Biology Reference
In-Depth Information
parental care optimally not only within
broods but also between broods, as
increased investment in any one brood
will reduce a parent's ability to invest in
future broods.
Fig. 8.2 shows the theoretical
optimal investment per offspring from
a parent's point of view. Increased
investment in any one offspring will
bring diminishing benefits, as the care
provided begins to saturate the
offspring's needs. Increased investment
will also bring increasing costs in
terms of reduced resources available
for other offspring, both in current and
future broods. There will be an
optimum where the benefits minus the
costs are at a maximum.
There is good experimental evidence
for this trade-off between the benefits
and costs of parental care, but the form
of the trade-off varies between species.
In some cases increased investment in
the current brood reduces the parent's
survival. In side-blotched lizards
Uta
stansburiana
, gravid females not only
have the extra mass of their eggs to
carry, but their distended abdomens
hinder their leg movements. When
some females had half their eggs removed surgically, they had improved locomotary
performance (measured on a treadmill) and were more likely to survive to produce
another clutch, probably because of reduced predation (Miles
et al
., 2000). In other
cases, increased investment reduces an adult's future fecundity, rather than survival.
When male common gobies,
Pomatoschistus microps
, were induced to invest more in
fanning the eggs in their nest (by reducing levels of dissolved oxygen in their aquaria)
they lost more mass and were more likely to abandon their next clutch (Jones & Reynolds,
1999). When male and female burying beetles
Nicrophorus vespilloides
were induced to
care for a large brood of larvae in their first breeding attempt, they subsequently
produced fewer larvae from future broods than those that cared for a small brood the
first time they bred (Ward
et al
., 2009). Similarly, when collared flycatchers
Ficedula
albicollis
were induced to increase their feeding rates to their current brood (by increasing
brood size), both males and females survived as well as control birds but they had
reduced fecundity the following year (Gustafsson & Sutherland, 1988).
Obviously, increased reproductive effort may involve greater exposure to predators or
compromise the breeder's ability to maintain its own body condition, and so reduce a
breeder's survival. But why would increased current reproduction sometimes reduce
only future fecundity? One possibility is that resources allocated to reproduction are at
B
C
Parental investment per offspring
Fig. 8.2
The optimal parental
investment per offspring from a parent's
point of view is where the Benefits
minus Costs are at a maximum.
Increasing investment brings diminishing
benefits as the offspring's needs become
saturated, but costs continue to increase
because every unit of continued
investment deprives other offspring
(current and future) of a parent's limited
lifetime resources for care.
Evidence for
trade-offs:
increased
investment may
reduce adult
survival or future
fecundity
