Evolution__3rd_Edition

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Theory predicts deviations from
50 : 50 sex ratios in certain special
cases
Seychelles warblers produce more
sons in some conditions, and more
daughters in others
CHAPTER 12 / Adaptations in Sexual Reproduction 339
sex ratio is among the best understood adaptations in life. Here is one example to illustrate
this area of research.
12.5.2 Sex ratios may be biased when either sons or daughters
disproportionately act as “helpers at the nest”
We looked in Section 11.2.4 (p. 298) at “helpers at the nest.” In some bird species, some
offspring remain at their parents’ nest after fledging. These offspring do not themselves
breed, but help their parents to rear the next brood of offspring. In some cases, mainly
male offspring act as helpers; in others, mainly female offspring do. Natural selection
can then favour a sex ratio other than 50 : 50, depending on the exact circumstances.
The Seychelles warbler (Acrocehalus secheliensis) is a bird that lives in the Seychelles
Islands, an archipelago 60 miles (100 km) or so north of Madagascar. Helpers at the
nest are seen in this species, and the helpers are mainly daughters of the reproducing
pair (88% of helpers are female). Sons mainly disperse to other territories after fledging.
Komdeur (1996) found that helpers had opposite effects on the reproductive success
of the nest, depending on the quality of the territory. (Komdeur measured territory
quality by counting samples of insects of the sort eaten by the warblers.)
On territories of high quality, the presence of 1–2 helpers increases the reproductive
success of the nest. But on territories of low quality, the presence of any number of
helpers decreases the reproductive success of the nest. The reason is likely that food is in
short supply and the food consumption of the helper herself reduces the food available
for the breeding pair and the nestlings. Even on a high quality territory, too many
helpers reduces nest success a nests with three or more helpers had a lower success
than if the helpers were absent. Again, the reason is probably competition for food. In
technical language, the two factors are called local resource competition and local
resource enhancement: the former refers to the case in which one gender of offspring
decreases parental reproductive success, and the latter to any case in which one gender
of offspring improves the local resources, for instance by bringing food to the young,
and increases parental reproductive success.
When helpers are net beneficial to the parents, natural selection favors parents who
produce more of the helping sex (daughters in the Seychelles warbler). When helpers
are net disadvantageous to the parents, natural selection favors the production of more
of the non-helping sex (sons in the Seychelles warbler). Komdeur (1996) found that
these predictions were found in reality. More sons were produced on low quality territory
and more daughters on high quality territory (Table 12.4a).
Komdeur also tested the theory by a translocation experiment. Certain pairs on
either low or high quality territories were moved to other islands, to territories of high
quality. The control pairs (moved from one high quality territory to another) continued
to produce extra daughters. But the experimental pairs moved from high to low
quality territories dramatically shifted from daughter production to son production
(Table 12.4b).
Komdeur did further experiments, and they provide further support for the theory.
However, the results of Table 12.4 are enough to illustrate the kind of evidence available,
although they do raise further questions. For instance, what is the mechanism by