Data from: Testis asymmetry in birds: the influences of sexual and natural selection

Gonad size and shape asymmetries are particularly common in birds.
Although some obvious size and shape differences between the left and
right testes in birds were first documented more than a century ago,
little is known about what influences the variation across species in
either the degree or the direction of these asymmetries. Here we show that
a left bias in size is the most likely ancestral state in most orders and
families, and that there is a weak but significant negative relation
between the degree of size and shape asymmetries. In extant species,
testis size and shape symmetries increase with the degree of sperm
competition (relative testes mass), but those relations are significant
only in those species with left bias in each of these traits. When space
is particularly constrained on the left side of the body cavity due to a
large gizzard (e.g. in granivores), the left testis is more elongated and
the degree of size symmetry is larger. Despite significant patterns,
relative testes mass and gizzard complexity together explain < 10%
of the variation in testis asymmetry in species with left biases. Thus our
analyses suggest that some other factor is largely responsible for the
evolution of gonad asymmetry in birds: 1) that a left bias in testis size
might be a correlated response to selection for a left bias in the
development of a single (left) ovary in females of most bird species,
and/or 2) that physiological efficiency due to the dramatic and rapid
increase in testis size of most species during the breeding season might
favour enlargement of one testis. Our conclusions highlight the need to
rethink and improve our understanding of the physiological processes
underlying the investment in gonads by male birds.