In this thesis, I investigate whether mothers of the polygynous fallow deer, Dama dama, bias their investment towards male offspring. I aim to answer the specific questions: (i) how is offspring fitness related to maternal investment?; (ii) do male fawns grow faster than female fawns during the period of maternal investment, and if so, why?; (iii) are male fawns more costly to raise than female fawns?; (iv) do fallow deer mothers adjust the sex of their offspring in relation to their ability to invest resources? To answer these questions I have collected and analysed data on reproduction of an enclosed fallow deer population and also performed an experimental study in which the level of investment was controlled for.
Heavier mothers gave birth earlier and to larger offspring which grew at a faster rate. Pre-winter body mass of fawns was also found to affect sub-adult body mass in both sexes indicating that maternal investment influences offspring adult body size and, presumably, their fitness. The effect of pre-winter body mass on sub-adult body mass was stronger in male than in female fawns. Male pre-winter body mass was also constantly higher than female pre-winter body mass. Presumably, this is because male reproductive success varies more with body size than does female reproductive success and, hence, there has been stronger selection for early growth in males than in females. Male fawns grew faster than female fawns both before and after birth, independent of their birth date, but there were no behavioral differences between the two sexes. Furthermore, the faster male growth could not be explained by mothers in better condition (heavier mothers) producing male offspring and mothers in poorer condition producing females.
When male and female fawns were given exactly the same amount and quality of milk by bottle-raising them, the difference in weight gain between bottle- and mother raised male fawns was significantly larger than between bottle- and mother-raised female fawns during the time when they solely consumed milk. It was also evident that bottle-raised male fawns suck harder and acquired milk at a more rapid rate than female fawns. These results suggest that male fawns receive more milk from their mothers than female fawns do.
Mothers with sons accumulated less body mass than mothers with daughters during the period from late pregnancy to the end of lactation which gives additional support for the conclusion that male fawns receive more milk from their mothers. The higher energetic cost of raising a male was also translated into a higher reproductive cost, as indicated by later birth date and lower pre-winter mass of offspring the following year. Pre-winter body mass the first year is related to adult body mass and, most likely, to reproductive success in fallow deer. Consequently, the higher milk production and the corresponding higher energetic cost of raising sons do affect the mother's future offspring's chances to reproduce and, thus, support theories of male-biased maternal investment.
Stockholm: Department of Zoology, Stockholm University , 1997. , 32 p.