Biology Reference
In-Depth Information
the male builds a foam nest and releases sperm before the female lays eggs. In these
cases, the 'opportunity for desertion' hypothesis predicts that males can desert first, but
nevertheless parental care is provided by the male. Therefore, male parental care
remains correlated with external fertilization independently of the order of gamete
release or opportunity to desert (Gross & Sargent, 1985).
Hypothesis 3: Association
Williams (1975) suggested that association with the embryos preadapts a sex for
parental care. For example, with internal fertilization the female is most closely
associated with the embryo and this may set the stage for the evolution of embryo
retention and live birth, followed by care of the young fry. With external fertilization, on
the other hand, the eggs are often laid in a male's territory and it is the male who is most
closely associated with the embryos. Defence of the territory in order to attract further
females becomes, incidentally, defence of the eggs and young, and therefore provides a
preadaptation for more elaborate parental care by males. Therefore, male care involves
fewer opportunity costs (lost matings) than in other cases, because a male that guards
eggs can still attract more mates. In fact, females sometimes prefer males that already
have eggs in their nest (Hale & St Mary, 2007). This hypothesis is the best predictor of
the data in Table 8.2. Male parental care is more common in territorial species and the
prevalence of male parental care with external fertilization results from the fact that
male territoriality is particularly common with external fertilization.
Male care in fish
is associated with
male territoriality
Parental investment: a parent's optimum
We now change focus, from broad comparisons across different taxa (birds, mammals,
fish) to consider what factors influence the amount of care a parent should devote to its
offspring. Robert Trivers (1972) introduced the concept of
parental investment
, which he
defined as 'any investment by the parent in an individual offspring that increases the
offspring's chance of surviving (and hence reproductive success) at the cost of the parent's
ability to invest in other offspring.' Parental investment will include any investment, such
as guarding or feeding, that benefits the eggs and young. Lifetime parental investment will
be the sum of all the resources a parent can gather in its lifetime and use for offspring care.
From a parent's point of view, what is the optimal parental investment per offspring?
The key point is that this will involve trade-offs, because whereas increased investment in
any one young will bring benefits to that offspring, there will be costs to the parent in
that it reduces the resources available for other offspring. These trade-offs will operate at
two stages. The first was recognized by David Lack (Chapter 1) and involves the trade-off
between offspring quantity and quality
within a brood
. If a parent spreads its limited
resources thinly among too many offspring, then few of them will survive. On the other
hand, if it uses its resources too generously among a small brood, then other parents will
produce more surviving young and will outcompete it over the generations. In theory,
there will be an optimal brood size to maximize productivity per brood. The second trade-
off, first recognized by G.C. Williams (1966b), involves that between investment in
current versus future broods
. To maximize its lifetime success, a parent needs to allocate
Investment
trade-offs within
broods and
between broods
