Biology Reference
In-Depth Information
Why do females invest more in offspring
care than do males?
Clearly females tend to be the caring sex, while males tend to compete for matings. We
can link this initially to gamete dimorphism, where small gametes (sperm) compete for
female resources (eggs). But why, after mating, do females still care more?
David Queller (1997) showed that there are two general reasons why females should
care more, both of which follow from common features of mating systems. These are
females mating multiply (polyandry) and sexual selection leading to skewed mating
success amongst males. In both cases, we must weigh up the relative returns to a male
from putting its resources (time and energy) into either helping raise a brood, or
attempting to obtain other mates.
Firstly, consider a mated pair, where there is a chance that another male (or males)
has fathered some of the offspring in the brood. When the paired male has to share
parentage of the brood, this reduces their genetic value to him, and so reduces the
relative benefit of staying to help. Put simply, there is little benefit from staying to help
raise the offspring of other males! Consequently, we would expect males to be less likely
to care in species where paternity is shared between multiple males.
Secondly, consider a species in which sexual selection leads to a skewed mating
success amongst males, with some males obtaining an above average share of the
matings. Queller (1997) realized that no matter how much greater their
potential
reproductive rate, the
average actual
reproductive rate of males must be exactly the
same as that of females, if the sex ratio is 1:1. This must be true, because each offspring
has exactly one mother and one father. However, consider the subset of males and
females who have succeeded in mating and so who have offspring available for care. If
most or all of the females breed, while only a fraction of the males are successful
(namely the strongest competitors) then the expected future reproductive success of
these few successful males will indeed be greater than that of the females. So these
males have more to gain by not helping and instead putting more resources into
obtaining other mates. Consider, for example, a group of 20 displaying male grouse in
which the largest and most vigorous male does all the mating (which will be with 20
females, if the sex ratio is 1:1). The expected reproductive success of this male is,
therefore, 20 times that of a female. This male should be less inclined to care than a
female because care would detract more from his potential future reproductive success
than it would from her success.
Therefore, the logic of Queller's (1997) second point is as follows: if females invest
more prior to mating, there will be competition among males for female investment; this
will lead to greater variance in male success (some males, the best competitors, will
have great success while others will fail to mate); these successful males will then be less
inclined to care after mating. As females provide more care, sexual selection on males
intensifies, so there is positive feedback, making it even less likely that males will care
(Kokko & Jennions, 2009). Box 7.1 summarizes some of the problems in measuring the
strength of sexual selection.
For males,
parental care is
often of less
benefit because
of paternity
sharing …
… and the
opportunity costs
of care are often
also greater for
males
