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A preliminary study of microscopic evidence for lactation in cattle
Andrew T. Chamberlain and Stephen T. Forbes
Published in J. Mulville and A.K. Outram (eds) 2005. The Zooarchaeology of Fats, Oils, Milk and Dairying.
Oxford, Oxbow, pp. 44-49.
Abstract
Pregnancy and lactation are periods of high calcium requirement due to the additional physiological demands imposed
on the mother by the developing foetus or infant. Some of the required calcium is released from the maternal skeleton
through an increase in bone remodelling. This study uses quantitative bone histomorphometry to seek microscopic
evidence for increased rates of bone turnover in lactating cattle. Microscopic thin sections were prepared from femurs
of modern dairy and beef cattle, and from archaeological cattle specimens of unknown husbandry regime. Modern
dairy cattle had a proliferation of small osteons of less than 100μ diameter, allowing a clear distinction to be made
between the modern beef and dairy cattle specimens. Analysis of the archaeological sample showed that some cattle
from historic and prehistoric periods resembled the modern dairy cattle in their osteon size distributions. These results
are consistent either with frequent pregnancy or with high volume lactation in these animals.
Introduction
In mammals, pregnancy and lactation are periods of high calcium requirement due to the additional physiological
demands imposed on the mother by the developing foetus or infant. A requirement for supplementary calcium is also
seen in deer during periods of antlerogenesis. Studies in a variety of mammalian species have shown that the
mobilisation of skeletal calcium during pregnancy, lactation and antlerogenesis is manifest in elevated rates of bone
turnover and a net loss of whole body bone mineral content (Chrisp et al. 1989; Baxter et al. 1999; Ott et al. 1999;
Vajda et al. 1999; Liesegang et al., 2000; Fiore et al. 2003; Manton et al. 2003). Together these studies show that the
mammalian skeleton is measureably responsive to physiological demands for calcium.
Calcium release from bone is mediated by osteoclasts and osteoblasts, bone cells which are responsible for bone
remodelling. Bone can be remodelled at the periosteal and endosteal surfaces, and also intracortically through
the development of secondary osteons. Secondary osteons are created by coordinated teams of cells which together
form a bone remodelling unit (Parfitt 1994). Bone remodelling units tunnel through cortical bone, resorbing
existing bone tissue and laying down new bone which is organised concentrically around Haversian canals (Fig.
1). The changes in structural organisation that result from bone remodelling are visible in microradiographs,
scanning electron micrographs, confocal microscope images and in thin sections of bone viewed under plain
or polarised transmitted light (Bell & Piper 2000; Schultz 2001). Intracortical remodelling has been implicated in
lactational calcium regulation. Vajda et al. (1999) found significantly greater numbers of newly-formed osteons
and resorption spaces in the femurs of lactating dogs, compared to non-lactating control animals, although they
did not report data on osteon diameters in their specimens.
Fig. 1. Schematic diagram of a forming osteon or bone
remodelling unit. Osteoclasts (bone resorbing cells) cut a
longitudinal tunnel through existing cortical bone. The
tunnel is subsequently infilled by osteoblasts (bone
forming cells), which lay down new layers of bone
arranged concentrically around a central space called a
Haversian canal.
Female domestic cattle are often intensively milked, but there have been few quantitative studies of bone
microstructure in either fresh or archaeological cattle bones and no previous research on microstructural changes that
might result from intensive milk production. Jowsey (1966) measured osteon diameters in the femurs of a range of
animal species, including four adult specimens of cattle. Cattaneo et al. (2001) recorded measurements of osteon
diameter in five adult cattle long bones. Both studies found that the osteons in their cattle bone samples averaged 240
to 250 microns in diameter, however the lactational status of the animals included in these studies was not known.
The aims of the present study were firstly to establish whether modern dairy cattle, which are bred and managed to
achieve a high milk output, show microscopic evidence of enhanced rates of bone turnover, and secondly to
investigate whether microscopic evidence for lactation can be found in archaeological specimens of cattle bone. This
study was initially undertaken by Stephen Forbes as a masters dissertation project at the University of Sheffield
(Forbes, 1994), and his preliminary work has been supplemented with data collected more recently by Andrew
Chamberlain.
Materials and Methods
Samples of cortical bone from the midshaft of the femur were obtained for four modern beef cattle and four modern
dairy cattle. The beef cattle included both males and females – though individual specimens could not be sexed - and
their estimated age at death was about 2 years old, whereas the dairy animals (females) were aged approximately
seven to ten years old. The breeds of cattle sampled were not known, nor was the husbandry regime except in the case
of the dairy cattle, which had grazed in open fields during the summer months and had been kept indoors during the
winter. Samples of three archaeological cattle femurs were obtained from a 1st century AD British Roman settlement
site and five cattle femurs and one aurochs femur were obtained from a British prehistoric cave site which has deposits
dated from the early Neolithic to the early Iron Age (Chamberlain 2001). None of the archaeological specimens
showed evidence of burning or other post-mortem modification.
Table 1. Details of Specimens. M = male, F = female.
Category
Number of Specimens
Sex
Age
Modern Beef Cattle
4
?M & F
Approximately 2 years
Modern Dairy Cattle
4
F
Approximately 7 to 10 years
Roman Cattle
3
?
Adult (2), unknown (1)
Prehistoric Cattle
5
?
Adult (1), <3.5 yrs (1), unknown (3)
Aurochs
1
?
<3.5 years
Transverse sections of the cortical bone of the midshaft of the femur were removed using a hand saw and then cut into
thin sections using a Leitz 1600 annular saw microtome. The modern material was cut directly (without embedding)
to a thickness of 30 microns. The archaeological material was cut to a thickness of between 50 and 100 microns,
using resin embedding for some of the most fragile specimens.
The thin sections were mounted on glass slides using Euparol adhesive and viewed under transmitted light with a
Scopeman video microscope and a Kyowa polarising microscope. Secondary osteons were defined by the presence of
a reversal (cement) line surrounding concentric lamellae and a Haversian canal (see Schultz 2001, for detailed
descriptions and definitions of bone microstructural features). The diagnostic features of the osteons were best seen
either under cross-polarised light, which reveals the characteristic extinction patterns of collagen fibre orientation, or
with plain light and a partially closed microscope condenser diaphragm which highlights the osteocyte lacunae and the
reversal line at the margin of the osteon (Figs 2-4). Both methods of illumination were used on each specimen to
ensure accurate identification of osteons.
Fig. 2. Microstructure of modern cattle femur, viewed under plain light. Width of
field approximately 1.5 mm. A layer of circumferential lamellar bone
approximately 100μ in width marks the endosteal surface on the right of the
image. Cortical bone with vascular channels and numerous secondary osteons
are also visible.
Fig. 3. Microstructure of modern cattle femur, viewed under polarised light.
Image dimensions as in Figure 2.
Fig. 4. Microstructure of aurochs femur, viewed under plain light. Width of field
approximately 1.5 mm, damaged endosteal surface visible towards the left. Three
large secondary osteons are located towards the top left of the field of view.
Measurements were obtained of 50 osteons per specimen, distributed throughout the anterior region of the bone
cortex, and each osteon was measured twice by recording the maximum diameter and a corresponding diameter
perpendicular to the maximum diameter. These two measurements were averaged for each osteon to give an estimate
of the typical average diameter of each osteon. Any osteon in which the maximum diameter was more than twice the
minimum diameter was exluded from the data set, as this indicates that the osteon is not running perpendicular to the
plane of the thin section (Jowsey 1966).
Results
A wide distribution of osteon diameter was observed in the modern samples of both beef and dairy cattle, but average
and modal osteon diameter was significantly smaller in the dairy cattle (mean = 136 μ, modal size category = 100-150
μ) compared to the beef cattle (mean = 203 μ, modal size category = 150-250 μ). A proliferation of small (i.e. < 100 μ
diameter) osteons was found only in the dairy cattle, and the shape of the frequency distributions of osteon diameter
allowed a clear distinction to be made between the two types of modern cattle in our sample (Figure 5).
Fig. 5. Osteon size distributions in modern dairy cattle (solid
lines) and modern beef cattle (dashed lines). Osteon numbers
are grouped in 50 micron intervals, 0–49, 50–99 etc.
The distribution of osteon diameters in the archaeological specimens followed a similar pattern to that of the modern
cattle, with two Roman and two prehistoric cattle closely resembling modern dairy cattle in their osteon size
distribution while the other specimens more closely resembled the modern beef cattle (Fig. 6). The modal size
category of the archaeological “non-dairy” cattle was 150-200 μ, somewhat smaller than in the modern beef cattle.
However, only a limited number of samples were studied, and further analysis is required to determine whether there
is a continuous spectrum of osteon size distribution or whether there are genuinely two distinct patterns among the
archaeological material. The single specimen of aurochs resembled the modern beef cattle profile: despite the large
overall size of the aurochs bone, its osteons are comparable in size to those of the largest of the modern domestic beef
cattle.
Fig. 6. Osteon size distributions in archaeological cattle and
aurochs. Vertical dashed lines indicate the centres of the
peaks of the distribution of osteon size for modern dairy and
beef cattle (see Fig. 5). Solid lines represent archaeological
specimens, with circular markers indicating Roman
specimens and square markers indicating prehistoric
specimens. The specimens are interpreted as breeding/
lactating cattle (left hand peak) or as non-breeding/non-
lactating cattle (right hand peak). The dashed line with solid
markers indicates the osteon size distribution in the single
specimen of aurochs.
Discussion
These preliminary results are interesting, although it should be noted that they are based on small samples that are
restricted in their geographic and temporal origins. Further work needs to be undertaken to establish whether the
differences in osteon size distribution reflect real differences in bone physiology between beef and dairy cattle, or
whether factors such as the breed of the animal, the management regime, the age at death of the animal and its
reproductive history have an influence on bone histomorphometry.
Although the modern dairy cattle in our sample were aged above 7 years we believe it is unlikely that small osteons
only appear in older animals, if this was the case then many larger osteons would still be visible as partially
remodelled fragments in our dairy cattle. The sex of the beef cattle was not recorded, but we believe it is unlikely that
there are systematic differences in osteon size between male and female animals, as significant sex differences in
osteon dimensions have not been found in humans (Pfeiffer 1998) and have not been reported in other mammalian
species.
At present we have little information about osteon size in different breeds of cattle: the single specimen of aurochs
(Bos primigenius) that we have studied possesses relatively large osteons, but these are nonetheless within the range of
osteon sizes observed in our sample of modern beef cattle. Cattaneo et al. (2001) did not find small diameter osteons
in their sample of five cattle bones (presumed to be from an Italian cattle breed), and their recorded osteon diameters,
which ranged from a minimum of 120 μ to a maximum of 420 μ, match well with our sample of beef cattle.
Similarly, we have little information about the dietary regimes and locomotor activities sustained by the cattle
included in this study, and it is possible that differences in animal management between beef and dairy herds may
contribute to the contrast in bone microstructure. However, the proliferation of small diameter osteons in modern
dairy cattle is consistent with the increased bone turnover associated either with frequent pregnancy or with high
volume lactation in these animals. The finding of a similar pattern in prehistoric cattle may indicate that these animals
were managed for intensive breeding and/or milk production, and this complements the emerging view that prehistoric
cattle may have been intensively exploited for dairy products (Copley et al. 2003).
Acknowledgements
Earlier versions of this paper were presented at the Osteon Workshop at the Institut für Anthropologie, Universität
Göttingen, in September 2001 and at the ICAZ conference, University of Durham, in August 2002. The comments of
referees are greatly appreciated.
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Andrew T. Chamberlain and Stephen T. Forbes. Department of Archaeology and Prehistory, University of Sheffield,
Northgate House, West St, Sheffield, S1 4ET
