There are trade-offs between current reproduction and other life history traits

As we've seen, when parents produce more offspring, their investment per offspring may decline. Such a decline can have various effects on the offspring, including reduced survival (as in lesser black-backed gulls) and reduced size (as in western fence lizards).

For example, a trade-off between current reproduction and survival has been documented in studies that examine how life history traits differ among species. In one such study, Ricklefs (1977) observed a trade-off between annual fecundity (as measured by the number of offspring raised to maturity) and annual survivorship in birds (FIGURE 7.15A). Trade-offs between reproduction and survival have also been observed within a species. For example, in the fruit fly Drosophila melanogaster, males spend more time and energy courting unmated females than they spend courting recently mated females. Partridge and Farquhar (1981) tested whether such differences in courtship activity affected the longevity of male fruit flies. Males were kept with eight virgin females per day or with eight previously inseminated females per day. In the absence of sexual activity, a male's life span is correlated positively with his size, so Partridge and Farquhar also recorded the size of each male. Among males of any particular

size, males kept with virgin females had a shorter life span than did males kept with inseminated females (FIGURE 7.15B), showing a cost (reduced life span) of sexual activity among males of this species.

David McIntyre

FIGURE 7.15 Trade-Offs between Reproduction and Survival (A) In a comparison of 14 different bird species, the annual survival rate declines as annual fecundity increases.

In (B), what is the average life span of male flies with a 0.8-mm thorax kept with virgin females? How does this compare with that of males of the same size kept with previously mated females?

(A after R. E. Ricklefs. 1977. Am Nat 111: 453-478; B after L. Partridge and M. Farquhar. 1981. Nature 294: 580-582.) View larger image

Similarly, evidence for a trade-off between current reproduction and growth has been found in mollusks, insects, mammals (including humans), fishes, amphibians, and reptiles (see citations in Barringer et al. 2013). A trade-off between reproduction and growth has also been observed in many plants, including Douglas fir trees (Pseudotsuga menziesii) (FIGURE 7.16). Note that by allocating resources to reproduction instead of growth, an individual will reproduce at a smaller size than it would if it had continued to grow and reproduced at a later time (when it was larger). Small individuals often produce fewer offspring than do large individuals, so this observation suggests that allocating resources to current reproduction might decrease an individual's potential for future reproduction. This trade-off has also received empirical support, as you can explore in ANALYZING DATA 7.1.

FIGURE 7.16 AReproductionversusGrowthTrade-Off Thethicknessofannualgrowth rings (a measure of growth rate) declines in Douglas fir trees that produce many cones. (After S. Eis et al. 1965. Can JBot 43: 1553-1559.) View larger image

ANALYZING DATA 7.1

Is There a Trade-Off between Current and Delayed Reproduction in the Collared Flycatcher?

Lars Gustafsson and Tomas Part (1990)* studied a population of collared flycatchers (Ficedula albicollis) on the Swedish island of Gotland.

1. Graph the average number of eggs (on the ó-axis) versus age (on the x-axis) for both early breeders and late breeders.

2. Do the results suggest that it would be advantageous for birds to delay reproduction until they were 2 years old? Explain.

3. Do the results indicate that allocating resources to current reproduction can reduce an individual’s potential for future reproduction? Explain.

4. These results were based on field observations. What are the limitations of such data? Propose an experiment to test whether there is a cost of reproduction in females that reduces their potential for future reproduction.

*Gustafsson, L., and T. Part. 1990. Acceleration of senescence in the collared flycatcher Ficedula albicollis by reproductive costs. Nature 347: 279-281.