Hind Foot Length: An Indicator for Monitoring Roe Deer Populations at a Landscape Scale

1. What they did

Roe deer live in forest and are hard to count, and counts are often wrong. Wildlife managers in France therefore use indicators that show how well a deer population is matched to its food and habitat. Many of these were designed for small areas of a few thousand hectares. The authors asked whether a simple body measurement, the hind foot length of fawns, could track the balance between deer numbers and their environment across a whole province.

The idea is that a deer’s hind foot grows fast after birth and stops growing early, so the length of a fawn’s foot should reflect the conditions the fawn grew up in. In the fenced Chizé Reserve, where deer numbers are well known, fawn hind foot length had already been found to fall as density rose.

• Collecting the feet. From 1990 to 2002, hunters in the Tarn province of southwestern France sent in the left hind foot of every roe deer legally shot (28,761 deer), with a card giving sex, estimated age (fawn or adult), and place and date of death. From 1996 onwards, age was checked by looking for a layer of cartilage at the end of the foot bone that only fawns have.
• Measuring. One experienced wildlife manager measured every foot with a wooden gauge, to the nearest millimetre, from the top of the heel bone (calcaneum) to the tip of the hoof.
• Dividing the province. They split the province into three regions by forest cover and altitude: deciduous forest in the north, farmland in the centre, and mountain mixed forest in the southeast.
• Density and climate. As a density index they used the number of roe deer shot per km² each year in each region, because no other reliable count existed. For climate they built a “hydric index” for spring and for summer, which rises when the season is wet and cool.
• Analysis. They analysed 9,770 fawns shot from 1993 to 2002, region by region. They corrected each foot length for the date it was shot, then tested whether yearly mean foot length was linked to density and to spring and summer climate. As a check, they also compared a low-density period (1993 to 1997) with a high-density period (1998 to 2002).

2. What they found

Basic patterns

• Fawns’ feet kept growing through the hunting season, by about 0.11 to 0.14 mm a day.
• Male fawns had slightly longer hind feet than females in all three regions, by about 3 to 4.5 mm.
• Foot length generally fell over the years in all three regions, with some ups and downs.
• The number of roe deer shot in the province rose from 373 to 4,999 a year. Shot-deer density rose from 1993 to 1999, then levelled off in region 2 and fell slightly in regions 1 and 3.

Density

• In the northern forest (region 1) and the mountain forest (region 3), fawns had shorter feet in years of higher density. In region 3, foot length fell by about 2.8 mm for each extra roe deer shot per km².
• In the farmland (region 2), where density was lowest, there was no clear link with density.
• Comparing the two periods gave the same picture: feet were 3.2 mm shorter in the high-density period in region 1 and 2.35 mm shorter in region 3, with no difference in region 2.

Climate

• Summer weather had no effect on foot length in any region.
• Cool, wet springs went with shorter feet in some cases: in region 2 for males only, with a weaker trend for males in region 3. In region 1, spring weather changed how strongly foot length responded to density; feet were most sensitive to density after dry, warm springs.

Regions

• The smallest fawns were in region 3, the mountainous, wettest and coolest region, which also had the highest density. The largest were in the farmland of region 2, which had the lowest density.

3. What they concluded

• Hind foot length of fawns is a useful indicator of density dependence in roe deer: it reflects how deer numbers relate to the food and habitat available, and can be used to follow population trends.
• The density index from hunting bags is only an approximation, but the authors are confident it reflects real trends, because the yearly figures and the two-period comparison gave the same results.
• The lack of a density effect in the farmland region may be because density there had not yet reached the level at which food becomes limiting.
• Indicators need to be followed over many years to show trends, and values should be compared over time within one area, not between areas, because each population responds to its own environment.
• Because males and females differ slightly, sex must be taken into account when comparing body measurements.
• A decline in foot length does not tell managers whether the cause is poorer habitat or more deer. The authors argue this matters less to a manager than having a reliable signal of density dependence.

6. Other technologies, skills or innovations

• Ageing by bone: the cartilage at the end of the metatarsus (the long foot bone) is present only in fawns and disappears at about 14 to 16 months, giving a reliable way to tell fawns from adults during the hunting season.
• A simple, cheap measurement: an experienced wildlife manager could measure about 150 hind feet in an hour with a wooden gauge.
• A forest cover index from a satellite image, smoothed over 1 km, used to map the regions.
• A hydric index (precipitation minus twice the temperature) as a stand-in for how much green food plants produce in spring and summer.

7. Implications for conservation (from the paper)

• Hunters across a whole province can collect hind feet easily, so the method gives information at a landscape scale, larger than the usual management units.
• Index methods like this are less time-consuming and costly than counting deer, and more useful for management decisions.
• Hind foot length should be monitored together with other indicators, such as the browsing index and the kilometric index (deer seen per kilometre walked), to give a full picture of a roe deer population and its environment.