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reproduction was seasonal. Further testing of this hypothesis requires more data from
throughout the vast geographic range of
Inia
, and a refinement of our definition of seasonal
changes in habitat (
e.g.,
measure not only differences in water depths, but the amount and
type of habitat lost and gained throughout a complete flood cycle).
Observations of mating offer little insight into seasonal reproduction of
Inia
, because
mating was rarely observed and difficult to classify. Mating by
Inia
was observed in all
seasons in Peru, during falling and low water in Bolivia, and not at all in Venezuela. In
general, categorizing an activity as mating was subjective. Little of the animals was visible
above the surface of the water and this made classification and description of behavior
difficult. It was impossible to determine how much of the behavior described as mating was
truly reproductive in nature, and how much was other social behavior such as play,
aggression, sexual behavior without copulation, or dominance displays. Caldwell et al.,
(1989) reported that heterosexual and homosexual behaviors were very common in captive
Inia
, regardless of season. Based on examination of dead animals
,
male
Inia
do not appear to
exhibit seasonality in reproductive condition (Best & da Silva, 1984).
Reproduction in odontocetes ranges from highly seasonal to year-round. Extreme
latitudes may influence reproduction via extreme seasonal differences in photo-period, water
temperature, and prey abundance. In many species, degree of reproductive seasonality is
related to geographic distribution; for example tropical
Stenella
reproduce year-round
(Barlow, 1984), while at high latitudes, reproduction in
Phocoena,
Monodon,
and
Delphinapterus
is highly seasonal (Leatherwood & Reeves, 1983; Read 1990). Reproductive
seasonality in some odontocetes however, such as
Tursiops,
has been shown to be flexible
and inherent to populations, rather than correlated with latitude (Urian et al., 1996). There is
little information on the seasonality of reproduction in other river dolphin species.
Lipotes
is
reported to mate in the spring and calve in the winter and spring (Zhou & Zhang, 1991).
Platanista
is thought to have a bimodal calving and reproductive season (Kasuya, 1972;
Shresta, 1989).
Pontoporia
is reported to give birth during the austral spring and early
summer of higher latitudes (Harrison et al
.
, 1981; Danilewicz, 2003), with year-round births
at slightly lower latitudes (Ramos, 1997).
Seasonal Mortality
Inia
are listed as vulnerable by the International Union for the Conservation of Nature
(IUCN, 2007). Mortality rates in the wild are unknown, and causes of natural mortality in the
wild include parasites and respiratory infections (Best & da Silva, 1993; da Silva & Best,
1996). There are no records of non-human predation on river dolphins, although caiman,
piranha, jaguars, and bull sharks occur throughout their range. In recent years
Inia
in Brazil
and Colombia reportedly have been killed for use as fish bait (Martin & da Silva, 2004b), but
we did not witness or hear of this occurring in any of the study sites during the time we were
there.
The low water season appears to be the period in which
Inia
are most vulnerable to
mortality from stranding, vessel strikes, intra-specific aggression, and fishery interactions.
Although strandings due to shallow water were not observed in the study areas, this
undoubtedly occurs on occasion, and has been reported in Brazil (Best, 1984) and Venezuela
(Kirk Winemiller, Texas A&M University, personal communication). Vessel strikes, although
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