Benefits of migration in a partially-migratory tropical ungulate

Nicolas Gaidet,Philippe Lecomte

doi:10.1186/1472-6785-13-36

Abstract

BackgroundPartial migration, where one portion of a population conducts seasonal migrations while the other remains on a single range, is common in wild ungulate populations. However the relative costs and benefits associated with the distinct strategies adopted by coexisting migrant and resident individuals have rarely been investigated. Here we compare the body condition of migrants and residents in a partially migratory population of impalas (Aepyceros melampus) in Zimbabwe. The study was conducted during two consecutive years with highly contrasted population densities (16.4 and 8.6 indiv/km2) due to harvesting.ResultsWe first identify a population substructure with a north–south sub-division in two spatial units related to distinct soils and vegetation cover. Impalas in the north range had a consistently higher diet quality and body condition than those in the south range. At the beginning of the dry season about one third of the individuals migrated from the lower (i.e. south) to the higher (i.e. north) diet quality range. This partial migration pattern was consistent between the consecutive years, and most individuals showed constancy to their moving strategy (migrant or resident). In both years, these migrants had a significantly higher body condition at the end of the dry season than the south residents that remained year-round in the lower diet quality range. Diet quality and body condition of impalas were higher in the year of lower density; however we did not detect any evidence for density-dependence in migration propensity, at the individual or population levels, nor in the benefit associated with migration.ConclusionsOur findings provide rare evidence for a significant relationship between body condition and seasonal migration strategy in wild ungulates in relation to a difference in the quality of resources acquired between distinct seasonal ranges. This study also constitutes rare evidence of partial migration in a tropical ungulate population.

Highlights

Partial migration, where one portion of a population conducts seasonal migrations while the other remains on a single range, is common in wild ungulate populations
Diet quality We found a seasonal reduction in diet quality (Figure 5) with a significant decrease in nitrogen concentration (FN) and a significant increase in fibre concentration (ADF) between the wet and the dry seasons (Anova, Season: F = 118.22, p < 0.001 for Faecal nitrogen (FN); F = 43.36, p < 0.001 for Acid detergent fibre (ADF))
We found a significant difference in diet quality between years, with a higher nitrogen and a lower fibre concentration in the second year when impala density was lower (Year: F = 191.15, p < 0.001 for FN; F = 49.00, p < 0.001 for ADF)

Summary

Introduction

Partial migration, where one portion of a population conducts seasonal migrations while the other remains on a single range, is common in wild ungulate populations. The relative costs and benefits associated with the distinct strategies adopted by coexisting migrant and resident individuals have rarely been investigated. Only a fraction of the population migrates between distinct seasonal ranges while the others remain sedentary year-round (i.e. partial migration [12]). This difference in strategy to track resources among co-existing migrant and resident individuals may contribute to shape individual variations in performance. Comparative measures of the relative performances of migrant and resident individuals have rarely been conducted to evaluate the costs and benefits associated with distinct migration tactics [17,18,23]

Methods

Results

Discussion

Conclusion