Letter to the editor |
Reply: Relationship between estrogen urinary levels before conception and sex ratio at birth in a primate, the gray mouse lemur
UMR CNRS-MNHN 5176, 4, avenue du Petit Château, 91800 Brunoy, France
E-mail: Martine.Perret{at}wanadoo.fr
Sir,
I thank Dr Cagnacci for his comments on my paper (Perret, 2005
). In this article, it was not in my mind to extrapolate the mouse lemurs’ results to humans but just to give a finding that could lead to further research. By the way, most of the criticisms exposed by Dr Cagnacci come from a faulty knowledge of the life history traits of mouse lemurs that are evidently very different from human ones.
One of the criticisms concerns female body mass and its possible role on sex ratio bias towards males. First, as indicated in the article, no relationship has been evidenced between the sex ratio of the litter and the body mass of the female (body mass versus litter type r = 0.113, n = 67, not significant). A separate analysis by age category gives the same results (r = 0.0, r = 0.1714 and r = 0.402 for young, adult and old females respectively, not significant). Dr Cagnacci suggested separating females by body mass group. This has been done for adult females divided into three groups according to their body mass: <80, <100 and >100 g, n = 37). All females demonstrated a significant sex ratio at birth biased towards males (G test = 13.7, df 2, P < 0.001) with no significant difference between the three body mass groups (G test = 0.15, df 2, not significant).
An effect on body composition (i.e. body mass, body composition, fat stores) on sex ratio bias is unlikely since all females in captivity are fed ad libitum, and thus, all females including the younger ones are in the best condition to invest in reproduction. Indeed, mouse lemurs undergo a pronounced fattening during autumn to go through the scarce winter season but also to invest in their reproduction during the following breeding season. Fat stores are used in the spring for pregnancy and lactation. In that way, the lower body mass of young females represented only a relatively lower fat storage owing to their young age at their first autumn. Lastly, in the field, the mean body mass of wild females is significantly lower (60–80 g) than in captive females. In wild females, an effect of body mass on litter production may exist but it has not yet been evaluated.
As underlined by Dr Cagnacci, discrepant results were described for the direction of sex ratio bias in women. Increasing sex ratio during the two world wars is contradictory to the decline in sex ratio during the recent wars or stressful situations. It seems easy to refer to behavioural components in one case (increased coital rate?) and to physiological mechanism in the other (stress, hormones?). What can really be ascertained is that records conducted in women do not allow to determine precisely which physiological/behavioural mechanisms underlie sex ratio bias in humans.
The hypothesis resulting from our study is that estrogen level during the follicular phase may be an important factor for biasing sex ratio, at least in mouse lemur. In this primate species, a complete ovarian rest is present during the winter (only primary follicles remain visible in the ovary) and in spring, in a very short time (
3 weeks), several primordial follicles develop to reach ovulation. Consequently, the impact of estrogen levels (gonadotrophin) is supposed to be more important than in women for who many secondary follicles are always present in the ovary. Our suggestion emerging from our results is not to state that a similar mechanism may explain sex ratio bias in human, but merely to test estrogen levels during the pre-ovulatory phase in addition to that present at ovulation time. This could be done in most primate species including humans and with reference to specific ovarian characteristics.
Reference
Perret M (2005) Relationship between urinary estrogen levels before conception and sex ratio at birth in a primate, the grey mouse lemur. Hum Reprod 20,1504–1510.
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