Smaller danios get the girls...

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Size is not everything for male Zebra danios (Danio rerio), according to a study to be published in a forthcoming issue of the journal Ethology.

Penelope Watt and coauthors found in their study that a small size for male Zebra danios is advantageous in some mating situations, because the enhanced manoeuvrability that comes with being smaller may enable males to get closer to the females during breeding.

In many species, dominant males are usually larger than subordinate males, but this is not the case in Zebra danios, where dominant and subordinate males are generally of the same size. The researchers showed that size does matter in Zebra danios, but not in the way anticipated.

The authors carried out a series of experiments to compare the paternity success of dominant and subordinate males in pairs that controlled for social status.  

They set up 50 groups consisting of four same-aged male Zebra danios and allowed them to establish a stable dominance hierarchy.

Once the dominant fish was identified, the dominant male from one group was paired up with another dominant male from a second group such that it was anticipated that one would become the dominant and the other the subordinate.

The two males were then placed in a tank within sight (but not in physical contact) with four females, and the identity of the dominant male within the pairing was established by behavioural observation.

The two males were finally allowed to mate with a single female for 10 minutes, and the parentage of the offspring produced were determined using eight microsatellite loci.

As expected, the authors found no statistically significant differences in either body or testes sizes between dominant and subordinate males, and a higher proportion of the offspring sired during the 15 matings that resulted belonged to those of the dominant males.

However, the authors found that the subordinate males were more successful at mating and gaining paternity when they were smaller than the dominant male they were paired with.  

The smaller subordinate males may also compensate for their decreased reproductive success relative to larger dominant males by mating more often, as previous studies have shown.

The authors hypothesise that small male Zebra danios may be more manoeuvrable than large males and so may be able to get closer to the female during spawning.

Alternatively, the authors posit because the small males are more persistent, females may choose to mate with them rather than to expend too much energy and time trying to avoid them.  

Whatever the true reason, the relative reproductive advantage conferred to smaller males may be one of the mechanisms by which variation in body size is maintained in Zebra danios.

For more information, see the paper: Watt, PJ, A Skinner, M Hale, S Nakagawa and T Burke (2011) Small subordinate male advantage in the zebrafish. Ethology doi:10.1111/j.1439-0310.2011.01953.x

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