Subject: Abstract: humpback whales

Michael Williamson (
Sun, 21 Apr 1996 16:19:18 -0400 (EDT)

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Date: Fri, 19 Apr 1996 20:26:00 EST
To: Multiple recipients of list MARMAM <MARMAM@UVVM.BITNET>
Subject: Abstract: humpback whales

Clapham, P.J.  1996.  The social and reproductive biology of humpback
whales: an ecological perspective.  Mammal Review 26: 27-49.

Existing knowledge of the social organization, mating system and
reproduction of humpback whales is reviewed to assess how our current
understanding of this wide-ranging marine mammal fits into the
predictive framework developed from ecological studies of more accessible
taxa.  The small unstable groups characteristic of this species on its
summer feeding grounds appear to be a function of an absence of predation
and of the patchy, mobile nature of most prey; the absence of territor-
iality and the minimal importance of kinship in associations are also
predictable consequences of the latter.  The mating system is similar to
both leks and to male dominance polygyny, in which males display (sing) or
directly compete (perhaps sometimes in coalitions) for access to females.
However, the rigid spatial structure characteristic of classic leks
is absent.  The mating system of this species is sufficiently different
to merit a novel category, and "floating lek" is proposed.  The widespread
distribution of females resulting from absence of both predation and
resources during the breeding season preclude simultaneous monopolization
by males of more than one potential mate.  Furthermore, these factors,
together with a male-biased operational sex ratio, minimize the possibility
of competition among females.  The intensity of intrasexual competition
among males conforms to predictions derived from information on testis
size and from expectation of future reproductive success.  Female choice
and, to a lesser extent, differential allocation of competitive effort
by males, appears likely.  Lack of interpopulation variation in social and
mating behaviour, and in general reproductive biology, is likely a response
to similarity of marine environmental conditions.  Year-to-year variation
in reproductive rates may be linked to variations in the abundance of
prey.  The invariably uniparous nature of female humpback whales is
to be related to the energetic demands of lactation, and the lower ratio
of available energy partitioned to reproduction that is characteristic
of larger mammals.  The reversed sexual size dimorphism of this species
may reflect different selective pressures on males and females.  Finally,
there is now evidence that, as in some other taxa, offspring sex ratiois
related to maternal condition.

Phil Clapham
Smithsonian Institution