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FIELDIANA ZOOLOGY

Published by
CHICAGO NATURAL HISTORY MUSEUM

Volume 34 NOVEMBER 12, 1952 No. 6

SECONDARY SEXUAL CHARACTERS

AND ECOLOGICAL COMPETITION
?

O> AUSTIN L. RAND

**"* CURATOR, DIVISION OF BIRDS

/X

^ Secondary sexual characters are usually considered to be bene-
tjjicial to the species by bringing the sexes together and by inhibiting
^mismating. However, there is the possibility that some sexual
dimorphism may be adaptive in intra-specific feeding. Lack (1944)
particularly, in recent years, has elaborated the thesis that two species
with identical feeding habits cannot occupy the same area and
habitat. If they do breed in the same area there must be some
difference in feeding habits. Somewhat earlier, Steere (1894, p. 419)
summed up the idea very well from his Philippine studies: "No two
species structurally adapted to the same conditions will occupy the
same area."

If individuals of closely related species can live in the same region
and habitat only if they reduce ecological competition by eating
mainly different foods, the question arises as to how this rule applies
to individuals of one species. Especially important is nesting time,
when, in some species, the male and female are restricted to a
relatively small area during the period of building the nest, incuba-
tion, and raising the young. Here ecological competition would be
at its highest. Sexual dimorphism is very common and is probably
mostly concerned with mating, but with the high potential of
dimorphism one might expect it to reduce competition for food.

There are numerous ways in which competition for food between
the sexes is limited.

Outside the breeding season, in many birds in northern latitudes,
the migration of the male and that of the female may be slightly
different. The males tend to remain farther north throughout the
winter, the females farther south. Thus, some intra-specific com-
petition for food is eliminated, and for a part of the year most of
each sex are living in a different environment, with different selection^

No. 698 65

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66 FIELDIANA: ZOOLOGY, VOLUME 34

factors acting on them. These differences in themselves could
initiate a sexual dimorphism.

There may be some sexual segregation during the period of
reproduction. There are some species, such as the old squaw duck
(Clangula hyemalis), in which, after egg-laying, the males desert
the females, and while the latter incubate and raise their broods on
fresh-water tundra ponds, the males live on salt water.

In the white-tailed ptarmigan (Lagopus leucunis) of the Rocky
Mountains, the males are said to frequent the lofty peaks near the
upper limit of vegetation, while the female is leading her brood of
young among the more luxuriant alpine meadows in the valleys
below, nearer tree line. In both these cases we have different
environmental conditions for the two sexes for part of the year.

According to Mcllhenny (1937), the boat-tailed grackle (Cassidix
mexicanus) of Louisiana segregates into flocks according to sex
during part of the year. The food habits vary somewhat according
to sex. The crayfish (Cambarus), he writes, is a favorite food of
the male.

In many species that are territorial, the resulting spacing out
of the pairs may have an effect in lessening intra-specific food
competition, though the importance of this aspect of the holding
of territory by a pair has been questioned (Nice, 1941). Another
result might be intensified intra-specific food competition for the
two members of the pair. Within a territory, however, there may
be specialization as to feeding areas, as in the St. Kilda's wren
(Troglodytes troglodytes), in which each parent tends to feed in
exclusive food patches not used by the other (Harrisson and Buchan,
1934).

Structural differences between the sexes are common. Perhaps
the commonest one is the frequent average difference in size, the
female being usually smaller although occasionally she is larger
(as hawks and owls). It assumes more interest when taken in con-
nection with Lack's conclusion (1944, p. 279) that when two closely
related species occupy the same habitat in the same region they
nearly always differ markedly in feeding habits and/or size. Here
perhaps we have a factor in the evolution of size differences between
the sexes. Storer (1952, p. 284), in discussing this aspect of the
problem in hawks, writes: "A sexual difference in size in raptors has
a selective advantage in permitting the existence of a denser popu-
lation by increasing the size range and hence the available numbers
of the prey." The difference in size of the sexes in other groups,

I u

RAND: SECONDARY SEXUAL CHARACTERS

67

too, may correlate with a difference in food preference and a lessening
of ecological competition for food.

In the boat-tailed grackle, there is some difference in food pref-
erence between the sexes, as stated above, and one method of feeding,
used by the female only, that Mcllhenny records (1937) is as follows:
In feeding, the female may hover close over the water to snatch
food from it. "This method of feeding is carried on only by the

FIG. 7. Western grebe (Aechmophorus occidentalis), showing sexual dimorphism
in bill shape; female above.

females, as the long, heavy tails of the males prevent any attempt
at flight feeding that entails hovering close to the water's surface."

Difference in bill shape is shown by the western grebe (Aechmo-
phorus occidentalis). The male has a straight, the female an up-
tilted bill. This difference (apparently little known) has not been
correlated with feeding or mating. But it is interesting when
compared with the condition in two other diving species, the com-
mon and the yellow-billed loon. The male western grebe has a
straight bill like that of the common loon (Gavia immer] ; the female's
is uptilted like that of the yellow-billed loon (G. adamsi). These
two loons are geographical representatives, and where their ranges
meet they are mutually exclusive, in different habitats. What may
be adaptive in two forms occupying different habitats in the loons,

68 FIELDIANA: ZOOLOGY, VOLUME 34

may be adaptive in the western grebes, in reducing intra-specific
food competition.

The classical case of structural difference between the sexes,
correlated with a difference in food habits, is that of the huia (Hetera-
locha acutirostris). The male has a straight, short beak, the female,
a long, curved beak. The birds are largely insectivorous, eating

FIG. 8. Huia (Heteralocha acutirostris), showing sexual dimorphism in bill
shape; female above.

many wood-boring grubs which the sexes may get in different ways.
The male with his stout, straight bill may dig into rotten wood for
hidden insects; the female probes into existing crevices and crannies.
Buller (1888, p. 7) records that he had a pair in captivity. Some-
times the male, after removing the decayed part of a log, was unable
to reach the grub, when the female "would at once come to his aid,
and accomplish with her long, slender bill what he had failed to do.
I noticed, however, that the female always appropriated to her own
use the morsels thus obtained."

SUMMARY AND DISCUSSION

The thesis that two closely related species of bird can occupy
the same habitat in the same area only if they differ markedly in
feeding habits or in size has two aspects: a short-term, every-day
one that reduces ecological competition for food, and a long-term,

RAND: SECONDARY SEXUAL CHARACTERS 69

evolutionary one, that forces apart two closely related species and
causes them to differ more and compete less.

Within a species, during at least part of the reproduction period,
two individuals of the same species live and feed in a more or less
limited area. The ecological competition for food would be as great
as if two individuals of different species, but with the same habits,
occupied the habitat in the same area. Some adjustment could be
expected. I have shown that differences between the behavior of
the sexes that reduce intra-specific competition for food do exist in
some cases. Sexual dimorphism is common, and though usually
considered as aiding the sexes in finding each other, and in avoiding
mismating, some dimorphism, such as the difference in size between
the sexes,|may also serve as adaptation for slightly different foods.
In a few cases sexual dimorphism is known to be correlated with a
difference in feeding habits. This seems to correlate with, and be
an extension of, the forces that promote reduction of inter-specific,
ecological competition for food. This is the short-term effect.

The long-term, evolutionary aspect would seem to call for factors
at work forcing the two sections of the dimorphic species (male
and female) apart. In those cases in which the two sexes live under
different environmental conditions part of the year, ordinary natural
selection could be at work during this period. However, other
factors, notably the necessity for coming together at least periodi-
cally, would seem to set limits as to how far these two forms could
drift apart. While many elaborate secondary sexual characters may
be concerned solely with mating, many less conspicuous ones may
actually be concerned with the reduction of ecological competition
for food between the sexes.

REFERENCES

BULLER, WALTER L.

1888. A history of the birds of New Zealand. 1, 2nd ed., 250 pp. Published
by the author, London.

HARRISSON, T. H., and BUCHAN, JOHN N. S.

1934. A field study of the St. Kilda wren (Troglodytes troglodytes hirtensis),
with especial reference to its numbers, territory and food habits. Jour.
Anim. Ecol., 3: 133-145.

LACK, DAVID

1944. Ecological aspects of species-formation in Passerine birds. Ibis, 1944:
260-286.

MclLHENNY, E. A.

1937. Life history of the boat-tailed grackle in Louisiana. Auk, 54: 274-
295.

70 FIELDIANA: ZOOLOGY, VOLUME 34

NICE, MARGARET N.

1941. The role of territory in bird life. Amer. Midi. Nat., 26, no. 3, 441-487.

STEERE, J. B.

1894. On the distribution of genera and species of non-migratory land-birds
in the Philippines. Ibis, 1894: 411-420.

STORER, R. W.

1952. Variation in the resident sharp-shinned hawks of Mexico. Condor,
54: 283-289.

UNIVERSITY OF ILLINOIS-URBANA