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.