Aging and improving reproductive success in horses: declining residual reproductive value or just older and wiser?
Cameron, EZ and Linklater, WL and Stafford, KJ and Minot, EO, Aging and improving reproductive success in horses: declining residual reproductive value or just older and wiser? , Behavioral Ecology and Sociobiology, 47, (4) pp. 243-249. ISSN 0340-5443 (2000) [Refereed Article]
In many mammalian species, female success
in raising offspring improves as they age. The residual
reproductive value hypothesis predicts that each individ-
ual offspring will be more valuable to the mother as she
ages because there is less conflict between the current
and potential future offspring. Therefore, as mothers age,
their investment into individual offspring should in-
crease. Empirical evidence for an influence of declining
residual reproductive value on maternal investment is
unconvincing. Older mothers may not invest more, but
may be more successful due to greater experience, al-
lowing them to target their investment more appropriate-
ly (targeted reproductive effort hypothesis). Most studies
do not preclude either hypothesis. Mare age significantly
influenced maternal investment in feral horses living on
the North Island of New Zealand. Older mares, that were
more successful at raising foals, were more protective
for the first 20 days of life, but less diligent thereafter.
Total maternal input by older mothers did not seem to be
any greater, but was better targeted at the most critical
period for foal survival and a similar pattern was ob-
served in mares that had lost a foal in the previous year.
In addition, older mothers were more likely to foal in
consecutive years, supporting the hypothesis that they
are investing less than younger mares in individual off-
spring. Therefore, older mothers seem to become more
successful by targeting their investment better due to ex-
perience, not by investing more in their offspring.