Biology Reference
In-Depth Information
ESS 3
:
female cares and male deserts
. This requires
wP
1
>
WP
0
, or the female will desert,
P
2
, or the male will care.
ESS 4
:
female cares and male cares
. This requires
wP
2
and
P
1
(1
+
p
)
>
>
WP
1
, or the female will desert, and
P
2
>
P
1
(1
+
p
) or the male will desert.
For given values of the parameters in the model, ESS 1 and ESS 4 can be alternative
possibilities, as can ESS 2 and ESS 3. For example, ESS 2 is most likely if the female can
lay many more eggs if she does not invest in caring (
W
>>
w
) and if one parent is much
better than none (
P
1
P
0
) but two parents are not much better than one (
P
2
≈
P
1
). This
situation probably applies to many fish, as discussed above, where the female tends to
desert and the male often cares. However, ESS 3 is an alternative possibility, especially if
a male who deserts has a much better chance of mating again; this may apply to some
species of birds and mammals. If two parents can raise twice as many young as
one (
P
2
>>
P
1
), or if the chance that a deserting parent will remate is small, then ESS 4
is the likely outcome, as in many species of passerine birds.
If you haven't spotted the flaw in the pay-off matrix in Table 8.3, then you are in good
company because it remained unnoticed for 25 years (Kokko & Jennions, 2003). The
problem, pointed out by Michael Wade and Stephen Shuster (2002) is that deserting
males gain extra offspring by mating with females who 'appear from nowhere', in the
sense that these females do not appear in the calculations of female fitness. Males,
therefore, have more total paternity than is possible from the number of offspring
produced by females. Wade and Shuster refined the model to show exactly where these
extra offspring might come from. For example, deserting males might steal some
paternity in care-giving male broods, or they might mate with females who have
deserted care-givers. These considerations complicate the pay-off matrix. Nevertheless,
the fundamental insight provided by Maynard Smith's model remains: the key to
understanding decisions by one sex is the decisions made by the other parent.
While Maynard Smith's model still provides a useful framework for thinking about
the evolution of parental care, the assumption that parents make independent decisions
(blind bids) over caring and deserting is unlikely to apply to most cases in nature. For
example, independent decision making would sometimes lead both parents to desert in
St Peter's fish, but at least one parent always cares (Balshine-Earn, 1995). More
realistic models need to incorporate sequential decision making and opportunities for
deserting first.
>>
Correcting a flaw
in the model
How much care?
There is also sexual conflict over how much care to provide. For example, a female
scissor-tailed sergeant fish would do best if the territorial male guarded all her eggs safely
through to hatching, but the male might do best to cannibalize some, or even all, of her
brood to maximize his success over all the clutches he will fertilize in his lifetime. Sexual
conflict will also occur during biparental care. We may not see obvious conflicts as a pair
of birds works hard to provision a hungry brood, but a simple experiment reveals the
underlying conflict. If either parent is removed temporarily, then the other parent often
increases its work rate. This shows that each parent has the capacity to work harder.
How then do cooperating parents come to an agreement over how hard each should
