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Life History Strategies Essay - UP677529
LIFE HISTORY STRATEGIES, RÜPPELL’S VULTURE (Gyps rueppellii) AND INCREASED
THREATS TO THE SPECIES’ SURVIVABILITY
Interpreting the way in which different organisms utilise specific traits to survive and reproduce can
be aided by the theoretical Darwinian Demon (Ferretti et al, 2005). An example of unlimited
exponential growth, this hypothetical organism would produce infinite offspring as soon as it was
born and would have ultimate levels of fitness, in a conjectural situation where evolution was
wholly unimpeded (Hughes et al, 2002). In reality, various factors influence the life history
strategies, or traits an organism possesses and utilises, in order to maximise its survivability
(Mueller et al, 2001). The theory of life history strategy is founded on the idea that available
resources in an environment are limited and certain characteristics such as, the reproductive
lifespan of an animal, age of sexual maturity and the quantity and size of progeny produced are
also deliberated (Roff, 2007). As resources available to an individual are finite in almost any given
environment, the amount of energy and time given to one life strategy will reduce the amount of
time and energy available for another (Bamford et al, 2009). As with Darwinian Theory, it is the
specific combination of adaptations and traits displayed by a species that determine its overall
survivability, and the extent to which a species can produce numerous, viable offspring (Antor et al,
2007).
In his book, Ecological Adaptations for Breeding in Birds (1968), evolutionary ecologist Lack, D.
advocated the theory that reproductive yield in avian species reflected their various adaptations to
environmental factors, including available resources and predation risk, with subsequent studies
adding further credence to the concept (Silby et al, 2012; Roff, 2007). Silby et al (2012),
propagated that calculable trade-offs in energy allocation between different avian species would be
the result when environmental confines were placed on energy supply, and similarly, when
physiological constraints limited metabolic energy expenditure. The result of this study, along with
separate investigations into demographic stochasticity and population viability in various avian
species by Robert et al (2004) and Roff (2007), suggest that (i) as parental care in a species
increases, reproductive output decreases, thus indicating the energy exchange between egg
production and hatchling nurture and development, (ii) a high mortality rate is compensated for by
a species investing in a high rate of reproduction output, (iii) and the amount of energy given to
reproduction is dictated by mass-specific metabolic rate, indicating that with an increased body
mass, reproductive rate usually decreases. This theory is significant when applied to the Rüppell’s
vulture (Gyps rueppellii) (Brehm, A., 1852), as the monogamous parent birds are an iteroparous,
annually breeding species, that invests heavily in raising only one to two chicks in a single year
(Ramíreza et al, 2011). G. rueppellii is not sexually dimorphic and with a wingspan up to 8.5 feet
wide, both males and females can reach up to nine kilos in weight (Ruxton & Houston, 2004).
Consequently, the idea that an increase in body mass results in reduced reproductive output could
indeed be applied to G. rueppellii (Weber et al, 2009).
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Expanding on the concept of life history strategies, MacArthur & Wilson (1967) hypothesised that
selective pressures drive evolution in two distinctly separate directions (Coulson et al, 2010). Using
ecological algebraic terminology, (r)–selection (r representing high production or turnover),
generally indicated a species that displayed traits such as small body mass, early maturation, short
generation time, high fecundity rates and wide progeny distribution (Cardona et al, 2009). Whereas
(K)–selected species (K representing the carrying capacity of a population), commonly exhibited a
large body mass, later age of maturation, longer generation time and extensive parental care given
to fewer offspring for a longer period of time (Grande et al, 2009). Through their work on The
Theory of Island Biogeography (1967) MacArthur & Wilson theorised that although different
species may physically co-inhabit the same environment, that particular environment may affect
each individual species in an entirely different way, encouraging wholly different traits and life
strategies (Lancaster et al, 2008). They suggested that (r)-selected individuals were more likely to
face an unpredictable or fluctuating environment, such as that of a small prey species for example,
which would render competitive traits among a species unnecessary, and instead favour high
reproduction rates (Londei, 2010). Conversely, (K)-selected species, would exist around maximum
carrying capacity in a relatively stable environment, requiring a species to adapt in order to
successfully compete intra-specifically for limited resources (cite). In (r)/ (K)-selection theory, G.
rueppellii could be identified as a predominantly (K) – selective species (Virani et al, 2012).
With G. rueppellii, parent birds equally share incubation responsibilities for up to 58 days, and after
hatching, both birds provide parental care for an additional 90 day period until the chick is able to
survive unassisted (Virani et al, 2012; Rose & Charlesworth, 1980). G. rueppellii have been
recorded travelling distances of up to 150 kilometres away from a nest site in order to procure food
for their chicks, and will store this food in their crops to regurgitate to the chick upon their return
(Satheesan & Satheesan, 2000). Although as a (K)-selected species adult G. rueppellii are
competitive inter and intra-specifically, to avoid predation and maximise infant mortality the species
is colonial, nesting on cliffs in mated pairs up 100 strong in close proximity (Margalida et al, 2003).
While G. rueppellii invest a significant amount of energy into rearing often only one chick, they
increase their species continued survival rate by adopting a perennial breeding lifestyle as a
monogamous pair, and will often reproduce every year, as long as there are sustainable resources
to allow for breeding (Oro et al, 2008). Virani et al (2012) monitored a nest site of G. rueppellii in
Kwenia between 2002 and 2009, and found that breeding was intrinsically related to rainfall in the
previous year, and in turn, to ungulate migration of the Mara-Serengeti, which provided vital
foraging ground for the scavenging species.
G. rueppellii are a long lived species, often reaching an age of 50 years old in the wild (IUCN,
2013). The species does not reach sexual maturity until approximately five years old, insuring the
animal has more chance of reaching optimal fitness and an ability to obtain food successfully
before breeding (Anadón et al, 2010). Developing adequate scavenging skills ensures adult G.
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rueppellii are able to provide optimal levels of parental care to their progeny (Johnson et al, 2006).
G. rueppellii frequently encounter other species of vulture, as well as jackals (Canis aureus) and
spotted hyenas (Crocuta crocuta) at carcasses, and a number of intra and inter-specific
interactions have been documented (Durieza et al, 2012). Mediation of competition via a trade-off
in traits can enable co-existence between different species, and Pennycuick (2008) identified
certain mechanisms between G. rueppellii and other vulture species, that reduced intra-specific
competition. Social vultures of the Gyps genus, dominate carcass feeding sites when migratory
ungulates are present in the dry season, which in turn causes smaller species such as the white
headed vulture (Trigonoceps occipitalis) to employ an entirely different feeding and breeding
strategy to avoid direct competition with larger vultures of the Gyps genus (Carrete et al, 2006). T.
occipitalis lay their eggs between June and August, whilst G. rueppellii and Lappet-faced vultures
(Torgos tracheliotos) for example, lay their eggs between February and May, and so raise their
young at different times, reducing the potential for intra-specific conflict (Pennycuick, 2008).
However, T. tracheliotos and G. rueppellii commonly coincide in raising their young and Kendall et
al (2012) propose that Lack’s theory of adaptation could explain how these two species avoid
conflict. T. tracheliotos are solitary birds which do not nest in colonies and avoidance of
competition between G. rueppellii and T. tracheliotos could primarily be due to a larger body mass
in T. tracheliotos encouraging successful niche-separation (Selva & Fortuna, 2007).
The inter-specific relationship between G. rueppellii and C. crocuta is also noteworthy, as it
suggests commensalism, or facultative symbiosis on the part of G. rueppellii, with the morphology
of C. crocuta being better adapted for tearing openings in flesh which the beaks of G. rueppellii are
less well modified for (Durieza et al, 2012). Consequently, G. rueppellii are documented as waiting
for an opportunity to feed on the cavities in carcass flesh provided by C. crocuta (Kendall, 2013).
G. rueppellii has evolved to occupy an ecological niche as an obligate scavenger, totally
dependent on carcasses for food (Shivik, 2006). Large individual body mass is selected for
generationally if availability of carrion is infrequent, as it is for G. rueppellii particularly in the wet
season, which allows the species to survive on body reserves in the periods between discovering
food falls (Kendall et al, 2014). Subsequently, flight patterns and behaviour are also altered
evolutionarily, and a heavy body mass necessitates soaring over flapping in flight, reducing speed
and as a result, the energy costs of long distance transport (Ruxton & Houston, 2004). Due to this
specialisation in high altitude soaring as a low energy form of flight, G. rueppellii have evolved
away from an agile morphology that would facilitate hunting prey of their own (Koenig, 2006).
Niche separation and specialisation such as that employed by G. rueppellii, have made them and
related species successful worldwide (Thiollay, 2006). Vultures occupy a position in the ecosystem
that is analogous to no other species, and they play an essential role in food cycles by improving
nutrient recycling and the stability of populations in their environment (Selva & Fortuna, 2007). The
important role vultures play ecologically was highlighted by the inadvertent poisoning of millions of
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Indian vultures (Gyps indicus) and other vulture species across Asia in recent decades, as a result
of livestock being administered with the veterinary drug Diclofenac, a non-steroidal anti-
inflammatory drug (NSAID) (Cuthbert et al, 2007). The decline of vulture species as a result of this
abiotic/anthropogenic factor has had a significant knock-on effect to the environment and the
human populace (Anderson et al, 2006). Carcasses, once utilised as food by vultures, rot in fields
and contaminate drinking water (Cuthbert et al, 2007). In the absence of vultures, species such as
rats and dogs have partially replaced them at carcasses, but the digestive abilities of these species
are hugely inefficient by comparison to vultures, who’s highly acidic, specialised digestive system
has the ability to kill any pathogens during transit (Kendall & Virani, 2012). Thus, diseases such as
anthrax and rabies are on the increase in Asia, and it is thought that over 30,000 people die of
rabies in India alone every year as a direct result of vulture decline (Kendall & Virani, 2012).
Similarly, use of NSAIDS in livestock, as well as pesticides, over hunting and persecution for trade
of meat and body parts in recent years in Africa, has seen vulture numbers significantly fall
(Thiollay, 2006).
With these new environmental pressures, the life history strategies employed by G. rueppellii that
successfully established them in their ecosystem may no longer be viable for the species’
prolonged survival, unless conditions are restored. Hypothetically, if the risk is not eliminated for G.
rueppellii, it may be more sustainable for this (K)-selected species to adopt some (r)-selected traits
to increase survivability, such as an increase in the number of breeding events and egg clutch size,
for example. However, continued monitoring of significant G. rueppellii nest sites, as well as
breeding and reintroduction programmes by conservationists are currently attempting to restore
previous environmental stability, as threats to the species survival are comparable to a stochastic
event, and it is unlikely the species could adapt to the changing environmental circumstances in
time.
Word count: 1638
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