Evolution__3rd_Edition

300 PART 3 / Adaptation and Natural Selection
Costs are hard to measure
empirically
Table 11.2
The survival of young Florida scrub jays in nests where the helpers had either been
experimentally removed or left undisturbed. The offspring in the experimental groups had
lower survival during and immediately after the period of parental care, but not at the egg
stage. These results are for 1987; the post-fledgling difference was similar in 1988, but the
pre-fledging difference was not. Modified from Mumme (1992).
survival rate was increased somewhere between two- and fivefold if helpers are present.
Let us use the total figure, for survival to day 60, to calculate whether natural selection
favors helping. The survival of an average young scrub jay to day 60 is increased from
7% to 35% if helpers are present: the difference is 35 − 7 = 28%. We divide this by two to
find the benefit of helping per helper: 28/2 = 14% = b.
The cost of helping is more difficult to estimate. The cost is equal to the reproductive
success that a helper would have had if it had not helped. We can make an upper and
lower bound estimate. The lower bound estimate is zero if the helper had been unable
to breed independently. This may be close to the true value of c in the saturated habitat
of Florida scrub jays. It is thought that one of the main advantages of staying at the
parental nest is the chance either of inheriting the territory or budding off another territory
at the edge of it: most new territories are formed in this way. A young jay may
have to stay at home in order ever to be able to breed independently. An alternative,
higher bound estimate of the cost is the reproductive success of pairs without helpers.
The justification of this estimate is that if the helper had bred by itself it would lack
helpers (which are mainly derived from earlier clutches) and thus achieve the success of
an unhelped pair. The cost of helping is then 7%, the chance of survival to day 60 of an
egg in a nest without helpers.
To apply Hamilton’s rule in this case we have to notice that the helper’s choice is
between producing sibs and producing its own offspring. It should help its sibs if:
r sib b > r off c
Experimental groups Control groups
(helpers removed) (helpers present)
Initial sample size 45 63
% survival from egg to hatching 67 68
% survival from hatching to fledging 30 63
% survival from fledging to day 60 33 81
% survival from egg to day 60 7 35
where r sib is the relatedness to a sib and r off is the relatedness to its own offspring, both of
which are 1 /2 if the sibs are full sibs. (The small difference from the rb > c version given
above arises because there we imagined changes to the survival of altruists and recipients.
The relatedness of an altruist to itself is necessarily 1 and the cost is implictly multiplied
by this. Here the altruism affects the numbers of two kinds of offspring a its own
..