The costs
of care
As with
foraging, caring for offspring by parents creates a series of potential
trade-offs. A guarding parent often has reduced opportunity to feed, which may
reduce later gamete production (e.g., in salmons, ricefishes, and livebearers;
Blumer 1979). Male Smallmouth Bass, Micropterus dolomieu, fan their
nests 24 h a day when they have eggs and young there and rarely leave the nest
to feed; mortality among males during their first breeding season can exceed
90% in some lakes (Wiegmann & Baylis (1995). Gillooly and Baylis (1999)
measured change in the wholebody composition of male Smallmouth Bass in the
field across an 8-day parental care period. They found that nestguarding males
lost an average of 3% of lean mass, a potentially significant amount given that
breeding occurs shortly after fish come out of the winter starvation period.
Other fishes known to suffer energy loss, decreased growth, delayed
reproduction, compromised immune function, or higher mortality due to parental
care include Three-spined Sticklebacks, cichlids, other centrarchids,
cardinalfishes, and gobies (e.g., Chellappa & Huntingford 1989; Lindstrom
2001; Okuda 2001).
Some
costs can be overcome in part if the male eats some of the eggs, a phenomenon
known as filial cannibalism (Fitzgerald 1992; Manica 2002; DeMartini
& Sikkel 2006); Smallmouth Bass males, however, do not eat their young.
Such cannibalism is known in at least 17 teleost families, with starvation
avoidance by the adult the most likely cause (Manica 2002). Feeding while
guarding young is relatively rare in mouth-brooding species. Another cost
incurred when nest guarding is lost opportunity to spawn, which compromises
future reproductive output. The decision of when to abandon current progeny
will therefore be influenced by how much a parent’s guarding activities can
reduce mortality in the current clutch versus what opportunities for breeding
exist in the near future (Perrone & Zaret 1979; Sargent & Gross 1994).
Short breeding seasons, scarce additional mates, and short lifetimes would
favor parental care of existing offspring over searching for additional
spawning opportunities. Females can exploit this dilemma by preferring males
that are already guarding eggs (see above).
Caring
for young also carries predation risks. Brood defense may reduce predation on
the young but simultaneously increases the parent’s exposure to predators.
Guarding parental sticklebacks, Pumpkinseed Sunfishes, and gobies take more
risks as their offspring grow, indicating that the value of the brood can
increase relative to parental survival during the parental care phase (Colgan
& Gross 1977; Pressley 1981; Magnhagen & Vestergaard 1991). Finally, an
inverse relationship often exists between degree of care and number of eggs
produced. Pelagic egg scatterers produce hundreds of thousands or millions of
tiny eggs that they abandon, whereas species that participate in extensive
parental care characteristically produce relatively small clutches of dozens to
a few hundred larger eggs. Highquality care may only be possible when small
numbers of young are produced. The ultimate evolutionary product, however, is
how many offspring make it into the next breeding generation. The existence of alternative
tactics within and among species attests to the fact that no single
reproductive system is universally optimal.
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