In grey mouse lemurs from the dry deciduous forests in western Madagascar each female is receptive for a single night per year. For male mouse lemurs this is a stressful time. The main question in a research project conducted by Nina Schwensow and Simone Sommer of the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin and their colleague Manfred Eberle from the Leibniz Institute for Primate Research in Gφttingen was: What will happen during this special night?
Up to fourteen males visit a single female and females copulate with up to seven different males at maximum. But who will become father?
The surprising answer: The selection of successful sperms must be influenced by internal body mechanisms after copulation. The female’s mate choice was not driven by any preferences and the males could not enforce copulation due to the fact that both sexes are similar in size and body weight (60 gram).
“For a female, pregnancy and lactation is very costly”, say Nina Schwensow and Simone Sommer, biologists from the IZW. “For females, the input of energy is far more effective when the offspring’s constitution and health status is based on genetically superior immuno competence.” Within the scientific community it is widely assumed that females choose males very carefully to maximise the offspring’s probability of survival. Here the so called MHC-genes (major histocompatibility complex) are important. Genes of this group are important components of the body’s immune system; they identify disease agents and activate immune reactions. There is good evidence that individuals recognise the MHC-constitution of conspecifics by odour. With this information females might select the most promising partner.
But this mechanism does not seem to be used by grey mouse lemurs (Microcebus murinus), because copulation takes place without pre-selection of sexual mates. In their single receptive night of the year females do not seem to be choosy. The scientists did not find any differences by comparing accepted and refused sexual mates concerning the MHC-gene setting or any physical attributes.
But: Which of the male mouse lemurs finally became the offspring’s father was clearly linked to immune genes. “The fathers definitively had more variable MHC-genes than the potential males, chosen in randomised simulations”, Schwensow adds. “Furthermore, the father’s MHC-genes were highly different form those of the mothers, which could lead to an optimum in immune competence for the offspring.” The grey mouse lemur’s mate choice is post-copulatory, driven by internal selection processes. For the first time, the hypothesis of so called cryptic post-copulatory mate choice could be proven for a wildlife primate population.
For scientists the reason for this form of mate choice is not clear. Presumably they are highly linked to the mating system, because the male mouse lemurs turn up superior in high numbers during the mating day. Project leader Simone Sommer says:” Presumably it is too costly for females to refuse several highly motivated males”. Promiscuity in connection with post-copulatory selection seems to be the most practical way to optimise the genetic constitution of the offspring.
Detailed results were published recently in the Proceedings of the Royal Society of London.
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