VARIATION OF SECONDARY SEX-RATIO IN BIRDS AND OTHER TETRAPODES - THE CASE OF COLIUS-STRIATUS NIGRICOLLIS (COLIIFORMES)

A review shows that in tetrapods the sex of individuals may depend upo n both genetic and environmental factors according to biochemical meca nisms currently studied in Amphibians and Reptiles. Some bird species are known to display extraordinary sex-ratios at birth, globally or ac cording to population, year, season, status of female or of breeding g roup, and even ''quality'' of male mate. In some cases a relationship between egg-laying sequence and sex of hatchlings was documented. It i s most often suggested that such skewed sex ratios in birds might depe nd upon maternal conditions, notably trophic or/and physiological ones , but such correlations prove to be quite intuitive, and actual decisi ve mecanisms are still unknown. This problem is dealt with studies of Speckled Mousebirds Colius striatus conducted in two populations with opposite biogeographical conditions: an expanding one near Makokou in North-eastern Gabon forests (1973-1977), and a relatively insulated on e, in Yaounde area, Cameroon, where empty places were rare for colies (1982-1985). In the first case, males always outnumber females at any age but in the studied sample it was necessary to include young up to 10 months of age in order to obtain a significant bias (N = 51). In th e second case, fledgling sex ratios were recorded during a 2-year peri od in 7 breeding groups. Trophic qualities and reliabilities of the ma in home ranges of these groups were evaluated during 13 months. Global ly, females outnumber males, though the difference is highly significa nt only when data of an extraordinary group are removed (N = 105 - 17) . The main home range of this last group which uniquely had male-biase d broods was an exceptionally rich 60 ha area (a botanical garden), Th e comparison and discussion of all data highly suggest that extraordin ary good feeding conditions can result in male-biased secondary sex ra tios in this species, and that undernourished groups can produce super numerary females, In such a social species psychophysiological status of breeders, even helpers, could also be crucial (see, B group case hi story in Cameroon). In reality, these relationships would be very diff icult to find in nature, except in exceptional habitats like those sel ected for this research, because they are not evident at wide-scale po pulation levels and because this colonizing species most often lives i n heterogeneous and/or fast-changing areas around human settlements, C onclusions about three a priori hypotheses about sex ratio adjustments (i.e., maternal control of ovogenesis, differential mortality of embr yos, and environmentally provoked sex reversal) and the very unique me tabolism among birds of this fruit-eating and folivorous species sugge st that the temperature experienced by the embryo would determine its sex. This reptilian ecobiological scenario is discussed after ecophysi ological, ecobehavioural, and ecobiogeographical data available for th e Speckled Mousebird. This novel proposal would be easy to test with c aptive bred colies or even other birds or mammals (e.g. Marsupial, Mol e-rat) with skewed sex ratios documented at birth according to trophic /metabolic, even climatic conditions, Such epigenetic sex ratio adjust ments would have important ecogenetical and biogeographical consequenc es, particularly in colonizing group-living species.