The offspring number and fitness trade-off, however, is perhaps best not viewed in isolation. Rather, if we combine it with the CR trade-off, we can turn to another of our types of life history question and ask: 'How is it that particular clutch sizes, or particular sizes of seed crop, have been favored?'
Lack (1947b) concentrated on the the Lack clutch size trade-off between offspring number and fitness and proposed that natural selection will favor not the largest clutch size but a compromise clutch size, which, by balancing the number produced against their subsequent survival, leads to the maximum number surviving to maturity. This has come to be known as the 'Lack clutch size' (Figure 4.29a). A number of attempts, especially with birds and to a lesser extent with insects, have been made to test the validity of this proposal by adding eggs to or removing them from natural clutches or broods, determining which clutch size is ultimately the most productive, and comparing this with the normal clutch size. Many of these have suggested that Lack's proposal is wrong: the clutch size most commonly observed 'naturally' is not the most productive. Experimental increases in clutch size, in particular, often lead to apparent increases in productivity (Godfray, 1987; Lessells, 1991; Stearns, 1992). Nevertheless, as is so often the case, Lack's proposal, whilst wrong in detail, has been immensely important in directing ecologists towards an understanding of clutch size. A number of reasons for the lack of fit are now apparent and two are particularly important.
First, many of the studies are likely to have made an inadequate assessment of the fitness of individual offspring. It is not enough to add two eggs to a bird's normal clutch of four and note that six apparently healthy birds hatch, develop and fledge from the nest. How well do they survive the following winter? How many chicks do they have themselves? For example, in a long-term study of great tits (Parus major) near Oxford, UK, beyond the Lack clutch size
Figure 4.29 (a) The 'Lack clutch size'. If the fitness of each individual offspring decreases as total clutch size increases, then the total fitness of a clutch (the product of number and individual fitness) must be maximized at some intermediate ('Lack') clutch size. (b) The mean observed number of young recruited per nest ± SE relative to experimental manipulations (additions to, or removals from, clutches) in great tits. The curve is the polynomial RECRUITMENT ~ EXPTL MANIP + (EXPTL MANIP)2. (After Pettifor et al., 2001.) (c) However, if there is also a cost of reproduction, then the 'optimum' clutch size is that where the net fitness is greatest, i.e. here, where the distance between the cost line and the 'benefit' (total clutch) curve is greatest. (After Charnov & Krebs, 1974.)
-4-3-2-10 1 2 3 Experimental manipulation
Maximum net benefit
Fitness of that clutch
Optimum Clutch size
✓'"^Cost of reproduction
Optimum Clutch size whereas 'addition' nests were more immediately productive (10.96) than control nests (8.68), which were more productive than removal nests (5.68), recruitment (i.e. survival of offspring to become breeding adults themselves) was highest from the unmanipulated clutches (Figure 4.29b).
Second, perhaps the most important omission from Lack's proposal is any consideration of the cost of reproduction. Natural selection will favor a lifetime pattern of reproduction that gives rise to the greatest fitness overall. A large and apparently productive clutch may extract too high a price in terms of RRV. The favored clutch size will then be less than what appears to be the most productive in the short term (Figure 4.29c). Few studies have been sufficiently detailed to allow the cost of reproduction to be taken into account in assessing an optimal clutch size. In one, bank vole females (Clethrionomys glareolus) were treated with gonadotropin hormones, inducing them to increase their reproductive allocation to a larger litter (Oksanen et al., 2002). Treated females were considerably more productive at the time the litters were born, and a very small but none the less significant increase was maintained in the number of offspring surviving to the following winter. However, the treated females also paid a significant cost for their increased reproductive efforts: higher mortality during nursing, decreased body mass gain and a decreased probability of producing a subsequent litter. A second study, on kestrels, is discussed below (see Section 4.13).
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