Fellow birders,
The Spotted Sandpiper will soon be arriving on the shores of
Virginias lakes, rivers and streams, and while it is very difficult to
definitively identify the sex of these birds in the field, you can be very
confident that the first birds which you see are females. This is because the
Spotted Sandpiper is one of only a few species of bird or any other animal in
which the parental roles are reversed and the primary mating system is classic
polyandry, in which each female has several mates and no male mates with more
than one female. Unlike most species of birds, female Spotted Sandpipers arrive
on the breeding grounds earlier than the males and establish breeding
territories which they defend vigorously from intrusions by other females,
occasionally by physical combat. If a female successfully attracts a male to
her territory, she will lay a clutch of four eggs. After the clutch is
complete, the male, not the female, incubates the eggs and raises the young,
aided b
y high
levels of a pituitary hormone, prolactin, known for its promotion of parental
care and commonly found in high levels in females. The female is thus freed
from the burden of incubating eggs and caring for offspring, and may then
proceed to attract one or more additional males, each of which will incubate
and tend a clutch on the females territory. A polyandrous female Spotted
Sandpiper may mate with up to four males, each of which hatches and raises a
brood.
The Spotted Sandpipers highly-unusual breeding system is not
shared by its cogener, the closely-related and very similar Common Sandpiper of
Eurasia, which typically forms monogamous pair bonds.
As a group shorebirds evidence a wide variety of mating systems,
many of which include a high degree of paternal care, and a high degree of
sexual dimorphism in favor of the female, particularly with respect to size and
in some cases plumage. Perhaps the best known examples in this regard are the
phalaropes, which are perhaps better known than the Spotted Sandpiper for their
reversed parental roles because of their more conspicuous reversed sexual
dimorphism. (Like the Spotted Sandpiper, female phalaropes are appreciably
larger than males, but unlike the Spotted Sandpiper female phalaropes are
noticeably more colorful than males in breeding plumage, a reversal which
confused early biologists and led Audubon to mislabel males and females in all
of his phalarope plates). Like the Spotted Sandpiper, m ales and females of the
three phalarope species reverse the normal parenting roles. Unlike the Spotted
Sandpiper, however, the phalaropes are generally monogamous, with
no mo
re than 5-10% of females practicing polyandry. Certain other birds which
exhibit paternal care are strictly monogamous, which further indicates that
male parental care does not necessarily result in the practice of classic
polyandry.
No one really knows why the parental roles of the Spotted Sandpiper
are reversed and the female Spotted Sandpiper typically practices classic
polyandry when most species of birds reproduce with biparental care and social
monogamy, presumably because this is the most successful system for raising
young. Nor is it known why in the case of birds classic polyandry as a primary
breeding system is restricted to a few species in a few major groups, which
consist primarily of the very primitive and flightless tinamous and ratites
(rheas, emu and cassowaries), certain shorebirds (including the Spotted
Sandpiper and to a lesser extent the phalaropes in the family Scolopacidae,
jacanas, painted-snipes and the Plains Wanderer, all of which are members of
the order Charadriiformes), the Buttonquail, a small, Ol d World family of
birds which resemble quail, and the Black Coucal, an Old World species of
cuckoo. Two members of the plover family also sometimes exhibit polyandry, t
he Eur
asian Dotterel and Kentish Plover (which is known as the Snowy Plover in North
America). With the exception of the Black Coucal, all of these species have
precocial chicks which can be cared for by one parent. Classic polyandry also
recently has been found in certain woodpecker species, such as the
Greater-spotted and Lesser-spotted Woodpeckers of Eurasia, which, like the
Black Coucal, have altricial chicks.
In some birds it is likely that exclusive paternal care is the
ancestral parenting system and that it has been preserved in some lineages.
This appears to be most strongly the case for the tinamous and ratites and
unlikely in the case of all other birds except possibly the shorebirds. Some
authorities believe that paternal care is ancestral in certain shorebirds,
while others believe that biparental care is ancestral in shorebirds and that
paternal care developed early in the Scolopacida (sandpipers and their allies),
with subsequent transitions from male care to female care or biparental care
and even back to male care. In any event, because female Spotted Sandpipers are
hormonally and behaviorally equipped to competently perform all parental
duties, which suggests a history of biparental care, it is believed that
paternal care evolved from biparental care in the Spotted Sandpiper.
One factor which must be considered in evaluating the cause of
paternal care and classic polyandry in a given population of birds is the ratio
of males to females in that population. In The Birders Handbook A Field
Guide to the Natural History of North American Birds (1988) by Ehrlich, Dobkin
and Wheye, the authors describe a long-term study of a breeding population of
Spotted Sandpipers on Little Pelican Island, Leech Lake, Minnesota and note
that a chronic shortage of available males developed as females first arrived
on the breeding grounds and, after their subsequent arrival, males were taken
out of the breeding pool because they were sitting on nests. Although this
account emphasizes the chronic shortage of breeding adult males in the stud y
and how such a shortage limited the production of additional clutches by
females, it appears that a shortage of breeding adult males is an effect, not a
cause, of classic polyandry in the Spotted Sandpiper. This is be
cause
female Spotted Sandpipers likely would not be able to commonly engage in
polyandrous matings unless there actually was some surplus of breeding adult
males in any given population, which generally appears to be the case across
the range of the Spotted Sandpiper according to the account of the Spotted
Sandpiper in the Cornell Labs The Birds of North America Online (Poole and
Gill, editors) (multiple indicators point to a moderate, not chronic, local
surplus of males on breeding grounds) (emphasis supplied). This also is
consistent with the recent finding of polyandry in the Lesser-spotted
Woodpecker. In this case, polyandry was only found when the sex ratio was
male-biased, and with female bias there were two cases of polygyny, the other
form of polygamy, in which males have multiple mates. Male-biased sex ratios
also were found in a recent incident of classic polyandry by the Kentish
Plover.
Although a surplus of breeding adult males may be a factor in the
development of polyandry, it cant possibly explain it because this is the
general pattern for most avian species, of which only 1% use classic polyandry
as their primary breeding system. (Adult males exceed females in many species
because of factors which result in a higher mortality for females than males,
which include longer post-breeding dispersal by females of many non-migratory
species and longer migration by females of many migratory species, which are
suspected to result in greater mortality, as well as differences in body size
and/or reproductive roles, with incubating females being particularly
vulnerable to predators.) It thus seems that any theory of classic polyandry
must depend on a lot more than just the ratio of males and females.
In J. David Ligons The Evolution of Avian Breeding Systems (Oxford
Ornithological Series 1999), which is an excellent source of information on all
avian breeding systems, the author does not set forth a comprehensive theory
which explains the development of classic polyandry but does discuss various
traits which are present in the few shorebirds and other birds which practice
polyandry. These include (i) sex-reversed size dimorphism, (ii) male parental
care, (iii) the need for only one parent to tend the young and (iv) the ability
of the female to lay successive clutches. The author also emphasizes an
ecological factor in the development of polyandry: access to breeding areas
that are unusually rich in food.
Recently, a researcher proposed that classic polyandry has evolved
in certain bird and fish species in the three main steps noted below.
· First, mainly or only male parental care evolves for any of several
reasons. There are several theories for the evolution of paternal care in
shorebirds but none are well understood and additional studies are needed. One
theory is that if food resources are often sparse and the breeding season is
short, as in Arctic-breeding birds, the female will not have enough energy to
both produce and incubate the eggs and the assumption of incubation
responsibility by the male will allow the female to survive and th e population
to remain viable. Another theory is that if the risk of nest predation is high,
as it often is in the Arctic, the reproductive success of both members of a
pair may increase if the male takes on most of the incubation and thus allows
the female to produce another clutch. While these theories may help explain the
higher level of paternal care exhibited by many species of shorebird which
breed in the far North, they do not seem to explain why parental ro
le rev
ersal developed in the widespread Spotted Sandpiper, which is one of the few
species of shorebird which breeds in temperate areas, including a substantial
part of the continental United States, and has a longer breeding season than
most other shorebirds. Unless, perhaps, paternal care developed in the Spotted
Sandpiper while its breeding range was restricted to the far North and before
it spread to temperate areas, if this is in fact what happened. A third theor y
for the development of paternal care is that males of certain species may be
better equipped than females to incubate alone because of the reduced energy or
other female reserves resulting from egg laying. This may be a factor in the
development of parental role reversal in shorebirds because they lay eggs that
are large and unusually shaped relative to those of most other birds and thus
are potentially costly to reproduce (which helps explain why shorebird clutches
typically contain exactly four eggs). Whatever the
reaso
n for parental role reversal, this reversal is not sufficient in and of itself
to produce classic polyandry, as noted above.
· Second, females become able to produce a larger clutch than a male
can accommodate. This can happen, for example, by evolution of smaller body
size in the male. This also can happen by female production of more or larger
eggs as a result of the evolution of larger body size or the availability of
food-rich resources for a sufficiently long period during the breeding season
as a result of a habitat shift, novel foraging methods, or both. Both such
changes may have been factors in the developm ent of the classic polyandry
exhibited by the Spotted Sandpiper and to a lesser extent the phalaropes, which
exhibit reversed size dimorphism and have shifted to more food-rich aquatic
habitats such as freshwater ponds, lakes and streams during the breeding
season, whereas few other Scolopacids have done so.
· And third, females compete to take advantage of higher fecundity and
lay a clutch for a second male while the first mate is still caring for his
clutch, which allows the competitively-successful female, free from the
responsibility of incubation and care of the young, to have more offspring,
thus spreading classic polyandry and its associated secondary sex differences,
such as larger and more ornamented females than males. The ultimate result
the production of more offspring presumably is the reason why a female would
choose to pursue the costly option of mating again instead of foraging.
See Andersson, Evolution of Classical Polyandry: Three Steps to Female
Emancipation, 111 Ethology 1 (January 2005), which is available for free on the
Internet.
A much less elegant theory for the development of classic polyandry
in the Spotted Sandpiper is ultimately suggested by Ligon in The Evolution of
Avian Breeding Systems. After evaluating all of the various factors which may
have contributed to the evolution of classic polyandry in the Spotted
Sandpiper, the author concludes by noting the distinct possibility that it
simply may have been due to chance.
Gerry Hawkins
Arlington, VA
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