Defining a mating system of a cryptic deer species

The understanding of the interplay of movement, behaviour and physiology that biologging offers has applied relevance for a range of fields, including evolutionary ecology, wildlife conservation and behavioural ecology. In recognition of this, the Journal of Animal Ecology has an upcoming Special Feature on Biologging  (submissions due 20th September).

A benefit of biologging is that it provides the opportunity to study animals that are notoriously difficult to directly observe. Assistant Professor Aaron Foley (East Foundation / CKWRI-TAMUK) explains how a combination of movement patterns, energetics, and temporal parentage assignment were used to describe the mating system of the white-tailed deer in south Texas.

White-tailed deer are the most abundant large mammal in North America. Despite being plentiful, deer are notoriously difficult to directly observe for extended periods of time because of their crepuscular behavior and preference for closed habitats (forests, brush, etc.). As a result, some aspects of their behavior are poorly understood, especially their mating system. There were two general beliefs associated with mating behaviors of male deer that were largely based on anecdotal observations. First, large antlered, dominant mature males sired all offspring. Secondly, it was assumed that males searched far and wide for receptive females.

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It was long assumed that mature males sired all offspring. (Photo: Randy DeYoung)

The first insight into the deer mating system was via genetic parentage analyses; the results were surprising and did not support the assumption that a few dominant mature bucks sired most of the offspring. Breeding success was distributed among many different males of different ages and young males (1–2) years old collectively sired ~30% of offspring. The finding that young males were consistently successful was puzzling because it was well known that males establish a dominance hierarchy prior to the rut. Additionally, young males are physically immature; thus, should be allocating energy towards skeletal development rather than breeding activities. So how were young males siring a third of offspring?

The next step was to investigate movement patterns of males. We affixed GPS radio-collars to 102 males in south Texas and found that movement rates increased 2–4 fold during the rut. Even though movement rates increased, most males did not wander widely in search of females. Three search tactics were identified; residents, nomads, and revisitations. Revisitiation was the primary tactic used by mature males; the tactic entailed revisiting multiple small focal areas every 20-28 hrs. Because oestrus is ~24 hrs and females often exhibit high site fidelity, we inferred this tactic was a way for males to periodically assess receptiveness of females. This tactic appears to be a profitable strategy as indicated by the finding that mature males sired ~70% of offspring.

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A male white-tailed deer exhibiting flehmen behavior which is used to assess receptiveness of females. (Photo: Randy DeYoung)

Most young males were not using the revisitation strategy. Yearlings were mainly residents, remaining in a single focal area for the duration of the rut, whereas most 2-year-old males were nomadic; adopting a search strategy optimal for locating sparse resources. While these findings contributed to our knowledge of the mating system of white-tailed deer, we still were unable to figure out how young males were consistently able to sire offspring.

In a recent paper in the Journal of Animal Ecology, we investigated the timing of breeding success by assigning parentage of foetuses with estimated conception dates. The results were revealing; breeding success of young males were mainly limited to the 2-week long peak rut when the number of receptive females were at the apex. Additionally, most offspring sired by young males were shared paternities. In other words, young males were unable to maintain a partnership with a female for her entire oestrus. Whether young males were easily displaced by a mature male or were practicing kleptogamy, trends in temporal breeding success suggested a low effort strategy, perhaps to minimize energy output while maximizing energy intake. In the same paper, we also measured body mass loss in two ways; total body mass loss and modeled body mass loss from mate-search (costs of locomotion). Males, regardless of age, were modeled to lose 4–5% of body mass due to locomotion. Empirical body mass loss was 0%, 12%, and 18% for 1, 2, and 3+ year old males, respectively.

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Yearlings (left) and 2-year-olds (front right) males generally are subdominant to mature males (center); thus, female access is limited except during peak rut when most females enter oestrus and cannot be monopolized by mature males. (Photo: David Hewitt)

The differentials between the two values of body mass loss provided great insight into how males were allocating their time during the rut. Because yearling males were modeled to lose body mass but did not actually lose body mass, they must have spent significant time foraging during the rut instead of searching for mates. This is probably why most yearlings were using the ‘resident’ strategy as a way to minimize energy expenditure while breeding opportunistically. Two-year-old males must have reduced foraging during the rut because total body mass loss exceeded costs due to locomotion. But because of the competitive landscape (≥40% of males in the population were mature), 2-year-old males likely did not have great success accessing females and exhibited a nomadic mate-search strategy. As a result, 2-year-old males were likely limited to opportunistic matings as revealed by the high proportion of shared paternities during peak rut.

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A male white-tailed deer scent-checking a female. (Photo: David Hewitt)

By using a combination of temporal parentage assignment, movement patterns, and energetics, we were able to describe the mating system of white-tailed deer without relying on direct observations. Further, our studies partially debunked two myths associated with mating behavior of male white-tailed deer which highlight the value of testing hypotheses rather than relying on incomplete observations.

More Info:

DeYoung, R. W., Demarais, S., Gee, K. L., Honeycutt, R. L., Hellickson, M. W., & Gonzales, R. A. (2009). Molecular evaluation of the white-tailed deer (Odocoileus virginianus) mating system. Journal of Mammalogy, 90: 946-953.

Foley, A. M., R. W. DeYoung, D. G. Hewitt, M. W. Hellickson, K. L. Gee, D. B. Wester, M. A. Lockwood, and K. V. Miller. (2015). Purposeful wanderings: mate search strategies of male white-tailed deer. Journal of Mammalogy, 96: 279-286.

Foley, A. M., D. G. Hewitt, R. W. DeYoung, M. W. Hellickson, M. A. Lockwood, and M. J. Schnupp. (2018). Reproductive effort and success of males in scramble-competition polygyny: Evidence for trade-offs between foraging and mate-search. Journal of Animal Ecology, DOI: 10.1111/1365-2656.12893.

 

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