Acta Chiropterologica, 8(2): 509-521,2006 "L ISSN 1508-1109 © Museum and Institute of Zoology PAS

Age related variation in the energy costs of torpor in Daubenton's bat: effects on fat accumulation prior to hibernation

TOMASZ KOKUREWICZI and JOHN R. SPEAKMAN2

:Agricultural University of Wroclaw, Department ofZoology and Ecology, Kozuchowska 5b, 51-631 Wroclaw. Poland; E-mail: kokllr@ozi.ar.wroc.pl

2Aberdeen Centre for Energy Regulation and Obesity, Department ofZoology, University ofAberdeen, Aberdeen AB24 2TZ, Scotland, United Kingdom and Rowett Research Institute, Bucksburn, Aberdeen,

AB21 2SB, Scotland, United Kingdom

Insectivorous bats in their first year of life generally deposit less fat prior to hibernation than older bats of the same species. In the present study we explored the energy expenditures of first-year (sub-adult) and older than one year (adult) Daubentons bats (Myotis daubentonii) during torpor and their patterns of roost site selection and fat accumulation in an artificial roost site, removing from the equation the effects of differences in aerial foraging behaviour by feeding them on non-aerial prey (mealworms). Sub-adult bats had oxygen consumption during torpor that averaged 2.75 x greater than adult individuals. In an artificial enclosure in which bats could fly freely and choose whether to roost inside or outside of a hollow brick, sub-adults gained body mass at a significantly lower rate (67.8 mg x day') than adults (l00.3 mg x day"), despite being fed non-aerial prey .mealworms). The difference in rates of mass accumulation (32,5 mg per day) far exceeded the theoretical influence of different metabolic rates (7 mg x day1) in torpor. Despite lower rates of mass gain in this artificial situation, sub-adults ultimately achieved the same mass accumulation as adults because they continued to accumulate fat for a longer period, an option that might be unavailable to them in the wild as feeding conditions deteriorate. The rate of body mass accumulation was positively correlated with the time spent utilising the brick roost site, but utilisation of this site did not differ significantly between age classes. These data support the hypothesis that differences in the accumulation of fat between age classes may reflect in part differences in expenditure as well as differences in food intake, but the contribution of differences in metabolism during torpor are relatively small.

Key words: Myotis daubentonii, oxygen consumption, hibernation torpor, fat accumulation

INTRODUCTION Thomas, 2003) and birds (Bryant, 1997). At the same time as they face increased en­

During winter in the temperate and arc­ ergy demands, however, insectivorous spe­

tic zones ambient temperatures decline on cies often experience a reduction in food

average by 20-40°C below the peak levels supply because their insect prey become

observed in summer (Oliver and Fairchild, less active at lower temperatures. In re­

1984). This reduction in temperature results sponse to the potential imbalance of ener­

in a large increase in the energy demands gy intake and energy expenditure, small

that are experienced by small endothermic insectivorous mammals, such as bats,

mammals (Speakman, 2000; Speakman and have evolved a number of strategies which

510 T. Kokurewicz and J. R. Speakman

include seasonal acclimatization, migra­tion (Davis and Hitchcock, 1965; Strelkov, 1969; Petit, 1998) and hibernation (Hall, 1832).

During hibernation bats remain in tor­por for protracted periods, normally last­ing 4-20 days, although sometimes up to 70 days (Twente, 1955; Twente and Brack, 1985; Twente et al., 1985). These periods of torpor are interrupted by periods of arousal, during which time body temperatures return to euthermic levels for several hours. Total energy utilization during hibernation is thus a combination of the expenditure during tor­por and that during periods of arousal (see Humphries et al., 2002). It has been esti­mated in several hibernators that 70-80% of the total costs of hibernation can be attrib­uted to the periods of arousal (Kayser, 1965; Thomas et al., 1990b; Thomas, 1993), with the balance being expended during torpor. Bats store fat prior to entering hiberna­tion (Krzanowski, 1961; Ransome, 1968; Ewing et al., 1970; Kunz et al., 1998) and gradually utilize these reserves throughout the hibernal period. In addition, in some regions, the winter climate may be suffi­ciently mild that insects occasionally be­come active at sufficient densities that bats can exploit them. In these areas, bats are regularly observed feeding on milder evenings throughout the hibernation peri­od (White, 1789; Ransome, 1968; Avery, 1985). The exact function of these flights remains a matter of debate. Although some authors suggest that flights serve to sup­plement the energy supplied by fat reserves (e.g., Burbank and Young, 1934; Krza­nowski, 1961; Stebbings, 1966; Ransome, 1968; Roer, 1969; Avery, 1985; Brigham, 1987; Whitaker et al., 1997), others, suggest

. that the feeding during flights serves only to cover the costs of the flights, which are consequent1y energetically neutral, but allow the bats to replenish depleted water reserves (Speakman and Racey, 1989;

Thomas et aZ., 1990a; Thomas and Geiser, 1997).

In theory, the balance of energy expendi­ture (torpor plus arousals) and the supply of energy from the fat reserves (perhaps supplemented by winter feeding) defines whether a bat will potentially survive the winter or will starve. It has previously been established, from banding studies, that little brown bats (Myotis lucifugusi in their first year of life have significantly greater mortality than bats older than one year (Davis and Hitchcock, 1965) and that this mortality is primarily due to prema­ture depletion of their stored reserves, rather than from predation or parasite infec­tion. Even if increased predation upon hi­bernating sub-adult bats was the proximate cause of death it was probably caused by prior depletion of their fat reserves (Ko­kurewicz, 2004). The precise reasons why first-year bats are more likely to deplete their reserves and die are uncertain. How­ever, an important aspect of this problem is probably the fact that first-year bats generally enter hibernation with lower fat reserves than older bats (Ransome, 1968; Ewing et al., 1970; Kokurewicz, 1990, 2004; Jones and Kokurewicz, 1994; Kunz et al., 1998).

The reasons why bats in their first year of life have lower fat reserves at the onset of hibernation are unclear. Sub-adult bats may be inefficient at echolocation (Konstanti­nov, 1989) and also at manoeuvring during flight (Hughes et al., 1995), which may make them less capable at capturing insects (Jones and Ransome, 1993; Kunz, 1974). However these disadvantages may be offset to some extent by their favourable wing­loading which reduces energy demand of flight (Hamilton and Barclay, 1998; Hoying and Kunz, 1998; Stem and Kunz, 1998; McLean and Speakman, 2000). Alternati­vely intra-specific competition between first-year and older bats over diminishing

food S1.:~

intake ::­An :::.

bats apt-~

sites ar ; muse c: 1

ski, 19'= If sub-a ; por eitl.e: priate r: ­mcapacr tially ar,

One fae:.­isation . .­tive torr need fe: growth. :­ally bec.: growth .. are 6-1 skov, I G,

sub-adu.: the time :-' to depos: nation. '., od whe.: ­Growth :,:. gy expe:'~

ture dur.:, bility orr as are ot c;

Our .::. that firs:­gy dema-, and that :. ences in ­test thi s :-. penditur, rect cak ; the pane:­adults ar..;

enclosu-­which \\ location __ tured. \\e :: fat accu:r..

511 Age related variation in the energy costs oftorpor

.ood supplies may restrict the potential food ntake in the younger bats.

An important aspect of fat deposition by ::>ats appears to be the selection of cool roost sites and the utilisation of torpor to min­mise daily energy expenditure (Krzanow­ski, 1961; Speakman and Rowland, 1999). .f sub-adult bats are less able to utilise tor­oor either because of problems with appro­oriate roost site selection or a physiological .ncapacity to utilise torpor, this could poten­.ially affect their ability to deposit fat stores. One factor which may compromise the util­.sation of torpor in sub-adults is that effec­:ive torpor may be incompatible with the need for the animals to complete somatic growth. In vespertilionid bats, young gener­ally become volant at 3--4 weeks of age, but growth is generally not completed until they :lre 6-10 weeks old (Nyholm, 1965; Kur­skov, 1981; Burnett and Kunz 1982). Thus sub-adult bats may continue to grow during the time period that they are also attempting to deposit fat stores in preparation for hiber­nation. Moreover, this is also the time peri­od when they undergo moult (Kunz, 1974). Growth and moult may elevate resting ener­gy expenditures and the levels of expendi­ture during torpor, thus impairing the capa­bility ofthe bats to deposit as large fat stores as are observed in older bats.

Our a priori hypothesis, therefore, was that first-year bats would have greater ener­gy demands during torpor than older bats and that this difference might explain differ­ences in fat accumulation prior to winter. To test this hypothesis we measured energy ex­penditure of sub-adult and adult using indi­rect calorimetry. In addition, we followed the patterns of fat accumulation of sub­adults and adults maintained in an outdoor enclosure and fed on non-volant prey, which would not require sophisticated echo­location and f1ight techniques to be cap­tured. We predicted that if the differences in fat accumulation reflected only ontogenetic

processes linked to the capture of volant prey, the differences in fat accumulation would be abolished when bats were fed non-volant prey. However, if differences in fat accumulation reflected patterns of ener­gy expenditure as well as energy intake, then differences between age groups would be independent of food type.

As far as we are aware, this is the first study to directly address the issue of age­related variation in the metabolic rate of tor­por bats and its consequences for fat accu­mulation. Previous studies have been hin­dered by the lack of suitable criteria for es­tablishing the ages of bats after growth of the major wing bones, and fusion of the epiphyses is complete (Anthony, 1988). Daubenton's bat (Mvotis daubentoniii is a small vespertilionid bat that ranges from Portugal and Ireland in the west to eastern China, Korea and Japan in the east (Bogda­nowicz, 1994). Like other temperate-zone vespertilionids growth of the forearm and fusion of the epiphyses in Daubenton's bat is complete within 3--4 weeks of birth (Ny­holm, 1965; Kurskov, 1981). However, first-year Daubenton's bats retain a small black spot on their lower lip until they are one year old (Richardson, 1994; Geiger et al., 1996). It is possible, therefore, to distin­guish bats entering their first hibernation from older bats (second or later hibernation) without having to band individuals in the previous summer. This unique ageing crite­rion opened up the opportunity to compare the energy demands during hibernation in natural conditions (Kokurewicz, 1999, 2004) and patterns of fat accumulation of first-year and older bats.

MATERIALS AND METHODS

Origin and Husbandry ofBats

Because of the endangered status of bats in the UK we were only able to obtain a permit to study a total of 18 individuals. We selected a sub­sample of these (6 older bats and 6 first-years) for

,

512 T. Kokurewicz and J. R. Speakman

5l

measurements of energy demands in torpor. Bats were taken into captivity under licences from Scottish Natural Heritage (SRAB: 04:93) and Countryside Council for Wales (BSR: 1:93). According to the li­cence conditions, all animals were released after the experiments at the places of capture. All the bats were captured during late September and early October. Two individuals, one adult and one sub-adult male, were captured in north-east Scotland (Highland) (57°03 'N, 2°54'W), eight individuals (4 adult ¥ ¥, 1 sub-adult ¥ and 3 adult 00) in Central Scotland (56°23 'N, 4°lTW) and eight individuals were cap­tured at two locations in Wales (51 °35'N, 2°59'W: 1 adult 0, 2 sub-adult ¥ ¥ and 4 sub-adult 0 0 and at

038'N, 2°39'W: I adult 0). Following capture, bats were banded with num­

bered metal forearm rings (Mammal Society) and im­mediately trained to feed on mealworms (Tenebrio molitor). Bats were kept in natural photoperiod and temperature in an enclosure measuring 5 x 3 x 2 m on the roof of the Zoology Department. University of Aberdeen (57°N). They were provided with a clay brick with three holes inside, placed in the enclosure to roost in during the day. Once the bats had learned to feed, they were provided with free access to both water and mealworms placed in several different con­tainers located on the f100r around the enclosure.

Torpid Metabolic Rates

Oxygen consumption was measured using an open-flow respirometry system as previously de­scribed for hibernal bats (Speakman et al., 1991: Speakman and Rowland, 1999). Before being placed in the respirometry chamber (volume 802 ml) bats were weighed to the nearest 0.01 g using a top-pan balance (Sartorius Ltd). A grooved wooden wall lo­cated on the inside of the respirometry chamber al­lowed the bats to hang naturally. A container of drink­ing water was provided inside the chamber. The respirometry chamber was housed in the dark at 5°C in a temperature controlled incubator (Matsui). Ambient temperature was recorded every 60 s by thermistor probes linked to a data logger (Grant Instruments, Squirrel) and oscillated with an ampli­tude of ± 1°C and a period of about 70 min. Meas­urements were made at a single ambient temperature since our previous studies ofthis species had indicat­ed a broad range of independence of torpid metabo­lism from ambient temperature between 2 and 9°C (Speakman et al., 1991).

Due to the small amount of oxygen used by torpid bats (Hock, 1951) air flow in the chamber was slow (30-80 ml x min") and controlled by a flow con­troller (R2-A: Applied Electrochemistry Inc.). Silica

gel was used to dry the incurrent and excurrent air­f1ows. Oxygen content of the excurrent air was meas­ured in the first channel of oxygen analyser (Applied Electrochemistry Inc., S-3A). The second channel measured the oxygen content of air pumped from the room to provide a continuous reference level for am­bient %02 , The difference in oxygen concentration between two channels was logged at 40 ms intervals. Records were averaged over periods of 1 min and oxygen consumption (ml O2 min') was calculated from the difference in oxygen content multiplied by the upstream airflow to minimise error in the estimat­ed energy expenditure (Koteja, 1996; Speakman, 2000).

Measurements of oxygen consumption of single bats usually lasted 24 h. At the beginning of each measurement bats were active and consumed large amounts of oxygen, but after 1-2 hours they became torpid. These patterns did not differ between adult and sub-adult individuals and all bats entered torpor for a period of around 20-22 hours. Oxygen consumption in torpid bats was calculated over the period with the lowest 10 min of oxygen uptake. A total of approxi­mately 1300 hours of respirometry was performed on 12 individuals. Each individual was measured on five separate occasions and the data reported here are means for each individual. Timings of the measure­ments over the mass accumulation period were ran­domised with respect to age of the individuals. We converted oxygen consumption to energy expenditure by assuming that the bats predominantly mobilised fat (Morris et al., 1994) with an energy equivalence of 19.66 J ml O2•1.

Roost Preference

A clay brick with three holes in it was placed in the enclosure and provided a roost site that the bats coul d occupy during the day. Ambient temperature inside the brick was recorded at 3-minute intervals by a thermistor probe linked to a data logger (Grant Instruments, Squirrel) placed in one of the holes. blocked from both sides and consequently inaccessi­ble to the bats. A second probe was attached to the wall of the enclosure to measure the external ambient temperature. Bats generally roosted inside the brick or outside on the wall s of the enclosure. Since bats emerged to feed each night they made a choice each night about where they roosted the fol­lowing day. To investigate the age related roost pref­erences we recorded the locations of bats each day. and summing the information across 18 days of ob­servation.

Ambient temperatures were measured continu­ously for 18 days (12-29 October 1993) with shon

breaks (ca. : the compute- :C~ .

18 days duri-: i :,' : and 16:00 h. :- ~ . enclosure we~e , .

ring number" On three:"':

days of expe::::' : ­tures of the c:c:' (±O.loC, tip c:.:-:-: 1.5 em). Dur.r, .. individual \\:0, '::­

suIted in unec..': handling and :':: -,

60 s. Compa-i- -: recorded amt-:::: identify the t::e~

that were mea,.:::: ent temperarr-,

Body Mass ,c: ...

The samp.e : to water drop, -. pour on the :c..:: and one adult ~.:.

Observations :':"': of November. B.::. vals. All mass .:.: 14:00 and 16:1 balance (Sarto-: .. '- : recording the 2 minutes.

Statistical Ar ..

Normalitv Shapiro-Wilks : ted to the reiat: length and oXYi=-::: . ergy expenditure pendent differe::.. : groups analysis.:.: mass as covaria.e was used for .:.'

TABLE 1. The erc,-: .:

at 5°C; x ± SD. '::: :

Fea:.:'

Forearm length. ::. -:- . Body mass (g) Oxygen consum:::. ­Energy expendi~~::e

513Age related variation in the energy costs oftorpor

breaks (ca. 1 hour per day) for downloading data to the computer and resetting memory of the logger. On 18 days during the observation period, between 14:00 and 16:00 h, the roosting locations of all bats in the enclosure were checked by noting their positions and ring numbers.

On three of these occasions, on 1st, 14th and 17th days of experiment we measured the body tempera­tures ofthe bats in = 12) using a thin thermistor probe (~0.1 °C, tip diameter 0.5 mm, inserted to a depth of 1.5 em). During the 14th day observations one adult individual was taken to the respirometry, which re­sulted in unequal sample size. For each bat the time of handling and taking measurements ofTb never exceed 60 s. Comparisons between Tb and simultaneously recorded ambient temperatures (TJ were used to identity the thermoregulatory status of bats. All bats that were measured were torpid (within 5°C of ambi­ent temperature).

Body Mass Accumulation

The sample was composed of 18 individuals. Due to water drops resulting from condensation of va­pour on the fur, records concerning one sub-adult and one adult bat were excluded from the analysis. Observations lasted from early October to the end of November. Bats were weighed at three day inter­vals. All mass determinations were made between 14:00 and 16:00 h to the nearest 0.01 g using top-pan balance (Sartorius Ltd). For each bat the time spent recording the ring number and weighing lasted ca. 2 minutes.

Statistical Analysis

N ormalily of distribution was tested by use of Shapiro-Wille's W-test. Regression equations were fit­ted to the relationships between body mass, forearm length and oxygen consumption (ml O2 rnin") and en­ergy expenditure (mW). To investigate the mass inde­pendent difference in metabolic rate between age groups analysis of covariances (ANCOVA) with body mass as covariate was applied. Mann-Whitney U-test was used for comparisons between medians of

temperatures in the enclosure and in the brick, while z-test was applied for evaluating the age related roost preferences. Rates of body mass accumulation of in­dividuals from the different age classes were com­pared using measurements of body mass at 3 day in­tervals in a repeated measures ANCOVA. Overall gra­dients of accumulation for each individual were aver­aged over the accumulation period and differences be­tween the age classes compared using ANOVA.

RESULTS

Metabolic Rate During Torpor

Forearm length and body mass (n = 12) were normally distributed (Shapiro-Wilks W-test; W = 0.9, P > 0.05 and W = 0.9, P > 0.05, respectively). The average fore­arm lengths and body masses of adult bats were slightly greater than the sub-adult bats (Table 1) but the differences were not statis­tically significant (F = 3.70, dj = 10, P> 0.05 and F = 2.47, df = 10, P > 0.05, respectively). There was no significant rela­tionship between energy expenditure dur­ing torpor and body mass neither in adult (r = -0.57, F = 1.91, d.f. = 4, P > 0.05) nor in sub-adult individuals (r = 0.53, F = 1.56, d.f. = 4, P > 0.05). Mass independent dif­ferences in oxygen consumption and energy expenditure between compared age groups were significant at P < 0.01 (Al\COVA: age effect F = 18.81, d.f ~ 10; see Fig. 1). In average first-year bats had energy expen­diture 2.65x greater than adult individuals (Table 1).

Roost Preference

The median temperature in the enclo­sure was 8.5°C (SD = 2.17, min-max =

TABLE 1. The effects of age on energy expenditure and oxygen consumption during torpor in 12 ,\1. daubentonii at 5°C; x ± SD, and sample sizes (in parentheses) are shown

Feature Adults (6) Sub-adults (6) All individuals (12)

Forearm length (rnrn) 37.6 ± 1.29 36.4 ± 0.89 37.0± 1.24 Body mass (g) 8.52 ± 1.025 7.54±1.146 8.03 ± 1.157 Oxygen consumption (m! min") 0.008 ± 0.0061 0.022 ± 0.0160 Energy expenditure (mW) 2.78 ± 1.996 7.37 ± 5.230

----'----------------------~----------

• •

••

SI4 T. Kokurewicz and J. R. Speakman

0.045 --,-------------------------, change :!

c 'E6' 0.035

E •c a 0.025 ~ E :::J o(f) c8 0.015 c Q) OJ

~ •• o o 0.005 ~o

6.5 7.5 8.5 9.5

Body mass (g)

similar ", average. ing in :­found ::-. bats SP~-"I

sub-adli> = 5.21. :-:-: x = F­n = 7J. ::­cally Si;l P > O.cJ~

greater :-'j

decrease .;

Body Fe: FIG. 1. Oxygen consumption during torpor at SoC in pre-hibernal M daubentonii as a function of body mass Adults (individuals older than one year) are shown as open circles and sub-adults (individuals in their first yea; Three 1

of life) as solid circles of expe-.;: perature"

0.36-16.6, n = 8,103) and in the brick was Whitney U-test). The temperature recordec sions (c.=.­8.8°C (SD = 2.08, min-max = 0.36-16.6, inside the brick was slightly higher than tha: slightly :-. ; n = 8103). This difference was small but recorded outside during the night (Fig. 2!: adult b:::c statistically significant at P < 0.001 (Mann- however, the major pattern of temperature c = 9.S. "

~" = 10.'

12 -,--------------------------, ;1 = 6. re',,:";

11 ! ~ it e! 10

i !:::J

10 Qj

9 !~0..

E 2 C ~222 • ~~ Q)

:0 81~~~~~ ~. ~ ~~ E « 7

6+------,------,---,--------,--------,--------1

o 4 8 12 16 20 24 Time of day (hours)

FIG. 2. The diurnal pattern of variation in ambient temperature inside a hollow clay brick Ce) and outside ;: brick but within an enclosure CO) in which M daubentonii were maintained during the pre-hibernation per' _ Bats could roost either inside the brick or outside in the main enclosure. The temperatures at each time _­averaged across 18 days of measurements. At some times the points overlap. The probe in the brick"

isolated from the roosting bats and was not affected by their body temperatures

----- --------

• •

• •

515 Age related variation in the energy costs of torpor

change throughout the day was remarkably similar in the alternative roosting sites. On average, half the bats were found roost­ing in the brick and the remainder were found in the enclosure. In average adult bats spent more days inside the brick than sub-adult individuals (adults, x = 1J.2, SD == 5.21, min-max = 1-17, n = 9; sub-adults, X: = 10.43, SD = 4.12, min-max = 4- 16,

71 == 7), but the difference was not statisti­cally significant (z-test, t = 0.33, df = 14, P> 0.05). Bats used the brick as a roost in greater numbers as the ambient temperature decreased (Fig. 3).

Body Temperature

Three times, on 1st, 14th and 17th days of experiment we measured the body tem­peratures of the bats. In two of these occa­sions (day 1st and 17th) adult bats had slightly higher body temperatures than sub­adult bats (adults, x = 10.2, SD = 0.32; x = 9.8, SD = 1.02, n = 6; and sub-adults, x = 10.0, SD = 0.29; x = 9.7, SD = 0.72, 17 = 6, respectively), while in one (day 14th)

it was higher in sub-adult bats (sub-adults, x = 10.2, SD = 1.16, n = 6; and adults, x = 9.8, SD = 0.59, 11 = 5). In all three cases the differences were not statistically significant (Mann-Whitney U-test, Z day l st = -1.39, Z day 14th = -0.55 and Z day 17th = -0.24, P> 0.05).

Body Mass Accumulation

Adults gained body mass at a significant­ly greater rate than sub-adults (ANCOVA: gradient effect F~ 4.68, df = 14, P < 0.05). The gradients of the respective patterns of mass accumulation revealed that adults gained mass an average of 100.3 mg (SD = 13.3) per day while the sub-adults achieved a rate only 67% of this at 67.8 mg (SD =

7.9) per day. The difference in the rate of mass accumulation was thus 32.5 mg per day. On average adults accumulated mass for 18 days, and thus deposited an average of 1.80 g while the sub-adults continued to accumulate mass for another five days and ended up with a mass accumulation of 1.56 g, which did not differ significantly

18

:t2 16 ..Q ~14 (3

.s 12 OJ c ~ 10 o o'- 8 2 (1)

::: 6 o • ~ 4 •E ~ 2

o -'- ---r--, ---,-----,- ----,------- ­

4 5 6 7 8 9 10 11 12 Ambient temperature ee)

FIG. 3. Relationship between average daily temperature (Ta) and number of 111. daubentonii in artificial shelter. The curve shows the least squares fit regression y = 21.37 - 1.303., (r = -0.50, F = 4.75, d.]. = 14.

P < 0.05)

T. Kokurewicz and J. R. Speakman

T-\BLE 2, The effects of age on mass dynamics in captive M daubentonii during the pre-hibernal perio.. The mean ± SD, and sample sizes (in parentheses) are shown. Significant differences at P < 0,05 between tl.. age classes are indicated by *

Features Adults (9) Sub-adults (7) All individuals (161

Forearm length (mm) Initial body mass (g)* Final body mass (g)* Daily accumulation (mg day:')" Number of days of accumulation Mass increase (g)

37,5 ± 1.28 7.71 ± 0.627 9.51 ± 0.891

100.3 ± 13.3 17.8±6.18 1.80 ± 0.24

36.3 ± 0.88 6.68 ± 0.421 8.24 ± 0.783 67.8 ± 7.9 22.7 ± 5.09 1.56±0.18

37.0± 1.26 7.28 ± 0.748 9.35 ± 1,073 76.9 ± 7.2 19.8±6.12 1.52±0.16

from that in the older bats (Table 2). Across n = 12, equivalent to a consumption o: all individuals, there was a significant posi­ 0.115 ml 02 glh') and was similar to the tive relationship between the increase in average levels recorded previously for fOL: body mass and the number of days the in­ British vespertilionid bats measured me: dividual bats were observed roosting in the ambient temperature range 0-10= C the brick roost (r = 0.51, F= 4.9, d.f = 14, (x = 6.17 mW, SD = 3.39, n = 182 ­P < 0.05; see Fig. 4). Speakman et al., 1991). In contrast, in th;

little brown bat (Myotis lucifugusy, a Nortr

DISCUSSION American congeneric species (Findley ar.; Jones, 1967; Findley, 1970) metabolic ra.e

Metabolic Rates ofTorpid Bats reached a minimum values of 0.03 ml 0 glh' at 2°C (Hock, 1951) and 0.02 ml 0:

Mean energy expenditure during torpor glh' at SoC (Thomas et al., 1990b). Littl; in Daubenton's bat at an ambient tempera­ brown bats have almost identical bod­ture of SoC was 5.08 mW (SD = 4.47, masses to Daubentons bats, yet the average

o4 =­

-9 ~

U)

Kl 3 E • 0 >. 0"0 0

~2 <lJ U)

co <lJ

g 1

• 0

0 5 10 15 20 Days spent in brick shelter

F1G. 4. Relationship between the mass increase and the number of days that individual M daubentonii spen: the brick shelter. The curve shows the least squares fit regression: y = 1.14 + 0.091x (r = 0.51, F = .: ,

d.f. = 14, P < 0.05). Adults (individuals older than one year) are shown as open circles (0) and sub-ad; ' (individuals in their first year of life) as solid circles (e)

517 Age related variation in the energy costs of torpor

oxygen consumption in M daubentonii recorded in our study was nearly six times higher than observed in the same tempera­ture in M lucifugus (Thomas et al., 1990b). Even restricting the analysis to include only adult bats resulted in a mean metabolic rate three times greater. than reported in -'v1. lucifugus. Henshaw (1970) found that BMR in M lucifugus was higher in Novem­ber than in March. Our measurements were taken in October and November, which might be related to the seasonal effect re­ported by Henshaw (1970). However, our previous estimates of energy expenditure in torpid M daubentonii (Speakman et al. 1991) during mid-winter were also higher than recorded in M lucifugus from North America (Hock, 1951; Thomas et al., 1990b). Several factors could explain these differences, however, one potentially signif­icant factor influencing the higher metabol­ic rates during torpor in M daubentonii may be the maritime climate of the UK, where bats arouse periodically from torpor and feed. To retain sensitivity to variations in environmental stimuli, bats may need to re­main at higher metabolic rates than are rou­tinely observed in M lucifugus which gen­erally roosts in localities where winter feed­ing is not possible.

Recent investigations on torpor patterns of hibernating eastern chipmunks (Tamias striatus - Humphries et al., 2003; Munro et al., 2005) evidenced that the maintaining high body temperature is compatible with the size of fat reserves and the ability to re­fill them through feeding. It is possible that higher metabolic rates of sub-adult than adult Daubenton's bats might be a ther­moregulatory strategy correlated with the differences in their fat reserves and the pos­sibility of winter feeding.

Effects ofAge

First-year Daubenton's bats had torpid metabolic rates that were on average 2.75

times higher than older bats measured at the same temperatures. Body masses of adults were greater than the sub-adults by about 15%, probably due to the greater stores of fat deposited by adults prior to hibernation. Although fat is typically considered to be metabolically inert, these differences in fat reserves cannot explain the reported differ­ences in metabolic rates. There are at least two other possible explanations for the higher metabolic rates of the sub-adult bats. First these bats were possibly still growing during this period and sustaining the meta­bolic apparatus necessary for growth may be incompatible with a reduced metabolic rate comparable to that observed in older bats that had completed growth. Alternati­vely the ability to regulate body temperature at low levels may be limited in physiologi­cally immature first-year individuals. Sub­adult bats in the respirometers may have maintained their body temperatures at high­er levels above ambient than adult individu­als, leading to their greater metabolic rates. Unfortunately, we could not test this hy­pothesis directly since it was not possible to remove the animals from the respiro­metry chambers fast enough to measure their undisturbed body temperatures. In the enclosure there were no significant differ­ences in body temperatures between adults and sub-adults. However, since these body temperatures were measured in late after­noon at the high point of the diurnal ambi­ent temperature cycle (see Fig. 2) they prob­ably do not reflect the capacities of individ­uals to reduce their body temperatures to minimal levels.

Using average values that we measured for metabolic rate during torpor we mod­elled the impact of such differences on the potential extent offat accumulation by adult and sub-adult bats, assuming that they had equal resting metabolic rates, flight meta­bolic rates, food intakes and times spent in torpor. If both adults and sub-adults spent

518 T. Kokurewicz and J. R. Speakman

16 hrs of each day in torpor the difference in their expenditures over this period would be equivalent to 0.6 kJ. Assuming an energy density for bat fat of 39.6 kJ/g (Ewing et al., 1970), this difference in expenditure would be equivalent to the daily deposition of approximately 7 mg of fat. Clearly, under some conditions, the ability of adult bats to reduce their metabolic rates during torpor to lower levels than sub-adult individuals could have an impact of the rates at which fat is accumulated.

Body Mass Accumulation

During the period over which measure­ments were collected M daubentonii first accumulated body mass and then started to lose it. This presumably reflected a shift from the pre-hibernation period when bats were accumulating fat reserves to full hiber­nation during which time the reserves were mobilised. This shift in strategy was not precipitated by a decline in food supply be­cause the bats were given ad libitum access to food throughout the entire period, al­though as temperature declined the utilisa­tion of this resource by the bats declined. The rate at which sub-adult bats accumulat­ed body mass was only 67% of the rate achieved by adult individuals. This differ­ence occurred despite the fact that the bats were fed a non-volant prey which presum­ably did not require sophisticated echoloca­tion or flight skills to be captured and in­gested. Perhaps sub-adults ate less food than adults in spite of not being restricted to do so by their echolocation and flight skills. An alternative hypothesis, however, is that sub-adults accumulated mass at a lower rate than adults because of their greater meta­bolic rates during torpor, which limited their capacity to achieve a positive energy balance, thereby accumulating fat. This latter interpretation is consistent with the respirometry studies we performed which

demonstrated that first-year bats in torp: ~

have greater metabolic rates than older ba:; under constant temperature conditions However, the different rates of mass aCCL­mulation between adult and sub-adult ba.. (32.5 mg x day') was considerably greare: than the difference expected in fat accumt lation from the observed metabolic rate. in torpor (7 mg x day") indicated by o;:

modelling if they spent 16h of each day : torpor. This suggests either that the mass ac­cumulation we observed was not comprise; solely of fat or that other factors unde­pinned the age related difference in rates c: mass gain. One possible factor was the roes: site selection strategies of the adult and sut adult individuals, however, adult individt als did not preferentially use this site, sug­gesting their greater rates of mass accumL­lation were not related to selection of th, roosting site.

In captivity, sub-adults continued to ac­cumulate body mass for longer than adul.: and ultimately their body mass accumuls­tions did not differ significantly. This rna have been influenced by the continuor availability of food in the captive situatio; In the field such an opportunity to prolor, fat accumulation might not present itself a­weather conditions deteriorate and inse.: prey become scarce (Kunz et al., 199~

Overall the main conclusion from our stuc is that differences in the abilities of SU

O

.. _

adult and adult bats to conserve energy du:­ing torpor may contribute to their lowe: mass accumulation prior to entering hibe:­nation, but the effect is relatively minor.

ACKNOWLEDGEMENTS

TK was supported by an EU TEMPUS exchar.z scholarship during the completion of this work. \'., wish to thank Wieslaw Bogdanowicz and Tim Ne; for many useful comments on earlier versions of :-. manuscript. We are grateful to Abigail Entwistle, Jc: Messenger (Vincent Wildlife Trust) and John Hadd: (Central Scotland Bat Group) for their invaluable he 0

519 Age related variation in the energy costs of torpor

n collecting bats. We are grateful to Scottish Natural l leritage and The Countryside Council for Wales for licenses to capture and hold the animals in captivity,

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Received 03 March 2006, accepted 14 September 2006