Mongoose inbreeding maintains social system?
In the Biology Letters of the Royal Society, H. J. Nichols, M. A. Cant, I. Hoffman and J. Sanderson are the authors of a foul calumny against the mongoose race! Working at Bielefeld University in Germany and Exeter and Liverpool John Moores University, they have discovered incest in the family. It is no joke, as several human populations have discovered in the past. The genetic disadvantage of inbreeding depression in the banded mongoose is countered by survival gains in remaining with the social group. Because of this close sociability, banded mongoose are basically very large meerkats,Suricata suricatta, which we all know have great social survival skills.
Breeding between close relatives, as the authors say,
unmasks harmful recessive alleles. Even females in this species mate with strangers when the frequent group fights give them a chance to copulate, very briefly! In theory the inbreeding would (1) increase inclusive fitness, and (2) increase cooperative benefits. A large genetic dataset was therefore prepared for Mungos mungo groups with a mean number size of 18.
With all females allowed to breed (unlike the meerkat), the animals remain in their natal groups throughout. Pups are reared communally so that mating after adolescence is often with familiar individuals, though siblings may not be recognised. If a new group forms, a cohort of females from a group meet with a cohort of males from another. Older groups with small numbers would have high levels of inbreeding.
20 microsatellite markers marked out parentage for some group members while the frequency of female breeding in the group was noted in the 14 groups found in Ugandas Queen Elizabeth National Park. This study took 16 years with animals habituated to observation within a 5m distance. Every few days, the whole life cycle could be tracked and every dispersal or breeding event recorded.
The results were analysed for 516 pups, 64% of which had been born to females that used resident males as fathers. 18% were born to females mated with extra-group males and 18% were born to females that had dispersed from the group. Most of the latter pups were fathered by the resident males (ie. 77 out of 95 pups.) As a group aged, relatedness of parent pairs increased significantly, but smaller groups did not follow that expected pattern.
Inbreeding appears to be regular in the banded mongoose. Detailed analysis reveals 8.5% of cases with close inbreeding and a further 16.7 % of moderate inbreeding in the pedigrees. Prairie dogs and meerkats have similar patterns of inbreeding but these few species stand out in the mass of less <2>sociable mammals. Father/daughter incest was the most common (there were 8 examples), as mothers often die before their sons reach a normal breeding age of up to 4 years old. No mother/son breeding was recorded in 170 observations. Other mammals avoid this inbreeding by dispersing the females at adolescence or mating beyond the group. Why does this mongoose inbreed. It could be the extremely high mortality rate within the new groups studied here. They suffered treble the normal deaths. Violence is also the answer to outbreeding for females, as their groups are always at war.
The importance of this paper lies in the use of these rare examples of
inbreeders to discover how the genome and the social system cope with the strain. Obviously, there are benefits, but it is extremely useful to know if and how the alleles that would reduce individuals fitness are dealt with in the species. It would be useful for conservation if the famed Malagasy fossa were to be investigated to see just how sociable or inbred its system of mating is. These magnificent predators are a giant mongoose, with severe conservation worries.