1. The relationships between female body mass (WWal)i, tter size (m), juvenile growth
rate (G) and mass at weaning (W20) were examined by monitoring natural litters in
29 greater white-toothed shrews, Crocidura russula (Hermann 1780). The trade-offs
between m and G or W20 were further investigated by manipulating litter sizes: each
of seven females reared four litters of 2, 4, 6 and 8 offspring.
2. Offspring mass at weaning (W20) exhibited a large variance, most of which could
be attributed (ANCOVA on manipulated litters) to two effects: a litter-size effect, and a
female individual effect, referred to as 'female quality'.
3. Litter size explained 68% of the variance in W20 among manipulated litters (linear
regression). The limited milk supply was probably responsible for this effect, because
litter size depressed growth rate during the first half of the lactation period (G1), but
not during the weaning stage (G2).
4. Among non-manipulated litters, litter size correlated positively with maternal body
mass (Wa), so that large females tended to produce small juveniles. This correlation
between m and Wa is seen as the result of a body-mass dependence in the cost of
raising a litter of a given size, during either pregnancy or lactation.
5. Differences in 'female quality' explained 16% of the variance in W20 among manipulated
litters. This factor did not affect GI and may thus relate to differences among
offspring of different females in their rates of processing milk and/or external food
during late lactation.
6. 'Female quality' was independent of both body mass and litter size: larger females
did not produce larger offspring when controlled for litter size, while higher-quality
females did not produce larger litters.
7. Our results support the hypothesis that most variance in adult and juvenile body
masses is non-genetic, and stems from the trade-off between litter size and offspring
size.