Energetics of lactation in domestic dog (Canis familiaris) breeds of two sizes

Comparative Biochemistry and Physiology Part A 125 (2000) 197–210

Energetics of lactation in domestic dog (Canis familiaris)breeds of two sizes

Michael Scantlebury a,b,*, Richard Butterwick b , John R. Speakman a

a Department of Zoology, Aberdeen Centre for Energy Regulation and Obesity, Uni6ersity of Aberdeen, Aberdeen, AB24 2TZ, UKb Waltham Centre for Pet Nutrition, Waltham-on-the-Wolds, Melton Mowbray, Leicestershire, LE14 4RT, UK

Received 31 May 1999; received in revised form 15 October 1999; accepted 4 November 1999

Abstract

The energetics of lactation was measured in two breeds of domestic dog during peak lactation. Labrador Retrievers(30 kg) had larger litter sizes than Miniature Schnauzers (6 kg). During the 7-day experimental period, Labrador pupsincreased more in mass than Schnauzer pups, both absolutely and relatively. Consequently, the energy demands of thelitter, relative to maternal metabolism, were higher in Labradors than Schnauzers. Milk composition and gross efficiencyof milk production were not significantly different between breeds and the costs of lactation were fuelled by increases infood intake. Metabolisable energy intake was higher than predicted in Labradors, but lower than predicted inSchnauzers. These patterns differ from interspecific expectations, which would predict larger animals to reproduce moreslowly, have smaller litter sizes, and invest less energy in reproduction. © 2000 Elsevier Science Inc. All rights reserved.

Keywords: Domestic dog; Lactation; Energetics; Deuterium; Isotope; Water turnover; Allometry; Body composition; Milk composition

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1. Introduction

The functional significance of differences inbody size across mammals has received consider-able attention in the literature (Kleiber et al.,1961; Bonner et al., 1965; Linzell et al., 1972;Harvey et al., 1985; Kozlowski et al., 1997). In thecontext of reproduction, larger animals generallyexhibit reduced reproductive rates (Bonner et al.,1965; Krzyzanowski et al., 1975; Clutton-Brock etal., 1983), increased lengths of gestation, increasedweaning times and take longer to reach maturity

(Sacher et al., 1959; Millar, 1977; Western, 1979;Sacher and Staffeldt, 1999). Other variables, suchas food intake, milk production and offspringgrowth rate tend to be greater in smaller mam-mals when expressed relative to adult mass(Kleiber et al., 1961; Oftedal and Gittleman, 1989;Calder, 1996). As relatively less milk is producedin larger animals, less food is required to producethe milk, as a proportion of adult body mass.

The effects of body mass on reproductive en-ergetics are normally compared between groups ofhigher taxa (Millar, 1983; Millar and Zammuto,1983; Harvey et al., 1985) because intraspecificvariation in body mass is small compared withinterspecific differences (Harvey et al., 1989).However, the effects of mass in these comparisonsare confounded by phylogenetic and ecological

* Corresponding author. Tel.: +44-1224-272879; fax: +44-1224-272396.

E-mail address: [email protected] (M. Scantle-bury)

1095-6433/00/$ - see front matter © 2000 Elsevier Science Inc. All rights reserved.PII: S 1 0 9 5 -6433 (99 )00175 -0

M. Scantlebury et al. / Comparati6e Biochemistry and Physiology, Part A 125 (2000) 197–210198

covariation. An intraspecific study, however,would have the advantage of being phylogeneti-cally distinct (Felsenstein et al., 1985) and inde-pendent of ecology. It would be particularlyworthwhile if a species could be found with alarge variation in body mass. The domestic dogCanis familiaris has the largest range in adultbody mass of any mammalian species (Burger andJohnson, 1991). This variation provided an op-portunity to examine the differences in reproduc-tive energetics between animals whose principaldifference was body mass.

Although some energy additional to the mainte-nance level is required during gestation, lactationis the most energetically demanding period ofmammalian reproduction (Hanwell et al., 1977;Millar, 1979; Oftedal and Iverson, 1987; Gittle-man et al., 1988; Reilly et al., 1996). This periodof maximal energy intake is likely to be a keydeterminant of life history as it defines theamount of resources available to produce off-spring (Sadleir, 1984; Weiner, 1987). Conse-quently, observations were made during peaklactation in domestic dogs (Paragon et al., 1993).

The aims of this study were to obtain quantita-tive information on the mass, water and energytransfer between mothers and offspring duringlactation in dog breeds of different masses, andinvestigate whether the intraspecific relationshipswith body mass were consistent with (interspe-cific) allometric predictions. In particular, weaimed to measure the following variables: littersize, body mass change in adults and offspring,food intake and faecal production, apparent di-gestibility of food, water kinetics, milk intake,milk composition, body composition of adultsand offspring and the gross efficiency of lactation.

2. Materials and methods

2.1. Animals

Twelve Labrador Retrievers (‘Labradors’) andsix Miniature Schnauzers (‘Schnauzers’) plus lit-ters (totalling 77 and 21 pups, respectively) wereused in this study. Experiments took place overperiods of 7 days between 24 and 30 days postpartum that spanned the period of peak lactationfor both breeds. Animals were bred and reared atthe Waltham Centre for Pet Nutrition (WCPN),Melton Mowbray, UK.

2.2. Diets and housing

Adults were fed a commercial diet (Waltham®

Formula® Expert Growth) from the onset of ges-tation until the offspring were weaned. The dietconsisted of 26.9% crude protein, 14.4% fat,44.8% nitrogen free extract, 7.4% water and 6.5%ash. The metabolisable energy content of the dietwas determined by complete faeces and urinecollections and was 15.6 kJ/g (WCPN unpub-lished data). Food and water were supplied indeep stainless steel bowls that were too high forpups to drink or eat from. Food intake wasdetermined by weighing (Sartorius F150 balance90.1 g) the food bowl at 08:00 h each day.Mothers were housed with their litters in individ-ual pens with constant access to outside runs andfresh water (Loveridge, 1994). Food intake andfaeces collections were for 7 days. Urinary losseswere calculated by subtraction of the metabolis-able energy intake and apparent faecal losses fromthe gross energy intake.

2.3. Analytical methods

Samples of food, faeces and milk were analysedfor crude protein (nitrogen×6.25 for food andfaeces, nitrogen×6.38 for milk) (McDonald etal., 1981) by the combustion method (DUMAS,LECO FP428 analyser), fat by acid hydrolysisand extraction with petroleum ether (Soxhelet,Gerhardt system), water by drying to constantweight at 105°C, and ash by baking at 550°C toconstant weight. The gross energy contents offood and faeces were determined by adiabaticbomb calorimetry (Parr 1271, Auto Calorimeter).Milk energy density was derived from the proxi-mate composition and was estimated using theequation:

Energy (kJ/g)= [(fat(g)×9.11+protein(g)×5.86

+sugar(g)×3.95)/100]

×4.186/100 (1)

(Oftedal, 1984a,b; Perrin, 1958).

2.4. Body composition analysis

Body composition was determined by dual en-ergy X-ray absorptiometry (DXA) (Hologic QDR1000/W Densitometer, Hologic, Inc. WalthamMA) (Mazess et al., 1990; Jebb et al., 1997).

M. Scantlebury et al. / Comparati6e Biochemistry and Physiology, Part A 125 (2000) 197–210 199

Adults and pups were DXA scanned twice, onceat the beginning and once at the end of theexperimental period. Offspring and adults werescanned using the infant whole body and adultwhole body programmes, respectively (Brunton etal., 1996; Picaud et al., 1996), providing measure-ments of bone mineral density (BMD, g/cm2),bone mineral content (BMC, g), lean tissue (g)and fat (g). Water content (g) was calculated aslean tissue×0.73 (Pace et al., 1945) in adults andlean tissue×0.80 in offspring (Sawicka-Kapusta,1974). Individual scans took approx. 10 min dur-ing which the animals were anaesthetised (0.1ml/kg Vetalar, 0.1 ml/kg Rompun).

2.5. Milk collection

Milk samples were collected at the same timeeach day to minimise any effect of circadian vari-ation in milk composition (Oftedal, 1984a). Threemilk samples were collected from each female at1500 on days 24, 28 and 30 post partum. Pupswere removed from their mothers for an hourprior to milking. Milk was collected by manualexpression immediately after an injection of oxy-tocin (Intra-muscular, 10 IU for Labradors and 5IU for Schnauzers). All nipples were milked andall glands were emptied as completely as possibleproducing 40–80 ml of milk per sample (Oftedal,1984a). Milk samples were stored in rubber-sealedglass containers at −80°C until analysis. Milkingtook approx. 20 min after which the pups werereturned to their mother.

2.6. Drinking water

Drinking water of adults was measured fromthe drop in the water level of a header tanksupplying the water bowl, accurate to 0.1 l. Thisprovided an upper limit of the amount of freewater drunk as it included any water that wasspilt. Evaporation was discounted as a possiblesource of error because previous validation stud-ies had shown evaporative water losses from thetank and drinking bowl were less than 100 ml/week (mean 80 ml/week, S.D. 10.1, n=4). Pupshad no access to water and only consumed milk.

2.7. E6aporati6e water loss

Adults were placed in whole body gas analysis(WBGA) chambers [internal dimensions were

1.5×1×1 m for Labradors and 1×0.75×0.75metres for Schnauzers] for 1 h at some pointduring the experimental week. Animals were ha-bituated to these chambers prior to entry. Thechambers had been purpose built for analysis ofdietary gasses such as methane and hydrogensulphide (Papasouliotis et al., 1993), and consistedof an airtight transparent glass door with ply-wood sides to which an open circuit airflow wassupplied. The air inlet was cooled (20°C) and thehumidity controlled (40–50% RH) prior to enter-ing the chamber by a water trap. Airflow wasadjusted so that increases in temperature andhumidity were approximately the same for eachanimal. Three internal thermohygrographs werefitted at various locations inside the chambers andin the outflow. One hour was sufficient for tem-perature and humidity to equilibrate at flow ratesof approx. 100 l/min. Evaporative water loss ofthe animals was measured by the difference inwater contained in the air of the outflow with andwithout the animal inside the chamber. Water losswas calculated using information on atmospherichumidity, temperature and pressure (Webb et al.,1995) from standard Smithsonian tables (Weast etal., 1964).

2.8. Total body water turno6er

Deuterated water (99.9% D2O, MSD IsotopesInc) was used to determine body water space (Nd)and water turnover rates (rH2O) in adults andoffspring (Nagy et al., 1980). Doses, administeredIV, were 7.0 ml for adult Labradors (n=12), 2.5ml for adult Schnauzers (n=6) (approx. 0.1 g/kg)1.0 ml for Labrador pups (n=36) and 0.9 ml forSchnauzer pups (n=10) (approx. 0.5g/kg). Six ofthe 12 Labrador litters were used for maternalbody water measurements (litter sizes 5, 7, 7, 6, 7,5) and six were used to measure offspring (littersizes 6, 6, 6, 7, 8, 7). Similarly, four Schnauzerlitters were used for maternal body water mea-surements (litter sizes 2, 3, 2, 4) and three littersfor measurements of offspring (litter sizes 4, 4, 6).The calculations assume that all substances enter-ing the animal are labelled at background levelsand there is no recycling or re-entry of deuterium(Lifson et al., 1966; Nagy, 1980; Oftedal, 1984a;Oftedal et al., 1993b). Adults and offspring werelabelled in separate experiments and only half theoffspring of any litter were labelled so that theeffects of isotope recycling could be corrected for.


We have previously shown that recycling of iso-topes is unlikely to cause significant errors inmaternal DEE calculations, but may cause anunderestimate of pup water turnover by up to10.9% (Scantlebury et al., 2000). We took sevenblood samples per mother and seven samples perpup (1 per day). Blood samples were vacuumdistilled into Pasteur pipettes (Nagy, 1983). Hy-drogen, obtained by reacting the resulting distil-late with LiAlH4 (Speakman, 1997; Ward et al.,2000), was used for the determination of 2H:1Hratio using a mass spectrometer (Optima, Micro-mass IRMS, Manchester, UK). Water turnoverwas calculated using a dedicated computer pack-age, which took into account the effects of smalldeviations from 24 h for the sampling intervalsand the effects of mass changes on body waterpool size over the experimental period. Totalbody water efflux (Lifson et al., 1966) in adultsand pups was calculated according to theequation:

rH2O=kdNd×F (2)

where F is the fractionation factor of the isotopeand determined by:

F= [pE× (0.941)+ (1−pE)× (1.0)] (3)

where pE is the proportion of evaporation of theisotope. Zero pE would imply a fractionationfactor of 1.00, whereas 100% pE would imply avalue of 0.941 (Speakman, 1997). There is uncer-tainty about the magnitude of in vivo fractiona-

tion in humans and animals (Schoeller et al.,1986; Speakman, 1997). However, based on ourmeasurements of evaporative water loss in dogs(Figs. 1 and 2), a value of 25% pE was used. Eq.(2) reduces to:

rH2O=kdNd[(0.941+3)×0.25]. (4)

2.9. Calculation of milk intake from deuteriumelimination in labelled pups

Milk intake (MI) (g/day) per offspring wascalculated as:

MI=100× (TWI+1.07FD+0.42PrD)

/(%WM+1.07%FM+0.42%PrM

+0.58%SM) (5)

(Oftedal et al., 1987; Oftedal and Iverson, 1987;Perrin, 1958) (Abbreviations according to Oftedalet al., 1993a) in which total water intake (TWI)=(rH2O+G); where rH2O is the daily waterturnover calculated by deuterium elimination andG is the daily increase in body water as a result ofoffspring growth. FD and PrD are fat and proteindeposition (g/d) and%WM,%FM,%PrM and %SMare percentages of water, fat, protein and totalsugar in the ingested milk respectively. Milk com-position data for each female was averaged overthe time period. Rates of fat and protein deposi-tion in pups were measured by changes in theDXA estimates of body composition. Gross effi-ciency of lactation was calculated as:

Efficiency=EMILK/[(EFIL+EM)−EFIP] (6)

in which EFIL was the net energy intake duringlactation, EM was the energy obtained from massloss during lactation, EFIP was the net energyintake post lactation and EMILK is the milk energyduring lactation (English et al., 1985).

2.10. Statistical methods

Means are displayed with standard deviations.Paired t-tests were used to analyse changes overthe experimental period in the same individualsfor body mass, BMC, lean and fat tissue. Twosample t-tests were used to examine differencesbetween breeds in food intake, faecal output,apparent absorption and dry matter digestibility.Analysis of covariance (ANCOVA) was used totest for differences between pups; body mass andlitter size were entered as co-variates with breed as

Fig. 1. Bone mineral content (BMC) plotted against bodymass in 10 non-lactating Labradors (closed symbols) and 10lactating Labradors (open symbols). There was a significantdecrease in BMC in the lactating individuals.


a categorical factor. One pup per litter wasselected at random for inclusion in the analysis toprevent pseudoreplication. Statistical analyseswere performed using Minitab 5.2 software (Ryanet al., 1985).

3. Results

3.1. Animals

Adult Labradors were significantly heavierthan Schnauzers (2-sample t16=22.85, PB0.001)(Table 1). Within a breed, there was no signifi-cant change in mean body mass over the 7 dayperiod (paired t11= −2.21, P=0.054, for Labra-dors and t6=1.58, P=0.19 for Schnauzers).Labradors had significantly larger litter sizes thanSchnauzers (2-sample t16=4.83, P=0.0013)(Table 1). Litter size varied in Labradors fromfour to nine pups and in Schnauzers from two tosix pups. We created divisions within breeds of‘large’ and ‘small’ litter sizes corresponding tofour to six and seven to nine pups in Labradorsand one to three and four to six pups inSchnauzers. There was no significant difference inbody mass for either breed between females withlarge and small litter sizes (2-sample t10=0.77,P=0.47 for Labradors and t4=0.82, P=0.47for Schnauzers).

Labrador pups were heavier than Schnauzerpups at 24 days (2-sample t66=18.28, PB0.001)and pups of both breeds increased significantly inbody mass from 24–30-days-old (paired t50=19.86, PB0.001 in Labradors and t16=6.13,PB0.001 in Schnauzers) (Table 2). These in-creases were significantly different between breedswhen the effect of body mass was not taken intoaccount (two-sample t66=10.71, PB0.001, datapooled across all pups), but mass change was notsignificantly correlated with pup body mass (AN-COVA: F1,9=1.40, P=0.267), breed (F1,9=0.18,P=0.679) or litter size (F1,9=1.07, P=0.327)when mass was included as a co-variate.

3.2. Food intake

Although Labradors consumed more food thanSchnauzers (2-sample t16=7.11, PB0.001)(Table 1), there was no difference in food intakebetween breeds when body mass was included as

a co-variate (ANCOVA: F1,15=3.87, P=0.068).The gross energy intakes of Labradors andSchnauzers during lactation (27367 kJ/day and6505 kJ/day, respectively) (Table 1) were greaterthan that measured during the period pre-breed-ing (7430 (S.D. 873) kJ/day and 2506 (S.D. 336)kJ/day, respectively). Labradors with larger littersconsumed significantly more food than those withsmaller litters (1074 g/day, S.D. 461 for 4–6pups, 1611 g/day, S.D. 264, for 7–9 pups, 2-sam-ple t11=2.56, P=0.038). There was no signifi-cant difference in food intake between theSchnauzers that had small and large litters.

3.3. Faecal production

Labradors produced more faeces thanSchnauzers (2-sample t16=7.65, PB0.001)(Table 1) and the dry matter had a greater energycontent (2–sample t16=3.27, P=0.004) (Table3). Mean faecal energy losses represented 6.1 and5.4% of the gross energy intake of Labradors andSchnauzers respectively. Labradors with large lit-ter sizes produced more faeces than those withsmall litter sizes (mean 244 g/day, S.D. 65 forlarge littered individuals and mean 159 g/dayS.D. 23 for small littered individuals, 2-samplet11=3.01 P=0.024).

3.4. Apparent dry matter digestibility

There was no significant difference in apparentdry matter digestibility between breeds (2-samplet16=0.66, P=0.54) or within breeds between in-dividuals with large and small litter sizes (2-sam-ple t10=1.13, P=0.30 in Labradors andt4=0.43, P=0.71 in Schnauzers) (Table 1).

3.5. Adult body composition

Although Labradors contained significantlymore BMC, lean and fat tissue than Schnauzersbecause they were larger, there was no significantdifference in percent BMC, lean tissue or fatbetween Labradors and Schnauzers (Table 3).Equally, there were no significant differences inbody composition for either breed between indi-viduals with large and small litter sizes. The onlysignificant change in body composition over theexperimental period, was a decrease in fat contentof the Labradors (paired t11= −2.42, P=0.039)(Table 3). However, BMC in the lactating moth-


Table 1Mean and standard deviations (S.D.) of energy balance taken for adult dogs

Energy balance Labradors (n=12) Schnauzers (n=6)

MeanMean S.D. PaS.D.

Body mass (kg) 6.4730.26 0.11 B0.0013.303.57 1.400.90 0.00Litter size 6.42

0.751−0.525 0.111 0.074 0.88Chg (body mass) (kg)2411439* 345 150 B0.001Food intake (g/day)

6505 28284170 B0.001Gross energy intake (kJ/day) 273675382 2340Metabolised energy intake (kJ/day) B0.00122448 3760

45 2165 B0.001202*Faeces produced (g/day)10973437* 739 340 B0.001Faecal energy output (kJ/day)

394 184Urine energy output (kJ/day) B0.0011485 4740.82 0.080.10 0.880.81Apparent digestibility

8151303 389 212 0.0024Milk produced (g/day)7715 4346 2304 1132 0.0024Milk energy output (kJ/day)

61% Lactation efficiency 63

a P values denote a significant difference between Labradors and Schnauzers.* Denotes a significant difference between large and small litter sizes. Data for body mass (kg), litter size, change in body mass Chg

(body mass) (kg), food intake (g/day), gross energy intake (kJ/day), metabolised energy intake (kJ/day), faeces produced (g/day), faecalenergy output (kJ/day), urine energy output (kJ/day), apparent dry matter digestibility, milk produced (g/day), milk energy output(kJ/day), lactation efficiency (%).

Fig. 2. Flow diagram showing mass, water and energy transfer in a lactating Labrador and offspring. Arrow sizes are proportional toflow rates. Unknown quantities are denoted by dotted arrows.


Table 2Mean and standard deviations (S.D.) of kinetic measurements taken for adult dogs

Labradors (n=12)Kinetic measurements Schnauzers (n=6) Pa

Mean S.D. Mean S.D.

0.261.261.54 B0.0015.32Drinking water (l/day)Evaporated water (l/day) 1.11 0.40 0.38 0.21 0.0017k(d) (per h) 0.01193 0.00306 0.00783 0.00101 0.022

18.80N(d) (l) B0.0010.743.692.59B0.0011.1 3.4 0.7N (DXA) (l) 18.0

5.40 2.80Total water turnover (l/day) 0.68 0.000.1862% Body water 2576

a P values denote a significant difference between Labradors and Schnauzers. Data for drinking water (l/day), evaporated water(l/day), kd (deuterium elimination rate, h−1), Nd (body water space estimated by deuterium dilution, litres), N(DXA) (body water spaceestimated by DXA, litres), total water turnover estimated by deuterium elimination (l/day) % body water calculated by deuteriumdilution.

ers was significantly lower than that measured innon-breeding individuals (unpublished data, AN-COVA: F1,17=38.11, PB0.001, with body massas a co-variate, Fig. 1).

3.6. Pup body composition

There was no significant difference in percentBMC between Labrador and Schnauzer pups.However, Labrador pups contained relatively lessfat and more lean tissue than Schnauzer pups(Tables 4 and 5). There were no differences inbody composition between individuals of eitherbreed that came from large and small litter sizes.All pups increased significantly in BMC and leantissue during the experimental period, althoughonly the Labrador pups increased significantly infat (Table 5).

3.7. Water kinetics and milk intake

3.7.1. Drinking waterAdult Labradors drank more water than

Schnauzers (2-sample t16=8.54, PB0.001) (Table2). However, these differences were not differentwhen mass was entered as a co-variate (AN-COVA: F1,16=1.80, P=0.198). There were nodifferences in the amount of water used betweenmothers that raised large and small litters (t11=1.62, P=0.16 for Labradors and t6=0.46, P=0.69 for Schnauzers).

3.8. E6aporati6e water loss

Evaporative water loss was greater in Labra-

dors than Schnauzers (2-sample t16=4.24, P=0.0017) (Table 2). These differences were notsignificant, when mass was entered as a co-variate(ANCOVA: F1,10=0.37, P=0.557). There was nodifference in evaporative water loss between indi-

Fig. 3. Flow diagram showing mass, water and energy transfer


Table 3Mean and S.D. of body composition taken for adult dogs.

Body composition Labradors (n=12) Schnauzers (n=6) Pa

Mean S.D. Mean S.D.

2% BMC 0.270 0.320.076% LEAN 382479

19 4% FAT 16 2.8 0.082BMC (g) 652 80 129 24.6 B0.001LEAN (g) 22474 1516 4191 1141 B0.001

5335 1460FAT (g) 999 439 B0.0010.27 0.476.733.98Chg (BMC) (g) 12.5

Chg (Lean) (g) 0.6863−202716−50−16 63 0.057Chg (Fat) (g) −224** 293

a P values denote a significant difference between Labradors and Schnauzers.** Denotes a significant change over time (24–30 days post partum). Data for % BMC (percent bone mineral content), % LEAN

(percent lean tissue), % FAT (percent fat tissue), BMC (bone mineral content, g), LEAN (lean tissue content, g), FAT (fat tissuecontent, g), Chg (BMC) (change in bone mineral content from 24 to 30 days of lactation), Chg (LEAN) (change in lean tissue content),Chg (FAT) (change in fat tissue content).

Table 4Mean and S.D. of faeces composition taken for adult dogs

Faeces composition Labradors (n=12) Schnauzers (n=6) Pa

Mean S.D. Mean S.D.

72.7 2.1 66.2 1.4 B0.001% Water0.00021.08.2% Ash 1.16.0

0.810.71.3 0.0269.6% Crude protein2.0 0.0010.31.7% Crude fat 0.1

10.1 1.2% Nitrogen free extract 12.9 0.5 B0.001Dry matter energy (kJ/g) 17.0 0.7 16.2 0.5 0.004

a P values denote a significant difference between Labradors and Schnauzers. Data for percent water, percent ash, percent crudeprotein, percent crude fat, percent nitrogen free extract, dry matter energy (kJ/g).

viduals that raised large litters and small litters(2-sample t11=0.23. P=0.83, data for Labradorsonly).

3.9. Total body water turno6er

In all cases the log converted enrichments of2hydrogen above background decreased linearlyand were correlated with time over theexperimental period. The r2 values averaged99.5% in both breeds. Adult Labradors had largerdeuterium dilution spaces (Nd) than Schnauzers(2-sample t16=13.47, PB0.001), greaterdeuterium elimination rates (kd) (2-samplet16=3.05, P=0.022) and higher water turnoverrates (rH2O) (2-sample t16=5.26, P=0.0033)(Table 2). Independent body water spaces

measured by DXA in Labradors and Schnauzers(Table 2) were 4 and 7%, respectively less thandeuterium dilution spaces. These differences areconsistent with isotope based calculations of bodywater producing overestimates of between 5 and10%, because of additional isotope pools(Speakman, 1997). There were no significantdifferences in either Nd, kd or rH2O betweenindividuals of either breed that reared large littersand small litters.

Deuterium dilution spaces in Labrador pupswere greater than Schnauzer pups (2-samplet66=9.55, PB0.001) (Table 5). In contrast toadults, deuterium elimination rates were notsignificantly different between breeds (2-samplet66=0.11, P=0.91). Water turnover rates weregreater in Labrador than Schnauzer pups(2-sample t66=5.99, PB0.001). DXA estimates


Table 5Mean and S.D. of measurements taken for offspring

Schnauzers (n=17) P*Labradors (n=51)

S.D.MeanS.D.Mean

a

Body mass (g) B0.001520 1608602350241** 87Chg (body mass) (g) 69** 45 B0.001

Total litter mass (g) 15060 2300 310 810 B0.001Body compositionb

2 0%BMC 2 0 0.067%LEAN 84 2 80 4 0.0001

2 B0.00118 4%FAT 1446 13BMC (g) 13 4 B0.001

LEAN (g) B0.001109674405197159166 B0.001117345FAT (g)

Chg (BMC) (g) 6.89** 4 3.03** 4 0.001591 54.2** 91 B0.001Chg (LEAN) (g) 221**

15.3** 30Chg (FAT) (g) 12 26 B0.001Water kinetics and energy balancec

0.005379 0.00095k(d) (per h) 0.005613 0.00211 0.911710 319 660 106N(d) (ml) B0.001

327 561 95 B0.001N (DXA) (ml) 1665B0.00147 84 20Water turnover (ml/day) 195

109127Milk consumption (g/day) 0.0024203 601201 752Milk energy intake (kJ/day) 645 352 B0.001

6% Body water 7773 9

a Body mass (kg), change in body mass Chg (body mass) (g), total litter mass (g).b Body composition; % BMC (percent bone mineral content), % LEAN (percent lean tissue), % FAT (percent fat tissue), BMC (bone

mineral, g), LEAN (lean tissue content, g), FAT (fat tissue content, g), Chg (BMC) (change in bone mineral content from 24 to 30 daysof lactation), Chg (LEAN) (change in lean tissue content), Chg (FAT) (change in fat tissue content).

c Water kinetics and energy balance: kd (deuterium elimination rate, h−1), Nd (body water space estimated by deuterium dilution, ml),N(DXA) (body water space estimated by DXA, ml), water turnover calculated by isotope elimination (ml/day), milk consumption(g/day), milk energy intake (kJ/day), % body water calculated by deuterium dilution.

* P values denote a significant difference between Labradors and Schnauzers.** Denotes a significant change over time during peak lactation (24–30 days post partum).

of body water space in Labrador and Schnauzerpups were 4 and 10%, respectively less than thedeuterium dilution spaces.

3.10. Milk intake, milk composition and milk en-ergy output

Milk intake was greater in Labrador thanSchnauzer pups (2-sample t66=3.70, P=0.0024)(Table 5). There were no differences in kd, Nd ormilk intake between those individuals that camefrom large or small litters. In addition, there wereno differences in milk composition betweenbreeds, within breeds with different litter sizes, orbetween day 24 and day 30 of lactation withinindividuals. Milk contained on average 21.2 (S.D.1.33)% dry matter, 8.23 (S.D. 0.66)% crude

protein, 8.11 (S.D. 1.91)% fat and 4.86(S.D.1.42)% sugar. Milk energy averaged 5.92(S.D. 0.60) kJ gross per gram.

4. Discussion

4.1. Animals

There is a positive relationship between littersize and body mass across dog breeds (Robinsonet al., 1973), in contrast to the general mammalianpattern (Blueweiss et al., 1978). However, gesta-tion length (Krzyzanowski et al., 1975), oestruscycle length, weaning times (Evans and White,1988) and age at first breeding are remarkablyuniform across dog breeds. This is surprising


given the convincing allometric patterns for thesetraits across mammals of similar size ranges, withlarger animals generally reproducing more slowlyand investing less overall resources into reproduc-tion (Hanwell et al., 1977; Calder, 1996). Hence,one might expect the (intraspecific) patterns ofenergy allocation observed across dogs of differ-ent masses to differ from expected (interspecific)allometric variation. Figs. 2 and 3 show the mass,water and energy transfer in lactating Labradorsand Schnauzers, respectively (24–28 days postpartum).

4.2. Food intake

Metabolisable energy intakes (MEI) duringpeak lactation were similar to the maximum pre-dicted rates of energy assimilation. MEI’s ofLabradors were 29% higher than the Weiner(1989) prediction and 13% higher than the Kirk-wood (1983) prediction. In contrast, Schnauzerswere 14% lower than the Weiner prediction and18% lower than the Kirkwood prediction (Fig. 4).Hence, Labradors appeared to be working harderduring peak lactation than Schnauzers, indepen-dently of their mass. These increases are consis-tent with Labradors having the larger litters. Thefact that differences in food intake between largeand small litters were only observed in the Labra-dors may reflect the larger litter sizes of Labra-dors. The extra cost of lactation was met byincreases in food intake of 300–400% in Labra-dors and 200–300% in Schnauzers, in agreementwith other studies on small mammals (Lonnerdalet al., 1981; Lochmiller et al., 1982; Glazier, 1985;Hammond et al., 1992. In contrast, when foodsupply is intermittent, or when lactation is neces-sarily brief, some large carnivores such as speciesof pinniped and bear cover some or all of thecosts of lactation by utilising body reserves (e.g.Arnould and Ramsay, 1994; Lydersen et al., 1996,1997; Oftedal, 1993; Oftedal et al., 1993a). This islikely to depend on the size of the mother, withrelatively larger mothers being able to contributemore resources or fast for longer periods (Bowenet al., 1992; Fedak et al., 1996).

4.3. Body mass and body composition

There was no change in mass for adults ofeither breed during peak lactation. This is consis-tent with previous studies in which the costs of

lactation are met by increases in food intake(Meyer et al., 1984; Oftedal, 1984a; Kienzle et al.,1985). Nevertheless, there was a significant de-crease in the body fat of the Labradors. The lossin the larger breed is consistent with the idea thatlarger mothers may be better able to utilise bodyreserves during lactation (Bowen et al., 1992;Fedak et al., 1996), and it is tempting to regardthis fat loss as a possible limit to energy assimila-tion. However, the large amount of variability inthe data and the relatively minor loss of fat(0.74% reduction in body fat of the Labradorsover the 7 day interval) indicate this loss is un-likely to be biologically significant. Althoughthere was no loss of BMC during the week ofpeak lactation, lactating mothers had significantlyless BMC than non-breeding individuals (Fig. 1).Maternal BMC was therefore likely to have beenused to support offspring skeletal growth (Meyeret al., 1984; Fukuda et al., 1993). In contrast toadults, pups of both breeds increased in bodymass during the experimental period. Lean tissuegrowth accounted for 90% of the mass increase inLabradors and 80% in Schnauzers (Table 5, Figs.2 and 3). By comparison, offspring of some spe-cies of pinniped, with a necessarily short durationof lactation, are able to grow rapidly and 50–80%of the increase in mass is fat (Meyer et al., 1984;Oftedal et al., 1993b; Lydersen et al., 1995, 1996,1997).

Fig. 4. Log-converted metabolisable energy intake (MEI)against log-converted body mass (kg) for 12 Labrador Retriev-ers (closed symbols) and five Miniature Schnauzers (opensymbols). Predicted relationships are given as lines. The solidline is the Kirkwood (1983) prediction and the dotted line isthe Weiner (1989) prediction. Labradors had higher andSchnauzers lower MEI’s than predicted.


Fig. 5. Metabolic mass ratio (LMM/MMM) plotted againstlog-converted body mass (kg) for 12 Labrador Retrievers(closed symbols) and six Miniature Schnauzers (open sym-bols). The higher ratio in Labradors compared to Schnauzersindicates a higher demand of the litter relative to maternalmetabolism.

milk (Oftedal, 1984b). Typically canids have highmilk energy outputs, of which the domestic doghas one of the highest (Oftedal and Gittleman,1989). Communal rearing of offspring might haveallowed the evolution of large litter sizes whichsubsequently induced high milk energy outputs(Moehlem, 1989; Oftedal and Gittleman, 1989).These milk energy demands might be expected todiffer, following selection for variations in bodymass, with resulting litter size changes. Expressionof milk energy output relative to maternalmetabolic mass (MMM) [(M0.75): M=maternalbody mass, 0.75 is the allometric exponent] is oneway of correcting for size-related differences inenergy yield (Oftedal, 1984b). Relative energy out-puts calculated in this way were 598 kJ/kg0.75 inLabradors and 567 kJ/kg0.75 in Schnauzers. Thesevalues are similar, but Labradors maintainedslightly higher milk energy outputs independent oftheir mass. Milk energy output can also be ex-pressed in terms of litter metabolic mass (LMM)(allometric exponent is 0.83), which provides anestimate of the relative energy demands of thelitter (Gittleman et al., 1987; Oftedal and Gittle-man, 1989). Energy output per LMM was 591kJ/kg0.83 in Labradors and 731 kJ/kg0.83 inSchnauzers. The ratio LMM/MMM indicates thedemands of the litter relative to maternalmetabolism. It explains approx. 97% of the vari-ance in maternal energy production per kg0.75 andis valid for species that vary greatly in body mass(Oftedal, 1984b). LMM/MMM, was 1.01 inLabradors and 0.78 in Schnauzers, which indi-cates Labradors have a higher demand of thelitter relative to maternal metabolism thanSchnauzers (Fig. 5).

In summary, the pattern of energy allocationduring reproduction in Labradors and Schnauzerswas different from the predicted interspecific pat-tern, which predicts larger animals to reproducemore slowly, have smaller litter sizes and investless energy in reproduction. In the current obser-vations, although milk composition and gross effi-ciency of milk production were not significantlydifferent between breeds, Labradors had a rela-tively higher milk energy output than theSchnauzers and invested a greater amount of re-sources in reproduction. The costs of lactationwere met by increases in metabolisable energyintake, which was higher than predicted in Labra-dors and lower than predicted in Schnauzers.

4.4. Milk composition

Milk production estimates in the current studywere similar to the 1690 g/milk per day in aGerman Shepherd (66) and 1060 g/milk per day inBeagles at day 26 of lactation (Oftedal, 1984a).Although milk composition varies greatly betweenspecies (Jenness et al., 1970; Linzell et al., 1972),there appears to be little difference between dogbreeds (Russe, 1961). Milk composition changeswith period of lactation (Thomee, 1978; Lon-nerdal et al., 1981; Oftedal et al., 1993a; Lydersenet al., 1995), which teat and how completely agland is emptied (Oftedal, 1984b). In the currentstudy, milk composition was similar to that mea-sured in beagles at a corresponding stage of lacta-tion (Oftedal, 1984a). We found no difference inmilk composition between Labradors andSchnauzers measured during the same period oflactation or any evidence of more dilute milkbeing produced by mothers of larger litters (e.g. incotton rats, Sigmodon hispidus) (Rogowitz et al.,1995). Therefore, milk composition is unlikely tobe a source of disparity in lactation energeticsbetween these two breeds.

4.5. Milk energy output

In mammals, the magnitude of milk energyoutput roughly parallels body mass, with largerindividuals producing more energy per day in


Acknowledgements

The work was funded by a grant from WalthamCentre for Pet Nutrition to the University ofAberdeen. We are grateful for the help and sup-port of all those who helped in this project. AtWCPN these included Ivan Burger, Derek Booles,Tom McCappin, Amanda Hawthorne, WaringHynds, David Poore, Matthew Gilham, Pat Nu-gent and David Lawson. We would like to thankSally Ward for valuable comments on earlierdrafts of the ms and Peter Thomson for the massspectrometry. Calculations of water turnover weremade using a dedicated program; Lemen &Speakman, 1997 http://www.natureware.com/double.htm.

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Energetics of lactation in domestic dog (Canis familiaris) breeds of two sizes