Inbreeding alters sexual traits of house mice

Researchers at the Konrad Lorenz Institute of Ethology (KLIVV) at the University of Veterinary Medicine, Vienna recently published a study in which they investigated two common hypotheses on primary and secondary sexual traits using male wild-caught house mice (Mus musculus musculus). They found that inbreeding influences the expression of primary and secondary sexual traits. According to the scientists, this is the first experimental evidence that the courtship songs and sperm quality of house mice are both influenced by inbreeding, though, contrary to what they expected, that these sexual traits were not positively or negatively correlated, i.e., male courtship song did not provide an honest indicator of their fertility nor was there a trade-offs between these traits.

Foto: Bettina Wernisch

Two hypotheses on the evolution of sexual traits

The Fertility Indicator Hypothesis states that the expression of secondary (pre-copulatory) sexual traits, such as courtship behavior or songs, by males provides reliable indicators of their sperm traits (primary or post-copulatory traits) and predicts positive correlations between these pre- and post-copulatory traits. In contrast, the sexual allocation trade-off hypothesis claims that males make trade-offs between investments in primary and secondary sexual traits. However, whether or not males face such trade-offs may depend on their quality or condition. ‘If the sexual traits of males are condition-dependent, and males in good condition are thus better able to invest into both pre- and post-copulatory traits than males in lower condition, then both hypotheses are possible,’ says study co-author Dustin J. Penn.

Only two generations of inbreeding change sperm quality and courtship behavior

To test these hypotheses, the researchers manipulated the genetic quality of wild-caught male house mice through experimental inbreeding and tested whether inbreeding affects primary or secondary sexual traits. "First, we recorded the courtship behavior and vocalizations of the males when they were in contact with females, Then we measured the males' reproductive organs and the quality of their sperm, as well as the expression of four genes associated with spermatogenesis. We then tested whether these pre- and post-copulatory traits correlate," reports study first author Doris Nicolakis.

The scientists found no difference in the number of courtship calls, but inbreeding altered the vocal repertoire and reduced other courtship behaviors in male house mice. ‘Inbreeding also had a negative effect on testes mass as well as on sperm quantity and quality,’ says Nicolakis.

However, the scientists found no evidence to support either the fertility indicator hypothesis or the sexual allocation trade-off hypotheses. Study last author, Sarah M. Zala comments: ‘We saw no consistent correlations between pre- and post-copulatory traits, either positive or negative, independent of inbreeding.’ However, the authors explained why studies are needed in the wild or naturalistic conditions, as the expected correlations may not occur in laboratory settings.

The study “Pre- and post-copulatory traits are affected by experimental inbreeding, but they are not correlated” by Doris Nicolakis, Maria Adelaide Marconi, Kerstin E. Auer, Dustin J. Penn und Sarah M. Zala was published in “BMC Biology”.

Scientific article

Scientifc contact:
Doris Nicolakis MSc.
Konrad-Lorenz-Institut für Vergleichende Verhaltensforschung (KLIVV)
Veterinärmedizinische Universität Wien (Vetmeduni)
Doris.Nicolakis@vetmeduni.ac.at

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