After removing duplicate samples from each population, detected either from photographic identification or genotype identity, a total of 972 identified individuals were included in the analyses. The mean numbers of alleles per locus (A) were 12.91 for Gabon and 13.09 for Madagascar. The largest number of alleles (21) was found at locus GATA417 for Madagascar; the smallest (4) was recorded at locus EV1Pm for both sampling sites. No significant differences were found between the observed heterozygosity (Ho=0.76±s.d.=0.01 for Gabon, Ho=0.75±s.d.=0.00 for Madagascar), and the heterozygosity expected under Hardy-Weinberg assumptions (He=0.76±s.d.=0.05 for Gabon, He=0.75±s.d.=0.05 for Madagascar). Deviation from HWE was rejected, and there was no evidence of LD (p<0.05) in the 110 pairwise tests performed.
The low estimate obtained for the average PI (PI=1.123×10−12) allowed us to conclude that any match recovered when searching genotype data for between-site matches would indicate inter-oceanic migration of an individual. One sample collected in 2000 in Antongil Bay, Madagascar, matched a sample collected two years later off Loango National Park, Gabon (, ; PIsib=3.114×10−5 and PIpof=5.382×10−9 for Madagascar, PIsib=2.811×10−5 and PIpof=5.640×10−9 for Gabon). Both samples were collected from a male individual and shared a locally rare mtDNA lineage (frequencies: p=0.008 in Madagascar and p=0.006 in Gabon, H. C. Rosenbaum et al., unpublished data). A photographic comparison has not yet been attempted for datasets from these two areas. However, a comparison of dorsal fin characteristics in photographs taken at the time of sampling confirmed the genetic match.
Figure 1 Humpback whale sampling areas and migration routes around Africa. Stars indicate the locations where the two matching samples were collected on July 17th 2000 in Antongil Bay (16°00′ S, 49°55′ E), Madagascar, and on August (more ...)
In Madagascar, this individual was accompanied by a female and analyses of genotypes putatively identified her as his mother (relationship log(ratio)=1.673, p
<0.001; ; and see electronic supplementary material for kinship analyses). Other behavioural data show that humpback whale calves remain with their mother for their first year, and occasionally for their second year (Glockner-Ferrari & Ferrari 1984
; Baker et al. 1987
; Clapham & Mayo 1987
). Therefore, when sighted in Gabon two years later, this individual could have been a three- or four-year-old juvenile, as the mean age at attainment of sexual maturity in male humpback whales is five years (Clapham 1992
; Chittleborough 1955
Gene flow estimates from Migrate based on seven sampled humpback whale populations indicated that the eastern South Atlantic and the southwestern Indian Oceans are expected to exchange approximately 35 migrants per generation (Nem) in each direction (95%CI=27.1–45.1). However, the same analysis estimated that only approximately one individual per generation (Nem=0.8, 95%CI=0.6–1.3) is expected to migrate from Madagascar to Gabon.