Perhaps not apparent at first sight is the restricted availability of water for wildlife
in cold regions. Not only are many of those regions deserts, as their rainfall is low,
but during winter the moisture is available only as snow, and valuable energy is needed
to melt it. Arctic mammals go to some lengths to conserve water. Caribou recycle
nitrogen to reduce the formation of urine, thereby conserving water.The distribution of many North American birds is limited at northern latitudes by
season length, the number of days available for breeding above a certain tempera-
ture. This is another aspect of temperature limitation. However, the southern bound-
ary is limited by day length, the number of hours available for feeding themselves
and their young (Emlen et al. 1986; Root 1988).
Seasons are highly predictable in the northern temperate latitudes of North
America and Eurasia, and many birds and mammals have evolved a response to
proximate factors(i.e. the immediate factors affecting an animal), particularly day
length (photoperiod), which trigger conception and result in the production of
young during optimum conditions. Such conditions are the ultimate factors(i.e. the
underlying selection pressure) to which an animal is adapted by breeding seasonally
(Baker 1938).
Increasing photoperiod determines the start of the breeding season in many bird
species (Perrins 1970), while declining photoperiod triggers the rut in caribou; the
rut is so synchronized that most conceptions occur in a mere 10-day period starting
around the first day of November (Leader-Williams 1988). Moose and elk also have
highly synchronized birth seasons (Houston 1982), which suggests photoperiodic
control of reproduction.
Among tropical ungulates only the wildebeest is known to respond to photo-
period. In southern Africa it uses solar photoperiod to synchronize conceptions, but
near the equator where solar photoperiod varies by only 20 minutes in the year it is
cued by a combination of lunar and solar photoperiod (Spinage 1973; Sinclair 1977).
In variable environments with less predictable seasons, as in the tropics and arid
regions, animals tend not to use photoperiod to anticipate conditions but rather adjust
their reproductive behavior to the current conditions. Thus, tropical birds begin breed-
ing when the rainy season starts, responding to the increase in insect food supply
and the growth spurt of the vegetation (Sinclair 1978). In some arid areas such as
Western Australia the seasonality of rain is relatively predictable but its location is
not. Emus (Dromaius novaehollandiae) there travel long distances searching for areas
that have received rain (Davies 1976), as do male red kangaroos (Macropus rufus)
(Norbury et al. 1994).
Most ungulates produce their young during the wet season in Africa and South
America, but put on fat before giving birth. This fat is then used during lactation,
the period when the energy demands on the female are highest (Ojasti 1983; Sinclair
1983). Therefore, nutrition in the seasonal tropics becomes both the proximate and
ultimate factor determining the timing of births. An example is provided by the lechwe
(Kobus lechee), an African antelope that lives on seasonally flooded riverine grass-
lands (Fig. 7.3). During the peak of the floods animals are confined to the less
preferred surrounding woodlands. The greatest area of flood plain is exposed at the
low point in the flood cycle and it is at this time, corresponding with greatest avail-
ability of food, that births take place. In Zambia the peak of births occurs in the dry
season 3 months after the rains; in the Okavango swamp of Botswana it occurs inDISPERSAL, DISPERSION, AND DISTRIBUTION 97
7.4.3Range limited
by day length and
seasonality