kitchener, Ontario). Water depth was maintained at I.0 m for beaver and 0.75 m for mink and muskrat. Rocks of varying size were placed on the bottom of the tank to serve as a natural substrate. The water was changed as required (usually once every 24 h) to maintain reasonable clarity for viewing. Animals used in the research were live-trapped either by project or government personnel. They were housed in the University of Guelph’s animal holding facility and the herhivores maintained on pelleted feed and carrots (muskrat), apples (muskrat and beaver) and Popul~s tremuloidrs (beaver) and the mink on commercial mink food. Anaesthesia was induced in beaver with Rompun (Cutler *Present University.

address: Pullman.

Wildlife Biology, WA 99164. USA.

Washington

State 249

Laboratories, IUC Mississauga, Ontario) (2 mg/kg) and Rogar/STB, London. Ontario) Ketamine (Ketaset, (IO mg/kg) by intramuscular injection. Atropine (0.6 mg) was given intramusculaily to control bradycardia. Following induction each beaver was intubated with a 4.5 mm non-cuffed endotracheal tube. Anaesthesia was maintained with halothane and oxygen delivered via a Bain circuit. Anaesthesia was induced in muskrat and mink with ketamine (30 mg/kg) and acepromazine (0.05 mg) (Atravet, Ayerst Laboratories, Montreal, Quebec) by intramuscular injection and maintained with halothane and oxygen delivered via a face mask on a Bain circuit. The animals were placed in dorsal recumbency and the ventral chest prepared for surgery. After draping, a midline incision (approximately 4&5.0cm long) was made over the sternum and a subcutaneous pocket developed on the ventral abdomen. caudal to the xyphoid. The ECG transmitter was located in the pocket and the leads sutured to the chest, one on either side of the heart on the lateral chest wall. Care was taken to minimize the amount of muscle tissue between the heart and the terminal portion of the ECG leads. Transmitters operating in the 87-92 mHz range and supplied either by Wyoming Biotelemetry (Longmont, Colorado) (15.4g) or by Ken Jinde, electronics technician, College of Biological Science, University of Guelph (14. I g) were implanted. The animals were allow,ed to recover from surgery for at least I2 h and usually 24 h before being placed in the tank. Heart rate was monitored by receiving the transmitter signals on either a metal or flexible wire dipole FM antenna mounted above the water surface. and then converting these to audio or visual analogue output on a Sony Model STA30 FM/AM Tuner or a Konigsberg TR l-3 single channel ECG receiver/demodulator receiver. a Telequipment SSYA oscilloscope, and a Gould Brush 220 2-channel chart recorder. In some animals a Krohnhite Model 3550 Filter was used to help remove muscle potentials. The behaviour of the animals was observed by use of a video system consisting of two Panasonic WV-1050 TV cameras. a Sony Model 3800 videotape recorder (VTR) and a monitor (either Sony Video Motion Analyser SVM 1010 or Sanyo VM 40920). Heart rate was determined for each of the following behaviours-for beaver: resting, diving, swim/ float. and grooming; for mink: resting, diving. swimming, walking and grooming; for muskrat: resting, diving, swim-

FREDERICK

250 Table

I. Average

F.

GILBERT and NORMAN GOFTON

heart rate values (*SE)

for beaver

and mink tested in the aquatic

tank

Behavior Species

Diving

Swimming

Beaver Mink

67 f 35(23) 163 f 75 (9)

125 + 41(23) 263 k 145(11)

Number

of animals

Walking

where the same animal was represented in both behaviour categories otherwise statistical significance was determined by one-way analysis of variance (Zar. 1974). RESULTS

Heart rate values were obtained for 24 beaver (6 juvenile males, 3 juvenile females, 8 adult males and 7 adult females). No significant differences were found between age and sex groups. However, juvenile beaver had a significantly higher (P < 0.05 paired t-test) heart rate for grooming than resting, a difference not found in adult beaver. The average diving heart rate was 67 beats/min which was about 55 per cent of the heart rate for other monitored activities (Table I). Swimming/floating behaviour resulted in a significantly higher (P < 0.01 paired t-test, P < 0.05 ANOVA) heart rate than resting behaviour. Mink appeared to show bradycardia when forced to dive as the average overall heart rate for this behaviour (163 beats/min) was significantly (P < 0.005) below the next lowest rate (261 beats/min) (Table I). However, only two of these dives lasted more than 17 set (most lasted only 5-7 set) and in these two cases the heart rate nearly returned to pre-dive levels, 202 beatsimin for a 34 set dive and 257 beats/min for a 38 set dive, before the end of the dive. Considerable individual variation occurred with base-line differences between animals for given behaviours of over 200 beats/min. Muskrat showed a bradycardic response when diving (112 beats/min). Heart rates were consistently higher for all behaviours for animals tested at 0°C compared with the warmer water temperature of I l-12 C (Table 2). The heart rate for diving was about 47 per cent of the rates for the other three behaviours (P < 0.005).

2. Average

Grooming

116 f 38 (23) 261 + 146(3)

264 k 168 (5)

121 * 45(19) 274 f 119(11)

given in brackets,

ming and grooming and then averaged by individual and sex, and age category. Paired t-tests were used for data sets

Table

Resting

heart

rate values (*SE) water temperatures

DISCUSSIDK

The physiological significance of heart rate measurements showing bradycardia in animals when underwater has recently come into question (Butler & Woakes, 1975; Kanwisher et d.. 1981). The work of these authors suggests that the decrease in heart rate may have a considerable stress or fear component. A similar finding has been made for some terrestrial mammals (e.g. Smith et ul., 1981). Diving studies where restrained animals are forcibly submerged therefore may not be indicative of the actual physiological response the animals would show under natural circumstances. The work with mink would seem to support this conclusion as the animals, when diving, first showed a pronounced bradycardia but the longer they stayed under the water the closer the heart rate came to normal levels. These animals were reluctant to dive except when disturbed by an observer so the initial bradycardia was likely to be a fear response. This observation, combined with the strong evidence for ‘wet drowning’ in mink (Gilbert & Gofton, 1982) indicates this species is not aquatic but rather riparian. Biological studies of this species (e.g. Gerell, 1970; Sinclair et d.. 1974) tend to confirm this. Irving & Orr (1935) gave the resting heart rate of a single beaver of lOO/min. Clausen & Ersland (1970) gave values of 90-I 10 beatsimin for beaver above water. In both these studies the beaver were restrained but the values were comparable to the 115 beats,/min for resting behaviour found in this study. As beaver seldom voluntarily dived for longer than I min in our study our free dive values could not be compared with the earlier studies, Drummond & Jones (1979)examined bradycardia in muskrats for both restrained and unrestrained dives. The unrestrained animals had average heart rates of 54 beatsjmin shortly after diving, falling to 27 beats; min at 40sec. This was considerably lower than the

for muskrat tested of 0 C and 12-C

in the aquatic

tank

Behavior Diving

Temperature

100 * 51 (7) 139 + 71(3) 112 k 65 (10)

12 c O-C Combined ANOVA (O‘C vs 12’C) Number

F 1,8 = 4.12 F,,,,,, = 5.32

of animals

Swimming 238 + lOO(9) 263 k 55 (4) 246 f 89(13) F ,,,, = 0.86 F,, .,,,, = 4.84

given in brackets.

Resting

Grooming

225 + 66(9) 265 + 91 (4) 238 f 80(13)

224 f 81 (9) 267 + 87(4) 237 -f 89(13)

F,,,,

F ,,,,

= 3.20

= 2.93

of

Heart

rate for beaver,

112 beats/min recorded as an average for the muskrat in the current study. In addition, the average of approximately 240 beats/min recorded for other behaviours is considerably lower than the 310 and 266 beats/min for unrestrained and restrained muskrats in Drummond & Jones’s (1979) study. There was some indication from our work that animals undergoing longer dives had more pronounced bradycardia but the strict proportionality shown by Drummond & Jones (1979) did not occur. Energy expenditure in mammals has been generally correlated with heart rate (Webster, 1967; Johnson & Gessaman, 1973; Holter et al., 1976). However, the large discrepancy between individuals in heart rate witnessed in our study would probably preclude using average species’ values for determination of energy expenditure related to behaviour. It was apparent that activities such as grooming or swimming tended to cause higher heart rate values in the same animal than quiescent behaviours such as resting. Also, the increased heart rate values for muskrat studied at 0°C were not unexpected as the lower critical temperature for this species is about 10°C (McEwen et al., 1974). This means that individual energy budgets could probably be compiled using heart rate data as has been done for sheep (Webster, 1967) and deer (Mautz, 1980). Also, it appears that water activities are considerably more energy expensive for mink than muskrat and that the larger size (1100 vs 950g) of the carnivore (Hemmingsen, 1960) does not result in relatively lower heart rates for terrestrial activities either. McNab (1980) suggests that food habits of mammals directly influence basal metabolic rate and we would expect a herbivore to have a higher specific metabolic rate and hence energy expenditure than a comparatively sized carnivore. Again this was not the case and it would appear that morphometric and behavioural differences (McNeil1 and Lawton, 1970) may more than offset such general energetic considerations at least for these two species. Brown & Lasiewski (1972) showed that the metabolism of cold stressed weasels was 5SlOO percent higher than species of comparable weight. They suggested the difference was a function of the long, thin body shape of the weasel. Mink have a body configuration similar to the weasel, and Farrell & Wood (I 968) showed female ranch mink had a higher maintenance requirement than would be predicted from uork with other species of similar weight. They suggested that maintenance energy requirements could be expected to vary with body weight raised to a power close to I.0 rather than the normal 0.73 because of the higher activity levels in this species. The heart rate data from our study supported these earlier energetic observations. A~li,lo~~/ed~erllenrs--We would like to thank Dr Lavigne. Zoology Department, for reviewing the manuscript. Drs S. Thompson and C. Hunt assisted with the surgery and L. Henderson, J. Adamczewski, L. Buckland, N. McNamara, M. Cunningham and K. McCormick all assisted in the acquisition of the heart rate data. Funding was provided through a Department of Supply and Services contract to the Federal Provincial Committee for Humane Trapping.

mink and muskrat

251 REFERENCES

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