Population Density and the Economics of Territorial Defense in a Coral Reef Fish

Abstract

We propose that in some species, local population density can strongly affect the economic defendability of a mating territory. This is so because the numbers of females and potentially interfering males determine allocations of time and energy to reproduction and defense. At low densities, allocations to defense should be small and territorial mating success should initially rise with local density, reflecting the supply of females. If defense takes priority over mating, higher population densities can create a situation in which the time or energy devoted to defense against other males detracts from allocations to reproduction. Thus a point is reached where territorial mating success declines with increasing density, as a function of the number of nonterritorial males. We investigated these hypotheses by recording changes in the daily mating success of territorial males of the bluehead wrasse (Thalassoma bifasciatum) following experimental manipulations of local population size and composition. On large reefs, where mating population densities are already high, territorial mating success varied inversely with changes in overall population density and with changes in nonterritorial male numbers only; changes in female numbers had little effect. Thus at higher densities the demands of defense appear to be more important in determining mating success than the supply of available mates. Territorial mating success varied directly with population density changes only on the smallest experimental reff, where there were few nonterritorial males. The reduction of mating success at higher densities was correlated with a decreases in both the time spent in courtship and the efficiency of courtship itself. The effect of population density should be particularly important in species with short breeding periods and where the male contributes relatively little time or energy to each mating.

Chapter 5 Insights for Behavioral Ecology from Behavioral Syndromes

“…Here, because behavioral types are easy to identify, studies have indeed examined frequency-dependent fitnesses associated with the different types. But even here, few experimental studies have manipulated the relative frequency of these AMTs (Bleay et al, 2007;Warner and Hoffman, 1980). A possible explanation for this disconnect between decades of theory and empirical work is the fact that until recently, relatively few studies have quantified individual variation in behavioral type in order to identify "hawks" and "doves," a prerequisite for experimentally creating groups with different mixes of hawks and doves.…”

Diversity and flexibility of sex-change strategies in animals

“…Where local conditions favour resource defence polygyny, most males are derived by adult sex change [12,22]. By contrast, when high densities make resource defence polygyny uneconomic, individuals usually choose to become male before maturation (so-called primary males; Box 1) [12,22] and never change sex as adults.…”

“…Where local conditions favour resource defence polygyny, most males are derived by adult sex change [12,22]. By contrast, when high densities make resource defence polygyny uneconomic, individuals usually choose to become male before maturation (so-called primary males; Box 1) [12,22] and never change sex as adults. Differences in the relative frequency of sex change can occur on neighbouring reefs, indicating that individuals are choosing a life-history tactic that involves adult sex change, or not, depending on the mating conditions that they experience in that habitat patch.…”

“…Similarly, in species that exhibit resource defence polygyny, a large female will usually change sex following the disappearance of a territorial male [35] because it greatly increases her RV. Territorial males can spawn with O50 females per day [22], so the advantage to changing sex when a dominant position becomes available can be significant.…”

Earlier sex change in infected individuals of the protogynous reef fish Thalassoma bifasciatum

“…As fertilization rates are generally high in this species (Marconato et al 1997), the number of eggs released by a female can serve as a proxy for her current fitness. After a 1-2 month larval period (mean 49 days), fish recruit to a reef and stay there for the rest of their lives (Warner 1975;Warner and Hoffman 1980b;Victor 1986). Individuals start life as either female or male, and display a yellow and brown initial phase (IP) coloration.…”