Keeping an eye on the use of eye-lens weight as a universal indicator of age for European wild rabbits

Abstract

Accurate methods for age determination are critical to the knowledge of wildlife populations' age structure and, therefore, to their successful management. The reliability of age estimation may have profound economic and ecological consequences on the management of the European wild rabbits, Oryctolagus cuniculus, in its native and introduced range, where it is a keystone species and a major pest, respectively. As in other mammal species, European rabbits' age is often estimated using the Gompertz relationship between age and lens' weight. The growth rate formula has been developed based on data collected from European rabbits introduced in Australia, where a single subspecies (O. cuniculus cuniculus, Occ) is present. However, this curve has never been validated in the species native range, the Iberian Peninsula, where two subspecies (Occ, and O. c. algirus, Oca) coexist naturally. In this study, we tested the relationship between age and lens' weight using 173 Occ and 112 Oca wild rabbits that were surveyed in two experimental facilities in Spain. Our findings show that, in the native range, the published growth curve formula fits well Occ but not Oca data. Therefore, we recommend using the formula reported in this study to estimate the age of Oca (Lens dry weight = 240 × 10(−64.9/(Age+32))). This study supports Oca rabbits' distinctiveness revealed by previous studies, which suggests that management interventions should be applied to protect this subspecies whose distribution range is very narrow and whose populations seem to be declining. More broadly, our findings point to the importance of testing the suitability of growth curves defined for other species with different genetic forms as occurs in the European wild rabbit case.

Improving the method of lens mass preparation for age assessment in the European brown hare (Lepus europaeus)

“…Therefore, with the application of the first and third modified methods, the time required for obtaining the analyses is 5 days, with the fixation duration in formalin being 48 hours. In most of the available studies that dealt with determining the age of hares, the fixation time was longer and ranged from 3 days (Yamada et al 1990, Pintur et al 2006, one week (Caboń- Raczyńska & Raczyński 1972), one month (Keith and Cary 1979), two months (Vaquerizas et al 2021), to 5 months (Andersen & Jensen 1979). Additionally, the third modified method involved the use of a 6% formalin solution, while the formalin concentration in the aforementioned studies was higher (10%).…”

“…This time is also shorter compared to most available studies on hare age determination. Drying time ranged from 3 days at 80°C (Yamada et al 1990) and 3 days at 37°C (Pintur et al 2006), 4 days at 90°C (Andersen & Jensen 1979), 5 days at 80°C (Keith and Cary 1979), all the way to 14 days at 85°C (Vaquerizas et al 2021). On the other hand, in the study conducted by Flis et al (2019), the drying time was shorter (24 hours at a temperature of 100°C), following the methodology of Méres et al (2013).…”