in some vertebrate species males and females form long-term bonds and their survival and fecundity become highly dependent on each other -- in others species males and females lead almost completely independent lives -- for examples, compare Tundra Swans and Red Deer

in Tundra Swans an individual's fecundity depends on a long-term relationship with a mate

20-30,000 Tundra Swans spend the winter on Lake Mattamuskeet in eastern NC -- the species breeds in Arctic tundra around world in Canada, Alaska and Russia -- winter in three distinct regions (California, NC and VA, Netherlands and England) -- NC birds come from Canadian and probably Alaskan breeding grounds -- breeding pairs migrate together with their 4-month old cygnets -- young become independent in their first year -- unmated birds find mates during winter (as do other ducks and geese) -- pairs stay together "until death do us part" -- sexes are indistinguishable externally

study of birds wintering in England has given us detailed information about their demography -- time 0 in the life cycle is set at 4 months of age when cygnets appear on wintering grounds with their parents -- males and females have similar annual survival (0.66 in their first year, 0.87 thereafter)

swans first find mates at an average age of 2.4 years (range 1-9) but do not produce their first young until age 5.1 years on average (range 2-10) -- average breeding success (cygnets/adult) increases steadily from age 3 through at least age 9 to a level of about 0.6 cygnets/adult/year

loss of a mate results in lower breeding success -- only about 60% of birds that lose a mate find another within one year -- new partners seldom raise cygnets in their first year together -- breeding success continues to increase after a new partner is found for at least 5 years -- so most of the increase in breeding success from age 3 to 9 is a result of the pair's experience together -- rather than the individuals' physical maturity

in comparisons of equally experienced pairs, average number of cygnets per year correlates with the male's weight -- male's weight influences the pair's ability to win encounters with other families over access to the best feeding areas -- large females tend to select large males for mates

Red Deer illustrate the opposite pattern -- males and females lead almost entirely separate lives -- demography of the sexes differs strikingly -- females stay in small groups with their mothers and other female relatives -- one-year-old males leave their maternal groups and associate with each other in "bachelor groups" -- males eventually compete with each other for access to female groups ("harems") during the rut in October -- population on the Scottish island of Rhum has been studied since 1968 by Tim Clutton-Brock and his colleagues -- island acquired by the Nature Conservancy in 1957 -- no predators present

for females (hinds), variation in reproductive success depends mostly on survival of calves during their first winter -- calf survival depends on calf's weight in autumn before winter begins (not on birth weight) -- autumn weight depends on its birth date in spring -- and thus on its mother's condition and weight during winter and spring -- hinds in small groups that live in good grazing areas have better condition, calve earlier, and have more surviving calves -- reproduction has a cost for hinds -- hinds that raise young in one season are more likely to die in the following winter than those that do not -- hinds sometimes skip a year particularly after they raise a male calf (males grow faster and consume more milk)

for males (stags), variation in reproductive success depends mostly on number of females fertilized -- mating requires defense of groups of females against rival males -- reproductive success depends on fighting success -- fights sometimes cause injury (note lower age-specific survivorship of males compared to females after age 9) -- fighting success correlates with weight and antler size -- these measures at age 6 correlate with the same measures at age 16 months (in second summer after birth) -- and these measures at 16 months in turn correlate with birth date and weight -- and ultimately with mother's condition and weight

immediate determinants of survival and fecundity are different for males and females . . . but, for both sexes, survival and fecundity in the long run depend on mother's condition before giving birth