Ecological conditions experienced by offspring during pregnancy and early post-natal life determine mandible size in roe deer

Abstract

Population dynamics studies and harvesting strategies often take advantage of body size measurements. Selected elements of the skeletal system such as mandibles, are often used as retrospective indices to describe body size. The variation in mandibular measurements reflects the variation in the ecological context and hence the variation in animal performance. We investigated the length of the anterior and posterior sections of the mandible in relation to the conditions experienced by juveniles of 8-10 months of age during prenatal and early postnatal life and we evaluated these parameters as ecological indicators of juvenile condition as well as female reproductive condition in a roe deer population living in the southern part of the species range. We analyzed a sample of over 24,000 mandibles of roe deer shot in 22 hunting districts in the Arezzo province (Tuscany, Central Italy) from 2005 to 2015 per age class. Mandible total length in juveniles is equal to 90% of total length in adults. In this stage of life the growing of the mandible's anterior section is already completed while that of the posterior section is still ongoing. Environmental conditions conveyed by forest productivity, agricultural land use, local population density and climate strongly affected the growth of the anterior and posterior sections of the mandibles. Conditions experienced both by pregnant females and offspring played an important role in shaping the length of the anterior section, while the size of the posterior section was found to be related to the conditions experienced by offspring. Temporal changes of the length of the anterior section are a particularly suitable index of growth constraints. Anterior section length in fact differs according to more or less advantageous conditions recorded not only in the year of birth, but also in the previous year. Similarly, the sexual size dimorphism of the anterior section of the roe deer mandible can be used to describe the quality of females above two years of age, as well as habitat value. Hence the anterior section length of the mandible and its sexual size dimorphism are indexes that can provide cues of population performance, because they capture the system's complexities, while remain simple enough to be easily and routinely used in the majority of European countries where roe deer hunting period extends from early autumn to late spring.

Fig 1. Measurements on roe deer mandible.

a: total length measured from the anterior margin of the alveolus of I₁ to the posterior margin of the processus angularis; b: length of the anterior section measured from the posterior margin of the alveolus of C to the posterior margin of the alveolus of P₄; c: length of the posterior section measured from the posterior margin of the alveolus of P₄ to the posterior margin of the processus angularis.

Fig 2. Mandible growth.

Mandible total length in relation to the age class of 24,972 roe deer (12,026 females and 12,946 males) legally shot during the annual harvest (August 1^st-September 30^th and January 1^st-March 15^th) from 2005 to 2015 in Arezzo province (Tuscany, Central Italy).

Fig 3. Mandible sections growth.

Length of the anterior (A) and posterior (B) section of 2,161 mandibles of juveniles (8–10 months; 1,176 females and 985 males), collected during the 2013–2015 hunting seasons in the Arezzo province (Tuscany, Central Italy), in relation to the Julian date (1^st January-15^th March).

Fig 4. Sexual size dimorphism (SSD).

SSD in the length of the anterior and posterior section of 2,161 mandibles of juveniles (8–10 months; 1,176 females and 985 males), collected during the 2013–2015 hunting seasons in the Arezzo province (Tuscany, Central Italy), in relation to the Julian date (1^st January-15^th March).

Fig 5. Mandible sections length.

Length of the anterior (A) and posterior (B) section of 2,161 mandibles of juveniles (8–10 months; 1,176 females and 985 males), collected during the 2013–2015 hunting seasons (1^st January-15^th March) in the Arezzo province (Tuscany, Central Italy), in relation to the dressed body weight of adult females (≥ 2 years). R² is reported for each regression line.

Fig 6. Mandible anterior section as an index of growth constraints.

A: Length of the anterior section of 2,161 mandibles of juveniles (8–10 months; 1,176 females and 985 males) in relation to the forest productivity (fPAR) in March-October during the 2013–2015 hunting seasons (1^st January-15^th March) in the Arezzo province (Tuscany, Central Italy). SSD is reported for each year between error bars and differences between sexes were reported (t test). B: Dressed body weight of 2,161 juveniles (8–10 months; 1,176 females and 985 males) in relation to the dressed body weight of adult females (≥ 2 years) during the 2013–2015 hunting seasons (1^st January-15^th March) in the Arezzo province (Tuscany, Central Italy). Differences between sexes were reported (t test).